ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3...

162
PROCESSES IN BIOLOGICAL VISION: including, ELECTROCHEMISTRY OF THE NEURON This material is excerpted from the full β-version of the text. The final printed version will be more concise due to further editing and economical constraints. A Table of Contents and an index are located at the end of this paper. James T. Fulton Vision Concepts [email protected] April 30, 2017 Copyright 2004 James T. Fulton

Transcript of ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3...

Page 1: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

P R O C E S S E S I NBIOLOGICAL VISION:including, ELECTROCHEMISTRYOF THE NEURONThis material is excerpted from the full β-version of the text. The finalprinted version will be more concise due to further editing andeconomical constraints. A Table of Contents and an index are locatedat the end of this paper.

James T. FultonVision Concepts

[email protected]

April 30, 2017 Copyright 2004 James T. Fulton

Page 2: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 1

1Released: April 30, 20172Autrum, H. ed. (1979) Comparative Physiology and Evolution of Vision in Invertebrates: A. InvertebratePhotoreceptors. NY: Springer-Verlag3Bernhard, C. (1965) The Functional Organization of the Compound Eye. NY: Pergamon Press4Laughlin, S. (1981) Common principles for vertebrate and invertebrate visual systems. In Handbook ofSensory Physiology, Vol. VII/6B, “Comparative physiology and evolution of vision in invertebrates,” Autrum,H. ed. NY: Springer-Verlag pg. 2635Altshuler, D. et. al. Xxx Evol. Ecol. Res. vol. 3, p 7676Neumeyer, C. & Arnold, K. (1989) Tetrachromatic colour vision in goldfish and turtle. Xxx In

3 Description of the Retina1

3.1 Introduction

A major finding of this work is that the retinula of the Insecta and some Mollusca eyes do not employopsin as a substrate for the appropriate chromophores, which remain the rhodopsin()’s of the retininefamily (section 3.6). The retinines are deposited directly on the villi emanating from the dendrites of thesensory receptor neurons. This finding has wide ramifications relating to the conventional concepts of thesensory receptors of Chordata. The opsin of these sensory receptors is used to control the orientation of theretinine liquid crystalline chromophores. It does not participate in the transduction process.

Innumerable representations of the retina appear in the literature, at all different scales and using every imaginableartistic technique. Most of them do not show an absolute scale or even the coordinates of the retinal area beingdiscussed. They virtually never show the direction and optical quality of the incident radiation or the location of thefocal plane of the optical system. When these parameters are used as a foundation of discussion, many sketches,photographs and electron micrographs are found inadequate. Some become interpretable from a differentperspective. The purpose of this Chapter is to provide a more comprehensive view of the retina, including a morecoordinated description of the different zones of the retina, and a broader understanding of the top level architectureof the visual signaling system pertaining to the retina. Recently, good progress has been made in determining thedistribution of different chromophorically sensitive photoreceptors in the retina. However, the results are stilllargely statistical.

Initially, this chapter will examine all photosensitive surfaces that contribute to the visual capabilities of an animal. In this context, the group of retinula found in the eyes of Arthropoda will be considered a retina. This provides auseful element in the development of an overall framework for discussing the retina.

Traditionally, the eyes of animals have been separated into two categories, vertebrate and invertebrate. Completebooks have been written based on this distinction2. Bernhard has provided a comprehensive study of the compoundeye3. Laughlin has provided an extensive comparison between these two categories4. However, this use of adichotomy is constraining. As discussed briefly in Chapter 1, there are clearly three categories of eyes that are wellaligned with the conventional phylogenic tree. These are the compound eye, and its prototypical simple eye, ofArthropoda, the complex eye with a direct retina of Mollusca, and the complex eye with a reversed retina ofChordata. Evolution has led to incongruities within this classification but they are minor–and illustrative. Thiswork will employ the above trichotomy instead of the conventional dichotomy.

The above trichotomy provides a much better framework for interpreting both the detailed form of the types ofphotoreceptor cells found in eyes and the degree of structure in the retinas of these phyla.

The above trichotomy of eye types should not be confused with the spectral capability of eyes. The fundamentalarchitecture of biological vision is tetrachromatic. Provision is made in the photochemistry of vision to formphotoreceptors of four spectral types and many superfamilies and families of animals are tetrachromatic. It is onlywithin phyla that one finds trichromats and they are of two distinct types. Arthropoda are generally shortwavelength trichromats, employing photoreceptors sensitive to the ultraviolet, short and medium wavelength spectralranges. Chordata are generally tetrachromatic, except in the physically larger animals and sometimes as a result ofevolution to satisfy an ecological niche. Tetrachromats have photoreceptors sensitive to the ultraviolet, short,medium and long wavelength regions of the visible spectrum. Many birds5, fish and rodents are known to betetrachromatic. Tetrachromaticity is common among fish6. However, many species exhibit tetrachromatic vision as

Page 3: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

2 Processes in Biological Vision

7Franceschini, N. (1985) XXX8Bowmaker, J. & Kunz, Y. (1987) Ultraviolet receptors, tetrachromatic colour vision and retinal mosaics in thebroun trout (Salmo trutta): age-dependent changes. Vision Res. vol. 27, no. 12, pp. 2101-21089Kouyama, N. & Marshak, D. (1997) The topographical relationship between two neuronal mosaics in the shortwavelength-sensitive system of the primate retina. Visual Neuroscience, vol. 14, pp. 159-16710Chan, T. Goodchild, A. & Martin, P. (1997) The morphology and distribution of horizontal cells in the retinaof the new world monkey, the marmoset Callithrix jacchus. Visual Neuroscience, vol. 14, pp. 125-14011Dacey, D. Lee, B. Stafford, D. Pokorny, J. & Smith, V. (1996) Horizontal cells of the primate retina: conespecificity without spectral opponency. Science, vol. 271, pp. 656-659

juveniles but frequently change to long wavelength trichromats with age (or more fundamentally, as the lens groupgrows and thickens). The larger members of Chordata, particularly the larger terrestrial members such as Human,have lost their ultraviolet sensitivity due to the thickness of the lens group serving the eye. It is not clear that theretinas of all large Chordata have completely lost their ultraviolet capability through evolution. Humans exhibit atleast some ultraviolet capability when the lens group is removed. The large chordates are long wavelengthtrichromats, having photoreceptors sensitive to the short, medium and long wavelength portions of the visualspectrum.

The data available for Mollusca is inadequate to determine their general capability.

3.1.1 Background

3.1.1.1 Order and tetra-chromaticity in the photoreceptor arrays of the retinas

By examining the trichotomy of animal retinas, there is a clear trend with regard to the statistical order of the arraysof photoreceptors. There also appears to be an order with respect to these arrays in many animals as a function oftheir maturity.

Franceschini has provided a striking example of the orderliness of the retinal array in Arthropoda7. This orderlinesseven extends to the lamina behind the retina. The orderliness is clearly traceable to the prototypical simple eye. Theorderliness is reminiscent of that of a crystalline structure.

Bowmaker & Kunz have described a similar level of orderliness in the immature brown trout8. They noted that theorderliness appeared to fall with age and surmised that the ultraviolet photoreceptors tended to disappear from theretina. Their images were primarily of small areas of the retina. Their spectrograms of the four chromophores ofvision, although somewhat limited at the extreme wavelengths and plotted on a normalized linear ordinate, are inclose agreement with those predicted by this work.

Kouyama & Marshak studied the statistics of two mosaics of retinal neurons in the primate retina9. The areas werein the mid-periphery, typically six millimeters from the macaque fovea, and consisted of about 150 photoreceptorsand about twice as many signal processing neurons described as bipolar cells. Unfortunately, they did not employoptical stimuli to excite the arrays. They employed staining of what were believed to be photoreceptors and bipolarcells associated with the short wavelength spectral channels of vision. Therefore, the arrays were not directlycorrelated with the spectral performance of the visual system. Chan, et. al. provide both morphology and statisticson the distribution of horizontal cells of the 1st lateral matrix in new world monkeys, with some comparisons with themacaque monkey10. They provide a picture of one horizontal cell, at 6.9 millimeters from the fovea, with a neuriticarborization of less than 50 microns in diameter but an axon that is 958 microns long. This length approaches themaximum reported for neurons processing signals in the electrotonic (analog) domain.

Chan, et. al. also reference the paper by Dacey defining two distinct types of horizontal cells that appear tocorrespond to the P-channel and Q-channel horizontal cells of this work11. The H2 horizontal cells respond to shortwavelength stimulation. Both the H1 and H2 type cells respond to mid-wavelength stimulation. The H1 type cellsrespond to both mid and long wavelength stimulation but do not respond to short wavelength stimulation. The H1cells generate Q-channel chrominance signals and the H2 cells generate P-channel chrominance signals according tothe nomenclature of this work. Chan, et. al. confirm the polymorphism of color vision in the marmoset basedprimarily on their ability to get it to make color matches following training. They claim trichromatism in females ofthat family. However, they claim the other animals are dichromats because they did not respond.

Page 4: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 3

12Kaneko, A. (1979) Physiology of the retina. Ann. Rev. Neurosci., vol. 2, pp. 169-191

Kaneko provides a more concise description of several types of horizontal cells based on their spectral sensitivity12. He also notes the high degree of spatial summation associated with these cells and their genealogy traceable back tothe S-potentials of Svaetichin in 1953. He describes so-called L-type horizontal cells which hyperpolarize inresponse to any spectrum of light and would correspond to the bipolar cells forming the R-channel (luminance)signals of this work. He also describes a biphasic C-type cell that is hyperpolarized by short wavelength light anddepolarized by long wavelengths. Unfortunately, the paper was in the form of a review and he did not quantify“long” in this context. The C-type cell could correspond to the horizontal cells forming the P-channel chrominancesignal of this work. He also described a waveform previously described by Svaetichin and also by Tomita. This“triphasic” C-type cell was hyperpolarized by spectral lights at both ends of the spectrum and depolarized by theintermediate spectral region. He also discussed the fact that the “horizontal cells” occurred in multiple layers of theretina. The “external horizontal” cells connected to photoreceptors and corresponded to the 1st lateral matrix of thiswork. The intermediate horizontal cells are probably bipolar cells. The “internal horizontal” cells appear tocorrespond to the 2nd lateral matrix. This organization may account for the triphasic character of the later cells. Cells in this layer are predominantly concerned with the spatial summation of signals from earlier cells. Subtractionof P-channel and Q-channel signals could generate a triphasic output although the ultimate purpose was to providespatial integration of information. Kaneko also reported intermediate horizontal cells that were axon-less(amercine).

Kaneko hints at the time-dispersal processing of information, at least among the amercine cells of the 2nd lateralmatrix. He also discusses the “atypical” spike signals found by some investigators when probing the 2nd lateralmatrix. Finally, he notes that as of 1979 the nature of any chemical neurotransmitter associated with thephotorecpetors had not been identified. He posits that it is not glutamate because of the abundance of this materialthroughout the neural system. He notes that other investigators have been suggesting GABA because of the reportsthat horizontal cells take up this chemical from a bathing solution and can also synthesize GABA.

The Chan, et. al. paper also provides considerable statistical information on both the size of horizontal cells and theirarborization. It also addresses the ubiquitous question of why some horizontal cells have “axons” and others do not. Clearly there are a variety of types of horizontal cells. Some appear to have short axons, some have long axons andsome have an axon sharing an outer coating with neuritic structures over a portion of their length.

The introductory material in the above paper by Kouyama & Marshak repeats much questionable conventionalwisdom. It highlights another significant relationship between the retinas of animals, are all photoreceptor arraysformed from and considered part of the brain or are they part of the peripheral nervous system. Most texts considerthe chordate retina to be formed of a multilayer extension of the neural tissue of the central nervous system. However, the photoreceptors of Arthropoda are frequently described as more peripheral in nature with their axonsextending to contact the structurally remote lamina. Mollusca is also frequently shown as employing photoreceptorswith minimal local neural support prior to long axons extending to a separate structure that is generally consideredpart of the brain. On the other hand, the chordate retina is usually shown as a multilayer laminate with considerablesimilarity to the laminate found in the chordate brain. Whereas, Houyama & Marshak identify this laminate asconsisting of three layers, it can be considered as five layers by separating the inner nuclear layer into a 1st laterallayer, a bipolar layer and a 2nd lateral layer (see Section 2.6). This brings the number of layers more in line with theconventional view of the LGN and cortical structures. Note the 2nd lateral layer is far less well developed in primatesthan it is in felines and other hunting members of Chordata.

It is important to note the tetrachromatic capability, and polarization detection capability designed into the basicretina of vision. This capability is frequently limited through evolution based on the local environment. In the paperof Bowmaker & Kunz, the fish, Salmo trutta, a member of Chordata, has little need for an ultraviolet capability as itmatures and moves into deeper water that ultraviolet light does not penetrate. Furthermore, as it matures, thethickness of its lens and corneal cover become too thick to transmit ultraviolet light effectively. It is possible that theultraviolet photoreceptors would atrophy. However, recent data from humans shows that the ultraviolet sensitivephotoreceptors remain active in the retina through old age. It is only the lens that restricts ultraviolet vision inhumans (See Section 17.2.4).

When the retinas of the chordates are examined, the orderliness of the photoreceptor arrays is seen to be much lessthan in Arthropoda. There appears to be a definite fractal appearance to the arrays when an area of several hundredcells on a side is examined. Frequently there appears to be a hexagonal grouping on a local basis within the overallarray. There are also additional local arrangements associated with the development of the fovea and foveola insome animals. These arrangements, which frequently are not circularly symmetrical, also appear to be evolutionaryresponses to environmental conditions.

Page 5: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

4 Processes in Biological Vision

13Kageyama, G. & Wong-Riley, M. (1984) The histological localization of the cytochrome oxidase in the retinaand lateral geniculate nucleus of the ferret, cat, and monkey. J. Neuroscience. vol. 4, no. 10, pp. 2445-245914Benardete, E. & Kaplan, E. (1997) The receptive field of the primate P retinal ganglion cell. VisualNeuroscience, vol. 14, pp. 169-185 & 187-20515Gouras, P. (1991) The perception of color. Boca Raton, FL: CRC Press16Laughlin, S. (1981) Common principles for vertebrate and invertebrate visual systems. In Handbook ofsensory physiology, Vol VII/6B “Comparative physiology and evolution of vision in invertebrates,” Autrum,H. ed. NY: Springer-Verlag, pp. 263-267

As a result of these modifications, the orderliness of photoreceptor arrays in chordate retinas cannot be easilydescribed. Attempts at description of these retinas quickly enter the field of complex statistical notation. Examplesof the trend in these statistics are found in Kageyama & Wong-Riley13. That article also hints at the near completebreakdown in the correlation between the statistics of the photoreceptor arrays and subsequent neural arrays of thesignal processing stage. The extremely high degree of spatial signal summation performed by the bipolar cells andlateral cells of the 1st lateral matrix destroys any one-to-one correlations based on physical geometry in the signalpath of chordates except for the cells of the foveola. The size of these spatial receptive fields at the ganglion levelhas been studied by Benardete & Kaplan, again falling back on statistics to describe their findings14.

The Benardete & Kaplan papers also introduce another interesting topic. When performing center-surroundexperiments, they used two different stimulus configurations. In one case, the center and surround shared a commonborder. In another, there was a (uncontrolled?) gap between the two stimuli. They do not discuss the uncontrolledvariable related to tremor that may have led them to these configurations. They did however, paralyze the opticalsystem of the animals under test. Whether they paralyzed the tremor as well as the larger saccades, which havedifferent origins within the brain, was not discussed. Although they used a color CRT monitor, they did not describeany measures to control or quantify the spectral content of their stimuli. Their papers used the M & P pathwaydesignations. There was no discussion of the P-channel and Q-channels related to the chrominance signals definedin this work.

3.1.1.2 Comparison of retinas of different phyla

Most of the literature does not associate the photoreceptors of vision directly with brain tissue. In Arthropoda, theaxons of the photoreceptors are shown traveling to a distinctly separate structure, the lamina for additionalprocessing15. In the case of Mollusca, there is less data available. However, it appears that the photoreceptors aresupported locally by at most one or two layers of neurons before the signals travel to the structures associated withthe animals brain. Only further study will determine whether these retinas can be considered to be made by materialintimately associated with the brain of the animal. If they are, the neural pathways from the retinas would beproperly described as association fibers within the brain. Otherwise, they would be considered peripheral neuralpaths. The case is more clear for Chordata. The retinas are extensions of the surface of the brain and both the opticnerve and optic radiation are clearly classed as association fibers. There is a significant difference in the topology ofthe retinas and the rest of the brain. In most of the brain, the association fibers originate and terminate in layer 4. Placing these terminals in a middle level of the brain tissue provides maximum oppportunity for interconnection withboth decoding and encoding circuits. In the case of the retinas, there is no decoding requirement. The ganglion cellsconnecting to the association fibers are found in the final layer of the retina.

Laughlin has provided a broad though dated comparison of retinas, and other elements of vision, between vertebratesand invertebrates in an index to a volume devoted to invertebrate vision16. Use of this dichotomy does not presentthe data for Arthropoda and Mollusca in the proper context and leads to concepts and interpretations not adopted inthis work, such as multiple spatial images being passed along the signaling system. However, the coverage isextensive and much of the data is useful. On page 5 of the same work, Autrum discusses conforming thenomenclature used in the study of the retinas and brains of arthropods and molluscs to eliminate confusion.

Sections 3.5 & 3.6 have been added to this chapter for further discussion of the ommatidia of members of Molluscaand Insecta respectively. These sections focus on the morphology and function of these ommatidia. Little data hasbeen uncovered relating to the metabolism and potential regeneration of these ommatidia and their componentphotoreceptor neurons to date (2016). Section 3.6 has become quite large because of the surprising diversity of theelements of the visual modalities of the millions of Orders, SubOrders and Families of Insecta. Some of thisdiversity has called for a taxonomy of Insecta based specifically on features within the visual modalities of variousspecies (Section xxx).

Page 6: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 5

17Kaneko, A. (1979) Op. Cit. pg. 18518Stell, W, (1972) The morphological organization of the vertebrate retina. In Handbook of SensoryPhysiology. Vol. VII/2. Ed. Fuortes, M. NY: Springer-Verlag. Pp. 111-213

3.1.2 A framework for discussion of the chordate retina

The remainder of this Chapter will focus on the chordate retina.

In 1979, Kaneko made the bold statement that “The main neuronal circuitry of the retina has now been made clear,but most of our understanding still remains superficial17.” He did this after providing tentative answers to only eightof the twenty questions Stell had placed on the table in 197218. To broaden this understanding and combine it withthe morphology of the retina requires a more sophisticated framework. It is suggested that this can be done byexamining the retina from three major perspectives, a morphological plan view, a morphological profile view, and asignaling architecture view.

3.1.2.1 The plan view perspective of the retina

The retina is far from a symmetrical structure. When discussing the plan view of a retina, it is important to considera global view of the whole retina as well as local views that are referenced with respect to the coordinates of theglobal view. It is not adequate to specify that a recording is made at a distance from the fovea. The angle must alsobe specified.

It is also of interest to make recordings of the various layers of the retina after staining. It is critical that suchrecordings specify the same dimensions as mentioned above. Considerable effort should also be made to specify theabsolute or relative location of the imagery in terms of the cross-section of the retina from Bruch’s membrane orsome other surface near the retinal pigmented epithelium (RPE) that is relatively stable. Recording as a function ofthe depth from the vitreous humor makes the results highly location dependent.

There are two primary problems with describing the plan view of a retina. First, it is a spherical surface that isdifficult to represent using a two dimensional figure. It is also frequently more useful to represent the plan view withrespect to object space, as it is normally done in optometry. If it is presented with respect to object space, the correctindex of refraction associated with the normal object space environment should be used. Observing this precautioninsures the data represents the functional characteristics of the retina. Notice that the angles in object space aresignificantly different from those of the terrestrial chordate retina in image space due to the difference in index ofrefraction between the vitreous humor and air. Failing to maintain the appropriate indexes significantly distorts theangles relative to the retina with reference to the pupil as measured in object space. Calculating distances on theretina using incorrect angles is counterproductive.

3.1.2.1.1 A panoramic view of the photoreceptors of the retina

In many experiments attempting to record the plan view of a retina, there is an interesting option. A panoramicimage of the retina covering 180 degrees or more in object space can be recorded over a swath of only ten degrees orless. The resulting image will show the effective sizes of the Outer Segments of the photoreceptors throughout thefield of the retina. They will appear quite small near the foveola and quite large near the periphery even though allphotoreceptors in a given retina are nearly the same size (typically within a factor of two). The larger apparentdifference is due to the variable focal length of the physiological optical system of the animal.

3.1.2.1.2 Global recording of other layers of the retina

Techniques are available for recording layers of the retina other than the photoreceptors. Such recordings areusually performed in-vitro after dissection of the ocular, and frequently after slicing the retina to make it lay flat. These processes severely distort the actual geometry of the retina relative to its performance in object space. Theresulting recorded data should be carefully related to the equivalent object space data. Alternately, the retina shouldbe stained, the vitreous humor replaced and the physiological lens system replaced so that a panoramic recording canbe made as outlined above. When discussing the layers of the retina, it is suggested that the layer notation discussedin Chapter 2 be used to help codify the database, recognizing that the 2nd lateral layer is poorly developed in humanretinas and probably in all primates optimized for an arboreal environment.

Page 7: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

6 Processes in Biological Vision

19Hubel, D. (1988) Eye, brain, and vision. NY: Scientific American Library

3.1.2.1.3 Local recordings of the plan view of the retina

The literature is replete with local recordings of the retina. Some are made through the physiological optical systemand some are made after dissection. For the human, a terrestrial chordate, the angles discussed with respect to theserecordings are seldom specific and are frequently misstated or calculated erroneously. If the recording is madethrough the physiological optics, the angles refer to object space and not image space. As discussed above, thesurface dimensions on the retina must be calculated with respect to the image space angles.

3.1.2.2 The profile view perspective of the retina

It is seldom useful to attempt to record a global view of the retina in profile because it is so thin relative to its overalldimensions. However, it is very useful to make local recordings. These can show the orientation of the OuterSegments of the photoreceptors relative to the pupil of the eye as well as define the density of neurons in thedifferent layers of the retina laminate as a function of location relative to the optical axis, the fovea or otherlandmarks. It is very important to indicate specifically what landmark was used for reference and where the imagewas taken relative to that landmark in the spherical coordinates of the retina (with specific reference to object orimage space). Linear dimensions relative to the retina are usually inadequate in defining a location on the retina forpurposes of understanding function. As mentioned above, it is suggested that the layers of the laminate be defined interms of those in Chapter 2 to support and maintain a consistent database. It is also suggested that measurements bemade relative to Bruch’s membrane or some other feature near the RPE that is stable relative to the layers.

Measurements that can be precisely located by others can support detailed investigations into the signal architectureof the brain. This architecture is poorly understood at this time beyond the simplest level. This work has found thatthe system makes use of certain spatial relationships in the signal processing of the retina to avoid the need for thebrain to employ trigonometric calculations. It is fairly obvious that if more detailed information concerning theretina was known as a function of location, the precise mapping of this type of spatial layout could be recognized. There are other mappings that are currently believed likely but cannot be recognized with the limited precision of thedata available on a global context. It is this level of detail that is needed to help understand the extensivearborization associated with the neurons of different layers of the retina. In many cases, this knowledge may notlead to defining sublayers of the retina. It may however lead to the description of multiple maps that representdifferent groups of neurons within a single layer.

3.1.2.3 The signaling architecture of the retina

The literature presents a pretty clear picture of the gross functional responsibilities of each major layer of thechordate retina. There are some differences in emphasis between the various major families (humans have a poorlydeveloped 2nd lateral matrix layer). It is the opportunity to extend the functional subdivision of these responsibilitieswhich calls for the type of spatial accuracy in data recording discussed in the above sections. As these studiesproceed, it becomes ever more important to observe and update the best available signaling architecture diagramsand detailed signaling schematics of the retina. It also becomes extremely important to recognize that the signalprocessing architecture employs both temporal, chromatic and spatial processing–frequently within the same layer ofthe retina. The so-called tri-phasic waveforms of Svaetichin and of Tomita may be examples of this. These havebeen recorded based on the spectral characteristics of large area stimuli. It has not been shown that these waveformsare considered spectrally important by the brain. They might actually be of significant spatial importance instead. Inthe absence of any broad band (black and white) photoreceptors in the animal retina, the signal processing must takeadvantage of the original spectral signals from the photoreceptors. If these signals are differenced, either in the 1st or2nd lateral matrices, and then further processed without concern for their spectral properties, spatial pre-emphasis ofimportant elements of the scene in object space can be obtained. This is very likely to be the situation found in bigand small felines. They are known to generate or extract signals in the cortex that show both angular preference andselectivity to the pitch of gratings at a particular orientation19.

Laboratory activity in the next few years should show great progress in understanding the still mind numbingcomplexity in the architecture of the retina of Chordata. However, a greater degree of precision in locating therelevant groups of neurons within a given layer of the retina will be required.

3.1.2.3.1 Expansion of spatial processing within the Top Level Schematic

Page 8: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 7

20Hubel, D. (1988) Op. Cit.21Kaneko,, A. & Tachibana, M. (1983a) Double color-opponent receptive fields of carp bipolar cells. VisionRes. vol. 23, pp. 381-38822Kaneko,, A. & Tachibana, M. (1983b) Double color-opponent receptive fields of carp bipolar cells. VisionRes. vol. 23, pp. 371-38023Dacey, D. Lee, B. Stafford, D. Pokorny, J. & Smith, V. (1996) Horizontal cells of the primate retina: conespecificity without spectral opponency. Science, vol. 271, pp. 656-65924Kaneko, A. & Tachibana, M. (1983a) Op. Cit.

Although Hubel, et. al20. and others have made strides in recording neural activity in the cortex of felines reflecting aresponse to spatial orientation and complexity within a scene, it is unlikely, based on the Top Level Architecture ofthe visual system that the processing associated with these responses originates in the cortex or LGN. It most likelyoriginates in what has been labeled the 2nd lateral matrix of this work. This matrix probably generates a series ofsignals, of both monophase and biphase character, that can be transmitted in compact form over the optic nervebefore being processed further by the engines of the old brain and the Cortex. As discussed in the sections on theVisual Block Diagram, this spatial encoding has not been explicitly presented in this work because of the limiteddata available concerning its specific architecture and signaling paths. These paths, carrying primarily correlatedspatial information, have been assigned the Z-channel designation (Z1, Z2, Z3, etc.) at this time pending furtherspecificity. Eventually, it is hoped that these channels can be correlated with the investigations of the above authors. The signals of the Z-channels are assumed to originate in the 2nd lateral matrix. However, the processing matrices ofthe retina have been shown as three distinct entities primarily for pedagogical reasons. As hinted at above, there isno guarantee that these matrices are discreet with respect to the layers of the retina. They may involve multiplegroups of neurons within one or more layers. It is not even clear yet whether this type of processing is concentratedin the various regions associated with the foveola, the periphery or both.

3.1.2.3.2 Subdivision of retinal layers or interdigitation

Up through the 1970's, the chordate retina was generally considered to consist of three neural layers. The interneurallayer, INL, was frequently described as the bipolar layer. With a total thickness of only about 50-60 microns, it wasvery difficult to subdivide this layer based on electrophysiological probing. More recently, finer probes and moreprecise positioning mechanisms have become available. Accompanied by a variety of staining techniques, theseimprovements have led to recognition of additional regions within the INL. Currently, this layer is seen to consist ofthree sublayers of neural cells,

+ the 1st lateral matrix layer consisting of what are generally labeled as horizontal cells and appear to be primarilyconcerned with generating chrominance signals,

+ the bipolar layer consisting primarily of bipolar cells concerned with the generation of luminance signals, and

+ the 2nd lateral matrix layer consisting of a variety of cells grouped around the designation Amacrine cells. Thislayer is poorly developed in the higher primates but appears to be primarily concerned with the generation ofappearance signals.

Beyond the above coarse subdivision of the signal processing layers of the retina, each of these sublayers exhibits avariety of functional cell types. The literature has suffered from a lack of a model in this area with many authorsproviding glimpses of the signals found in one sublayer based on the probing of a small number of samples. Someof the papers providing the best signal characteristics have assumed the simplest possible architecture21. Kaneko &Tachibana open their paper with the presumptive statement that “bipolar cells provide the only pathway fromphotoreceptor cells to ganglion cells.” Their analyses do not provide for or address any other type of cell. However,much of their data can be interpreted as involving the horizontal cells of another paper of theirs22 and those of manyother authors23. They do stress that the first paper deals specifically with “bipolar cells” which have highlycomplicated receptive fields. This condition typifies the problem of sorting out the wide variety of functional celltypes within these regions.

As an example of the difficulty of drawing conclusions concerning the architecture of the individual layers (orsublayers) of the signal processing region of the retina, Kaneko & Tachibana24 studied the receptive fields of 85“bipolar cells.” Parentheses appear around this term because their article did not recognize the existence of anyother type of signal processing cells nor did they precisely define what they meant by this designation. They did notdefine the location of their probe with respect to the retina and any fovea if present. They found that only one

Page 9: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

8 Processes in Biological Vision

quarter of these cells exhibited “double opponent receptive fields. Of these 15 represented “on-center cells” andthree represented “off-center cells.”

Kaneko & Tahibana presented interesting data, in the same paper, describing the performance of the off-centerdouble opponent cells as a function of stimulus spot size. This data clearly showed that the cells were involved in aspatial integration processing role. No data was offered as to whether the cells studied provided signals, eithersimultaneously or exclusively, to the chrominance or the appearance channels of the visual system. See Section3.4.2.2 for details.

3.1.2.3.3 Initial tabulation of signal processing roles within the retina

Although the majority of the interconnections between the photoreceptors of the signal detection stage and the initialneurons of the signal processing stage are pretty well understood, it will be a long time before the interconnectionsbetween the initial signal processing neurons and the ganglion cells is understood to any level of detail. Thissituation is exacerbated by several conditions. First, we do not have a clear conceptual understanding of the roles oftime, space and wavelength in the algorithms used in signal processing within the retina. Second, we do not know ifthe same algorithms are used in the foveola, the fovea and the periphery of the retina. It appears that the algorithmsvary significantly. Third, the input structures of the ganglion cells appear to form part of the signal processing stageof vision. These structures clearly perform a significant signal integration role prior to encoding.

Whereas it is easy to record signals as a function of spectral wavelength, it is much more difficult to record signalswith fine precision as a function of spatial location. Most spatial location data involve the integration of data fromretinal electrophysical probing in response to finite size (gross) spectral stimuli. On the other hand most attempts toobtain finer data have employed gratings or sharp edges in psychophysical, not electrophysical, experiments. Therehas been very little activity attempting to correlate the results of the probing of the retina with the form of the signalsactually transmitted to and perceived by the cortex. A probe signal that exhibits spectral characteristics but isprocessed by the signal projection stage as appearance information is not perceived in the same context as thelaboratory investigator might presume.

Because of the above problems, the literature contains a great deal of data employing a great variety of notation,much of it based on ordinal numbers, type 1, type 2, H1, H2, etc. or on other convenient designations. Correlatingthese designations over a period of 50 years, based on the limited specificity in the published articles, becomes quitedifficult. However, it is possible to discern classes of functional signal processing within the retina based on theliterature. These will be grouped in TABLE 3.1.2-1 for purposes of discussion.

Page 10: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 9

TABLE 3.1.2-1FUNCTIONAL CLASSES OF SIGNAL PROCESSING IN THE RETINA OF CHORDATA

OTHER THAN IN THE FOVEOLA AND WITHOUT CONSIDERATION OF TIME-DISPERSAL

Predominant layer Major Predominant % of Function(Designation in vision) role cell type layer

SIGNAL DETECTIONI. Molecular Spectral sig. gen. Photoreceptor 100 Transduce light into electricity and(Photoreceptor) normalize the signal amplitudes

SIGNAL PROCESSINGII. External granular Chrom. sig. gen. Horizontal High Compose the N, O, P & Q signals as (1st Lateral matrix) appropriate (typically associated with “H1”

and “H2” cells)

III. Pyramidal Lumin. sig. gen. Bipolar ? Compose the monophase R signal(Bipolar)

? Relay biphase signals from the 1st lateralmatrix to the ganglion cells*

IV. Internal granular Appear. sig. gen. Horizontal. ? Compose the currently poorly defined(2nd Lateral matrix) (Amercine) Zn signals describing specific spatial

properties (static and dynamic) of the scene

25** Perform spatial signal integration for as yetpoorly understood purposes

? Compose the “threat location” signals fortransmission to the Precision Optical Systemof the mid-brain***

SIGNAL PROJECTIONV. Ganglionic Encoding of Ganglion Threshold and encode monophase (Ganglion) analog signals signals from all sources

Encode biphase signals from all sources.

* In general, literary references to biphase or inverting bipolar cells are considered lateral cells in this table. There isan open question concerning whether some or all axons from the 1st lateral layer connect directly to ganglion cells. An alternate situation would require some bipolar cells to be biased for the transmission of biphase signals from the1st lateral layer to the ganglion cells.** The percentage is taken from Kaneko & Tachibana (1983b)*** Threat location signals describe the angular distance of a threat from the line of fixation in terms of the axes ofthe ocular muscle pairs.

Page 11: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

10 Processes in Biological Vision

25Flitcroft, D. (1985) The interactions between chromatic aberration, defocus and stimulus chromaticity. VisionRes. vol. 29, no. 3, pp. 349-36026DeVries, S. & Baylor, D. (1995) An alternative pathway for signal flow from rod photoreceptors to ganglioncells in mammalian retina. Proc. Natl. Acad. Sci. USA, vol. 92, pp. 10658-10662

3.1.3 Additional concerns in experiment design

All of the experimental effort to date associated with the organization of the retina has been exploratory. Lacking adetailed model with which to work, most of it has involved less than rigorous experimental design. The result hasbeen a large amount of data collected in the presence of uncontrolled variables. Under these conditions, it isunderstandable that most of the published data is statistical in character.

As Flitcroft has noted, “Several studies of the colour coding in cells of the primary visual cortex in primates havedescribed cells that have double opponent receptive fields (with references)25.” This condition is not limited to theprimates and does not originate in the cortex. The earliest cells of the signal processing stage exhibit both achromatic opponent and a spatial opponent characteristic that are not normally separated into independent variableswithin the experimental design. Beyond this dual character is the additional factor of timing, both with regard to thetime dispersal of signals due to their point of origination in the retina and to the impact of tremor on the conversionof spatial information into temporal signals. In many animals, there is also the parameter of polarization of theincident radiation to contend with. To understand the operation of the signaling system beginning at the retina, it ismandatory that these variables be recognized as independent and treated accordingly during experiments. Flitcroft’sanalyses all assume a linear visual system. This assumption essentially limits his analyses to small signal conditions.

3.1.3.1 Lack of a detailed model

DeVries & Baylor typified the experimental design problem when they opened their paper26. They began “Rodsignals in the mammalian retina are thought to. . . .” and then continued with: “A possible alternative pathwayinvolves. . . .” [underlines added] Their position is not unique. Without a comprehensive view of a large volume ofthe literature, it is difficult to define the signaling architecture of the retina. After such a study, it is clear that verymeticulous experimental design and parameter control is required. The model of this work can aid in this respect. DeVries & Baylor conclude with “evidence that an alternative pathway transmits signals to ganglion cells in parallelwith the classical RDB (rod-depolarizing bipolar) pathway.” That finding is compatible with the model of this work. However, it appears that a detailed understanding of the circuitry of the retina may still be beyond the state-of-theare. This is primarily because of our lack of a complete understanding of the fundamental signaling architecture ofthe visual system

3.1.3.2 Lack of consistent control of the motion of the eyes

The experimental literature provides a variety of experimental conditions relative to control of the eyes under test. In general, the procedures do not address the subject of tremor directly. In some experiments, the eye motions arehalted by drugs. However, there was no discussion of what motions were halted and what motions, if any, remain. Needless to say, tremor plays a large part in understanding the operation of the retina with respect to edges betweenvarious center and surround stimuli.

3.1.3.3 Lack of precise control of stimuli

A large amount of the experimental work has involved the use of wide spectral band stimuli that have not beencorrelated with the absorption characteristics of the chromophores of vision. Under these conditions, the signalsrecorded from the neural layers of the retina are hopelessly compromised before data reduction is begun. It hasbecome possible to obtain more discreet data in recent years with the use of tricolor cathode ray displays. However,many investigators have defined colors for their experiments that do not limit the color of the display to that of onlyone phosphor at a time.

Most experiments do not describe the edges of the stimulus fields. Without knowing the sharpness of the edgesinvolved, it is difficult to completely interpret the electrophysiological signals generated.

Page 12: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 11

27Wassle, H. & Boycott, B. (1991) Functional Architecture of the Mammalian Retina. Physiol. Rev. vol. 71no. 2 pp. 447-480

3.1.3.4 Importance of controlling the stimuli orientation

It has only recently been found that the direction of the light falling on photoreceptor cells is a critically importantparameter in signal generation. These cells exhibit two separate absorption spectra. The isotropic spectrum is due tothe absorption of the chromophores by an intrinsic non-functional mechanism. It always exhibits a peak absorptionnear 500nm regardless of the type of chromophore involved. A an-isotropic spectrum is realized when light isapplied to the cells along the long axis of the Outer Segments. This is the functional spectrum that differs in peakabsorption depending on the chromophore present. This is the explanation why observations made through the pupilof the eye give different characteristics for the individual photoreceptors than do experiments involving transverseillumination of individual cells.

The absorption characteristics of the photoreceptor cells are also affected by the F/# of the illumination applied tothem. The spectral differentiation of cells in the retinal mosaic becomes poorer if the pupil is dilated and is almostentirely lost if the ocular is dissected to allow flood illumination to be applied to the retina.

3.1.4 Matters specific to the organizational structure of the chordate retina

3.1.4.1 Matters of scale

The language used in the literature to describe the retina is quite broad ranging. This reflects the broad range ofbackgrounds of the investigators. When one attempts to provide a review of the overall field, the problem ofterminology is a difficult one. The comprehensive review by Wassle & Boycott27, although already dated, is bothan excellent source and an excellent example of this problem. It summarizes the three sets of names used to define“two” classes of ganglion cells of the macaque retina (pg. 449). It includes over 400 references. However, many ofthe figures are caricatures. To provide a comprehensive discussion of the retina requires both a comprehensive andconsistent vocabulary of independent discrete terms.

A first order example of the problem involves the terms, anatomy, morphology, histology and cytology. These termsrelate to the structure and shape of bodily parts. However, they apply at different levels of scale. The problem iscomplicated by the fact that animals vary so widely in scale. In this work, anatomy will relate to both morphologyand physiology at the visual system level. Histology will apply to the morphology and physiology at the lightmicroscope level. Cytology will encompass morphology and physiology at the electron microscope level. Cytologygenerally relies on the electron microscope for both adequate definition and magnification. Topography will applyto the external features of an element at any of the above levels.

Many investigators have tried to select especially large samples of a given cell type from among a group of animalsof a given scale. This procedure makes many laboratory procedures easier. However, one should keep in mind thata “larger” than average sample is necessarily an atypical sample. The investigator should be aware of theconsequences of this fact and make note of it in his dissertation.

Except for the two-dimensional extent of the complete retina, it is best described in terms of histology. To study theindividual cells of the retina, the coarser surface features, i.e., topography, can be described at the histological level. The finer surface features, and questions about the actual formation, structure and function of a cell require theelectron microscope and thus fall within the purview of cytology.

3.1.4.2 Fovea versus other terms

It serves no purpose to have different names for the area of most acute angular vision in the phylum Chordata. Thishas arisen where investigators have chosen arbitrarily to adopt a new element name rather than consult the literature. The most general definition of the word fovea does not specify its detailed shape, only that it involves a smalldepression. Prince has used the word fovea to describe a variety of features related to high acuity vision, usingadditional adjectives where needed. This is most appropriate. Reference to a “central area” in cats should bereplaced by fovea or fovea centralis. References to a “visual streak” in rabbits should similarly be replaced by foveaor elongated fovea. These terms would then be consistent with double fovea and other specific descriptors. Thefovea is seen to be a feature:

+ with a shape that is highly variable and species specific, possibly even family specific.+ with a location that can be defined in terms of the optical axes of the eye. This location is alsospecies specific.

Page 13: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

12 Processes in Biological Vision

With the term fovea applied to all members of the chordate phylum, the terms parafovea, perifovea, foveola, etc.take on their normal meanings independent of species or family. In this work, the inner zone of the fovea, known asthe foveola, takes on a more important role than previously. The photoreceptors of this zone enjoy a unique signalpath to the cortex.

3.1.4.3 Amercine cells

The designation of a lateral cell in the neural layer of the retina by the Greek descriptor, amercine (without an axon),based on its apparent topography was unfortunate. It left the audience with the impression that the cell had nofunctional axon. The fact that the axon and at least part of the dendritic structures shared a common external cellwall was not recognized for a very long time. Recently, synapses believed to be associated with an axon of anamercine cell have been discussed. This work will present the cytology and the functional characteristics of theamercine cell that should clarify this descriptor generated difficulty. There has been a tendency to describe a layerof the retina in terms of the label amercine. This is very unfortunate. It is important to note that many types ofneurons and many types of “amercine” cells have now been identified in this putative layer. Many have fullyarborized neuritic structures and identifiable axons.

3.1.4.4 Matters of architecture

The retina exhibits two separate and distinct architectural profiles that can be related to its plan and profile views. Each of these should be addressed individually in order to understand how the resultant neural signals are created fortransmission to the brain. As noted in Chapter 2, one of the features of this architecture is the introduction of time-dispersal into the signaling algorithm. The second is the introduction of tremor to allow the fundamental changedetection architecture of the photoreceptor cells to be used in an imaging mode. These elements of system designcan have a large impact on center-surround studies using large diameter stimuli. It also suggests that most signalrecording should be done in the S-plan of the retina before the data is applied to the threshold circuits of the ganglioncells. Finally, much of the information concerning the stimuli is carried in the time domain and good temporaldiscrimination must be designed into the test set.

In recent times, several investigators have extended the definition of the parvocellular and magnocellular pathwaysbetween the LGN and area 17 of the cortex to include the paths from the retina to the LGN. Unfortunately, thisnomenclature obscures the fact that the parvocellular paths include more than one class of signals. These P-channeland Q-channel signals are not only different, they are fundamentally orthogonal to each other. By definition, theyare not antagonistic to each other. In fact, in one quadrant of color space, they parallel each other. Good temporaland chromatic control of the stimuli, along with adequate temporal discrimination in the test set will demonstratethese facts.

3.1.5 Matters of photoreceptor cell classification

One can track the problems of classification as a function of time in the literature for many features in vision. As thestate of the art moved forward, definitions changed and were refined. This problem has been particularly chronicregarding the photoreceptors of the eye. It occurs to a lesser extent in the recent discussions regarding the amercinecells of the retina. Here, the surface features (topography) were used to describe both the morphology (shape andstructure) and the functional elements of the cell. The results have been less than adequate. Many investigatorshave extolled the value of morphology in explaining the physiology of parts of the eye, i.e., shape explainingfunction. At the present level of understanding, this method of inference has become a trap. Without a bettertheoretical base, using morphological features to infer physiological function at the level of cytology is very difficult. This is particularly true where the signaling function in vision has not been defined precisely.

3.1.5.1 Background--Rods and cones

Because the literature of the retina contains so many references to them, having a definition of what is a rod andwhat is a cone would be very useful later. This is especially true because so few precise definitions of a rod or coneappear in the literature. Similarly, there are few places that illustrate the range of photoreceptor cells (and possiblynon-photoreceptor cells) in the retina.

The following discussion is needlessly long but is presented in the interest of completeness. It willbe shown later that there is no functional (signaling) distinction between the morphologicaldesignations “rods” and “cones.” In fact there is no morphological distinction between the

Page 14: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 13

28Walls, G. (1942) The Vertebrate Eye. Bloomfield Hills, MI: Cranbrook Institute of Science29Detwiler, S. (1943) Vertebrate photoreceptors. NY: Macmillan pg. 4530Young, R. (1971) Hypothesis to account for a basic distinction between rods and cones. Vision Res. vol. 11,pp 1-531Rodieck, R. (1973) The vertebrate retina. San Francisco, CA: W. H. Freeman pp. 9-1032Crescitelli, F. (1972) A paradigm: the receptors and visual pigments of an Anuran. In Photochemistry ofVision. Vol. VII/1 Dartnall, H. ed. NY: Springer-Verlag pg. 25833Hart, W. (1992) Adler’s Physiology of the eye. St. Louis, MO: Mosby Year Book. pg. 58634Ebrey, T. & Koutalos, Y. (2001) Vertebrate Photoreceptors Prog Ret Eye Res vol 20(1), pp 49-9435Walkey, H. Barbur, J. Harlow, J. & Makous, W. (2001) Measurements of Chromatic Sensitivity in theMesopic Range Color Res Appl Suppl vol 26, pp S36-S42

inadequately defined concept of rods and cones.

More specifically, the designation “rod” is a euphemism for a putative photoreceptor exhibitingthe spectral sensitivity associated with scotopic vision. This spectral sensitivity is actually thelogarithmic summation of the S–channel and M–channel photoreceptors.

Only a few serious attempts to provide descriptions of rods and cones could be found by the author. Some of thebetter attempts include Walls28, Detwiler29, Young30, Rodieck31, Crescitelli32, and Adler33. Many of these authors tryto merge the morphology of the cells (or various parts of the cells) with the habitation of the animal to support aclassification duality. Summarizing their discussion is impossible because there is no common thread. The penchantof man to employ a dichotomy is the only idea that appears throughout the discussion.

Ebrey & Koutalos made an extensive effort to identify rods and cones in 2001 without success34. They reviewed theearly history of the concept and the evolution of the “Duplex Theory” of human vision. They noted, “There are atleast four points of view one can take in order to classify or group the vertebrate photoreceptors: (i) their visual pig-ments; (ii) the enzymes and other proteins associated with the phototransduction apparatus; (iii) their anatomy,structure, and topology, at both the light and electron microscopic level; and (iv) their electrophysiology. In addition,one could add to this list their psychophysics, which, unlike the first four must take into account not only thephotoreceptors themselves, but also processing at the post-receptoral and central levels of the nervous system.” Their analyses concentrated on the earlier writings on chemical and genetic processes. They did not resolve whichof the above categories correctly described the putative differences between rods and cones.

The emergence of the ability to resolve individual photoreceptors in the topography of the retina using OCTtechnology, specifically in the form of AOSLO, the demise of the concept of a rod is now confirmed. As noted inSection3.2.3.2, via a private communications, no rods were identified beginning in 1999. With the recent advancesin tremor compensation AOSLO, the absence of any rods in the retina of the human eye is now confirmed. Theintellectual concept of an achromatic and a chromatic set of photoreceptors, the rods and cones, is now totallyarchaic.

The ancient and totally archaic Duplex Theory of vision, with rods responding to low light levels andcones responding to high light levels, has been completely falsified and should be purged from alltextbooks in a timely manner. The Duplex Theory constitutes a blight on the teaching of biologicalscience.

Walkey et al. have recently discussed the concept of “rod intrusion” in spectral measurements at low light levels35. Their results were generally inconclusive, except to agree with one proposition of this work. “The results of thisstudy suggest that chromatic and luminance contrast signals are also processed separately in the mesopic range.”

3.1.5.1.1 The clinical attempt of Shultze to identify rods in 1860-70

Walls provided a good summary in 1942 starting from the work of Schultze in 1866. Schultze had worked incomparative ocular histology for more than 15 years and propagated a conclusion based on this work. He noted thatnocturnal vertebrates had a preponderance of “rods” over “cones” (or no cones at all); and that diurnal species hadmany “cones,” and might even lack “rods” entirely. He thus suggested the “rod” is the organ of scotopic (dim-light)vision and the “cone” is the receptor for photopic (bright-light) vision. Schultze then added a corollary that the“cone” alone is responsible for color vision; for in dim light colors are no longer discriminable and the world

Page 15: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

14 Processes in Biological Vision

36Fine, B. & Yanoff, M. (1972) Ocular Histology, A Text and Atlas. New York: Harper & Row

presents itself only in shades of gray.

Rodieck provided a different summary. According to Rodieck, Schultze originally described the retina as containingtwo types of photoreceptors in 1866. His definition was a static one. It described rods in the peripheral retina asexhibiting a cylindrical Outer Segment and a thin Inner Segment. Cones were described as having a tapered OuterSegment and a thicker Inner Segment. This view has been challenged often. Schultze also originated therelationship between the morphology of photoreceptors and their sensitivity. He found the retinas of nocturnalanimals had retinas virtually free of cones. He therefore proposed, based on no other evidence or data, that the rodssubserve vision in dim light and the cones subserve vision in bright light. It is not clear whether this definitionshould be restricted to the peripheral cells but it is the foundation of the Duplicity Theory of animal vision.

Schultze was wrong! He neither knew, or claimed to know, the dynamic range of the cells he was discussing. Thedynamic range of individual photoreceptors, in collaboration with the iris, is approximately 50,000:1. This rangenegates the need for two separate sensory channels based on illumination level.

3.1.5.1.2 Attempts at redefining the retina during the 1940-70s

As technology improved, particularly in light and electron microscopy, a whole series of new attempts at definingputative differences between rods and cones were made. Most of these relied upon morphology but additionalattempts were made to tie the morphological features to the observed psycho-physical features. Little attempt wasmade to tie any morphological features to physiological features.

Walls went on to express his own views: “The visual cells of vertebrates . . . were long ago given the names ‘rod”and ‘cone’--though with our superior modern knowledge of their phylogenetic ramifications and physiologicalcharacteristics we might wish that a more apt pair of names could be substituted for the traditional ones. In a givenretina containing both highly sensitive visual cells (rods) and relatively insensitive ones (cones), the high- and low-threshold cells can always be told apart; but the rod on one retina may resemble structurally the cone of another, ormay give evidence of having been recently derived from a cone-type in an ancestor of different habits.” Walls gaveno explicit statement about how they were told apart but he did develop several themes: “One of the mostnoteworthy peculiarities which cones may have is that presented by the cones of the greater portion of the humanretina, and also by some other placental mammals: the cone outer segment is a cylinder enclosed by a tubular processof the pigment epithelial cell opposite to it . . . No such arrangement is ever seen in rods. . . .” Unfortunately, Fine& Yanoff36 seem to take a contrary view when speaking about foveal cones: “Their relation to the pigmentepithelium here resembles that of rods elsewhere (i.e., they reach to the surface of the pigment cell and areenveloped by extremely delicate apical villi of the pigment epithelial cells).”

Page 16: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 15

37Dowling, J. (1965) Foveal receptors of the monkey retina: fine structure. Science. vol 147, pp. 57-5938Walraven, P. (1962) On the mechanisms of colour vision. Soesterberg, Netherlands: Institute for Perception.39Hogan, M. Alvarado, J & Weddell, J. (1971) Histology of the Human Eye, An Atlas and Text.Philadelphia: W. B. Saunders40Rodieck, R. (1973) The Vertebrate Retina, Principles of Structure and Function. San Francisco: W. H.Freeman41Ohman, P. (1975) Fine structure of photoreceptors and associated neurons in the retina of lampetra fluviatilis(cyclostomi). Vision Res. vol. 16, pp 659-66242Stell, W. & Harosi, F. (1976) Cone structure and visual pigment content in the retina of the goldfish. VisionRes. vol. 16, pp. 647-657

Fine & Yanoff expanded on their above words in an attempted to redefine the differences between rods and cones inman: “The foveal cones resemble the rods superficially. The foveal cones possess all the cytologic characteristics ofcones elsewhere except for their shape. Their outer segments are cylindrical and elongated like rods but possesslamella that are typical of cones. The lamella [as used in discussing photoreceptors] are of the ‘tight’ or closelyapposed type, with direct connections to the surface plasma membrane. Longitudinal furrows are absent. Theirsynaptic expansions are all typical broad cone ‘feet’ containing the characteristic multiple synaptic lamella [as usedin neurology].

The last two references only agree on two aspects. First, the outer segments of both cones and rods are cylindrical. Wald (1965) stressed this fact when he said “It has long been recognized that the outer segments of foveal cones arerod-shaped; they are attenuated cylinders about 50 microns long and about 1-2 microns wide.” Dowling wrote withconsternation in 1965 that cones were clearly rod shaped in the fovea37. He provided a side by side comparison of a“rod” and a “cone” from the same retina and provided many relevant dimensions. No structural difference can beseen in his pictures and there is no indication of where along the Outer Segment the pictures were taken. Dimensional differences were too small to be quantified at 162,000x. Second, the feet of rods and cones seemdifferent. However, they are careful to suggest there is some overlap in this area, some cones are occasionally seento synapse with a “summing” type of bipolar cell.

At the same time, Walraven and many others in the psychophysical community were dismissing the concept of a rodaltogether based on their experiments38. They generally defend block diagrams containing only three types ofspectrally sensitive photoreceptors and show the luminance response being calculated from the individual spectralresponses. No achromatic, broadband photoreceptor is required.

Note that two other large texts in histology fail to define and/or differentiate the structural features of rods and cones. Neither Hogan et. al.39 in 1971 or Rodieck40 in 1973 are explicit in this area. In 1975, Ohman was unable to definethis difference in respect to the river lamprey and finally concludes with: “Ultrastructurally both long and shortreceptors show characters that favour the impression of rods41.”

Stell and Harosi42 performed a comprehensive study of the goldfish. Following extensive morphological andelectrophysiological examinations, they described the structural features of both putative rods and cones but thenwere unable to find a single photoreceptor exhibiting a broadband spectral response. Instead, their microspectrophotometer measurements found all of the photoreceptors fell into one of three spectral bands with peaks near455, 530 and 625 nm.

In addition, Walls said: “The stalk-like portion of the inner segment [of cones] is highly contractile and hence iscalled the myoid (=muscle-like).” He then went on to illustrate six rods and labeled the myoid structure in three ofthe six. More specifically, he described the rod labeled d as the “common or ‘red’ (rhodopsin-containing) rod of theleopard frog; dark-adapted (i.e., with myoid contracted).” Next to it was shown the rod labeled e, “‘green’(Schwalbe’s) rod [of the leopard frog].” This contractile function may be important to note; it could account for thevarious body shapes of what are frequently reported to be cones. Is it possible, their body shape changes significantly in aspect ratio over a short time like other muscle tissue? Could their shape relate to how tense they arein holding the outer segments in alignment or position during or after phagocytosis?

Bernard (1900) and Cameron (1905) took a dynamic view of the photoreceptors. They believed they saw significantchanges in the cells with time. They proposed that cones were developmental stages in the formation of maturevisual cells and that there is no such thing as duplicity as proposed by Schultze. This position was reviewed andexpanded by Muntz in 1964. Based on more recent experiments measuring the rate of movement of individual disks

Page 17: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

16 Processes in Biological Vision

43Young, R. (1971) An hypothesis to account for a basic distinction between rods and cones. Vision Res. vol.11, pp 1-544Young, R. (1971) An hypothesis to account for a basic definition between rods and cones. Vision Res. vol.11, pp 1-545Weale, R. (1974) The Eye. Vol. 6 Davson & Graham ed. NY: Academic Press pg. 7646Crescitelli, F. (1972) in Photochemistry of vision, vol. VII/1 NY: Springer-Verlag pg. 258

toward the RPE of about 7.2 microns/day, it is clear that an Outer Segment of conical shape is unsustainable in aliving retina.

It is noteworthy that after more than 100 years, Young was moved in 1971 to offer a new hypothesis to separaterods from cones43. Young addressed the morphological difference between rods and cones from a morphogenicperspective only44. The use of the term “a” in his title is provocative. Using nuclear chemistry, he showed evidencethat the cells he described as cones did not continually generate new disks as did the cells he described as rods. Further, the cones did not participate in the phagocytosis process since they did not reach the area of the RPE cells. He also addressed the fact that immature rods exhibited a conical outer segment until they grew to the point wherephagocytosis removed the smaller diameter disks. Young did not address any physiological issues such as thespectral response of the cells he enumerated. His recommendation to retain the designation rods and cones anddefine cones as possessing a conical outer segment because of the large literature can only be relevant for themorphological community. He did not review whether these “cones” were electrophysiologically functional. Hislast paragraph suggests they might not be.

Weale45 gave the status of morphological cones a different twist while referencing Young, “Cones being conical,presumably because of their arrested development in most species, the mechanical problem is different from thatbelieved to exist in rods.” It appears that morphological cones are not a temporally stable physiological componentof a healthy visual system, at least in humans.

Ohman also addressed the question of phagocytosis in a species at the bottom of the vertebrate family. He found thesame dynamic situation. “It is known that rod, but not cone, discs are continually renewed at the base of the outersegment, transferred apically and phagocyted by the epithelial cells. . . .”

Additional support for the hypothesis that “cones” are immature or non-functional comes from the observation thatonly “rods” secrete IRBP, a necessary protein for the transport of chromophore molecules through the IPM. SeeSection 7.1.2.3.5.

Crescitelli46 was critical of the proposition by Muntz because it was based on indirect evidence; Muntz used an ERGof the b-wave to support his position. Crescitelli said the b-wave was not indicative of the performance of thephotoreceptor cells since it originated proximal to those cells. He did not comment on the quality of the evidencepresented in 1866 by Schultze.

Many caricatures appear in the literature attempting to illustrate the critical features of rod and cone outer segments. Variations in this ratio may occur due to growth, death or other factors related to the myoid and not yet recognized. The caricatures frequently focus on the apparent differences in lamella structure and their close association with anouter membrane of the outer segment if present. Most drawings show the content of the outer segments as consistingof many disks stacked like coins. Some show the material inside the outer segment being laid in as a blanket wouldbe folded, particularly near the connection to the inner segment where the lamella are presumably formed. As wewill see later when discussing the lamella, this difference appears to be a trivial one concerning the visual function. In addition, many drawings show the lamella enclosed by an outer jacket. This jacket takes on many forms,including complete absence. Clearly, the lamella are formed at the point of junction with the inner segment and inthis region the inner segment appears to provide a tubular shell from which the lamella are extruded. It is also clearthat at the outer end of the outer segment, the lamella are generally enshrouded by the pigment epithelium. What isdifficult to determine from the literature is whether there is an outer membrane associated functionally with the outersegment or whether various photomicrographs are showing one of these inner or outer sheaths associated with theadjoining tissue. If there is a substantial outer sheath which includes an end cap, then it must be explained how thelamella are able to get out of the sheath prior to phagocytosis. Alternately, it must be explained how an internal partof a cell is phagocytized without destroying the cell.

A different definition delineating a rod versus a cone may be (at least in man) related to the neural connection.

Page 18: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 17

47Nilsson, S. (1965) The ultrastructure of the receptor outer segments in the retina of the leopard frog (Ranapipiens)48Wassel, H. & Boycott, B. (1991) Functional architecture of the mammalian retina. Psychological Reviews,vol. 71, no. 2, pp 447-48049Kolb, H. (1991) Anatomical pathways for color vision in the human retina. Visual Neurosci. vol. 7, pp. 61-7450Ahnelt, P. Kolb, H. & Pflug, R. (1987) Identification of a subtype of cone photoreceptor, likely to be bluesensitive, in the human retina. J. Comp. Neurology. vol. 255, pp. 18-3451Loew, E. (1994) A third, ultraviolet-sensitive, visual pigment in the Tokay Gecko (Gekko gekko). Vision Res.vol. 34, no. 11, pp 1427-1431

Unfortunately this distinction only applies to two-dimensional pictures of three-dimensional structures. Based ontwo dimensional images, investigators frequently claim a cone foot is broad( in at least one plane) and makes asynapse-like junction with one or more dendrites of a single bipolar cell (frequently described as of the midget type). A rod foot is smaller and rounded and makes a junction with the dendrite of a “summing” bipolar (frequentlydescribed as of the centrifugal type or the diffuse type). Even this definition may only apply strictly to part of theretina. The more significant problems with this definition are two. The two-dimensional photograph is less thanadequate and the definition of the three different bipolar types involved is ambiguous.

An attempt to delineate rods from cones by Nilsson47 takes a different approach again. Based on electron-microscopy, he says the difference between them is related to the proximity of the disks in the Outer Segments. Heassociated the cones with closely packed double membranes, and rods with double membranes separated by 5-10nm. Unfortunately this definition is an incomplete one since the distance between disks is not constant over theirsurface. It is also unclear whether the electron microscope presented the external physical contour of each disk orwhether it presented a contour related to an area that was opaque to electrons.

We will not discuss here the further complications in Walls and others arising from double cones, twin cones,ophidian double cones, or double rods.

It must be pointed out that Wassle and Boycott have not served their audience well by propagating the old idea ofSchultze (1866) that the fovea is blind at night48. This position goes against reality (See “Some Reality Checks” inChapter 1).

In summary, a very wide continuum of photoreceptor cell types is found in the animal kingdom ranging from theshort and fat to the long and thin and including the double types not reported in man. Even the short types may notbe subject to unique classification due to their ability to change their aspect ratio through the contractile abilities ofthe myoid. All known mature photoreceptors exhibit a common outer segment that is cylindrical in basic form. Thepredominant current method of differentiating between “rods” and “cones” is by the synaptic connection they makewith the neural network of bipolar cells. Even this may not form a definitive situation, especially outside the fovealregion. The type of synaptic connection may instead be more indicative of the signal processing function exterior tothe photoreceptors, especially if the signal processing is self organizing during early post natal development.

3.1.5.1.3 Attempts at redefinition during the 1980-90s

Attempts have continued during recent times to extend the classification of rods and cones in order to classify thespectral performance of cones based on their morphological characteristics49. These attempts exhibit a surrealquality similar to that discussed earlier. Although there may be some morphological differences related to differentspectral types of photoreceptors due their location in an overall trichromatic (tetrachromatic) retinal array, thesefeatures are probably secondary and less than definitive. The position of Ahnelt, Kolb & Pflug in 1987 appears tostill stand50: “The cones differ in having different photopigments and different neural connectivity, but nomorphological differences with which to distinguish the three different spectral types have been reported.” That is,cones can be defined in terms of electrophysiological features, but not by their morphological features.

As recently as 1994, Loew found difficulty in delineating rods and cones in the Tokay Gecko. He noted thedifficulty of defining rods and cones at the light microscope level so as to agree with their definition at theultrastructure level51.

In 1996, Packer et al. repeated the protocol of Schultze using modern techniques by excising a piece of primateretina, flattening it and photographing patches of it using transmission microscopy using an incandescent lamp and

Page 19: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

18 Processes in Biological Vision

52Packer, O. Williams, D. & Bensinger, D. (1996) Photopigment Transmittance Imaging of the PrimatePhotoreceptor Mosaic J Neurosci vol 16(7), pp 2251-2260

three chromatic filters52. An objective lens collected to emerging light and projected it onto a CCD imager. Thefilters were centered at 440, 510 and 590 nm. Both human and macaque retina pieces were explored. There areserious questions about what they actually recorded and their discussion develops a variety of problems they wereunable to resolve.

While noting the photoreceptors were not parallel to the radiation from their light source when the retinal tissue wasperpendicular to the incident light, they tilted their custom microscope stage to maximize the image contrast,

“Some of the light that leaks out of the photoreceptor inner segments without traversing the photopigmentwill eventually be caught by the CCD, reducing the signal-to-noise ratio of the spectral measurements andmaking the distinction of small spectral sensitivity differences among the S, M, and L cones more difficult.To minimize this contrast reduction, bright spots corresponding to light emerging from cone outer segmentswere brought into focus using IR illumination to prevent photopigment bleaching. The cone optical axes werethen aligned with the optical axis of the microscope by tilting the stage until the amount of light beingtransmitted through the photoreceptors was maximized. Figure 2 shows that when the tips of the outersegments.”

“Some of the light that leaks out of the photoreceptor inner segments without traversing the photopigmentwill eventually be caught by the CCD, reducing the signal-to-noise ratio of the spectral measurements andmaking the distinction of small spectral sensitivity differences among the S, M, and L cones more difficult.To minimize this contrast reduction, bright spots corresponding to light emerging from cone outer segmentswere brought into focus using IR illumination to prevent photopigment bleaching. The cone optical axes werethen aligned with the optical axis of the microscope by tilting the stage until the amount of light beingtransmitted through the photoreceptors was maximized. Figure 2 shows that when the tips of the outersegments.”

They did not describe the wavelength of their infra-red source, but it appears to be short wavelength IR in order to becompatible with simple IR image converters. Such a source is effective in bleaching visual receptors, particularly ofthe M– and L– types. Their calculations of the number of photons applied to their photoreceptors indicates theywere also bleached considerably by their exposure levels, since the light adaptation function is very fast acting. Thequantum count applied to each photoreceptor and described in the caption to figure 4 (6 or 7 log photons) is verylarge for a photoreceptor accepted generally to be a quantum counter in the unbleached condition.

Their images, which resemble those of Schultze, show the purported rod outer segments to have a diameter of lessthan 1.5 microns at their extreme tip while the purported cone outer segments have diameters on the order of 3microns. As noted in Sections 3.6.2.3.3 & 4.3.4.2.1, a cylindrical waveguide of less than 1.5 microns is a veryinefficient projector of light at visual wavelengths. The 1.5 micron diameter bright spots are more likely glia orsome other type of biological cells. Packer et al. illustrates the problem in their figure 6.

A subsequent statement requires very careful interpretation,

“Figure 2 shows that when the tips of the outer segments are in sharp focus and the axes of the photoreceptorsare aligned, individual photoreceptors glow brightly. In fact, the intensity of the light emerging from theouter-segment tip was always greater than the intensity of the illuminating beam, showing that both rods andcones have optical gains greater than 1 and as high as 3. Thus, it is possible in this preparation to funnelincident light efficiently through the photoreceptors in relatively large patches of peripheral retina.”

As noted in Section 4.3.4.2.1, the disk stack forming the outer segment of the individual photoreceptor is a veryefficient absorber at its absorption wavelength when unbleached. As a general principle, virtually no light should bereaching the peripheral tip of the outer segment when the photoreceptors are unbleached. In analogy with the currentsilicon photosensors in cellphones, ipads, etc., the absorption coefficient of the individual biological photoreceptorsis greater than 90%. In the absence of bleaching, virtually no light should be emerging from the peripheral tip of thephotoreceptors. Their data suggests the photoreceptors were already significantly bleached during theirmeasurements. In addition, if their bright spots in figure 2 (right) are actually rods at their peripheral tip, their size atthe input aperture must be large enough to account for the noted optical gains. If approaching 3:1, the entranceapertures would be nominally overlapping. They concluded the discussion related to figure 4 with the statement,

Page 20: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 19

53Arikawa, K. et. al. (1999) An ultraviolet absorbing pigment causes a narrow-band violet receptor. . . . VisionRes. vol. 39, pp 1-854Livrea, M. (2000) Vitamin A and Retinoids. Berlin: Birkhauser Verlag pg 6455Wolfing, J. Chung, M. Carroll, J. Roorda, A. & Williams, D. (2006) High-resolution retinal imaging of cone-rod dystrophy Ophthalmology vol 113, pp 1014-101956Carroll, J. Choi, S. & Williams, D. (2008) In vivo imaging of the photoreceptor mosaic of a rod monochromatVis Res Epub doi:10.1016/jvisres 2008.04.00657Choi, S. Dobbel, N. Christou, J. et al. (2004) In vivo imaging of the human rod photoreceptor mosaic Inv OpthVis Sci vol 45, E-Abstract 2794

Unfortunately, despite some successes, most patches of retina yielded absorptances that did not exceed 0.05,as was the case for axial MSP. We found many locations, as shown in Figure 5, in which a bright cone in araw image of the retina did not correspond to a high absorptance in the transmittance image. Likewise, areasof high absorptance did not always correspond to bright cones in the raw image. Apparently, the outersegments of these cones were bent, as shown in Figure 6, allowing most of the light captured by the innersegment to leak out before traversing the photopigment.

At 5% absorptance, the photoreceptors are hardly more sensitive than photographic film, in clear contradiction totheir known performance compared to a silicon CCD.

Interestingly, figure 7 of Packard et al.shows photoreceptors that they label with question marks and notes, “Thequestion marks are just to the left of two cones with a yellowish hue similar to that expected of S cones.” Thesecorrespond to the UV photoreceptors discussed in this work, documented within the RPE elsewhere in this work, andpredicted to occur in the retina.

Those working with Arthropoda have never sought or found a variation in elements of the ommatidia (such as rodsand cones) based on spectral absorption or presumptive sensitivity range. Even where such a distinction isnecessary, in the case of polarization sensitivity, the distinction is limited to relative orientation between the outersegments of such cells. Recent authors have also found the arrangement of spectrally distinct cells to be randomwithin the overall retinal plane of Arthropoda53.

It is striking that all recent photomicrographs of in-vivo human retinas show a uniform array of photoreceptors withno sign of any morphologically identifiable subtypes (see Section 3.2.2.1).

In 2000, Livrea observed, “There has been a major difficulty in studying the cones due to lack of material.” Shethen discusses the considerable theoretical activity going on under the assumption that such functional elementsactually exist54.

3.1.5.1.4 Attempts at locating rods during 2000-11 & culminating in 2016

The recent work of the Williams team in imaging the in-vivo retina through the pupil using active optics has placednew emphasis on the spatial characteristics of the retina.. Using false color photographic techniques(L–chromophores reflect complementary pale bluish light), they have identified the various types of photoreceptors(see Section 3.2.3). In 2006, Wolfing et al. (including Williams) used the most sophisticated retinal imagingtechniques available to study the retina of a subject with “cone-rod dystrophy55.” No rods were identified in thatretina. After thousands of images using this higher resolution approach in several laboratories, the Williams teammade the following comments in 200856. “To our knowledge, there is only one report of in vivo images of rods fromthe normal human retina.” This report was made by the same team in 200457. The report was only an abstract of aconvention talk. It did not include any specifics or graphics. Their subsequent comment in 2008 is less thanconvincing. “The difficulty in imaging them, and the relative sparseness at retinal locations where they shouldoutnumber the cones nearly 10-fold, is consistent with previous data that rods are less effective waveguides thancones.” They did not suggest that there were large voids among the cones that were occupied by unimaged rods. Finally, they note, “This underscores the importance of imaging the retinae of individual subjects rather than makinggeneral assumptions about the achromat retina.”

Page 21: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

20 Processes in Biological Vision

58Carroll, J, Pircher, M. & Zawadzki, R. (2011) Introduction: Feature Issue on Cellular Imaging of the RetinaBiomedical Optics Express vol 2(6)59Dubra, A. & Sulai, Y. (2011) Reflective afocal broadband adaptive optics scanning ophthalmoscopeBiomedical Optics Express vol 2(6), pp 1757-176860Dubra, A. Sulai, Y. Norris, J. et al. (2011) Noninvasive imaging of the human rod photoreceptor mosaic usinga confocal adaptive optics scanning ophthalmoscope Biomedical Optics Express vol 2(7), 1864-1876

In 2011, the Optical Society of America supported a special journal issue on active optics technology58. It includedtwo articles by the Williams team59,60 along with many other papers, some of which are discussed in greater detail inChapter 18 of this work. The Williams team asserted they identified “rods” for the first time in-vivo in humansbased entirely on the location of their imagery at various eccentricities outside of the foveola. No spectralconfirmation of their assertions were provided. The first paper describes the configuration and benefits of theUniversity of Rochester scanning AO ophthalmoscope in considerable detail.

The Dubra et al. paper requires very close study. It contains a large amount of empirical data but does not includegraphical or detailed descriptions of their test configurations. Figure 2 is a particular problem because it is notemphasized that the left two frames show the light exiting the peripheral end of the photoreceptors after removal ofthe RPE from the test sample while the right panel shows the light reflected from the entrance aperture of the outersegments of the photoreceptors. Their discussion of the difference in the focal planes of the photoreceptors at theRPE interface is largely extraneous as this has no functional significance. Their discussion of the difference in thefocal planes of the photoreceptors at the entrance apertures of the photoreceptors is superficial as it does notdistinguish the focal planes of the four spectrally selective photoreceptor types. Nor does it describe the off-axisperformance of the optics of the eye (particularly with respect to chromatic aberration of the principle ray). Itappears the measurements reported by Packer et al. relating to the two left frames were made with a significantlybleached retina although Packer et al. did not explicitly recognize this fact and intimate the eyes were dark adapted. Their provision of imagery based on “linear” and “logarithmic” brightness data is laudatory as it highlights the majordifferences associated with these scales. Packer et al. did not obtain any spectral data supporting their designation ofrods versus cones in a specific image frame. Their figure 7 does appear to show the location of UV photoreceptors. They designate these cells by question marks and describe their pale color (similar to that of S-cones) under theirunspecified illumination (color temperature probably less than 5000K).

The 2016 paper by the Roorda team confirmed beyond a shadow of a doubt at the one-half micron resolution levelthat there are no broad spectral response achromatic receptors (rods) in the fovea of the in-vivo human retina(Section 3.2.3.6).

3.1.5.2 “Rods” and “cones” are not functional descriptors

Schultze was wrong in 1866! And the subsequent in-vitro histological work of Osterberg, Curcio et al have beensuperceded by the in-vivo imaging work of the Williams team and the Roorda team.

The concept of dividing photoreceptors into two distinct classes will not be used in this work except in thisChapter for four reasons.

1. It will be shown that the operation of the eye can be completely described without reference to cells exhibiting abroad, achromatic, spectral response (rods).

2. No photoreceptor has ever been displayed that exhibited such a broad spectral response in its operational mode ashas been attributed to rods.

3. Recent work (post 1997) in gene therapy has provided new information suggesting morphological “cones” are notfunctional in vision. See Section 4.6.4.1.2, Bennett and Redmund references.

4. The method of adaptation used in vision negates the need to define two distinct classes of photoreceptors basedon their range of stimulus sensitivity.

5. The work of the Roorda team using a state-of-the-art AOSLO during the 21st Century has demonstratedbeyond the shadow of a doubt there are no achromatic photoreceptors (rods) in the human retina (Section3.2.3.6).

Page 22: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 21

61Ma, J. Znoiko, S. Othersen, K. et. al. (2001) A visual pigment expressed in both rod and cone photoreceptorsNeuron vol. 32, pp 451-46162Hubbard, R. (1977) On the anatomy and physiology of the retina Vision Res vol. 17, pp 1247-1265

The rod-cone dichotomy will be supported in this chapter only to present data from other investigators found in theliterature. There is no functional distinction between photoreceptor cells that can be ascribed to theirmorphological shape. Only confusion would be served by editing the work of these other investigators to eliminatethe descriptors, rods and cones.

The ancient and totally archaic Duplex Theory of vision, with rods responding to low light levels andcones responding to high light levels, has been completely falsified and should be purged from alltextbooks in a timely manner. The Duplex Theory constitutes a blight on the teaching of biologicalscience.

The spectral performance of a given photoreceptor is sometimes confused because of an unusual feature of itsspectral absorption characteristic. It is anisotropic. When employed in an Outer Segment of the photoreceptor, thechromophores exhibit a very high absorption coefficient for light incident along the axis of the Outer Segment. Thisabsorption characteristic is its functional characteristic. The same chromophore in the same configuration willexhibit an alternative absorption characteristic to light incident perpendicular to this axis. This weaker characteristicalways exhibits a peak absorption near 500 nm. It is the same characteristic that will be obtained by measuringchromophoric material in a dilute solution in-vitro. This subject will be addressed in detail in Chapter 5.

The concept of dividing photoreceptors into three (or four) spectral classes based on morphology will not be usedin this work.

It will be shown below that, to the first order, all photoreceptor cells are functionally identical. The only differencerelated to their spectral performance is associated with the chromophore coating the disks of the Outer Segment. This difference is at the molecular level. It consists of a difference in physical location of the oxygen auxochromesfound along the conjugated carbon chain of the molecule. No other direct, first order, feature separates the conesinto classes relative to their spectral performance.

3.1.5.3 On the subject of “red rods” and “green rods”

Another oddity that occurs occasionally in the literature are the terms red rods and green rods. Ma et. al. haverecently provided an explanation of this situation61. They note that Hubbard translated a paper by Boll of 187762. Inthat paper Boll differentiated the physical appearance of two types of photoreceptors based on their apparent colorusing a light microscope. While the paper also used the terms rods and cones, in this case he was only speaking of adichotomy between two intermixed photoreceptor types. He described the majority as red rods and the minority asgreen rods. In such a case, the observed color is the complement of the absorbed light. It is not clear what lightsource Boll was using. Depending on the light and optics and his state of adaptation, there are two cases. If he useda sufficiently high color temperature source (prior to the invention of the electric lamp), and his optics weresufficiently good, the functional difference between these cell types would be in their absorption of blue or purplelight. Since there are not two distinct chromophores relating to the names blue and purple, it appears that both cellswere absorbing the light associated with an S–channel chromophore with some secondary factor causing a shadingthat resulted in a net reddish or greenish tinge. In the absence of sufficient short wavelength illuminant, he wouldhave observed the functional difference between the M– and L–channel absorbers. The M–channel absorber wouldlook reddish (magenta) and the L–channel absorber would appear a pale greenish blue. In either case, thisexplanation is in complete agreement with the findings of Ma, et. al. In that paper, the spectrally determinedS–channel cones and the morphologically determined “green rods” exhibit the same absorption spectra. They are infact the same electrophysiological entity. This leaves open the question of the role of the red rods. Are they also infact M–channel cones? If so, the designation “rod” can be stricken from the glossary of vision. Themorphologically identified red rods, would absorb maximally near 532 nm as expected for the M–channel cone. Ifthis is true, Ma, et. al. have documented the successful chemical isolation of all four chromophores of vision, thoseof the UV–, S–, M– and L–channel, using the salamander, Ambystoma tigrinum.. Chapter 5 and particularlySection 5.5.10.6 will discuss the fact that the actual chromophoric materials are the Rhodonines.

In the introduction to Boll’s paper, Hubbard quoted Muller (1851), “an individual rod can look alternately colorlessand colored, depending on whether it lies on its side or stands upright.” This observation recognizes the anisotropicproperties of the photoreceptor outer segment.

3.2 Morphology of the chordate retina

Page 23: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

22 Processes in Biological Vision

63Wassle, H. & Boycott, B. (1991) op. cit.64Boycott, B. & Dowling, J. (1969) Organization of the primate retina: light microscopy. Philos. Trans. R. Soc.Lond. B Biol. Sci. Vol. 255, pp. 109-194

The review of Wassle and Boycott63 in 1991 is an invaluable source of references for studying the retina. The articleitself is involved primarily in the morphology of the retina but it only includes two micrographs among myriads ofhand drawn caricatures; many of these caricatures include the expression “putative” in the caption. The object of thereview “is to survey current understanding of how the different neurons of the mammalian retina are arrayed andinterconnected to form functional units.” Although the article attempts to relate morphology to function, it is basedon morphology and does not review or include references to the signaling function. It does present archaic sketchespurported to show signal flow at the most basic level.

The most valuable picture of the cross-section of the retina remains that of Boycott and Dowling of 196964. It isreproduced in Figure 3.2.1-1 This is a picture from a phase-contrast light microscope. This image is takenapproximately 1.25 mm. from the center of the fovea in an unspecified direction. The callouts along the left marginhave been widely used in discussing the retina. However, a scale and additional nomenclature has been added on theright. The scale is shown starting from Bruch’s Membrane. Light approaches from the bottom in this figure and isbrought to a focus at the Petzval Surface. It is located quite near the junction of the inner and outer segments. Theinner nuclear layer is further subdivided into the Outer, Middle and Inner Matrix Layers to reflect the overallarchitecture developed in this work. The Outer Matrix Layer is populated by the morphologically designatedhorizontal cells of the first lateral processing matrix. The Middle Matrix Layer is populated by bipolar cells. TheInner Matrix Layer consists of the various cell types generally categorized as amercine cells and forming the secondlateral processing matrix. Although these layers may not be clearly subdivided morphologically, they arefunctionally. The previously designated inner plexiform layer is subdivided into an Inner Fiber Layer (closest to theINL) and consisting of axons, and an Inner Plexiform Layer associated with the dendrites of the ganglion cells.

Page 24: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 23

65Kolb, H. (2001) Web Vision an internet resource, http://webvision.med.utah.edu

Figure 3.2.1-1 CR Cross section through a human retina, about 1.25 mm. from the center of the fovea. Vertical scalehas been added to quantify the thickness of this retina. Labels on the right have been added to highlight other featuresand to subdivide the inner nuclear layer. A part of a blood vessel containing erythrocytes shows in the ganglion layer.Photographed by phase contrast microscopy. An arrow point to a “displaced” ganglion cell. Modified from Boycott &Dowling (1969)

The labels on the right are in general agreement with those of Kolb65. Wolken has also provided an alternate set of

Page 25: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

24 Processes in Biological Vision

66Wolken, J. (1986) Light and life processes. NY: Van Nostrand Reinhold, pg. 15467Brown, K. Watanabe, K. & Murakami, M. (1965) The early and late receptor potentials of monkey cones androds. In Cold Spring Harbor Symposia on Quantitative Biology, volume XXX, Sensory Receptors. ColdSpring Harbor NY: Self Published pp 457-48268Snodderly, D. Auran, J. & Delori, F. (1984) The macular pigment. II. spatial distribution in primate retinas.Invest. Ophthalmol. Vis. Sci. vol. 25, pp. 674-69Ahnelt, P. (1998) The photoreceptor mosaic. Eye, vol. 12, pp. 531-540

Figure 3.2.1-2 The fovea of monkey, Macaca, irus. Thespace between the dashed lines represents the field lensformed by the neural layer. The space between the lowerdashed line and the RPE shows the variation in the length ofthe Outer Segments with position in the retina. Modifiedfrom Brown, Watanabe & Murakami, 1965.

labels to those of Boycott and Dowling66. His drawing provided specific callouts for some of the details notrecognized by them but he did not separate the inner and outer segment layers. There appears to be an editorialinconsistency between his figure and some of his text related to layers seven and eight. He recognizes the presenceof both the 1st and 2nd lateral neuron layers along with the bipolar layer within his inner nuclear layer. However, helabels the various interconnecting fiber layers as molecular layers. Finally, the choroid layer is described asconsisting of black pigmented cells acting as an antireflection device. This is a species specific designation that maybe more appropriately associated with Bruch’s membrane.

As Walls did point out in 1942, the number of neurons in the 2nd lateral matrix of birds, the IML, may exceed thenumber of bipolar neurons found within the inner nuclear layer. This is not the case in humans where the number ofneurons in the second lateral matrix is minimal.

Brown, Watanabe & Murakami have provided several figures showing the variations in the relative thicknesses andcurvatures of the layers for the retina of the cynomolgus monkey, Macaca irus67. The figures represent crosssections of retina along a line from the parafoveal ridge to the optic disc. Their figure 2 is particularly useful. It isreproduced as Figure 3.2.1-2 and describes the profile of the retina in the vicinity of the fovea. The figure stressesthe extended length of the Outer Segments in this region. It also shows the curvature of the focal plane defined bythe demarcation between the inner and outer segments of the photoreceptors. Finally, it illustrates the variation inthickness of the neural layers in this area. This variation constitutes the field lens discussed in Section 2.4.4.

3.2.1 Anatomical Level

The retina in Chordata is an approximately sphericallining to the posterior two thirds of the ocular globe. The human retina can be taken as typical of thechordates except for scale. Whereas its thickness ismeasured as a few hundred microns, its surface areacan be measured in square centimeters. It can bethought of as a laminate as thick as three to five sheetsof copy paper and containing a large amount ofstructure. The retinal laminate fills the space betweenthe vitreous humor and the membrane of Bruch. Inmany species, the thickness of the laminate is notuniform and the relative positions of different layerswithin the laminate are not uniform. This variationmakes it difficult to obtain a picture or micrograph ofthe details of a specific layer of the retina. The figuresof Snodderly, et. al68. show how difficult it is to select alayer by embedding and then slicing a retina. Taking aslice near the fovea and parallel to the RPE-choroidinterface passes into and out of several individuallayers within distances of 50-100 microns. Thisproblem has contributed to the shortage of reliablemicrographs of the face of the Outer Segments or innersegments of photoreceptor cells.

Ahnelt has recently provided an overview of the retina at the anatomical level69. It must be read with care. Thiswork does not support many of the allegations presented in that paper. He assumes the optical axis of the eye iscoincident with the fixation point and perpetuates the morphological definition of rods and cones. Discussing cones,he stresses that “the conical tapering is not a consistent morphological feature of this cell type. A comparison of

Page 26: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 25

70McGeer, P. Eccles, J. & McGeer, E. (1987) Molecular Neurobiology of the Mammalian Brain. NY: PlenumPress. pp 35-3871Heller, J. (1976) Intracellular retinol-binding proteins from bovine pigment epithelial and photoreceptor cellfractions. J. Biol. Chem. vol. 251, no. 10, pp 2952-295772Heckenlively, J. (1988) Retinitis Pigmentosa, NY: J. P. Lippincott, pg 50-51

cones along a meridian of human retina shows considerable variation.” Additional, less than convincing, criteria fordetermining the function of a photoreceptor cell based on morphological features are included in the discussion. Thediscussion concludes with “So far no morphological criteria for direct differentiation of L- and M-cones have beenreported but there may be differing connectivities along their midget pathways.” A cross-section of the retina isintroduced based on the classical figure of Polyak.

3.2.1.1 The brain/blood barrier

The current vision literature typically considers the retina as part of the brain. This is based on the generally similarorganization of the layers of neurons in the neural laminate of the retina. However, it only briefly discusses a featureassociated with brain tissue, the brain/blood barrier. Section 4.5.1discusses the morphogenesis of the eye from aperspective that suggests the retina is not part of the brain but is a complex portion of the peripheral neural system. This interpretation is due to the specific properties of the neural layers and the RPE layer of the eye and theimportant space, the Inter-photoreceptor-matrix, between them.. This space is essentially exterior to the epidermis ofthe animal, although it becomes enclosed within the eye with the formation of the lens group. This external space,the very high rate of opsin production by the photoreceptor cells and the digestive processes associated with the RPEcells show little analogy with the neural tissue of the brain.

The concept of a blood-brain barrier is not widely detailed in the vision literature. While some describe the barrieras a layer of astroglial cells, others describe it as a special feature of the endothelial cells forming the walls of thecapillaries70.

Chen & Heller describe the cells of the RPE forming one membrane of the brain/blood barrier but they do notdiscuss any isolation between the neural retina and the vitreal humor and the retinal artery. Heller also discusses theblood/brain barrier in his introduction and discussion from the perspective of the conventional wisdom71. Thebrain/blood barrier is usually described as preventing both large proteins and many other smaller molecules fromentering the brain. As will be seen in this work, the situation is more complex in the eye. The “brain/blood barrier”appears to contain three components in the area of the retina. The cells of the RPE are tightly packed and doconstitute a general barrier between the choroid arterial system and the neural portion of the retina. They areparticularly efficient at controlling oxygen. However, the RPE cells do pass the chromophores of vision through thisbarrier and associate them with large binding and transport proteins within the IPM. These proteins andchromophores are prevented from entering the neural tissue of the retina by a secondary membrane, the OuterLimiting Membrane. This membrane may be discrete but it is more likely that it is represented by an closeleypacked interdigitated layer of photoreceptor and glial cells. On the opposite surface of the retina is the InnerLimiting Membrane that prevents material from the vitreous from entering the neural tissue. It may also preventlarge molecules from the neural vascular system from entering the neural tissue of the retina. There may be ananomaly in that Boycott & Dowling showed a part of a blood vessel within the ganglion layer of their micrograph ofthe human retina. It is possible that these small vessels are surrounded by their own individual brain/blood barrier.

The three membranes create two separate zones. One zone, the IPM, protects the chromophores from oxidationwhile they are being transported to the disks of the OS. The second zone, the INM, protects the neural tissue fromthe blood supply as in any zone of the brain and from both the chromophore material and the transport proteins. Aswill be seen below, this explanation is still too simple. There are a number of materials conveyed through the IPMin support of the neurological functioning of the photoreceptor cells. These materials may pass through the RPEcells. However, it is more likely they pass through and/or are manufactured by the glial cells that are locatedbetween the RPE cells or between the photoreceptor cells.

A key role of the membranes surrounding the IPM is to prevent any materials, particularly those containing oxygen,from attacking the highly labile chromophores of vision found within this chamber.

3.2.1.1.1 Membranes separating the laminates

Heckenlively72 discusses the morphological properties of Bruch’s membrane in detail.

Page 27: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

26 Processes in Biological Vision

73Records, R. (1979) Physiology of the Human Eye and Visual System NY: Harper & Row pg 29974Leggett, K. (2001) Ultrahigh-resolution OCT detects early-stage eye disease. Biophotonics International,Sept-Oct. pp 60-6275Drexler, W. et. al. (2001) Ultrahigh resolution ophthalmic optical coherence tomography Nature Medicine,vol. 7, pp 502-507 76Blanks, J. In Ryan, S. ed. (2001) Retina, 3rd ed. Vol. 1, St. Louis, MO: Mosby, Chapter 2, pg 48-49

3.2.1.2 Layers of the Retina and some statistics

Records has provided an informative description of the morphogenesis of the human retina73. Figure 3.2.1-3reproduces his figure after deleting the label for rods and cones and introducing Verhoeff’s “membrane.” Thismembrane is resolved in optical coherent tomography (OCT) at the two micron resolution level (Section 3.2.2). However, higher resolution would show that it consists of a concentration of pigment bodies and terminal bars in the1/3 of the RPE cells closest to the photoreceptor cells..

Determination of the dimensions of the retinal layers are usually performed in-vitro following preparatory steps thatfrequently impact the absolute dimensions of the specimens. However, excellent data is available from a number ofinvestigations. A new technique has just appeared, optical coherence tomography74,75. This technique based on lightinterferometry using extremely short pulses of light, can be used to examine the layers of the retina at differentlocations without surgical intervention. It may offer more precise in-vivo values in the future but is quite complex atpresent.

Blanks has recently provided a brief discussion of the non uniformities in the layers of the retina that highlight thefact that the overall visual system is divided into a series of zones76. The signals from these various zones must bereconciled in the cortex to achieve a perceived uniform field of view. Before considering her figure 3-15,reproduced from Stone, et. a., the reader is advised to review Section 2.6.1.2 of this work.

Page 28: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 27

77Werblin, F. & Dowling, J. (1968) Organization of the retina of the mudpuppy, necturus maculosus. II.Intracellular recording J. Neurophysiol. Vol. 32, pp. 339-355

Figure 3.2.1-3 Embryogenesis of the retina showing cellular origin of the various layers. Modified from Records, 1979.

3.2.1.2.1 The neural laminate

The neural laminate is the most anterior. It is the region between the inner limiting membrane and the outer limitingmembrane. It consists of a hydraulic bed for the exchange of nutrients and waste products. The neural laminatecontains the neural arterial network, supporting this hydraulic bed, and all of the soma and Activa associated withthe signal neurons of the retina. It also includes the soma of the photoreceptor cells. Sublayers of interconnectioncircuitry occur within this laminate of the retina. They are usually described as the fiber layer, the outer plexiformlayer, the inner plexiform layer and the optic fiber layer. The first three of these layers are complex matricessupporting the interconnection of dendrites and axons on an immense scale. Between these layers of circuitry andthe outer limiting membrane are several layers of neurons. Beginning with the outer limiting membrane, thesesublayers are the outer nuclear layer containing the soma of the photoreceptor cells, the 1st lateral layer, the bipolarlayer containing the bipolar cells, the 2nd lateral layer and the ganglion cell layer. Most, if not all, of the amercinecells are found in the 2nd lateral layer.

Occasionally, cells of a given type are reportedly found outside their normal domain (See [Figure 3.2.1-1]). Theseare rare and may involve unknown functions or misidentification. They are often defined as displaced ganglioncells or Dogiel cells and have been reported to occur in many retinas77. As shown elsewhere in this work, if thecells are being probed electrically and excessive capacitance is added to the circuit of the cell, it may go intooscillation. This may have occurred with a few cells in the inner nuclear layer. An alternate effect of capacitance isalso shown in the above paper. The effect of excessive capacitance compared with the resistive impedance level ofthe cell is evident in the spot waveforms for the horizontal and bipolar cells of figure 3. The waveforms are

Page 29: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

28 Processes in Biological Vision

78Steinberg, R. (1973) Scanning electron microscopy of the bullfrog’s retina and pigment epitherlium, Z.Zellforsch, vol. 143, pg. 451 [in Nolte, pg. 404]

Figure 3.2.1-4 CR Electron micrograph of the photoreceptorand nuclear laminates of the bullfrog. Chosen for clarityand believed similar to that of all chordates, includinghuman. From Steinberg (1973 ).

essentially noise free because of the RC time constant of the overall circuit. In the reported experiments, the authorssay the test circuit was always band-limited by the high distributed capacitance and resistance at the pipette tip, evenwith negative feedback used to reduce the apparent input impedance. They were using a field effect transistor at thetest set input with an impedance reported by the manufacturer as 1013 ohms and 10-12 farads. The test lead to thespecimen probably introduced ten times this amount of shunt capacitance. Both excessively smooth (noise free)waveforms and forced oscillation are signs of an inadequate test set.

The neurons found in this laminate can be subdivided into two major classifications. Those principally involved intransmitting an in-line signal from the photoreceptor cells to the optic nerve and those principally involved in lateralconnections between the in-line cells. The in-line cells are generally involved in signal manipulation aimed at signalsummation and subsequent distribution to a large group of orthodromic cells. The lateral cells are used primarily fordata processing involving signal differencing and correlation. Because of the proximity of these circuit elements andthe limited dielectric strength of the hydraulic matrix, the voltage between these various circuits is limited to lessthan 200 millivolts.

This laminate is within the optical path of the light approaching the photoreceptors and exhibits optical propertiesthat must be taken into account. It is considered an optical field lens in this work.

3.2.1.2.2 The photoreceptor laminate

The photoreceptor laminate contains both the inner and outer segments of the photoreceptor cells immersed again ina hydraulic bed. This bed is known as the Inter Photoreceptor Matrix (IPM). It is delimited by the barrier pierced bythe Inner Segments, the Outer Limiting Membrane (OLM), and the retinal pigment epithelium (RPE). This bed is inintimate contact with and supplied with material from the RPE laminate.

Figure 3.2.1-4 clearly illustrates the structure of both the photoreceptor and neural laminates of a bullfrog78. Thefigure is of a moment in time and was chosen for its clarity and is similar to those of all chordates, including man. Note the incompatibility of the structure labeled a cone outer segment with growth that proceeds continuously fromthe inner segment to the location of phagocytosis at the RPE laminate.

It is important to recognize that the neurons known asthe photoreceptor cells are neuro-secretory cells, as aremost if not all sensory cells. The Inner Segmentsextrude the protein substrate, generally identified asOpsin, into the IPM and form the uncoated disks of theOuter Segment. Chromophoric material is secretedfrom the RPE and transported across the IPM bydiffusion to the region of the IS/OS interface. At thatlocation, the chromophore condenses onto the substrateas a liquid crystalline film.

Note the structures labeled “cone outer segments” areshorter than the other outer segments. They are notconsidered mature functional Outer Segments in thiswork.

3.2.1.2.3 The RPE laminate

The third laminate consists of the retinal pigmentepithelium (RPE) and the arteriola network associatedwith the posterior of the retina. It performs two majorfunctions. A major function is to supply energy to theinner and outer segments of the photoreceptor cells viathe IPM located in the adjacent laminate. Bysupplying the active portions of the photoreceptor cellsfrom the RPE, the competition for resources is greatly

Page 30: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 29

79Boycott, B. & Dowling, J. (1969) op. cit.80Straatsma, B. Foos, R. & Spencer, L. (1969) The retina-topography and clinical correlations. in Symposiumon Retina and Retinal Surgery. St. Louis MO: C. V. Mosby (also in Hogan, et. al. (1971) Histology of thehuman eye. Philadelphia PA: W. B. Saunders pg. 401

reduced compared to the situation in the neural laminate. The diffusion bed of the RPE is not in the optical pathwayand it can be uniformly supplied by the capillaries. This minimizes the time constant of the hydraulic path servingthe very important accommodation amplifiers. These amplifiers are located along the sides of the outer segmentsand connect to the inner segments through the cilium collar.

A second major function of this arteriola network is to support the continual reconstruction of the Outer Segments ofthe chordate eye. A similar function may occur in the eye of other phyla if the animal has a long life span. Otherwise, the function may only be one of construction, without involving phagocytosis. In these other phyla, theequivalent of the RPE is not planar but appears to be found in the interstitial spaces between the retinula and therhabdom, or even the ommatidia as a whole. The equivalent structures are given names like outer or corneal pigmentcells. The pigment cells are so named because of their storage of chromophoric material in bulk form. This materialis eventually used in building (rebuilding) photoreceptor Outer Segments.

The RPE removes old disks from the Outer Segment, recycles their constituents and aids in the recoating of the newdisk substrates (protein) extruded by the Inner Segments. The process of removal is normally considered to involvephagocytosis. The details of this processing will be discussed in Chapter 7.

3.2.1.2.4 Laminate dimensions

Scaling from the figure of Boycott and Dowling79, the three layers in an adult human have thicknesses of;

Layer Thickness Relative to sheet of paper

Neural Laminate 310 microns 4 sheetsGanglion layer 60Inner nuclear 551st lateral layer ~5Bipolar layer var.2nd lateral layer var.Outer nuclear 25

Intercon. fibers 175 2 1/3 sheetsPhotoreceptor Laminate 40 microns ½ sheet

Inner Segments 15Outer Segments 22

RPE Laminate 18 microns 1/4 sheet

[Figure 2.2.2-4] shows the minimum thickness of the retina in the foveal area to be 200 microns for a rhesusmonkey. For more details regarding the thickness of the retina, see Hogan who reproduces the graph of Straatsma et.al80. for the human eye. Figure 3.2.1-5, also from Hogan, provides the definition of the various regions of theretina. The fovea is a small area (typically 350 microns in diameter) where the photoreceptors are particularlydensely packed.

Page 31: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

30 Processes in Biological Vision

Figure 3.2.1-5 CR A fundus photograph matched with ameridional light micrograph of the macular region. Thefundus photograph shows the foveola (a), fovea (b),parafoveal area (c) and perifoveal region (d). From Hogan(1971)

Page 32: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 31

81Stone, J. (1983) Parallel Processing in the Visual System. NY: Plenum pp 291-30582Pirenne, M. (1967) Vision and the Eye. London: Chapman & Hall. pg. 4

3.2.1.3 Other anatomical features

Some care must be taken in analyzing features at the anatomical level. What appear to be simple anatomicalcharacteristics may in fact be more complex than first perceived due to a more complex histological architecture. These differences will be highlighted below using material from Stone81.

3.2.1.3.1 The visual streak versus an elongated fovea

Stone stresses a concentration of ganglion cells frequently located along the horizontal axis of a chordate eye. Thisconcentration is frequently described as a “visual streak” at the anatomical level without any consideration ofwhether it is also present in the photosensing layers of the retina. On the other hand, an area of concentration ofphotoreceptors within the photosensing layer is frequently described as a fovea, sometimes including an even moreconcentrated region known as a foveola. Many of the figures in Stone indicate a peak density of ganglion cellscoincident with the area centralis or fovea. However, this characteristic is not borne out by micrographs of the areacentralis. While the average density of ganglion cells may be high in this general area, the instantaneous density atthe immediate location of the fovea is extremely low. Micrographs at the histological level show clearly that thereare in fact no ganglion cells overlying the fovea, at least in man and other anthropoids. This is shown clearly in[Figures 3.2.1-2 and 3.2.2-1] of this work. While a visual streak may be indicative (at a gross level) of a degree ofspatially oriented signal processing within the neurological laminate of the retina, it suffers two shortcomings. It isnot descriptive of the organization of the photoreceptor laminate of the retina and it is not representative of theganglion layer at the histological level.

An effort should be made to differentiate between a potentially elongated fovea, in a variety of species, located in thephotosensing laminate and a functionally separate concentration of ganglion cells along the horizontal meridian ofthe neural laminate of a retina. The former impacts the performance of the physiological optics of the eye. Thelatter more likely impacts the neurological signal processing of the eye subsequent to image detection.

3.2.1.3.2 The optical disk (or blind spot)

The most important nonfunctional feature of the retina is the optical disk or blind spot. The dimensions of theoptical disk were developed in Chapter 2. The disk is the region where all of the signaling circuits and supportfunctions enter the ocular orb and are separated to serve all functional areas of the eye. Little attention has beengiven to this feature in the literature. Pirenne has reproduced an early and limited drawing of the cross-section of thedisk82. The disk is three-dimensional; some of the elements within the optic nerve separate from it at three differentlevels. The arterial system separates and subdivides at the level of three different surfaces, the choroid, the RPElaminate and the neural laminate. Similarly, some motor nerves separate from the optic nerve at the choroid level forpurposes of controlling the objective optical group at the anterior of the eye. All of the sensory nerves associatedwith the retina come together as a group within the neural laminate and pass out of the eye through the optical disk. They form the major part of the optic nerve, a somewhat restrictive name for this element. Without photoreceptorsin the optical disk, the eye is completely blind in this region, an area larger than that subtended by six moonspositioned side by side. This fact forces the adoption of the idea that the visual system involves computationaloptics consisting of two elements. The first element is a short term memory of significant size. The second elementis a “fill program” not unlike a paint program used in modern desk top publishing and other computer programs.

Section 2.2.1 reviews the impact of the blind spots on the overall performance of the visual system.

3.2.1.3.3 The macula or macula lutea

The clinicians tend to describe two distinct regions as the macula or macula lutea. Many academicians do notrecognize any structure deserving these names. Clinicians prior to the middle of the 20th Century argued that therewas a distinctly colored region of the retina when viewed through the pupil. It was frequently described as from 8 to14 degrees in diameter (when referred to object space) and of a very slightly different color than the outer retina. Many relatively naive associates claimed they could not see any distinct shading of this diameter. More recently,other clinicians have defined the more distinct ring of dominant vascularization encircling the foveola (1.2 degrees indiameter) at a diameter of 2–3 degrees centered on the point of regard (nominal center of a circular foveola). There

Page 33: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

32 Processes in Biological Vision

83Miller, D. ed. (1987) Clinical light damage to the eye. NY: Springer-Verlag pp. 97-9984Snodderly, D. Auran, J. & Delori, F. (1984) The macular pigment. Invest. Opthalmol. Vis. Sci. Vol. 25 pp.660-673 and 674-68585Snodderly, D. (2013) http://www.sbs.utexas.edu/SnodderlyLab/gallery.html

is very fine vascularization extending from this ring in toward the smaller foveola. However, this finervascularization does not extend into the 1.2 degree diameter region of the foveola.

The foveal area of the retina in humans, and apparently other primates, exhibits a distinct yellow coloration whenviewed through the pupil in-vivo. The origin of this coloration is obscure. At mid-century, many considered it anoverlay on the vitreal surface of the neural layer of the retina and given the name macula lutea. The work of Polyakat that time and more recent photographs of cross-sections of the retina suggest the coloration is due to physicalconditions within the neural layer. Miller83 has provided excellent color images of the area, although he defines it asoccurring centered on the posterior pole of the human retina. In this work, the posterior pole is the point where theoptical axis penetrates the retina. The point of fixation is not at this point and it is generally agreed that the maculalutea is centered on the point of fixation. In the pictures taken with different colored light sources, the overall coloris seen to be due to a variety of spatially separate sublayers. A major question is whether the layers are indicative ofthe presence of a new localized material or the absence of neuron tissue in this area. This absence is well recognizedin the literature. It would be expected to reduce the amount of scatter associated with the light passing through theneural layer on the way to the most spatially sensitive part of the retina, the foveola. In the latter case, theconclusion would be that the color of the macula lutea is merely the basic color of the neural layer as a substrate inthe absence of neurons.

Wald made spectral difference measurements on the in-vivo retina and calculated the spectrum of a material on theassumption that it overlayed the retina. His conclusion was that the change in spectra was due to a material that wasprobably similar to retinol. He proposed, in his normal charismatic style, xanthophyll from the limited catalog ofchemicals with the desired spectrum available at that time. The assumption was that xanthophyll either reflectedmore light near XXX or absorbed light at complementary wavelengths.

Snodderly, el. al. have presented two papers exploring the “pigments” of the macular lutea of macaque retinas84. They included excellent pictures of a cross section of the fovea taken with narrow spectral bandwidth lights at 460and 525 nm, Figure 3.2.1-6. The lights also had very small spatial diameters, 12.5 and 7.5 microns diameter. Theresults show the retina to consist of various layers of different reflectance. These layers vary in thickness near thefovea and provide an alternate interpretation of the mechanism causing the appearance of the macula lutea. Themeasurements with the 7.5 micron aperture were also quite noisy, requiring additional smoothing in the dataprocessing. This would suggest the structure of the material of the neural layers was important.

Snodderly, et. al. confirmed that the pigment concentration was centered on the fovea. They also determined that itwas dichroic with the major axis of absorption oriented tangential to a circle centered on that fovea. They did notfind scattering to be a significant factor in their experiments. Their figure 9 of the second paper shows the pigmentonly extends to a radius of 750 microns, which corresponds roughly to the area they describe as the foveola. Themost significant absorption at 525 nm was in the Outer Segments of the photoreceptors as would be expected. Nomajor absorption could be associated with any individual layer of the neural laminate. At 460 nm, the situation wasquite different. The absorption within the outer fiber layer (labeled RA in their figures) and the inner plexiform layeris significant. It is also significant in the outer nuclear area at the center of the foveola. They also found significantdifferences specimen-to-specimen. They associated most of the absorption with the axons of the photoreceptorneurons. However, there imagery shows a clear association with the inner plexiform layer and the thin layerbetween the ganglion layer and the inner limiting membrane.

Snodderly’s laboratory has recently released more cross sections of healthy Macaque retinas85 that can be comparedto examples of diseased human retinas in Chapter 18.

Examining Figure 3.2.1-6xxx and [Figure 3.2.2-1] of the next section, a case can be made that the additionalabsorption in the region of 460 nm is due to the absence of neural tissue in the INM. This would leave a higherdensity of matrix material in these areas. The higher absorption would then be related to the material of the INM andnot the axons themselves. Based on the generally accepted caricature of Polyak, there is a lower density ofphotoreceptor cell nuclei at the center of the foveola, there are fewer interconnections in the outer fiber layer andouter plexiform layer in the vicinity of the foveola, and there are virtually no interconnections associated with the

Page 34: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 33

Figure 3.2.1-6 Cross-sections of a macaque retina taken in 460 nm (blue) and 525 nm (green) light. The foveola isrepresented by the flat region of each retina devoid of any vascularization from the layers labeled IN, IP and GC. It hasa nominal diameter of 350 microns in Appendix L. The darker regions near the foveola appear to be due to the absenceof neural material relative to the material of the inter-neural matrix, IN. See original art for optimum reproduction. FromSnodderly, et. al. 1984.

inner plexiform layer and the optic fiber layer in the region of the foveola. The material remaining in these areas isessentially that of the INM itself. This would suggest that the observed phenomena given the name macular lutearesults from an absence of neural material and not the introduction of an additional filtering component. It is moredue to the increased density of the matrix material than it is to the presence of axons, which exhibit a lower densityin these areas.

There has been no confirmation of the xanthophyll hypothesis in the literature subsequent to Wald. The imagery ofSnodderly, et. al.does not show any short wavelength absorption in the regions of high axon density beyond thefovea. Axons are normally considered colorless and are generally known as white matter when found in the brain.

This analysis suggests that the macula lutea is due to an optimization designed to minimize loss in resolution due toscattering of the incident light from the pupil and has nothing to do with improving resolution by raising apparentscene contrast. Such an increase in contrast would be obtained at the expense of sensitivity in the short wavelengthspectral region. It is further suggested that a macula lutea is an inherent feature of any retina exhibiting a foveolaand its presence is not related to evolutionary adaptation. Whether the net absorption associated with the macularlutea is associated with only one chemical constituent of the INM is a subject for further experiment.

3.2.1.3.4 The signal paths on the neural laminate surface

Although it is common in the literature to see maps of the vascular structure on the surface of the retina facing theaperture, it is less common to find maps of the neural paths coursing over that surface. The neural map containsmuch finer detail than the vascular map. Miller provides a picture but the neural paths are obscured by the large

Page 35: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

34 Processes in Biological Vision

86Miller, D. (1991) Optics and refraction. Vol. 1, edited by Podos, S & Yanoff, M. NY: Gower MedicalPublishing. Pg. 3.2487Fine, B. & Yanoff, M. (1979) Ocular histology: a text and atlas NY: Harper & Row. Chapters 6 & 1288Hart, W. editor (1992) Adler’s Physiology of the Eye, 9th ed. St. Louis, MO: Mosby Year Book pgs. 495 &617 89Hogan, M. Alvarado, J. & Weddell, J. (1971) Histology of the Human Eye. Philadelphia, PA: W. B. Saunders90Records, R. (1979) Physiological Aspects of Clinical Neuro-Ophthalmology Chicago, IL: Year Book MedicalPublishers, pg 50991Rodieck, R. (1973) The Vertebrate Retina San Francisco, CA: W. H. Freeman & Co. pp 253-254

number of vasculature included in the same scene86. Fine & Yanoff discuss this optic fiber layer in more detail87 asdoes Adler88. Most of these materials concentrate on the area near the fovea and do not provide a comprehensivetreatment of the organization of the neurons as they approach the Lamina Cribosa at the center of the “blind spot.” The Adler material shows the distinct division of the axonal paths into upper and lower sectors passing through thearea of the fovea and also shows the variable length axon lengths complimentary to Henry’s Loops of the opticalradiation. They do not discuss the multiple layer nature of the axonal paths. Oyster appears to provide the mostcomplete caricature of the Lamina Cribosa based on earlier data from Hogan, Alvarado and Weddell89. An equallyinformative caricature appears in Records90.

The material presented in Section 15.2.4 suggests that the arrangement of the neurons in the optic fiber layer is assignificant as the arrangement of photoreceptors in the Outer Segment Layer. There appear to be a number of neuralsubsystems overlayed and intertwined within the 100-micron thickness of the optic fiber layer. These subsystemssupport different functional requirements within the overall visual system.

Based on experiments partially dissecting the optic nerve, it appears the nerves of these subsystems are grouped asthey pass through the lamina cribosa and maintain a fixed organization for the length of the optic nerve.

The optic nerve contains a group of neurons dedicated to the foveola at the center of the fovea. It also containsgroups associated with the various quadrants of the visual field surrounding the foveola. There also appears to be agroup associated with a coarse mapping of the visual field into vertical and horizontal stripes. These groupings areused in the LGN to derive pointing signals that drive the eye muscles to bring threats to the center of the foveaquickly. By providing a neural map as above, the brain avoids the necessity of computing trigonometric functions inorder to derive pointing commands based on polar coordinates relative to the foveola.

3.2.1.3.5 The tapetum

The tapetum is an intriguing layer of cells on the opposite side of Bruch’s membrane from the RPE layer. Itprovides retroreflection of light back through the photosensitive layer of the retina. The initial absorption coefficientof the retina is typically greater than 80%. Therefore, the utility of this retro-reflection is marginal in terms ofoverall performance. It may offer an improvement of about 10% at the photodetection level. Such an improvementwould only be useful under scotopic conditions. At higher levels, it would be ignored by the adaptation function.

The effect is intriguing because of its apparent importance in the retroreflection from the eyes of animals observedby humans at night. It is important to recognize that just the specular reflection from the vitreous/neural surface ofthe retina contributes significantly to the observed phenomena. While the light passing through the retina, reflectingfrom the tapetum, and returning to the eyes of the observer has been reduced to only a few percent of the incidentlight, a similar percentage can be reflected by the vitreous/neural interface due to a change in index of refraction. The significance of the effect is that the total light leaving the neural layer is retroreflected by the optical system. This causes the effect to be orders of magnitude brighter than the Lambertian light scattered by equivalent areas ofthe rest of the animals body. The effect has little to do with the tapetum per se. The figure provided by Rodieck in1973 should be disregarded91. All RPE cells contain mobile concentrations of chromophores that may be of theappropriate size to cause some retro-reflection due to their own spherical shape. Thus, his “retinal tapetum” isgeneric to all eyes and is actually an incidental feature of the RPE layer. For further details at the histological level,see Section 3.2.2.1.1.

Page 36: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 35

92Blinkov, S. & Glezer, I. (1968) The Human Brain in figures and Tables. NY: Pergamon Press pp 118-12593Oyster, C. (1999) The Human Eye. Sunderland, MA: Sinaure Associates, Inc. Chapter 1594Detwiler, S. (1943) Vertebrate photoreceptors. NY: Macmillan pp.17-2595Perry, V. & Cowey, A. (1985) The ganglion cell and cone distributions in the monkey retina. Vision Res. vol.25, no. 12, pp 1795-181096Polyak, S. (1941) The Retina. Chicago, IL: Univ. Chicago Press. pg 161 97Polyak, S. (1957) The vertebrate visual system. Chicago, IL: University of Chicago Press pg. 276

3.2.1.4 The optic nerve

Blinkov & Glezer have provided the most detailed tabulation of the structure and contents of the optic nerve andcompared it with the properties of the auditory and olfactory nerves92.

3.2.2 Gross histology of the retina

Historically, histological studies have not been able to differentiate between cells of different spectral performance93. This is largely true today although a few studies of small areas have begun to suggest at least the statisticaldistribution of photoreceptors as a function of their spectral class.

Detwiler94 provides a comparative chart of the cross sections of the retinas of a variety of animals to a commonscale. The results are worthy of study but are printed at a small scale. The highlight of his discussion concerns thesignificantly more neurons of the second order, bipolar, horizontal and amercine cells, in the retina of birds. Theratio of inner nuclear layer to outer nuclear layer thickness is five or six in some birds compared to two or less inman.

Perry & Cowey have provided a wealth of information on the retinas (as well as information on the physiologicaloptics, although interpreted using Gaussian optics) of several families of monkey95. It is noteworthy that they omitany discussion of the presence and densities of rods in the retinas. A caution is appropriate in their use of the termmagnification factor of the retina. This term is used more like a convergence factor in the neural path than an actualmagnification factor associated with the physiological optics. Although their work is strictly histological, theyconclude that 80% of the ganglion cells are associated with the chrominance channels connecting to theparvocellular layers of the dorsal LGN.

The frequently reproduced caricature of the human fovea by Polyak is shown in modified form in Figure 3.2.2-196.The curved line representing the cross-section of the Petzval surface of the optical system is indicated by the arrow. The original sketch is asymmetrical. The center line marked A represents the center of the foveal pit formed by theInner Limiting Membrane. The center line marked B represents the center of the curved Petzval cross-section. Alsoshown is the nominal 350 micron width of the foveola and the approaching rays of the light passing through thepupil. The dashed rectangle is shown centered between the two center lines. The F/8 bundle represents the lightadapted pupil and the F/2 bundle represents the dark adapted pupil. From these overlays, it is seen that the opticalpath to the foveola is essentially free of neurons associated with the INL and the ganglion cell layer under lightadapted conditions. Nuclei of the photoreceptor cells remain in the optical path but are quite close to the Petzvalsurface. Their presence is a compromise between optimum optical resolution and metabolic support to the IS’s. Under low light conditions performance is further compromised by the incursion of INL neurons into the F/2 opticalbundle. Additional, although dated, comments by Polyak can be found in his later work97. It is worth noting thatneither of his figures exhibits a distinct layer that can be described as a macula lutea.

Page 37: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

36 Processes in Biological Vision

98Wojtkowski, M. Srinivasan, V. Ko, T. et al. (2004) Ultrahigh-resolution, high-speed, Fourier domain opticalcoherence tomography and methods for dispersion compensation Optics Express vol 12(11), pp 2404-242299Alam, S. Zawadzki, R. Choi, S. et al. (2006) Clinical application of rapid serial Fourier-domain opticalcoherence tomography for macular imaging Ophthalmology vol 113, pp 1425-1431100Choi, S. Zawadzki, R. Greiner, M. et al. (2008) Fourier-domain optical coherence tomography and adaptiveoptics reveal nerve fiber layer loss and photoreceptor changes in a patient with optic nerve drusen JNeuroophthalmol vol 28(2), pp 120-125

Figure 3.2.2-1 A caricature of the central one-third of the human fovea from Polyak labeled with the conventionalmorphological terms and overlayed with the optical parameters of the system. See text. The horizontal white dashedline (labeled Osterberg section) will be discussed in Section 3.2.2.3.

A new method of imaging the living retina in cross section has recently appeared and been improving very rapidly. Figure 3.2.2-2 shows an ultrahigh-resolution spectral OCT image of human macula from Wojtkowski et al98. Thisversion employs false color imaging to highlight specific features. The technique employs Fourier domain opticalcoherence tomography (FD-OCT) with numerical compensation for the dispersion of light within the biologicaltissue. Alam et al99. and Choi et al100. have provided similar examples of the FD-OCT technique applied to thehuman retina. The technique views the retina through the pupil and measures the time delay (convertible to distancetraveled by the light) associated with each axial layer of the retina. This new technique is the first to “resolve”Verhoeff’s membrane in a living human retina. Verhoeff’s membrane is actually a representation of the averageresponse from a more complex layer containing a variety of individual elements, including terminal bars formedbetween adjacent RPE cells (Section xxx). The representation represents the 1/3 of the RPE closest to the pupil. The remainder of the RPE and the closely associated Bruch’s membrane (BM) are represented by the feature labeledeither choriocapillaris (CC) or RPE/BM by different investigators. Note the longer outer segments directly belowthe pit of the foveola.

Page 38: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 37

3.2.2.1 The sensing laminate

The sensing laminate of the retina provides the best reference for describing the retina at the histological level. Itconsists of the space between Bruch’s membrane and the OLM. This laminate only includes three sub-laminatesand is readily visible through the neural laminate that is sufficiently transparent. The high degree of planarity ofthese layers and the lack of interconnections in the plane of the layers make them easily identifiable. The layeradjacent to Bruch’s membrane is the RPE laminate. The next laminate is the Outer Segment laminate and the finallayer is the Inner Segment laminate.

The sensing laminate is dominated by the large number of individual photoreceptors of the visual system and avariety of additional cells providing physical support, isolation and possibly other functions. Most of these ancillarycell types have received little attention. The laminate is easily subdivided into two layers, the Inner Segment layerand the Outer Segment layer. The Outer Segment layer is very fragile. The Outer Segments are usually sheared inhalf when attempts are made to separate the photoreceptor laminate from the RPE laminate. This type of shearing isalso the dominant form of damage in the pathological condition known as detached retina.

Because of the fragility of the Outer Segment layer, it is rare to find experimental histological results that involve theOuter Segment. The results invariably represent sections through the Inner Segment layer. It is also rare to find anydiscussion of the correlation between the parameters associated with the Inner and Outer layer in order to support thegeneralizations usually applied to the results relative to the actual Outer Segments.

Recently, Williams at the University of Rochester has been perfecting a method of studying the Outer Segments in-vivo using photographic techniques and selective spectral filters in the illumination path. This has provided retinalmaps at resolutions of better than 2 microns. These maps have provided the first maps defining the location ofindividual photoreceptor according to their absorption characteristics.

3.2.2.1.1 The RPE sub-laminate

At the histological level, the RPE consists of two separate planes of cells, the sheet of tapetum cells arranged in closeproximity to Bruch’s membrane. This membrane forms the boundary between the RPE laminate and the choroid. The retinal epithelium cells form a thick semipermeable sheet opposite to the tapetum and adjacent to the OLM.

Figure 3.2.2-2 Ultrahigh-resolution spectral OCT image of living human macula using 2nd & 3rd order numericaldispersion compensation. Nominal axial (vertical) resolution is 2.1 microns. Lateral resolution is poorer than 5 microns.Illumination was 144 nm wide FWHM centered on 850 nm. Alternate labels are shown as used by various investigators.Note asymmetric magnifications as indicated by the Plimsoll mark at lower right. VM = Verhoeff’s “membrane.” CC= choriocapillaris. BM = base membrane. From Wojtkowski et al., 2004.

Page 39: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

38 Processes in Biological Vision

101Kandel, E. Schwartz, J. & Jessell, T. (2000) Principles of Neural Science, 4th ed. NY: McGraw-Hill pg. 515102Torrey, XXX Pg. 480103Bedell, H. & Enoch, J. (1980) An apparent failure of a photoreceptor alignment mechanism in a humanobserver. Arch. Ophthal. Vol. 98 pp. 2023-2026 has a list of references104Oyster, C. (1999) The Human Eye. Sunderland, MA: Sinaure Associates, Inc. Chap. 15105Rodieck, R. (1973) op. cit. Pg. 354106Williams, D. Collier, R. & Thompson, B. (1983) Spatial resolution of the short-wavelength mechanism InMollon, J. & Sharpe, L. eds Colour Vision NY: Academic Press pg 485+

These two layers form the enclosed IPM which contains both the outer and inner segments of the photoreceptorcells. The IPM is thus isolated from the blood stream behind the RPE and from the vascular matrix of the INM bythe OLM. This protected environment is required to support the chemically delicate chromophores of vision.

The tapetum sheet can evolve to form a variety of functions depending on the animal. It is generally a passive layer. Normally, it can aid in the absorption of stray light that has passed through the retina. In some cases, it consists ofsmall groups of cells that act as a retro-reflector to direct light back through the retina. As seen in the case of themollusc, Pecten, the cells can also be used to form an optically coherent sheet of cells that form a reflecting opticalelement in a catadioptric lens system.

The retinal epithelium sheet is much more complex. Besides supporting the metabolic requirements of theaccommodation amplifiers of the photoreceptors, the retinal epithelium is an important factory supporting theoperation of the disks in the Outer Segment. This function is described in detail in Chapter 7.

3.2.2.1.2 Orientation of photoreceptors in the outer segment sub-laminate

It is usually tacitly assumed that the photoreceptors are arranged perpendicular to the surface of the retina. This is theway they are invariably presented in caricature101. The text by Torrey has inappropriately shown the retinal layer ofthe eye in cross-section as a matter of artistic license102. In fact, the photoreceptors are sheared over based on theirlocation in the retina so that their optical axis is essentially parallel to the principal ray approaching from the exitaperture of the objective optical group (See Section 2.3.1.2). This has been well documented by Bedell & Enoch103

although their caricature of the eye is mis-drawn. They show that all of the photoreceptors are aligned with thecenter of the exit aperture of the objective lens group. The caricature assumes paraxial optics for conditions as greatas 25 degrees off the axis. Although they have not labeled the rays in the caricature, they imply a ray 25 degrees offaxis in object space is also 25 degrees off axis in image space. The fact that all healthy photoreceptors have the axisof their Outer Segment (and if appropriate, the portion of the Inner Segment containing the ellipsoid ) parallel to theincoming light ray should not be dismissed. It has a significant impact on the overall sensitivity of the visual system,particularly at high angles relative to the optical axis.

3.2.2.1.3 Spatial parameters of the mosaics of the outer segment sub-laminate

A recent textbook by Oyster provides useful material related to the mosaic parameters of the retina104. However, thematerial should be considered in the context of a sparse array of data. Obtaining photomicrographs of specificmosaics in the retina is difficult, particularly with respect to the entrance apertures of the Outer Segments. Very fewmicrographs of human retinas appear in the literature showing the mosaic of the Outer Segments. Most micrographsshow the mosaic of a part of the Inner Segment mosaic. Differentiating between caricatures and photographs andbetween photographs of the Inner Segments versus the Outer Segments is important in this discussion. Rodieckprovides references to retinal micrographs of a variety of animals. The caricature of the human retina shown inRodieck105 is a modern interpretation, without definitive dimensions, of an ancient sketch by Schultze (1866). Itclaims to show a cross section of the outer segments. It should be noted in that sketch, Schultze implied that all ofthe rods and cones were of equal diameter, only their packing density changed due to the size of the inner segmentsof the cones. A more realistic photomicrograph has been presented by Williams et al106. Part of his figure isreproduced as Figure 3.2.2-3. The white dots are soma and the smaller more prevalent dots are axons, or thin inter-soma parts found between the inner segment (extrusion chamber) and the soma of a cell associated with themultilayer character of the outer nuclear layer, ONL in [Figure 3.2.1-1]. The figure shows the same spatialrelationships as Shultze but with a scale. Williams et al. assert it is a tangential section of the retina through theinner segment layer near the fovea of Macaca nemestrina. Thus, the elements shown in the Schultze sketch and the

Page 40: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 39

xxx micrograph have nothing to do with the division of the outer segments into two classes of photoreceptors.

Figure 3.2.2-4 reproduces a sketch from Shultze in 1866, reproduced in Pirenne, showing a possible fractal nature of

Figure 3.2.2-3 Tangential section through inner segment layer of Macaca nemestrina near the fovea stained with Procionyellow. The calibration bar represents 25 microns uncorrected for shirnkage, or about 9.6 minutes of arc. (It is assumedthat the shrinkage factor is 0.78 and that 200 microns corresponds to 1 deg visual angle for this species.) From Williamset al., 1983.

Page 41: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

40 Processes in Biological Vision

107Pirenne, M. (1967) Vision and the eye. London: Chapman & Hall. Plate 7.108Lauwerier, H. (1991) Fractals. Princeton, NJ: Princeton Univ. Press. pg 64-66109Hirsch, J. & Curcio, C. (1989) The spatial resolution capacity of human foveal retina. Vision Res. vol. 29,no.9, pp 1095-1101110Farber, D. et. al. (1985) Distribution patterns of photoreceptors, protein and cyclic nucleotides in the humanretina Invest Ophthal Visual Sci vol. 26, no. 11, pp 1558-1568

Figure 3.2.2-4 The human photoreceptor mosaic in thefovea centralis. x270, the outer diameter corresponds toabout 0.35 mm. Schultze states that the disposition of thereceptors is fully as regular as is shown by thefigure–provided the retina is in a sufficiently fresh state. bbrepresents the bodies or inner segments, arranged incurvilinear rows as a shagreen-like mosaic. In a, the pointedends, or outer segments, of the receptors are shown as theyappear when the microscope is refocused. The principle onwhich the receptors are arranged is shown by the lines cc.The figure is based on drawings made of several freshretinae. The black part of the drawing shows the retina asSchultze supposed it woud appear if the “pigment” were leftin position. Schultze, 1866.

the human retinal mosaic107. In this map, the size of the photoreceptors varies by less than a factor of two. AlthoughSchultze used a french curve to create a framework describing what he saw when examining a flattened retina, it ispossible that the actual framework is described by a specific fractal form associated with biological growth ofspherical forms and based on a form known as a spherical spiral108. Alternately, the fundamental pattern may be of asimpler repeating, and overlapping face-centered pentagonal pattern as discussed below.

Nolte continues the common practice in his figure 17-12, incompatible with his figure 17-6, of showing cross-sections of only inner segments by Curio. The functional elements are the mature outer segments extending from theinner segments to the RPE. These elements are all nearly identical in diameter and length. (C) of his figure 17-13shows the diameter to vary by less than 2:1 over a major part of the retina. This is in contrast to the proposal byHirsch & Curcio that the variation of center-to-center cone spacing exceeds 2.5:1 within two degrees of center109. However, these authors are actually measuring inner, not outer, segments. Their equation for the variation in thisspacing, and its applicable range are clearly inappropriate. It defines the impossible condition of a center-to-centercell spacing of zero at the center of the retina.

Farber, et. al. have shown two micrographs of a humanretina at different eccentricities110. While the article isentitled photoreceptors, the figures are clearly labeledinner segments. If the figures actually show an areawhere the inner segments (or nuclei) are arranged inechelon, the labels become misleading.

There is also the challenge of obtaining in-vivo versusin-vitro mosaic micrographs. At the present time, onlyin -vivo micrographs, taken by reflection

spectrophotometry, offer indisputable datawith regard to the chromatic sensitivity ofindividual classes of photoreceptors. Theyalso promise to provide information as towhether all structures photographed areactually participating in the perception ofvision or are associated with otherhousekeeping functions.

Figure 3.2.2-5 from Miller & Snyder shows what theylabel a mosaic of Inner Segments from both themonkey, Macaca fascicularis and the sharp-shinnedHawk, Accipiter striatus. These may actually be thefaces of the outer segments as viewed from the pupilside of the retina as seen in the recent in-vivoophthalmological work of Roorda & Williams (Section3.2.3). The scale added post experiment in (B) showsthe Inner Segments to be nominally two microns indiameter with a very small standard deviation in theright half of the picture. Some smaller cells, nominallyone micron in diameter, are seen in the left part of thepicture. There is also a change in nominal spacingbetween the cells in the left and right half of thispicture. The image for the hawk may be taken throughthe field plate formed by the neural laminate. This

Page 42: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 41

111Walls, G. (1942) The vertebrate eye. Cranbrook Institute of Science pg. 183112Snyder, A. & Menzel, R. (1975) Photoreceptor optics. NY: Springer-Verlag. pg. 45113Bowmaker, J. & Kunz, Y. (1987) Ultraviolet receptors, tetrachromatic colour vision and retinal mosaics inthe brown trout Salmo trutta. Vis. Res. vol. 27, pp. 2101-2108114Coxeter, H. (1961) Introduction to Geometry. NY: Wiley & Sons, Chap. 2, 3 & 4.115Hull, D. & Bacon, D. (2001) Introduction to Dislocations, 4th ed. Oxford: Butterworth Heinemann

Figure 3.2.2-5 CR Micrographs of foveal “cone” innersegments at fixation point of monkey, Macaca fascicularis(A) and deep fovea of hawk, Accipiter striatus (B) at thelevel of the ELM. Arrows added to mark discontinuities.From Miller & Snyder (1979)

may introduce differential distortion in the micrograph of up to 30%111. In (A), the photoreceptors are as closelypacked as possible for cylinders. They appear as hexagonal groups of seven photoreceptors locally. On a largerscale, the array exhibits minor dislocations and specifying the characteristics of the overall pattern is more difficult.

Some micrographs of retinas have shown “rods” of much smaller diameter interspersed among “cones.” Snyder &Menzel have discussed the optical properties of Outer Segments from the perspective of light pipes in detail112. It isimportant to note that an Outer Segment with a diameter less than the wavelength of light to which its chromophoreis optimized is a very poor acceptor of light. Structures in these micrographs with a diameter of less than one micronshould not be considered photoreceptor structures without overwhelming evidence.

Bowmaker & Kunz113 have provided some local areamosaics of fish as a function of age. They show thepattern of cells changes considerably with age andconfirm that trout are tetrachromatic at ages up toabout two years. The local pattern is repeated similarto the replication of the retinula in molluscs.

When reviewing micrographs of a retina, theinvestigator usually perceives a pattern to the cells. However, the pattern is usually more a work of art thana geometrical pattern designed for simple datamanipulation. It appears that science has not yetdiscovered the underlying fundamental pattern(s) usedto form retinas in animals.

3.2.2.2 Geometrical patterns in retinalarrays

The geometric layout of the retina is occasionallydescribed as based on triangles, more often onhexagons and most frequently on close spacedhexagons containing a center element (groups of sevencells). It is often suggested that the breakdown inorderliness frequently results in close spaced pentagonsbeing interspersed with the close spaced hexagons.

From a geometrical perspective, describing the arraysin terms of triangles, even equilateral triangles, isgenerally inadequate. A higher order description isneeded to avoid ambiguity. The descriptions used incrystallography are probably most appropriate. Coxeter has provided a good, and hard to find, introduction to two-dimensional crystallography andthe various faults found in that field114. The title of hisbook is misleading. It is an introduction to collegelevel geometry and requires vector algebra and differential equations as prerequisites to follow theanalyses. Hull & Bacon115 have provided anintroduction to dislocations in crystallography thatproceeds from Coxeter. Unfortunately, it movesrapidly into three-dimensional rectilinear structures. It

Page 43: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

42 Processes in Biological Vision

Figure 3.2.2-6 The proposed fundamental array used in thespherical human retina. The array is necessarily distortedwhen drawn on a flat surface. However, the plasticity of theouter segments accommodates this distortion in thelaboratory. See text.

divides the possible types of imperfections in a regular array (or lattice) into point, line, surface and volume defects. These defects disturb the locally regular arrangement of the atoms. The point defect can consist of a void in theregular lattice or an interstitial insertion. The line defect usually involves the introduction of a short row of elementsbetween two longer rows of a lattice. Alternately, it can consist of the removal of a short segment of a row, causingthe adjacent rows to distort. The most common surface defect is the grain boundary between adjacent grains withdifferent intrinsic alignments. The grain boundary can look like a simple slip line only affecting a narrow region oflattice elements or it can appear as a stretching of the adjacent lattices to fill a localized void. This stretchingintroduces other local distortions as well. Each of the above imperfections is readily recognized in the abovemicrographs of retinas. It is likely that interstitials are unusual in the morphogenesis of the retina and that voids aremasked by the plastic nature of the liquid crystalline structures of the outer segments.

While a close-packed hexagon is the most intuitive form of fundamental region and linear axes of translation are tobe expected in the retina based on the theory of flat lattices; however, the retina is not flat. It is based on a sphericalsurface. The actual basic non-overlapping geometricform on the surface of a sphere, consists ofintersecting, equal length, segments of great circlescalled geodesics. These are found to result in afundamental array based on a close-spaced pentagonalarray. The fundamental array consists of fivehexagons surrounding a pentagon, all face-centered (Chapter 20 in Coxeter). This fundamental array isshown approximately in Figure 3.2.2-6 as distorted byshowing on a flat surface. Each straight line is actuallya geodesic. The fundamental pattern consists of fivehexagons surrounding a pentagon. The centralpentagon is surrounded by three pentagons forming thecenters of adjacent overlapping fundamental patterns. The individual photoreceptor outer segments occupythe center of each geometric figure and the points ofunion of the individual vertices of the figures. Thediameter of these individual outer segments are foundto be generally round and of the largest diameterpossible. However, the plasticity of the outersegments may force some of them into pentagonal orhexagonal shapes. It is also possible for thisfundamental pattern to change scale to accommodatelarger diameter outer segments as a function of thedistance from the center of the overall pattern.

If a retina is flattened for study, one should not besurprised if an apparent hexagonal structure is noted. The human visual system appears quicker to recognizehexagons than pentagons. However, the significanceof this hexagonal appearance should not be over-emphasized. The individual elements are stillconnected (theoretically) by great circles, not straightlines and the fundamental pattern is a face-centeredpentagon.

This work will take the face centered pentagon as thefundamental pattern of the human retina. Such apattern contains five face-centered hexagonssurrounding a face-centered pentagon as describedabove. This is the fundamental pattern of abasketball.

Note that when projected onto a flat surface, the retinadoes not exhibit clearly defined major axes. While onemight associated major axes with the pair of hexagons located side by side in the figure, other similar pairs ofhexagons are at angles relative to this pair. This will be important when discussion the Nyquist frequency of theretina as a sampling mechanism in Section 16.6.3.

Page 44: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 43

116Deering, M. (2005) A photon accurate model of the human eye ACM Trans Graphics vol 24(3) 117Deering, M. (2005) http://delivery.acm.org/10.1145/1080000/1073243/p649-deering.pdf?key1=1073243&key2=5477055711&coll=&dl=ACM&CFID=15151515&CFTOKEN=6184618118Stewart, I. (2011) Mathematics of Life: Unlocking the Secrets of Existence. London: Profile Books pp40-49119Dubra, A. Sulai, Y. Norris, J. et al. (2011) Noninvasive imaging of the human rod photoreceptor mosaic usinga confocal adaptive optics scanning ophthalmoscope Biomedical Optics Express vol 2(7), 1864-1876

The use of the optical Fourier Transforms offers a unique and rapid method of studying the array parameters of aretina, even in-vivo. This technique will be discussed further in Section 16.6.3. By varying the size of theilluminated window on the retina, the parameters can be evaluated as a function of position and area on the retina.

Recently, Deering has provided a computer generated replica of the spherical human retina that he believes hascaptured the actual rules employed in morphogenesis116. Because the paper is in an obscure journal, it may be hardto retrieve. A more readily available copy was available on the Internet in 2007117. The synthesized retina isvirtually indistinguishable from recent retinal images from Roorda & Williams (Section 3.2.3).

Stewart118 has offered an opposing origin of the map of the retina from that of Deering. Figure 3.2.2-7 is based onthe biological rules for cell replication (phyllotaxis) developed by Hofmeister in 1868. Hofmeister’s generativespiral was later revealed to be a so-called Fermat spiral based on the “Golden Angle” of 137.5 degrees appearing inderivations related to Fibonacci numbers. While based on projections on a flat surface, it appears to explain thevarious swirl patterns frequently seen in images of the receptor mosaic. The sensitivity of the patterns to variationsfrom 137.5° are obvious.

Dubra and associates reproduced their data on theshapes of retinal elements with the implicit assumptionthat the cells should be present in hexagonal groupingsas frequently assumed119. Their mapping clearly showsthe presence of significant numbers of 5-sided as wellas 6-sided arrays. Their data was collected usingthrough the pupil imaging of living human retina. However, they did not account for the spatialdistortions associated with the anamorphic lens of thereal eye at positions off-optical-axis. Figure 3.2.2-8shows their figure 5 with their caption. Figure 3.2.2-7 Fermat spiral patterns. Left; spacing 137°,

just less than the golden angle. Middle; spacing 137.5°, thegolden angle. Right; spacing 138°, just greater than thegolden angle. From Stewart, 2011.

Page 45: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

44 Processes in Biological Vision

A Voroni diagram is defined with respect to a flat plane and the data from the retina is not strictly compatible withthe planar requirement. However, the concept is useful.

Quoting Wikipedia as a convenience, “In mathematics, a Voronoi diagram is a partitioning of a plane intoregions based on distance to points in a specific subset of the plane. That set of points (called seeds, sites, orgenerators) is specified beforehand, and for each seed there is a corresponding region consisting of all pointscloser to that seed than to any other. These regions are called Voronoi cells.”

3.2.2.3 Statistical parameters of the complete mosaic of the outer segment sub-laminateRE-OUTLINE

The availability of precise statistics on either the geometrical or chromatic parameters of the retinas of animals isvery limited. There are many first order estimates but very few statistically relevant estimates of these statistics. Obtaining the geometrical parameters appears to be limited by the lack of an adequate theoretical base on which tocollect the statistics. A fundamental fractal equation that can be modified for different situations is probably needed.

This section is complicated by the functional division between the region of the retina called the foveola and thesurrounding retina. The statistics of these two areas are not likely to be the same. They should be considereddistinct subsets of the mosaic statistics. A following section will discuss the statistics of the foveola particularly.

Another problem associated with evaluating the statistics of the foveola involves the processing of the data. There is

evidence that the pretectum attempts to treat all of the photoreceptors of the foveola as spectrally equivalent whenevaluating fine detail. It is only when highly selective illumination is used that the signals delivered to the pretectumare limited to those from a specific set of spectral receptors. It also appears that there is little requirement fororderliness in the individual spectral arrays in a system based on edge detection and color estimationg involvingnearest neighbors to the edges. As a result, it has been very difficult to quantify the performance of the foveola interms of the spatial responses and spatial frequency responses as a function of chromaticity. This is particularly truewhen the chromatic sources used have not been carefully chosen to maximize contrast. Williams addresses the data

Figure 3.2.2-8 Dubra’s analysis of the regularity of the peripheral photoreceptor mosaic. Shown in a is the 6-houraveraged image (logarithmic display) from subject JC_0138, taken at about 10/ temporal to fixation, collected using 680nm light and 1.1 Airy disk pinhole size. Color-coded Voronoi domains associated with each cell are shown in panel b),where the color indicates the number of sides on each Voronoi polygon (magenta = 4, cyan = 5, green = 6, yellow = 7,red = 8, dark blue = 9). Regions of six-sided polygons indicate a regular triangular lattice, while other color mark pointsof disruption of the mosaic. Panel c shows the color-coded Voronoi domains associated with just the cone photoreceptorsin the image. From Dubra et al., 2011.

Page 46: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 45

120Williams, D. Sekiguchi, N. & Brainard, D. (1993) Color, contrast sensitivity, and the cone mosaic. Proc.Natl. Acad. Sci. USA vol. 90, pp 9770-9777121Pum, D. Ahnelt, P. & Grasl, M. (1990) Iso-orientation areas in the foveal cone mosaic, Visual Neuroscience,vol. 5, pp. 511-523122Pirenne, M. (1967) Vision and the eye. London: Chapman & Hall, plate 7123Lee, B. (1996) op. cit. Pg. 634124Kageyama, G. & Wong-Riley, M. (1984) The histochemical localization of cytochrome oxidase...withparticular reference to retinal mosaics. . . . Jour. Neurosci. Vol. 4, no. 10, pp. 2445-2459125Chan, T. Goodchild, A. & Martin, P. (1997) Vis. Neurosci. vol. 14. pp. 125-140126Wassle, H. & Boycott, B. (1991) op. cit. Fig 2

in this area120.

The reader is cautioned to differentiate between mosaics of the actual outer segments and the much more commonlyreported mosaics of the inner segments (which are not photosensitive). There are considerable differences betweenthese two mosaics at the research level. Recently, a Fourier transform technique has appeared that is able todetermine the orderliness of the retinal mosaic, irrespective of chromatic content, in-vivo and very rapidly.

Pum, Ahnelt & Grasl121 have attempted to define the quality of the lattice found in the humans and primates using a“radarscope” presentation. Starting from the premise that the retinal mosaic should be a perfect close spacedhexagonal array of seven photoreceptors, they have provided first order statistics on deviations from this ideal. Theywere using stained Inner Segments and were operating in-vitro. The results provide an autocorrelation functionalong the various axes of the array and are similar to those found in diffraction studies of crystals. The presentationsare both technically and artistically interesting. They were able to clearly demonstrate a dislocation between twozones that involved a 12 degree shift in orientation and a definable change in packing density along specific axes. The simultaneous presentation of more than two zones resulted in the expected statistical characteristics that couldbe read directly from the presentation. They also presented some information attempting to define the distribution ofphotoreceptors of different absorption within the overall arrays.

They make the interesting statement that the Inner Segments display a tubular shape above the external limitingmembrane and reference Miller & Bernard, 1983. As discussed in this Section, Williams and his colleagues havefrequently recorded a tubular image of Outer Segments in-vivo.

Schultze122 provided a large scale map of the inner segments of a human retina that seems to show a fractal pattern. The collection of statistics about the chromatic sensitivities of the photoreceptors, as an overlay to the basic patternhas not been productive to date.

Because of the irregular arrangement of human photoreceptors, most investigators have attempted to determine thestatistical properties of the photoreceptor cells based on either rectilinear grids or by measuring the shortest distanceto adjacent cells, the number of cells within a given radius, etc. These statistics are helpful but not definitive.

Lee gives a set of statistics in his Table 1123. related to the topology of the retina but places them in a framework thatmay complicate their interpretation.

Kageyama & Wong-Riley124 provide a variety of statistical parameters concerning various elements in the retinas ofseveral animals. Their work was based primarily on staining studies related to cytochrome oxidase staining as anindication of activity level in the cells.

Chan et. al125. Provide a set of statistics for horizontal cells in two different primate retinas.

3.2.2.3.1 Statistical parameters of the complete mosaic(s)

Figure 3.2.2-9 illustrates the total number of photoreceptors per square mm in the retina of the rhesus monkey. Thedata is taken from a composite figure prepared by Wassle & Boycott126 based on earlier data from Wassle, et.al. andfrom Steinberg, et. al. Total photoreceptor cell densities are remarkably uniform, averaging 100,000-110,000 cellsper square millimeter for eccentricities between 1.0 mm and 10 mm from the fovea (without accounting for theoptical disk). The density of “cones,” in the original composite was shown as rising to more than 200,000 cells persquare millimeter for a small region within 0.175 mm. of the fixation point. Other details on how this data was

Page 47: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

46 Processes in Biological Vision

127Osterberg, G. (1935 ) Topography of the layer of rods and cones in the human retina. Acta. Ophthal. suppl.6, pp. 1-103128Pirenne, M. (1967) Op. Cit.

assembled appear in Wassle & Boycott. It appears that the rod density should be shown approaching an asymptoteat 150,000-160,000 instead of rising without a limit. This small region is often called the foveola and is equivalent toabout +/- 0.6 degrees of visual angle in object space.

Osterberg127 has provided some very old data that is still reprinted. It provides similar data for the human on a linearscale with eccentricities measured in object space angles out to 100 degrees. Unfortunately, the provenance ofOsterberg’s data was largely lost when Pirenne plotted it128. It has subsequently been reprinted without provenancein virtually every text on vision including Davson, 1962, Rodieck, 1973 &, Wandell, 1995.

The data of Osterberg was obtained by sectioning a human retina at a given depth below the RPE surface of thetissue. The result is a plan-view of the retina taken at a location shown by the dashed white line in Figure 3.2.2-1xxx. Note that within the foveola, the dashed white line only intersects outer segments which are all cylindrical inshape. Osterberg called these outer segments rods. Note that outside the foveola, the dashed white line intersectsthe inner segments of only some of the photoreceptors. It also intersects the axons of the photoreceptors where theinner segments are not intercepted. As a result, the plan-view image of inner segments and axons agrees with thatdrawn by Shultze in 1866, except his so-called rods are actually axons. The common wisdom associated with thisfigure must be reinterpreted. The result is shown in Figure 3.2.2-9. In the region between plus and minus twodegrees, the count represents the density of outer segments intercepted. In the region beyond plus and minus twodegrees, the two counts represent the number of axons intercepted and the number of inner segments intercepted. The ratio between the number of axons and the number of inner segments suggests this retina had an outer neurallayer (ONL) that consisted of an average depth of 10-20 inner segments. The caricature in Figure 3.2.2-1 xxx suggests the lower number. A reinterpretation of a caricature by Shultze presented in 1866 (and reproduced inPirenne in 1948) is shown at the lower left. It applies to the peripheral region of the retina on both sides of thefoveola, except in the blind spot. The elements he labeled rods are clearly axons in this caricature based on thelocation of the section. The figure suggests a higher ratio of axons to inner segments, near 30. Modern micrographsmay provide more definitive numbers.

Page 48: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 47

Figure 3.2.2-9 The reinterpreted photoreceptor and neural densities of Osterberg. The break in the line between the outersegments at zero degrees and plus two degrees is usually drawn to be symmetrical with the region between zero andminus two degrees. Outside of plus and minus two degrees, the same curve represents the fraction of intercepted innersegments. Lower left; a reinterpretation of a caricature by Shultze, 1866, showing a section taken through the outernuclear layer of the retina. See text.

The data of Osterberg, as interpreted by Pirenne, leads to different conclusions from that of Wassle & Boycott. Onthe other hand the data as interpreted here is in close agreement with that of Wassle & Boycott. Their data, for boththe temporal and nasal meridians, show a peak (nearly constant) total photoreceptor density of about 147,000 cellsper square mm. It is clear from the reinterpreted graphs above that the maximum value of rod and/or cone densitydoes not rise without a limit as might be inferred from the logarithmic plot of Wassle & Boycott.

Figure 3.2.2-10 also illustrates the ratio of photoreceptor cells in the retina of the rhesus monkey on both amorphological and a functional basis using a logarithmic vertical scale. Whereas Wassle & Boycott show the cone

Page 49: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

48 Processes in Biological Vision

129Lee, B. (1996) Receptive field structure in the primate retina. Minireview. Vision Res. vol. 36, no. 5. pp. 631-644

Figure 3.2.2-10 Ratio of rods, cones and total photoreceptorcells to ganglion cells in a primate retina (rhesus monkey).The data is a replot of a composite prepared by Wassle &Boycott based on morphological experiments. They onlydisplayed the cone to ganglion ratio and showed it turningdown near zero eccentricity. There are no ganglion cellswithin the foveola and the cone-to- ganglion cell ratio mustrise to a morphological asymptote in this area. This workdoes not recognize the dichotomy of “rods & cones” in afunctional context. Only the top line represents a functionalrelationship. On a functional basis, there is one displacedganglion cell for each photoreceptor in the foveola. “A”, inthe lower left points to this functional asymptote. Theoriginal densities were measured along the temporalhorizontal meridian, zero eccentricity represents the fovea.

to ganglion cell ratio as turning down at smalleccentricities, this is a poor assumption. Within thefoveola, there are no ganglion cells. A morphologicalratio with a denominator approaching zero mustapproach infinity in the limit. There are ganglion cellssupporting the foveola. However, they are displacedlaterally within the retina. The result is a ratio thatapproaches infinity for the morphological case afterdipping to a minimum at an eccentricity near 0.5-0.75mm. For the functional case, Chapter 13 will discussthe “straight through signal path” believed to apply tothe photoreceptors of the foveola. Using this concept,the functional ratio of photoreceptors to ganglion cellsapproaches 1.0 in the limit as the eccentricityapproaches zero.

As noted in Chapter 2, care must be taken in relatingthe density of photoreceptors to the spatial resolutionof the eye and to the degree of physical convergencerelative to the number of neurons in the optic nerve. The morphological concept of convergence, asexpressed in the ratios of the above figure, does notaccount for the use of time-diversity and spatialdiversity encoding in the visual system. Byemploying these concepts, the visual system is able toperform its normal functions without any loss ofsignificant information. The physiological optics ofthe eye controls the spatial resolution of the system toa much greater degree than does the density ofphotoreceptors. Thus, the curves presented as figure 5by Lee primarily describe the performance of theoptics of the eye and not of the types of cellsenumerated129.

The above graphical relationships between the peakvalues in human can be evaluated more precisely inFigure 3.2.2-11. This figure shows the size and celldensity near the fixation point of human. Unfortunately, that figure is labeled as a sectionthrough the inner segments of the retina. A sectionthrough the outer segments would be more definitive. However, if the section were through the area of theellipsoids, it would be descriptive of the entrance

apertures of the outer segments. One must question whether the figure was obtained from a planar slice obtainedusing a microtome. If so, the image may include various zones of the retina as can be seen from [Figure 3.2.2-1]. Aslice taken approximately 50 microns from the RPE would show Inner Segments in the central portion of the imageand soma in the outer portion. Similarly, a slice taken at 25 microns from the RPE would show Outer Segments inthe center and Inner Segments in the outer portion.

Most of the literature, shows the Inner and Outer Segments to have the same diameter close to the fovea. Osterbergalso shows that the first rods appear at approximately 0.16 mm. from the center of the fovea (0.13 mm. in the fixedretina). A more definitive change occurs at 0.05 mm. in the figure. Within this region, the very small innersegments are usually related to a “rod free zone.”

Page 50: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 49

130Roorda, A. Romero-Borja, F. Donnelly III, W. Queener, H. Hebert, T. & Campbell, M. (2002) Adaptiveoptics scanning laser ophthalmoscopy Opt Express vol 10, pp 405-412

Figure 3.2.2-11 CR Horizontal section through a region of a fixed human retina containing the fovea whose exact centeris at the intersection of the straight lines at the left. The section is through the inner segments. The actual dimensionsof the unfixed retina were 1.31 larger than indicated. Pirenne (1948) indicated that cones are open circles whosediameter enlarges toward the parafovea; rods are seen as small black dots. Using Hogan’s dimensions, the foveolaextends to 0.175 mm. from the center of the circle (0.13 mm on this scale). From Osterberg, 1935.

Roorda et al. have recently provided photoreceptor densities in the human retina using their new adaptive opticsequipped scanning laser ophthalmoscope (AOSLO)130. The data is a great improvement over earlier measurementsand assertions, Figure 3.2.2-12. No discussion of the role of rods and cones appears in their paper. Note, nomeasurements are provided from within the radius of the foveola, 0.6 degrees.

Page 51: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

50 Processes in Biological Vision

Correlating these and various other comments in the literature concerning this data on morphological photoreceptordensities with the corresponding statements regarding the sensitivity of the retina is difficult. There are many casualstatements in the scientific literature that the human eye is blind at night in the region of the fovea because of thelack of rods in this area. Such a statement is absurd. This area forms a circle with a radius of 3.2 degrees from thefixation point and has a diameter equal to nearly 10 moons. Does the statement include the region of the foveola aswell? Is blind the right term? Would a more precise statement be that the sensitivity of the eye in the region of thefovea is limited to that of the cones? Based on the familiar adaptation curves, the sensitivity of “cones”is about twoorders of magnitude less than that of “rods,” at night? This is also unrealistic. The photoreceptors of the fovea, andthe foveola, provide the same psychophysical threshold for night vision as any other photoreceptors in the same eye,despite location. They may even provide slightly higher sensitivity under certain conditions because of the greaterlength of their Outer Segments. This greater length is due to the curvature of the Petzval Surface in the fovea.

Although the literature frequently gives the impression that the density of photoreceptors is much higher in thefoveola than elsewhere, it is not in the human. The maximum density is restricted by the requirement that thediameter of the photoreceptor cell must be large enough, about 2.0 microns, to accept light without significant lossdue to diffraction/refraction effects. This figure also illustrates that the density of cells cannot be calculated withgreat accuracy when only a few cells are present per unit area.

Looking at the figures provided by Wassle & Boycott, Osterberg, and Pirenne as a group, it appears the cells in thehuman fovea are not less than one quarter the diameter of the peripheral photoreceptors and the density ofphotoreceptors in the fovea is less than 135 percent ( 147,000 divided by 110,000) of that in the parafovea.

Looking at the figure from Osterberg, it does appear there is a slightly higher packing density of cells in the regionfrom 0.1 to 0.175 mm. from the visual fixation point in the chemically fixed retina. This impression may be due to

Figure 3.2.2-12 Change in photoreceptor spacing with eccentricity. The circle symbols show the cone photoreceptorspacing as a function of eccentricity from the fovea. The long-dashed line shows anatomical data from Curcio et al. 1990and the short-dashed line show psychophysical estimations of cone spacing from Williams. From Roorda, 2002.

Page 52: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 51

131Cicerone, C. & Nerger, J. (1989) The relative numbers of long-wavelength-sensitive to middle-wavelength-sensitive cones in the human fovea centralis Vision Res vol 29(1), pp 115-128132Ahnelt, P. Kolb, H. & Pflug, R. (1987) Identification of a subtype of cone photoreceptor in the human retina.J. Comp. Neurology. Vol. 255, pp. 18-34133Williams, D. (1985) Aliasing in human foveal vision. Vision Res. vol. 25, no. 2, pp 195-205134Putnam, N. Hofer, H. Doble, N. Chen, L. Carroll, J. & Williams, D. (2005) The locus of fixation and thefoveal cone mosaic J Vision vol 5, pp 632-639135Yellott, J. (1982) Spectral analysis of sampling by photoreceptors: topological disorder prevents aliasing.Vision Res. vol. 22, pp 1205-1210136Williams, D. (1985) Op. Cit.

the printing/reproduction process. Cicerone & Nerger reproduce the Osterberg data in their figure 8131. Their figureconfirms the above comment that the density of photoreceptors is near 147,000 per mm2 in the foveola and drops toabout 1/3 of that density (cells rising to diametess of 165% of those in the foveola) beyond 0.2 mm from the centerof the foveola..

Ahnelt132 has provided a similar black and white micrograph of the human retina in the central foveal region butwithout notation as to what plane is shown. It appears to agree very well with the image by Osterberg. It shows thefoveola extending out to about 0.02 mm. from the center and a rapid increase in photoreceptor size (of about 2:1 indiameter) in the region from 0.02 to 0.05 mm.

Dubra, cited above, has provided recent statistics on the in-vivo human retina using adaptive optics techniques basedon the archaic rod-cone assumption.

3.2.2.3.3 Statistics of the foveola only

The foveola is a functionally distinct area of the fovea normally centered on the point of fixation. It is the criticalarea supporting the analysis, interpretation and eventual perception of the scene within the field of view of the eye. The foveola is generally circular with a diamter of 1.18 degrees. This diameter (equivalent to 175 cells) containsabout 23,000 photoreceptor outer segments. The outer segments associated with the foveola are connected byindividual direct neural circuits to the pretectum of the mid brain and the other elements of the Precision OpticalSystem. The POS plays the critical role of causing fine tremor motions used to analyze a scene within the two-dimensional correlator of the pretectum.

This area also exhibits a significantly different level of performance due to the diameter, spacing and possiblyeffective absorption length of the outer segments. Figure 4 of Williams, although showing a slightly smallerdiameter, illustrates this area under a given set of test conditions133.

Putnam et al. have provided very precise information about the foveola and the location of the point of fixation134. They show the point of fixation is not correlated with their center of the foveola calculations.

3.2.2.3.4 Major axes of the foveola mosaic

While many investigators have studied the orderliness of the retinal mosaic and discussed this orderliness in terms ofa close packed hexagonal array, results have been marginal. Yellott has used a Fourier transform technique toillustrate that there is no preferred axes of the chromophore containing outer segment mosaic in the foveola135. Hisdata and the Fourier transform technique are discussed in Section 16.6.3.

3.2.2.3.5 Limiting resolution of the foveola of the retinal mosaic

The conventional wisdom has always treated the retina as a static imager made up of an array of individualphotoreceptors. The assumption has been that the resolution of such an array is determined by the packing factor(typically close-packed hexagonal array) of that array. In the foveola, this packing is usually defined by acenter-to-center cell spacing of three microns (based on an outer segment diameter of about 2.0 microns). Thecalculation of the limiting response based on this spacing using the Nyquist criterion has always given a calculatedvalue of less than the measured psychophysical performance of the human eye. This has led to the concept ofhyper-acuity. Unfortunately, this concept is not well founded. In the recent measurements of Williams shown inFigure 3.2.2-13, the disparity between the calculated Nyquist limit for an assumed outer segment diameter of threemicrons and the actual measured performance, is more than four to one136. See the original article for the

Page 53: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

52 Processes in Biological Vision

Figure 3.2.2-13 A measurement of contrast sensitivity versus spatial resolution in the human eye. The contrastsensitivity was determined using a lateral shearing type unequal path interferometer. The data points should not beconsidered a simple measure of the conventional contrast function. Modified from Williams, 1985.

psychophysical conditions related to this data. Using a more precise treatment of the Nyquist criteria, a morereasonable fit between theory and performance can be obtained (see Section 16.6.3). Under this treatment, thediameter of the average outer segment is between 1.5 and 2.0 microns based on this measured data. The value of 2.0microns has been used as a nominal standard in this work. It is consistent with the available micrographs of thefoveola portion of the retina (shown earlier in this section). See Section 17.6.3 for an analysis of the test set used.

In the actual case of vision, the retina is not an array that is sampled periodically as in the case of a television sensor. Each of the photoreceptors in the foveola portion of the array is directly connected to the brain. Furthermore, theimage of the scene is scanned across this photoreceptor continuously by the tremor of the eye (during the analysisintervals between the larger saccades). It is this scanning motion that generates the electrical signal that istransmitted to the brain. The motion results in a signal that changes with time. It is this change in the signal from asingle photoreceptor that is of interest in determining the resolution of that channel. In this case, the Nyquistcriterion is applied to individual outer segments and not to the elements of the mosaic

The limiting spatial resolution of the retina depends on the diameter of the photoreceptors. For a nominal twomicron diameter photoreceptor, the spatial resolution of the system exhibits a high pass characteristic with a limitingresolution of 250 cycles/degree. These values are for a square wave test pattern not filtered by the physiologicaloptics of the eye. As in the case of any optical system, the response below the limiting resolution varies slightly fora sinusoidal test pattern. The impact of the physiological optics is always to convert the test pattern to a moresinusoidal form due to the response of the optics.

Figure 3.2.2-14 shows the limiting resolutions of the human retina calculated under both the imager assumption(added by this author) and the change detector assumption. It is clear that the change detector assumption applied toa change detector model correctly describes the limiting resolution of the human visual system. Pask & Stacey have

Page 54: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 53

137Pask, C. & Stacey, A. (1988) Optical coherence and Wolf’s theory for electromagnetic waves J Opt Soc AmA vol 5(10), pp. 1688-1693 doi: 10.1364/JOSAA.5.001688138Laties, A. Bok, D. & Liebman, P. (1976) Procion Yellow: a marker dye for outer segment disc patency andfor rod renewal. Exp. Eye Res. vol. 23, pp 139-148

calculated a similar value based on a single detector model137. Their value using a 3.2 micron diameter waveguidewas between 125 and 165 cycles/degree (see Section 4.3.4.2.1). There is no role for a hyper-acuity concept whenusing the change detector model and the Nyquist criterion.

The data points in the figure are only useful in determining the limiting performance of the system. The testconfiguration developed by Williams does not provide a meaningful estimate of the contrast (spatial) function (CSF). The data points shown in the above figure are interesting by themselves. They represent the results of a testemploying a sub-aperture illuminating test set with special coherent capabilities. See Section 17.6.3 for a discussionof the development of the CSF and the limitations of the Williams test configuration.

The performance of the physiological optics of the visual system is discussed in Section 2.4, with particularemphasis on the human optical system.

3.2.2.4 Statistical parameters of the chromatic mosaics of the outer segments

To date, the investigations into the chromatic aspects of retinal arrays must be considered exploratory. There is nodirect data connecting the data obtained by staining with the spectral response of the cells stained. Furthermore, allpapers on the subject to date have assumed the human retina is trichromatic, in spite of excellent data to the contraryavailable since the 1970's (see Section 17.2.2). The most successful techniques employ differential staining that isvery time and concentration sensitive. Whether the result is a bulk staining of the cell or a surface staining of thecell wall, or a material on the surface of the cell wall, is seldom detailed. In cases where the stain is introduced intothe vitreous humor, how the stain penetrates the outer limiting membrane is also of concern. Laties, et. al. haveprovided the broadest discussion that could be found in this area138.

This work takes the general position that all photoreceptors are identical in composition and surface coatings underquiescent conditions, except for the particular molecular structure of the chromophores. These structural differencesare only recognizable at the molecular level. The molecules are essentially isomers of Rhodonine and are verydifficult to differentiate based on conventional chemical procedures, such as staining. The Rhodonines do notinclude any amino acid groups in their structure.

Three results stand out from this study. All animals are theoretically capable of ultraviolet vision. All three phylaexamined in this study include animals capable of ultraviolet vision. The human retina is capable of detectingultraviolet light as demonstrated by aphakic subjects. However, the human optical system filters out nearly all of theultraviolet light projected toward the retina.

Two principal methods of collecting statistics on the chromatic characteristics of the retinal mosaic are common. The physiological method is to employ microspectroradiometry in a reflective mode to observe the chromaticvariation in the retina. The psychophysical method is to use short flashes of diffraction limited light sources toattempt to stimulate individual photoreceptors and record the perceived response of the individual. This methodgenerally suffers from at least two complications. First, the method does not control the tremor within the eye. Thistremor has a magnitude equivalent to at least the diameter of the typical photoreceptor OS. Second, the method doesnot recognize the differencing technique employed in the architecture of the visual system or the “paint” programused at the cognitive level of the visual system to determine the most likely chromatic content of an area.

The chemical staining method involves staining a retina either in-vivo or in-vitro and then analyzing it in-vitro. Todate, this method has not employed spectroscopic methods to identify the stained photoreceptor cells. This has beengenerally attributed to the fact the dyes used tend to exhibit spectrums that would interfere with the measurement ofthe target cells. The method typically involves introduction of a dye into the vitreous humor and the assumption thatthe dye passes through the outer limiting membrane to dye some portion of the photoreceptors that is related to theirchromatic performance.

Very little statistics exist on the chromatic characteristics of the photoreceptor population of a retina. It appears nosatisfactory criteria have appeared upon which to collect these statistics. So far, only one type of photoreceptor hasbeen isolated by the dye technique. This type has been inferred to be the S-channel photoreceptor based on theputative mean density of S-channel detectors (typically 8-10%) from psychophysical experiments. It is suggested,based on this work, that the prevalence of S– and L–channel photoreceptors are nearly equal. If so, this criteria is

Page 55: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

54 Processes in Biological Vision

139De Monasterio, F. McCrane, E. Newlander, J. & Schein, S. (1985) Density profile of blue-sensitive conesalong the horizontal meridian of Macaque retina. Invest. Ophthalmol. Vis. Sci. vol. 26, pp 289-302140McCrane, E. de Monasterio, F. Schein, S. & Caruso, R. (1983) Non-flourescent dye staining of primate bluecones. Invest. Ophthalmol. Vis. Sci. vol. 24, no. 11, pp 1449-1455

not useful.

As will be shown in the next two Chapters, all of the photoreceptors of vision are functionally the same. This is truefor retinas, retinulas and ommatidia. There are, however, at least three operational differences in photoreceptors:

+ The form of the axon terminal varies depending on the complexity of interconnection with the signal processingneurons. This feature is not directly associated with chromatic performance.

+ The Outer segments of the photoreceptors vary in diameter with location in the mosaic. This parameter probablyvaries in response to optimization designed to the meet environmental requirements of the animal. It does not appearto be related to chromatic performance.

+ The photo/piezo material of, i.e., the chromophoric coating on the disks of the Outer Segments, is spectrallyspecific. Simple methods of determining the spectral properties of this material would lead to a significant increasein knowledge about eyes of different species and eyes in general.

All eyes can conceptually populate their retina with photoreceptors coated with one of four chromophores. An equaldistribution would call for each type to be represented by 25% of the population. This is unlikely for a number ofreasons. First, it appears that the population is strongly influenced by temperature during the gestation of the animal. Second, most eyes exhibit a mosaic that is not easily defined in terms of four quartiles.

Because the lens of the eye of humans (and other large chordates) absorbs most of the ultraviolet light available insunlight, the performance of the overall visual system of humans is considered to be trichromatic. However, as iseasily demonstrated in aphakic eyes, the human retina is actually tetrachromatic. It exhibits a high sensitivity in theultraviolet spectral region between 300 and 400 nm.

3.2.2.4.1 Chromatic mosaics based on trichromatic

The majority of the work on isolating a specific subset of photoreceptors occurred during the 1980's. Subsequent tothat time, various investigators have assumed the earlier work was conclusive. This work involved seeking a stainthat would isolate the spectral varieties of photoreceptors. The investigations were based on the premise that thetrichromatic assumption applied to the retina as it did to the overall performance of the visual system. No attentionwas given to the possibility that there were ultraviolet photoreceptors in the retina of humans and other primates. The summary of the situation presented in de Monasterio, et. al. is informative139. It can be divided into two areas.

They first discuss the relative density of the labeled cells. In the introduction, they say; “These stainedphotoreceptors were identified as the blue-sensitive cones because of their characteristic retinal distribution, largeangular spacing, and different incidence in mammalian species (see Discussion).” In that discussion, they state;“Taken together, the preceding observations strongly support the admittedly indirect identification of the stainedcones as blue-sensitive.” This position is followed by the statement that; “We have not examined the spectralsensitivity of the stained retina to demonstrate a loss of blue cone function [because the dyes themselves absorbedblue light].”

While describing the procedure as involving a differential staining, under carefully controlled stain concentrations,they describe the stain as being introduced into the intravitreal space in front of the neural retina. As they say,“While the trans-retinal transport mechanism was not elucidated, the dye normally did not stain other parts of thecone soma or postreceptoral cells at the concentrations used. . . .” Under the proper conditions, they claimed theyachieved extracellular staining of all of the outer segment of all cones. They indicated their stains bind covalentlywith proteins as well as other compounds containing amino groups. In a slightly earlier paper by the same group,they describe the staining as at low concentrations as staining the extracellular compartment of the outer segment ofcones. . . .140” They provided no model or description of “the extracellular compartment.”

Page 56: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 55

141Kouyama, G. & Marshak, D. (1997) The topographical relationship between two neuronal mosaics in theshort wavelength-sensitive system of primate retina. Vis. Neurosci. vol. 14, pp. 159-167

Kouyama & Marshak141 provide a topographical relationship between two mosaics in the same retina, one of “bluecones” and one of bipolar cells associated with the blue cones. Their isolation of the nuclei of the photoreceptorsand the bipolar cells was based on staining techniques similar to those of the above authors. Kouyama & Marshakmake the assumption that the inferences of the above authors concerning wavelength sensitivity were correct. Theyalso rely upon the trichromatic assumption.

3.2.2.4.2 Chromatic mosaics based on tetrachromatic assumption

Both De Monasterio, et. al. and McCrane, et. al. were careful, except in the title, to stress the isolated subset ofphotoreceptors was only tentatively associated with the S–channel of vision. Many, more recent, studies have reliedupon both the above inferences and the trichromatic assumption to support their conclusions. However, an alternateinterpretation is available. First, any analysis based on statistics must recognize the presence of UV-channelphotoreceptors in the human and primate retina (see Sections 5.5.10 & Section 17.2.2) as well as the putativepresence of rods (if the rod-cone dichotomy is supported). If the retinas being examined were protected fromultraviolet radiation by the lens, there should be a subset of photoreceptors present that are in a different state ofadaptation, and/or operating point. These cells are likely to exhibit a different sensitivity to differential staining thanother operational cells.

The glutamates used to support the operation of all photoreceptors through electrostenolytic mechanisms at thesurfaces of the cells, contain amino acid groups. In general, these electrostenolytic mechanisms occur on the surfaceof both the inner segments and the dendrites found in the furrows of the outer segments. The aggregations ofglutamates in these furrows clearly qualify as “extracellular compartments” of the outer segments. If theUV–sensitive photoreceptors are non functional due to the suppression of UV light by the lens, it is likely that theconcentration of the glutamates will be higher in the immediate vicinity of these UV photoreceptors. Under thisscenario, the cells isolated by the stains introduced by these authors are more likely UV-channel than S-channelphotoreceptors.

There is a great need for conformation of the spectral sensitivity of the stain isolated photoreceptors, to includeexploration of the ultraviolet region.

Liang, et. al. make the interesting observation that the various photoreceptors show different absorptions in theirretinal recordings that are fixed with time. This would suggest that future laboratory efforts should attempt to mapnot only the location and size of the various chromophoric photoreceptors, but also the absorption of these cells. This could lead to the correlation of the absorption of these cells to their age or possibly how uniformly straight theyare ( a factor in their waveguide characteristics, Section 3.6.2.3.3).

Chapter 12 will show that such variations in absorption, detected by reflectometry, are easily removed by theadaptation amplifiers of the photoreceptors.

3.2.2.4.3 Statistical parameters of the individual spectral channel mosaics

Prior to 2000, no demonstrably precise method of spatially isolating the photoreceptors of a specific spectralsensitivity has appeared. Only one method has shown consistently good isolation of a subset of cells of unknownspectral sensitivity. That technique has involved differential staining of an undetermined feature of certain cells. Beginning in 2000, the use of adaptive optical techniques integrated into an ophthalmoscope have providedexceptionally good data on the spectral channel array parameters of the retina.

De Monasterio, et. al. have provided an excellent statistical analysis of the subset of photoreceptors isolated by theirstaining technique. They also provide a few comments on the crystallography they found and introduce tesselationand the concept of Voronoi (geometrical) regions. From their data they calculate several Nyquist frequenciesassociated with the arrays. They appear to use the designation “blue cones” in the captions to their figures whileavoiding that label in their text. They were careful to caveat the equations they show in their paper.

Recently, a number of papers have appeared discussing the distribution of the S-channel photoreceptors in thehuman retina. These must be carefully examined because they usually rely upon inference to determine what cellsare actually sensitive to S-channel radiation. Many of them rely upon staining techniques developed largelyindependent of any spectrophotometric work. The best results appear to come from microspectroscopy. These will

Page 57: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

56 Processes in Biological Vision

142Cicerone, C. & Nerger, J. (1989) The relative numbers of long-wavelength-sensitive to middle-wavelength-sensitive cones in the human fovea centralis Vision Res vol 29(1), pp 115-128143Gowdy, P. & Cicerone, C. (1998) The spatial arrangement of the L and M cones in the central fovea of theliving human eye. Vision Res. vol. 38, pp. 2575-2589144Otake, S. Gowdy, P. & Cicerone, C. (2000) The spatial arrangement of L and M cones in the peripheralhuman retina. Vision Res. vol. 40, pp. 677-693145Stabell, B. & Stabell, U. (1980) Spectral sensitivity in the far peripheral retina. J. Opt. Soc. Am., vol. 70, no.8, pp. 959-963146Neitz, J. & Roorda, A. (2000) Chromatic topography of the retina. J. Opt. Soc. Am. A. vol. 17, no. 3, pp 495-650

be discussed in Section 3.2.3.

Cicerone & Nerger have performed psychophysical experiments to determine the density and ratio of L and M conesusing the common trichromatic assumption142. They did use test wavelengths of 520 and 640 nm to separate theirM– an L– channel photoreceptors. Their model does not provide for either S–channel or UV– channelphotoreceptors in the fovea and is therefore of questionable value. A team at the University of California, Irvine,carried on the Cicerone & Nerger work and has determined that, to the first order, the chromatic photoreceptors inhuman are randomly arranged143,144. Their experiments were psychophysical, based on a Hering model and onlyexamined L and M cones. Their assertion that the distribution of the S cones in the human is well established in theliterature appears weak. Their subsequent use of the words “consensus” and “disagreement about the presence orabsence of cones in the human central fovea” suggest otherwise. No reference appears in these papers to tremor orits compensation. They do refer to microsaccades and the ability of the subject to suppress these throughconcentration. They also address the subject of hyperacuity without discussing its origins. One of their assumptionswas perfect fixational accuracy. They employed 50 ms exposure times in the periphery experiments (17 degreestemporally from fixation) and 200 ms exposures in the fovea. The stimulus was a square, one minute of arc on a sidefor the fovea, 0.86 minutes of arc for the periphery. Assuming a photoreceptor diameter equal to 14 seconds of arcand an average tremor excursion of 1.5 photoreceptor diameters at a nominal frequency of 30 Hz, the imageprojected on the fovea is expected to excite approximately 36 photoreceptors based on the assumptions of paraxialoptics, during the foveal exposure interval. Because of the length of the exposure, many cells near the periphery ofthe test image will be excited multiple times while those in the center remain excited for a majority of the interval. Use of a dim light further complicates the question of perception. The likelyhood of multiple triads ofphotoreceptors being excited adequately to be perceived during one trial appears to be quite high. The perceivedcolor under such conditions appears to be a matter of chance. In the case of the peripheral experiments, the variablefocal length of the optical system was not discussed. Whereas the photoreceptors are somewhat larger at thiseccentricity (as they indicated), the focal length is somewhat shorter. The resulting photoreceptor size in objectspace is approximately the same. By using a much shorter exposure time of 50 ms, the situation is somewhatdifferent. The same large group of photoreceptors will be illuminated. However, the image will only move aboutone third of a photoreceptor diameter in each orthogonal direction during exposure. The result appears to be thesame, the perceived color of the spot will be determined primarily by chance. Their conclusion that the distributionof spectrally different photoreceptors in the retina is random appears based heavily on the test design andinstrumentation and provides a different interpretation of the same effect observed by other experimenters. Thoseexperimenters report flash to flash variability of perceived color for a one minute of arc test stimulus, as cited insection 1.2 of the second paper.

Stabell & Stabell have presented information on the relative spectral sensitivity of the retina at 45 degrees temporallyfrom fixation in object space relative to the fovea145. They employed a variety of test methods used by others in anattempt to rationalize the data in the literature. Using a 2800 Kelvin source, the general findings were that the retinashowed the same spectral performance at both locations after accounting for the absorption of the macula. Thismaterial continues to imply the interdigitation of spectrally selective photoreceptors is uniform over the useful extentof the human retina.

Neitz & Roorda hosted a special issue on Chromatic topography of the retina146. However, the papers are notablylacking in graphic models. In Martin, et. al., as an example, the S-channel photoreceptors are represented by theirnuclei layer on the assumption that the nuclei layer is a monolayer. Their statistics are suspect based on thisassumption. The micrographs of Sections 3.2.1 & 3.2.2 show clearly that this is not the case. Several papersinvestigating the ratio of L– to M–channel photoreceptors provide ratios between the two that vary over a range of

Page 58: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 57

147Brainard, D. Roorda, A. Yamauchi, Y et al. (2000) Functional consequences of the relative numbers of L andM cones J Opt Soc Am A vol 17(3), pp 607-614148Roorda, A. Metha, A. Lennie, P. & Williams, D. (2001) Packing arrangement of the three cone classes inprimate retina Vision Res vol 41, pp 1291-1306149Hofer, H. Carroll, J. Neitz, J. Neitz, M. & Williams, D. (2005) Organization of the human trichromatic conemosaic J Neurosci vol 25(42), pp 9669-9679150Calkins, D. Schein, S. Tsukamoto, Y & Sterling, P. (1994) M and L cones in macaque fovea connect tomidget ganglion cells by different numbers of excitatory synapses Nature vol 371, pp 70-72151Wassle, H. & Boycott, B. op. cit pp. 449-454

ten or twelve to one. Such a range clearly shows the lack of an understanding of the architecture involved and theresulting problem, as suggested by Brainard, et. al. The papers are clearly exploratory, take no account of theultraviolet photoreceptors found in human retinas (Section 17.2.2.3) and rely upon an inadequate adaptation protocol(Section xxx curve showing wavelength versus chromatic adaptation level xxx). Because the number ofUV–channel photoreceptors may exceed the number for the L–channel, reported ratios of L– to M– may be quitequestionable. While some of the papers hint at the return of the S–channel as a contributor to the luminousefficiency function, the tests were still run at very low color temperatures (typically 2850 Kelvin). The Brainardpaper did introduce the luminous efficiency function as involving a logarithmic sum of the L– and M–channelsignals as a function of wavelength. Alternately, it relied entirely on perceived responses to define a “uniqueyellow” at 574.7 to 576.8 nm. Their discussion of the relationship between photoreceptor spectra and genetics didnot recognize the existence of UV–photoreceptors in the human eye.

The use of adaptive optics equipped ophthalmoscopes has provided unprecedented data on the statistics of thehuman retina. References to the operation of these devices are provided in Section 18.4. Three significant papershave been published by the same basic team147,148,149. As noted in the titles of these papers, the authors did notconsider the presence of UV–channel photoreceptors. Their spectral channel isolation protocol assumed that theS–channel photoreceptors could be isolated from the M– and L– channel photoreceptors before these two channelswere further isolated. It can be assumed the UV–channel photoreceptors were combined with the S–channelphotoreceptors in these studies. The M – and L–channel photoreceptors were separated with an error rate on theorder of a few percent. In each of these cases, an area of the retina at one degree eccentricity was explored. Thusthe following comments may not apply to the foveola. However, they probably do based on the paper by Calkins etal150. The statistical tests used were described in detail.

The results of these studies were clear. “In all eyes, the M and L cones are arranged randomly. This gives rise topatches containing cones of a single type. In humans, . . , the arrangement of S–cones cannot be distinguished fromrandom.” The clumping of photoreceptors of a specific spectral sensitivity can lead to very large differences in theratio of L– to M– photoreceptors in a given area. The variability of this ratio is significant in confirming thearchitecture of the stage 2 signal processing within the retina (Section 13.5.3). The ratio varied from 1.1:1 to 16.5:1in the Hofer et al. paper.

The method of describing the statistical results varies among the above paper. However, in the general situation, thepercentage of UV– and S– photoreceptors combined is on the order of 5.5% of the total. After subtracting the UV–and S– photoreceptors, the percentage of L–photoreceptors has a mean of about 60% but varies from 27% to 94%among eight subjects. The remainder were M–photoreceptors.

If the investigators had been aware of the presence of UV–photoreceptors in the human eye, separation of the UV–and S–channel photoreceptors could have easily been accomplished using the same type of differential adaptationused to separate the M– and L–channel photoreceptors.

The fact the four statistically random arrays (including their clumps) are inter-digitated makes the description of theoverall arrangement very difficult. It also makes it very clear that the photoreceptors of color vision are not arrangedin an organized lattice of triads or tetrads.

3.2.2.5 The neural laminate RE-OUTLINE

This is the most complex laminate at the histological level. The neurons of each sublayer have immensely complexdendritic input and axonal output structures which are difficult to trace in detail. Interpreting the traces is alsoimpossible without a clear understanding of how they interconnect and the characteristics of the signals carried bythese connections. Wassle & Boycott151 have provided a large group of drawings showing the morphology of manydifferent types of neurons found in these sublayers. They also provide information on the spatial position and

Page 59: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

58 Processes in Biological Vision

152Shepherd, G. (1974) The synaptic organization of the brain : an introduction. New York : Oxford UniversityPress153Hubel, D. (1988) Eye, Brain and Vision. NY: W. H. Freeman pg. 52154Schmitt, F. (1979) in 4th Study Program in Neurosciences. Cambridge, Mass. : MIT Press, pp. 9-10

density of these different types.

3.2.2.5.1 Signal paths through the laminate

Many authors have presented conceptual drawings of the signal flow through the neural laminate without correlatingthese paths with the characteristics of the signals involved and the capability of the various cells to accommodatethese signals. The caricatures of Shepherd in 1974152, 1978, & 1979 are frequently reproduced in other works,including Shepherd of 1988. Hubel in 1988 presented an artistically attractive caricature153. Both the Shepherdversion of 1974 and the Hubel version illustrate the apparent synapse of lateral cells, which exhibit nomorphologically identifiable axon with the dendrites of bipolar cells or ganglion cells. They explained this situationby defining a new dendro-dendritic synapse. They also define bidirectional synapses at one junction between twocells of different type. These assumptions allowed them to define a typical signal path made up of typical elementsto provide plausible solutions to a variety of primarily psychophysical tests.

More recently, Daw has presented similar caricatures showing fewer directions of signal flow. Whereas earlierauthors attempted to show a composite signal flow diagram, Daw divides his caricatures to define additional signalflow possibilities. His caricatures rely on two different types of cone bipolar cells and two different types ofganglion cells. These can be interconnected in a variety of ways to provide plausible solutions to the results ofprimarily psychophysical tests. His models depend on the ideas of “ON” and “OFF” responses in an apparentachromatic signal environment. No color specific data is offered in his model. There is little data in the electro-physical literature supporting both a depolarizing bipolar cell and a hyperpolarizing bipolar cell. The methodologyused by Daw calls for both sign conserving and sign reversing signal transmission at different synapses. He alsoimplies both sign conserving and sign reversing bipolar cells.

The above caricatures leave many questions unanswered. None of the authors mentioned above offers anyexplanation about how the signals are amplified, inverted, or otherwise manipulated within or between neurons. Unambiguous electro-physical data supporting the existence of any bipolar cell with a depolarizing output would bemost interesting.

There are a variety of papers in the literature assigning the name bipolar cells to cells exhibitingeither depolarizing or hyperpolarizing output signals based only on the fact that the cells werelocated within the inner neural layer. For a variety of reasons, the authors did not confirm that thecells they were reporting on were physiologically bipolar cells and not lateral cells of either the 1st

or 2nd lateral matrix. These cells share a common morphological area and great precision isrequired to separate them on morphological grounds.

By examining the electro-physical data available on all of the neurons of the retina, defining the characteristics of thesignals accepted and produced by each of them is possible. This includes the quiescent voltages and currents at eachnode, the polarity and amplitude of the output signals, and whether the output signals are electrotonic or pulse innature. By studying the simpler neurons, determining how they operate internally is possible. See PART C.

With specific knowledge of the operation of the neuron in hand, an additional look at the problem of defining boththe signal flow and the signal manipulation leads to a more definitive component organization. Defining a“fundamental signal path” as opposed to a typical signal path is the best way to accomplish this. The Fundamentalsignal path is a theoretical construct found only in the foveola. It is a straight through path involving a singlephotoreceptor cell, a single bipolar cell and a single ganglion cell. Schmitt154 described this configuration as athrough-projection with long axon Golgi Type I neurons. Once this fundamental signal path is understood, defininga second path, a fundamental difference signal path, is possible (using short-axon Golgi Type II neurons). This is apath involving two photoreceptors, two bipolar cells, one lateral cell, and one ganglion cell. Once these signaldiagrams are understood, expanding the ideas to any degree required to account for other signal paths in the retina isquite easy.

The next level of neural circuit complexity in the retina is yet to be clearly understood. It involves very complex

Page 60: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 59

155Dacey, D. & Lee, B. (1995) Physiological identification of cone inputs to HI and HII horizontal cells inmacaque retina. Invest. Ophthalm. & Visual Sci. vol 36, S3

summation and differencing mechanisms involving the signals from hundreds of adjacent photoreceptors. In earliertimes, these interconnections were studied in a piece-meal fashion. These early studies employed stimulation patterns based on simple color patterns in a center-surround configuration. While these early color studies tended toisolate the diameter of the foveola, they provided little insight into operation of the visual system. More recentstudies exploring the contrast improvement capabilities of the visual system offer additional insight. These studieshave generally involved algorithms of the type first developed by Edwin Land in conjunctions with his RetinexTheory of Color Vision (Section 17.3.5.8). They suggest the primary purpose of these broader interconnections areto improve the contrast performance of the system in the presence of large intensity variations in the overall scene. These interconnections may be found primarily among the amercine cells of the retina. However, the minimalpresence of these cells in the human retina suggests the interconnections are found mainly among the horizontal cellsof the retina.

Several underlying principles must be understood before the signal paths can be described. The understanding that iskey is that every neuron contains at least one biological semiconductor amplifier, known as an Activa. The Activa isan active three terminal biological semiconductor device. Furthermore, every “gap junction” synapse forms anActiva. These devices can be analyzed using the same tools used for analyzing solid state semiconductor devices,transistors. Furthermore, recognizing that each lateral cell contains a neuritic structure and an axonal structurepackaged within a single tubular cell membrane, known morphologically as a dendrite, is necessary. The overallconfiguration of a lateral cell is developed in Chapter 9. The most important observation is that each lateral cell hastwo input structures and one output structure. One input structure provides a sign conserving signal path and oneinput structure provides a sign reversing signal path. The morphological bistratified input structures in lateral cellshave been documented by Dacey & Lee155. A similar bistratification has also been reported in ganglion cells by thesame authors in 1994 (See Section 13.4.5).

By assuming that all photoreceptor cells provide a hyperpolarizing output signal in response to illumination, that allbipolar cells are sign conserving amplifiers, and that lateral cells can provide both sign conserving and sign reversingsignal paths, two different classes of signal are presented to the ganglion cells. One is a monophasic signal of thesame sign as that of the photoreceptors. The other is a biphasic signal that goes positive in response to illuminationof one of the photoreceptors and goes negative in response to illumination of the other photoreceptor. This situationcalls for two types of ganglion cells, one that responds to a monophasic signal and one that responds to a biphasicsignal.

Note that the signals defined here do not involve inhibition. The process involves simple subtraction.

The signals from the ganglion cells are collected into the optic nerve and travel to a variety of locations in the brainas illustrated in Section 2.6.1. It will be shown that the ganglion cells that respond to monophasic input from thefoveola connect to an area of the Pretectum within the brain. Those from outside the foveola connect to themagnocellular region of the LGN in the brain. The ganglion cells that respond to biphasic inputs from the foveolaalso connect to an area in the Pretectum within the brain. Those from outside the foveola connect to theparvocellular region of the LGN of the brain. It appears that the Pretectum is optimized for analyzing compleximagery brought to the fixation point of the eye.

Based on the above, the caricature from Hubel can be redrawn as in Figure 3.2.2-14 to illustrate both achromaticand chromatic inputs and to eliminate the need for a dendro-dendritic synapse. Before describing the circuits of thisfigure, some common points can be summarized;

+ All neuron cells are three terminal biological semiconductor structures.+ All gap junctions are three terminal biological semiconductor structures.+ All interconnections between cells are feed-forward circuits. + All bipolar cells are functionally alike. + There is no demonstrated need for external feedback circuits.

Page 61: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

60 Processes in Biological Vision

Figure 3.2.2-14 Caricature of the signal paths found in the retina of the Chordate, with nomenclature associated withthe human. See text for explanation of the individual numbered situations. A; axon. D; dendrite. P; poda. These arethe external terminals related to the internal terminals of the Activa found in each neuron. Para; parasol type ganglioncells which accept monophasic inputs but only generate a series of action potentials when the input is above a thresholdlevel. Midget; midget type ganglion cells which accept biphasic inputs and generate a series of action potentials whenquiescent. A highly modified version of a caricature in Hubel.

This basic configuration can provide, depending on the chromophoric character of the two photoreceptors, both“ON” signals, i.e., action potentials in response to illuminance, and more complex signals. These more complexsignals can be described as “ON-Center, OFF-surround” for the achromatic case or “ON-Blue, OFF-Red,” for thechromatic case. By providing additional lateral cells connecting to adjacent photoreceptor channels, describingsignals such as “ON-Center-Blue” and “OFF-Surround-Red” is possible. Additional interconnection can producesignals such as “ON-Surround-Parallel lines,” etc.

Every neuron contains at least one 3-terminal Activa. Leads to these terminals appear on the surface of the neuron.

Page 62: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 61

Two of these are the synapses associated with the dendrites and the axons. A lead to the third terminal may appearat the cell surface as a de-minima contact with the surrounding fluid matrix or it may be represented by synapses ona poditic structure that resembles a dendritic structure.

For the straight through cell types (photoreceptor cells, bipolar cells and most ganglion cells) the third terminal ofthe Activa and therefore the cell, is not shown explicitly in caricatures. However, many caricatures in the electro-physiology literature show a current emanating from the Inner Segment of a photoreceptor cell, at some sort ofterminal, and re-entering the Outer Segment.

For the lateral cells, the horizontal and amercine cells, the third terminal should be shown explicitly since it is usedas a signal input point. In the above picture, the dendritic, axonal and poditic structures are indicated by the lettersD, A and P respectively.

In addition, it is important to recognize that psychophysical experiments involving a center and a surround mustspecify whether the center is coincident with the fixation point or not. Different signal paths are involved forsituations where the center is not at the fixation point.

3.2.2.5.2 Sign conserving amplifiers found in the luminance channels

#1 At the fixation point, there are straight through signal channels such as that labeled #1. The photoreceptordelivers a current to the bipolar cell through a gap junction. The bipolar cell acts as a signal repeater and deliversessentially the same current to the parasol type ganglion cell. Here, there is no sign reversal between the input andthe output at either a cell or a gap junction. Although it is the current that is the signal, most investigators prefer tomeasure the voltage at a circuit node. They prefer to speak of this voltage as hyperpolarizing if it is increasingrelative to the quiescent value and depolarizing if it is decreasing relative to the quiescent value. The signal at thegap junction between the photoreceptor and bipolar cells and between the bipolar and ganglion cells arehyperpolarizing. The ganglion cell is normally quiescent, but creates a series of action potentials with a timebetween pulses that is inversely proportional to the signal intensity, after reaching an initial threshold level. Thetime before reaching this threshold is usually labeled a latency interval. A parasol cell associated with aphotoreceptor in the foveola transmits its signal via the optic nerve to the Pretectum. A parasol cell related to part ofthe retina remote from the foveola transmits its signal to the magnocellular region of the LGN. In the idealizedstraight through channel, the signal at every point along the chain will exhibit a spectral characteristic that is thesame as that of the individual photoreceptor. This is a true chromatic signal channel.

Signals delivered to the Pretectum from the foveola are used to analyze and identify precise features of the target. This analysis is only possible in conjunction with the motions of the target relative to the line of fixation of the eye. This motion is normally supplied by the small saccades.

#2 Case #2 involves the summation of the signals, in the form of currents, at two points. Signals from manyphotoreceptors are summed at the dendritic structure of one or more bipolar cells. Signals from many bipolar cellsare then summed at the dendritic structure of a parasol cell. Here, there is no sign reversal between the inputs and theoutputs at either a cell or a gap junction. This type of circuit is generally found outside the foveola and particularlyin the periphery where the ratio of photoreceptor cells to ganglion cells can reach 10 or more. It can provide veryhigh sensitivity to large low contrast, or small high contrast targets in image space. The signal passed along thechain may exhibit spectral characteristics resulting from the summation of currents from photoreceptors of differentchromatic types. The weighting given to the different spectral types is generally unknown. Therefore, the spectralperformance of these signal channels can vary from that of a pure chromatic channel to that closely resembling thephotopic or scotopic spectrums. The parasol type ganglion cell creates a series of action potential pulses andtransmits this signal over the optic nerve to the magnocellular region of the LGN.

3.2.2.5.3 Amplifiers for both sign conserving and sign reversing paths

#3 A horizontal cell is used in case #3 to assemble the signals from many photoreceptors. The resultant signal isthen passed to a midget ganglion cell that encodes the biphase signal for transmission to the parvocellular region ofthe LGN. Here, two dendritic structures are shown connecting to a variety of photoreceptor cells in both thesurround and the center. In addition, additional signals are being collected at the poditic terminal through the sameprocedure. Signal currents collected at the dendritic terminal are summed without sign reversal and passed to thenon-inverting input terminal of the Activa. Signals summed at the poditic terminal are also summed without signreversal, but they are passed to the sign inverting input terminal of the Activa. The output signal from the Activa isan algebraic difference between the signals from the two input structures. It is a biphase signal relative to itsquiescent value. In addition, the signal applied to the inverting input terminal may be amplified. Therefore, theoutput signal is of the form:

Page 63: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

62 Processes in Biological Vision

156Dacey, D. & Lee, B. (1995) Physiological identification of cone inputs to HI and HII horizontal cells inmacaque retina. Invest. Ophthalm & Visual Sci., vol 36, S3 157Lee, B. (1996) Minireview: Receptive field structure in the primate retina. Vision Res. vol. 36, no 5, pp. 631-644158Blakemore, C. & Vital-Durand, F. (1986) Organization and post-natal development of the monkey’s lateralgeniculate nucleus. Jour. Physiol. vol. 380, pp. 483-486

A = D -k*P where k is the amplifier gain of that signal path. A is the output signal at the axon D is the input signal applied to the dendritic structure P is the input signal applied to the poditic structure

This signal is passed to a midget ganglion cell that can accept such a biphase signal. This type of ganglion cellcreates a series of equally spaced action potentials without any input signal. In the presence of an input signal, thespacing between the action potentials is increased or decreased as a function of the biphase input signal. Care shouldbe taken to recognize that the output pulse train passed to the brain is not biphasic. Only the information it carries isbiphasic when recovered in the Pretectum or LGN of the brain.

#4 Situation #4 is very similar to that of #3 but the signals are summed at the outputs of various bipolar cells. Inaddition, the morphology of the amercine cells frequently gives no hint of the location of the axonal output. Theaxon is shown as next to and enclosed within the same cell wall as the dendritic structure, i.e., an internal cellmembrane is separating the two structures for functional purposes. As in the previous case, the signals collected atthe dendritic structure are passed to the non inverting Activa terminal. The signals collected at the poditic terminal,without sign reversal, are passed to the sign reversing input to the Activa. The signal at the output of the Activa andpassed to the axon is biphasic as in the horizontal cell case. This signal is passed to a midget ganglion cell that isable to accept a biphase signal. It is treated the same as in situation #3.

#5 This situation is provided to highlight an additional capability. If a dendritic structure of either a horizontal oramercine cell is formed such that it bypasses certain cells in the surround to reach cells farther removed, it canprovide highly tailored inputs to the brain. Thus, a pair of low contrast lines in object space with a width of fourpixels each will provide an output signal to the brain that is larger than that from a single line of the same width buthigher contrast.

The situations highlighted in #3 to #5 are frequently associated with lateral cells that are described as bistratified. This nomenclature is completely compatible with a cell with two input structures, one connecting to the signconserving (non-inverting) signal input of the Activa, and the second one connecting to the sign reversing (inverting)signal input of the Activa. Dacey & Lee156 have provided several beautiful caricatures of lateral cells with this form.

The types of signals defined in situations 1-5 have all been measured at the S-plane of the retina in a variety ofanimals. An additional situation may exist if some bipolar cells can accept signals at their signal inverting podaterminals. For that class of bipolar cells, the output signal would be biphasic compared with a quiescent level andwould normally be passed to a midget ganglion cell that can handle a biphasic signal. Although the psychophysicalcommunity frequently assumes such a cell in their models, the electro-physical literature either does not report or isvery ambiguous on the existence of this type of cell.

Only a few of the possible situations are highlighted above. Many other situations can be defined that account forsuch specializations as the reported capabilities of the cat to differentiate ruled patterns with different orientations. Specifically, it provides a framework for the work of Lee157 and others who are trying to understand the perceivedresolution of the visual system based on measurements made at the level of the LGN. Although the title of the abovemini-review by Lee implies a concentration on the retina, it is actually concentrated on the computational opticsinvolved in both the retina and the LGN. It does not appear to consider the signal paths between the foveola and thePretectum. Figure 5 of that paper, originally from Blakemore & Vital-Durand158, is interesting in that it provides agraph of spatial resolution as a function of eccentricity as measured at the LGN level. The graph displays a widespread in data points in the region of low eccentricity. It should do this if many signals from the foveola are reroutedto the Pretectum. The graph is consistent with the data of Wassle & Boycott and the data of Osterberg presentedearlier. The ratio of ganglion cells to photoreceptor cells does track the perceived resolution at the LGN althoughthe horizontal scales are different. One graph used eccentricity measured in mm. across the retina. The other isprobably using the eccentricity angle referred to object space. The data of Osterberg used angular eccentricity

Page 64: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 63

159Valberg, A. Lee, B. Kaiser, P. & Kremers, J. (1992) Responses of macaque ganglion cells to movement ofchromatic borders. Jour. Physiol. Vol. 458, pp. 579-602160Heynen, H. & van Norren, D. (1985) Origin of the (L)ERG in the intact macaque eye. Vis. Res. vol. 25, pp.697-715

referred to object space.

Combining all of the above data, the spatial resolution of the sensing layer of the human (primate) retina falls byonly about 20% at an object space eccentricity of 30 degrees. It is still 80% or more of the peak value. However,the spatial performance of the complete eye falls much more drastically for two reasons, the optics of the eye hasmuch poorer resolution at this eccentricity and the signal processing within the retina is far less capable beyond thefoveola. As a result, the spatial resolution measured in the LGN is reduced to about 10% of the peak value. Thesestatements are consistent with the photoreceptor to ganglion cell ratio presented in [FIG 3.2.2-3] by Wassle &Boycott.

Many temporal and chromatic effects noted in Lee’s paper are also compatible with the above framework. One ofthe important facts highlighted by the differencing equation given above relates to the measurement of temporalfrequencies at the LGN level in response to stimuli that are poorly defined spectrally. By holding the illuminancelevel constant and changing the spectral content, it is possible to change the temporal frequency characteristicsmeasured at the LGN quite drastically. Alternately, it is possible to choose a stimulus with the proper chromaticspectrum so that the difference signal path terminating in the parvocellular region of the LGN will show zeroresponse to significant changes in illumination. This result implies a passband of zero hertz, i.e., no signaltransmission due to this stimulus even though the eye is fully functional. Valberg et. al.159 performed a similarexperiment involving the magnocellular region of the LGN but using a moving transition between two coloredregions in object space.

Although the signal paths of the retina are all direct coupled in the electrical sense, great caution must be taken indesigning test instrumentation. The highly nonlinear gain characteristic of the initial photoreceptor transduction andtranslation circuits must be taken into account. These circuits remove nearly all indications of the absoluteillumination level applied to the various photoreceptors from the signal path.

Heynen & van Norren160 have provided very detailed probe data on the signal found at different levels within the in-vivo retina of the macaque monkey. They appear to expand the data base provided by Svaetichin, Tomita, and othersworking in the 1960's, considerably. This data is very helpful in confirming the signal path architecture developedabove. However, without a putative architecture, the data is very difficult to reduce. Additional discussion of thesignal circuitry of vision will be found in Chapter 11.

3.2.2.5.4 Cell configurations within the laminate

The cell structures found within the neural laminate take on an amazing complexity and variety. Because of thisvariety, analyzing the operation of these cells in detail is very difficult. Dacey and Lee have provided manybeautiful caricatures of these cells that provide fertile material for thought. The minireview by Lee referenced earlierhas an excellent bibliography current to 1996. Unfortunately, the review becomes entangled in the old discussionbased on whether cells exhibit a difference signal related to blue-yellow or blue-green. Time will not be spent hererationalizing the data in that paper by reinterpreting all comments in terms of S-channel minus M-channeldifferences.

3.2.2.5.5 Axon sizes within the Optic Fiber Layer

The most exposed surface of the neural laminate is the optic fiber layer. This layer is nominally 100 microns thickover the surface of the neural laminate closest to the vitreous humor. Between this layer and the vitreous humor isthe Inner Limiting Membrane. Most of the larger vascular elements supporting the neural layer are located betweenthe Inner Limiting Membrane and the vitreous humor.

With axon diameters typically in the 5-12 micron range, the Optic Fiber Layer is able to support many layers ofneural axons as they course toward the Lamina Cribosa. Little information could be found that attempted to map theindividual subsystem layers defined above. Experimentation is needed to inject dyes into the various groups ofneurons in the optic nerve and trace the transport of those dyes into the neural laminate.

Page 65: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

64 Processes in Biological Vision

161Kolb, H. (1991) Anatomical pathways for color vision in the human retina. Visual Neurosci. vol 7, pp. 61-74162Hogan, M. Alvarado, J. & Waddell, J. (1971) Histology of the human eye. Philadelphia, PA: W. B. Saunderspp. 508-522

Figure 3.2.2-15 CR Capillary bed in the neural laminatebehind the macular region. The area shown correspondsclosely to the parafovea of the eye. The capillary free zone(a) corresponds to the foveola and is the pattern seen bymost people in the first afterimage. From Hogan.

3.2.2.5.4 Cell sizes within the laminate

Collection of reliable data on the size of neurons and neuron elements is difficult. Few investigators observe enoughcells of an individual proposed type to gain statistically relevant size information. Frequently, the proposed celltypes appear to overlap to a considerable degree. Kolb161 provides a variety of dimensions and ratios that can beilluminating. Of particular interest is the claim that the ratio of ganglion cells to photoreceptors in the fovea isbetween two and three to one. Further distinction between the foveola and the fovea, as defined herein, would beeven more enlightening. Unfortunately, the paper also includes the recitation of a long list of conventional wisdomin the Introduction and a large amount of speculation involving the words must, should, likely to be, etc. Ingeneral, these speculations appear to be consistent with the findings of this work when the circuits involved arerecognized to be analog in nature.

3.2.2.5.4 Hydraulic elements within the laminate

The neural laminate is a densely packed region of living and signaling tissue, including the soma associated with thephotoreceptor cells. It must be adequately supplied with nutrients. If it is not adequately supplied, deterioration inthe signaling capabilities of the visual system can be expected to occur rapidly. The neural laminate can bedescribed as a homogeneous hydraulic bed impregnated with a large number and wide variety of individual livingcells and supported by a complex network of hydraulic conduits. These conduits all emanate from the main arteryentering via the optic nerve. Their size decreases continuously as the lineal distance from the optic nerve increasesalong the conduit. This leads to a more limited supply capability with distance. Figure 3.2.2-15 illustrates thesituation and is taken from a comprehensive description given by Hogan162. The image is in a sense a doubleexposure because it shows both the arterial and the venous systems overlaid. Although the individual conduits arecontinuous and cannot truly be modeled as a lumped constant system, using a simplified lumped constant model forstudy in such a reticulated system is possible. The supply capability at a given point can then be described in termsof an RC time constant associated with the impedance of the channel feeding the nearest reservoir and the capacityof that reservoir. The hydraulic bed also has an intrinsic time constant. Because of this situation, the signalingcapability of each neuron of the retina exhibits a temporal response that can be described by at least two timeconstants describing the hydraulic system in its immediate vicinity.

The competition for nutrients is particularly importantin the region of the fovea. Each neuron must competefor nutrients and energy with other neurons in thevicinity. Because of the lack of conduits in theimmediate vicinity, the capability of supplying thesenutrients is largely controlled by the time constant ofthe hydraulic bed. This limitation is one of theprincipal causes of tunnel vision ( a symptom offatigue) and the after image effects associated with theeye. This is particularly obvious regarding the afterimages delineating the foveola from the rest of theretina.

3.2.2.6 Subdivisions of the neural laminate

The literature supports the subdivision of the neurallaminate into a single series of sub-laminates on bothphysiological and morphological grounds. Using theterminology of Boycott & Dowling, the neurallaminate extends from the Outer Limiting Membrane(OLM) to the Inner Limiting Membrane (ILM). Indoing so, it includes the nuclei of the photoreceptorcells found in the outer nuclear layer, all of the signal processing cells of the inner nuclear layer and the signalprojection neurons of the ganglion cell layer and all of the interconnections between these cells.

Page 66: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 65

163Euler, T. Schneider, H. & Wassle, H. (1996) Glutamate responses of bipolar cells in a slice preparation ofthe rat retina J Neurosci vol. 16(9), pp 2934-2944164Wassle, H. Levick, W. & Cleland, B. (1975) The distribution of the alpha type of ganglion cells in the cat’sretina J Comp Neurol vol. 159, pp 419-438

Figure 3.2.2-16 Caricatures of bipolar neurons within the retina of rat. The labels are conventional. It is proposed thebipolar neuron labeled RB is actually associated with the R-channel of vision and found in the peripheral retina.Dendritic input (only) from a variety of photoreceptors. It is proposed the bipolar cells labeled CB, serve a multitudeof functions associated with the chromatic and spatial signal processing channels of rat vision. The two arrows on theleft denote the stratification of the neurites of amecrine cells into their dendritic and poditic ramifications. Scale bar,60 μm. From Euler, et. al., 1996.

The inner nuclear layer can be further expanded. The names suggested for these sub-laminates in this work aredescriptive. In order, the 1st lateral laminate supports lateral matrix signal processing based primarily on the class oflateral cells known as horizontal cells. The neurites of neurons in this layer connect directly to the axons ofphotoreceptor cells. This sublayer consists of multiple arrays of neurons as will be discussed in Section 3.4.2. Thedata does not show whether these arrays within the sub-laminate are layered. The axons of the most proximal ofthese arrays probably connect directly to ganglion cells. However, the data is not conclusive in this area. Thesecond sub-laminate contains primarily bipolar cells and is therefore labeled the bipolar laminate. The dendrites ofthe bipolar cells connect primarily with the axons of photoreceptor cells and their axons connect directly withganglion cells and probably also neurites within the 2nd lateral laminate. The 2nd lateral laminate supports additionallateral matrix signal processing. As suggested, it appears that most of the neurites of these cells connect to bipolarcell axons. However, the literature is not decisive on this point. This laminate may also contain multiple arrays ofneurons that may or may not be arranged as sub-sub-laminates. The axons of at least the most distal of these arraysconnect to the neurites of the ganglion cells.

Euler & Wassle have recently provided a set of caricatures describing putative bipolar neurons of two types163. While based on the chemical theory of the neuron and largely exploratory, it is useful in understanding the paths ofbipolar neurons through the retina. While they define bipolar cells believed to serve both cone and rod signal paths,it is notable that the resting output potentials of the two types are statistically the same (rod-based, –45 mV ±13 mV;n = 21 and cone-based, –49 mV ±10 mV; n=38). Their method of cell identification relied primarily on thegeometry of the photoreceptor axon pedicle. Figure 3.2.2-16 reproduces their figure 1.

This work has not found any documented case of functional external feedback proceeding toward the distal part ofthe retina (negative feedback) from any neurons in any of the above layers.

3.2.2.6.1 Spatial parameters of the mosaics of the ganglion cells in cat

Wassle, et. al. have provided good caricatures and statistical data on the mosaic of “alpha ganglion cells” in theretina of the cat164. The data shows the cells aggregate in the location of the fovea (diameter of under three mm) ofthe cat as they do in human retinas (compare their figures 6A and 6B). They did note a slight aggregation along thehorizontal and vertical axes through the fovea. They did not define the spectral performance of these cells but noted

Page 67: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

66 Processes in Biological Vision

165Williams, D. MacLeod, D. & Hayhoe, M. (1981) Punctate sensitivity of the blue-sensitive mechanism.Vision Res. vol. 21, pp. 1357-1375166Ahnelt, P. Kolb, H. & Pflug, R. (1987) Identification of a subtype of cone photoreceptor, likely to be bluesensitive in the human retina. J. Comp. Neurol. Vol. 255, pp. 18-34

their large size relative to other ganglion cells (described as beta and gamma types). Their method of identificationwas limited to cytological features.

3.2.3 Fine histology of the photoreceptor layer of the retina

There have been a wide variety of experiments performed to determine the spectral performance of the human eye asa function of spatial position. As hinted at above, these have been performed at all levels of spatial resolution, thegross level relating to large areas of the retina, experiments involving regions from 1-2 degrees down to 15 minutesof arc, experiments examining the spatial arrangement of at most 25 chromatically mixed photoreceptors, andattempts to define the location of individual chromatic photoreceptors.

There are a variety of spatial maps purporting to show the spectral performance of the retina at spatial resolutions ofabout one degree in object space. Unfortunately, these have usually been prepared based on psychophysical datataken using wideband spectral filters that are not well correlated to the actual absorption spectra of the eye. Morerecently, some data has appeared based on the use of specular sources for the test image but these frequently employa broadband surround lighting that is again poorly correlated to the spectra of the chromophores of the eye.

At the next level of spatial resolution, Williams, et. al165. have provided a set of maps of chromatic sensitivity using aflashing spectral target, of 1.1 arc minutes diameter, and a broadband surround. Separate maps were obtained forseveral individuals under the same conditions. The variability of these maps with location suggests the difficulty ofusing statistical measures to define the spatial performance of the eye to either achromatic or colored lights. Theflashing target was programmed to form an 11 x 11 element array 50 arc minutes on a side and centered on thefixation point. The grid spacing was therefore 5 arc minutes. The maps obtained are very useful but it is not entirelyclear what they represent. As discussed in summary in Chapter 1, the combination of the luminance andchrominance signal processing channels of vision and the spatial motion of the line of fixation must be consideredwhen determining what the visual system perceives. Only then is it possible to estimate what the system willrecognize and report in a psychophysical experiment. These considerations lead to an entirely different explanationto one situation reported perplexedly by that team. Speaking of the area immediately surrounding the fixation point,they report: “Test flashes at location throughout this region appeared white and sharply defined whereas those fallingin the more sensitive outlying regions appeared violet and diffuse . . . ” They concluded that this was evidence of noshort wavelength photodetectors in the center of the fixation zone generally defined as the foveola. In the context ofthis work, the results reported would be highly subject to the difference in intensity between the test target and thebackground and to the spatial signal encoding by the signal manipulation stage of the retina as well as the spatialencoding of the signal projection stage leading to the visual cortex. In the most obvious example, whenever thesignal intensities were such that the logarithmic difference, representing the integral of the flux on eachphotoreceptor times the adaptation amplifier gain, between them was zero, no chrominance data was transmitted tothe visual cortex and the subject would report the test target as white. At other levels, the test target would initiallybe reported as the same color but less saturated than the background surround, then white and eventually violet.

The above experiments did not attempt to illuminate or resolve individual photoreceptors. Attempts to determine thestatistics of the retinal mosaic at the photoreceptor level have remained elusive. The introduction to the paper byAhnelt, et. al166. suggests the problem and the state of the art. They do not recognize the possibility of UV sensitivephotoreceptors in humans (following the archaic pattern of quoting the elementary proposal of Young in 1802-03),although documented in aphakic patients. They also stated in 1987 that: “The cones differ in having differentphotopigments and different neural connectivity, but no morphological differences with which to distinguish thethree different spectral types have been reported yet.” They proceed to postulate that a morphologically definablegroup of photoreceptors they have studied are likely to represent the blue-cones of vision. The group they propose isunique in several characteristics. However, they are also unique in one critical parameter. As noted, these cells arenot long enough to reach the RPE. This fact has several consequences. Because they are not in contact with theRPE, they may not be fully operational. They may be either juvenile or pathological. What is more important, theiranalysis does not recognize the growth dynamics of normal photoreceptors. Because of the normal growth rate ofthe disks of the Outer Segment, they cannot exist for more than about 2,000 hours, the time for a normal disk toprogress from the extrusion point within the Inner Segment cup to the phagocytosis point within the RPE. As aresult, a “short photoreceptor” is not in a stable configuration. If the cells of the proposed subgroup were normal

Page 68: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 67

167Rodieck, R. (1998) The First Steps in Seeing. Sunderland, MA: Sinauer Associates168Curcio, C. Sloan, K. Kalina, R. & Hendrickson, A. (1990) Human photoreceptor topography J Comp Neurolvol 292, pp 497-523169Curcio, C. Allen, K. Sloan, K. Lerea, C. Hurley, J. Klock, I. & Milam, A. (1991) Distribution andmorphology of human cone photoreceptors stained with anti-blue opsin. J. Comp. Neurol Vol. 312, pp. 610-624

cells in a real eye, they could not be found again or identified after a few weeks.

A collage on page 42 in the introductory text of Rodieck167 is being referenced in primary papers without adequatedescription. A serious reader should research the underlying papers.

This theory proposes that there are no significant differences between photoreceptors at the morphological orcytological level. There is a functionally identifiable difference in the isotropic absorption spectrum of thechromophores and this difference is caused by a difference at the molecular level of the chromophores (only). Thisidentifiable difference is in the length of the conjugated chain between the two auxochromes. Unfortunately, thisdifference is physically masked by the configuration of the liquid crystal when operational. It is probably onlymeasurable in the crystal via molecular resonance spectroscopy.

A variety of microspectrophotometry, MSP, techniques have been used to determine the characteristics of individualphotoreceptors in various animal retinas. However, they have also failed to provide conclusive mosaic maps orstatistics related to such maps. The task is difficult. For axial MSP, the background for the measurements is notwell defined. There may be red blood cells forming the background. Alternately, the chromogens located in theRPE may form an undefined or mixed background. Attempts to mark suspected photoreceptors for subsequent MSPhave failed until recently because the illumination was applied to the individual photoreceptor cell transversely. Theresult was always the isotropic spectrum of the chromophore present. This spectrum, although not used in vision,always has a peak at 500 nm. Very recently, correct anisotropic spectrums have been obtained using axial MSP onindividual photoreceptors. However, this has not yet led to a method for characterizing a large piece of or the entiremosaic.

3.2.3.1 The archaic representations of Curcio et al. showing inner segment ellipsoids

This section can be discarded. The recent work of the Roorda team during the 21st Century demonstrates in-vivo at the sub-micron resolution level that there are no rods in the human retina. xxx]

Curcio et al (1990)168 have followed the in-vitro procedures of Shultze and later Osterberg. They have preparedwhole mount retinas and then sliced through them parallel to their surface layer (at the exterior limiting membrane)in order to expose various surfaces within a specific lamina (relative to their reference surface and not relative to thePetzval surface). The results are primarily images of inner segment layers which are more stable physically andeasier to obtain than cross sections of outer segments. Their figure 1, and their text, also make it clear that the outersegments are generally skewed relative to their reference plane (between 20 and 45 degrees in the figure).

Curcio et al. make no effort to identify their rods and cones by spectral means. They rely entirely on theirassumption that cones are larger than rods at the level of the ellipsoids of the cells. No reference is made to theproperties of the outer segments. Their images lack detail at the micron level due to their use of Nomarskidifferential interference contrast microscopy combined with focusing at an intermediate level within the neuralmatrix at the level of the ellipsoids.

Curcio, et. al (1991)169. addressed the distribution problem differently. They review the previous art and then say in1991: “Many of these methods are inferential, and cells are presumed to be blue cones by virtue of the similarity oftheir distribution to that suggested by visual psychophysics (page 611).” Although they were careful to preserve thespatial fidelity of their test retinas, their approach was also inferential. They also note, “morphologically identifiedcones are 3-fold or more numerous than immuno-cytochemically labeled cones.” (page618). Their conclusion onpage 622 is also interesting. They say, “Our results for immuno-cytochemically labeled human blue cones areconsistent with a nonrandom distribution outside the rod-free zone.” They lumped all photoreceptors other thanthose associated with the S–channel under the heading R/G cones (based on the conventional assumption that theretina is only trichromatic. They employed materials that were thought to stain certain cells preferentially. Using amore detailed model, many alternate conclusions can be drawn from their approach. They determined that a circlecould be drawn encompassing the central 100 microns of the foveal center that did not contain any cones. However,

Page 69: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

68 Processes in Biological Vision

170Ahnelt, P. (1998) The photoreceptor mosaic. Eye, vol. 12, pp 531-540171McCrane, E. de Monasterio, F. Schein, S. & Caruso, R. (1983) Non-flourescent dye staining of primate bluecones. Invest. Ophthal. Vis. Sci. vol 24, no. 11, pp 1449-1455172Roorda, A. & Williams, D. (1999) The arrangement of the three cone classes in the living human eye.Nature, vol. 397, Feb 11, pp. 520-522173Roorda, A. & Williams, D. (1999) Probing the amazing human retina Biophotonics International, May/June,pp. 40-41174Liang, J. Williams, D. & Miller, D. (1997) Supernormal vision and high resolution retinal imaging throughadaptive optics. J. Opt. Soc. Am. A vol. 14, no. 11, pp. 2884-2892 175Roorda, A. (Private communication, June 1999)

their figure 3 suggests that the “blue-cone” and the “red/green cone” classes they have identified suffer from some ofthe same problems as those of Ahnelt, et. al. The photoreceptor shown appears either juvenile or pathological. Theirpaper makes a herculean effort to sort out the conflicting statements, and anomalous situations reported in theliterature about various classes of photoreceptors. However, their results are primarily diagrammatic and statistical.

Both the Curcio et al (1990) and Curcio et al. (1991) papers note they were not photographing the photoreceptorouter senments. In figure 1 of the 1990 paper, frames B & D are of the ellipsiod within the inner segments. Theirfrequently referenced figure 1, frames (B) & (C), of the 1991 paper do not show the cross section of the outersegments of the photoreceptors. They are slices through the inner segments of whole mount retinas near theellipsoid, (B) just vitread to the junction of the outer and inner segments and (C) near junction with the myoid. As aresult, the figures are similar to that of Schultze in 1866. The large elements tend to be ellipsoids containing thenucleus of the cell, and the small elements tend to be smaller diameter portions of the inner segment or “axon”segments leading to the nuclear layer. Only their frame (A) actually shows outer segments.

Ahnelt170 reiterated the fact, in a 1998 review, that “So far no morphological criteria for direct differentiation of L–and M– [photoreceptors] have been reported but there may be differing connectivities along their midget pathways.”

Their work is also based primarily on the location of the “inner segments” of the photoreceptors. It is not necessaryfor the inner segments of a photoreceptor to be in the optical path associated with the center of the foveola for theouter segments to still be present within this diameter (See Section 4.3.1) . Therefore, there conclusion that there areno blue outer segments within a 0.35 degree diameter area, presumably centered on the point of fixation, may bequestioned. It is surprising that they did not reference the work of McCrane, et. al171. McCrane, et. al. providebeautiful pictures of inner segments stained by procion yellow and other dyes. However, they again rely uponinferential evidence that their pictures correlate with a presumed average of 7% blue cones within a retina. Theirfigures also focus on the inner segments of the photoreceptors although they make the interesting observation that“Procion yellow stains the extracellular compartment of the outer segment of cones, . .”

Curcio, et. al. make a number of observations concerning the orderlines of the S–channel photoreceptor array.

The 1991 paper of Curcio, et. al. and McCrane, et. al., and their references generally, fail to show that the innersegments of short wavelength photoreceptors are different, in any way, from the inner segment of otherphotoreceptors. The stained inner segments could just as well belong to the UV cells of the retina. This would beespecially likely if these cells had degenerated from lack of stimulation by UV light (See Section 17.2.2).

3.2.3.2 The more recent work of Roorda & Williams showing the retinal face

Roorda & Williams172,173 have recently provided small area photomicrographs of in-vivo human retinal mosaics from“normal” trichromats. They used an available test set described in Liang, et. al. It used adaptive optics tocompensate for the distortions encountered when attempting to focus a camera through a non spherical opticalsystem174. Although they were aware of the natural tremor, it does not appear that they compensated adequately forit.175 The resulting pictures appear slightly out of focus. Each photoreceptor is imaged as a cylindrical structure. There are a variety of reasons why such an image should arise. One of them is because of the tremor of the eye andthe exposure interval employed. There is some concern as to whether they took adequate steps in experimentaldesign to account for any UV sensitive photoreceptor channels (see discussion of the aphakic human eye elsewherein this work) in the retina and for any chromatic reflection from the retinal epithelium. They published pseudo-colorimages of a small part of the human retinal mosaic eccentric to the fixation point, within the fovea, and within the

Page 70: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 69

176Roorda & Williams, Op. Cit.177Pallikaris, A. Williams, D. & Hofer, H. (2003) The reflectance of single cones in the living human eye IOVSvol 44(10), pp 4580-4592178Christou, J. Roorda, A. & Williams, D. (2004) Deconvolution of adaptive optics retinal images J Opt SocAm A vol 21(8), pp 1393-1401179Yang, J-Z. Nozato, K. Saito, K. Williams, D. & Roorda, A. (2014) Closed loop optical stabilization anddigital image registration in Adaptive Optics Scanning Laser Ophthalmoscopy Biomed Optics Exp vol 5(9), pp.3174-3191 https://doi.org/10.1364/BOE.5.003174 180Bowmaker, J. & Kunz, Y. (1987) Ultraviolet receptors, tetrachromatic colour vision and retinal mosaics inthe brown trout (Salmo trutta): age-dependent changes. Vision Res. vol. 27, no. 12, pp. 2102-2108

macular region. The caption to the black and white pictures in their Figure 1 is informative: “The mosaic wasilluminated with a 4-ms flash (1 degree diameter, ~0.3 μJ) through a 2 mm. entrance pupil. Light of wavelength 550nm was used to maximize absorptance by L- and M-cone photo-pigments. Images were obtained with a 6-mm exitpupil at a retinal eccentricity of one degree nasal from the foveal center, which is located to the left of the image. For each retinal location, about 50 images taken over 5 days were averaged.” The color images presented in twodifferent publications have been cropped differently making comprehensive interpretation more difficult. The 4-msexposure time is marginal in the presence of a nominally 20-ms period, 50 Hertz (30 to 90 Hertz range in literature),tremor. With a peak amplitude of 20-40 arc sec for the tremor, approximating 1 to 2 photoreceptor diameters, a 4-msexposure would be expected to result in a blurring of the edges in the photographs of about 1/4 to 1/8 of aphotoreceptor diameter. The blur in the imagery presented in Figure 3.2.3-1 is consistent with the above temporaland geometric ratios. The additional blurring associated with averaging of 50 individual recordings was notaddressed by this author. In the figure, A & B are subject JW temporal and nasal retina, respectively at one degreeof eccentricity from the fixation point. C is subject AN’s nasal retina, at one degree of eccentricity.

One of Roorda & Williams obvious conclusions was that the retina in this area did not represent a mosaic of threeuniformly spaced and interdigitated photoreceptor types. In fact there is no indication of uniformity in the mosaicassociated with a given chromatic channel as depicted in their paper. The authors did not address the presence ofany achromatic photoreceptors (“rods”) in their imagery in the paper. However, Roorda stated in the privatecommunication that they did not observe a single “rod.” In a second communications, the authors confirmed that“their measurements were made at a retinal angle of one degree and that all of the color centers are cones and thattherefore no rod receptors appear in the views.” It must be pointed out the data presented by Wassle & Boycott andshown in [Figure 3.2.2-3 ] calls for a number of achromatic “rods” equal to the sum of all chromatic “cones” at thisposition in the primate retina! Their data is apparently based on morphological analyses of in-vitro retinas, probablyInner Segments, and not functional analyses of in-vivo retinas. Roorda & Williams did not address the question ofany UV sensitive photoreceptors in the human retina.

It should be noted that the experiments of Roorda & Williams would not require compensation for tremor ifperformed on many animals that do not employ tremor. It would also be useful to perform these experiments onsome animals, such as cats, who appear to be able to control the muscles generating the tremor.

Roorda & Williams176 have employed a retro-reflective spectrophotometric approach (70 nm wide filters for the 550nm source) to the mapping of a small portion of the in-vivo human retina and have provided some limited statisticson the distribution of some photoreceptor Outer Segments that are useful. As discussed above, a redefinition of theirexperimental procedure following their exploratory effort could lead to better and more complete data. Until such atechnique is perfected, the statistical parameters of all retinas will remain elusive.

The Williams team have continued to be very active during the early 2000's and considerable improvement in theirtechniques have resulted177. The work of Christou, Roorda & Williams is of particular relevance178. Although theirability to record the mosaic of photoreceptors has improved immensely, they still have not recorded the presence ofany “rods” in these mosaics. Their recent paper employed off-line processing to cancel out eye tremor179.

It appears clear from the exploratory work of Roorda & Williams, and all of the above statistical work, that the retinais not organized in an orderly array of interdigitated photoreceptor arrays based on chromatic sensitivity. The brainuses a different method of determining color rendition than the concept generally used by man in the arts, creating ofthree separate images related to the colors RGB or CYMK. This method will be discussed in more detail later in thiswork.

At the sub-array level, Bowmaker & Kunz180 have studied small groups of photoreceptors, primarily in fish,morphologically in order to determine if there is a fundamental array in the photoreceptors of Chordata similar to the

Page 71: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

70 Processes in Biological Vision

Figure 3.2.3-1 CR [Color] Pseudo-color images of thehuman retina taken by reflective spectrophotometryfollowing selective bleaching. See text for details. FromRoorda & Williams, 1999.

arrays found in the individual ommatidia ofArthropoda. Although their results do not concentrateon the lattice structure for the chromaticphotoreceptors present, they did find a deterioration inthe lattices with aging in young trout which theyattribute to the loss of UV-sensitivity of these animalswith age. Whereas, their caricatures generally showeight photoreceptors, of varying sizes grouped about acenter photoreceptor, most other authors tend todescribe a fundamental mosaic of six nearly equalsized elements grouped around a central element–ahexagonal unit. The difficulty with many of thesecaricatures is that they are generally based onmicrographs of sections through the inner segments ofthe photoreceptor cells and not sections through theouter segments.

3.2.3.3 Putative arrangement ofphotoreceptors in the human eye

If the same architectural concept used in Arthropoda,and probably Mollusca, is extended to Chordata, itwould be expected that the photoreceptors would bearranged in a capsular arrangement based on a close-packed hexagonal array of seven cells each. Underthis assumption, there are two challenges. One is toidentify, or specify, the spectral characteristic of eachcell by location in the array. The other is to establishsome estimate of the regularity of the resultingcapsules. There is also the associated question of howthese various cell arrangements support the subsequentsignal processing used to support the perceptions oflightness and chromatic content.

In consonance with this work, and the majority of thework summarized above, no space will be reserved inthe putative human retina for “rods.”

Some of the above investigators have made estimatesof the ratios of spectral photoreceptors, but most havenot. The reason is the apparent large variation withposition in the retina (and possibly with the techniqueused to identify different cells). None of the previousinvestigations have considered the presence of UV-sensitive cells in the human retina. Table 3.2.3-1summarizes some of the available estimates, alongwith those from this work.

Page 72: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 71

181Roorda, A. (1999) Op. Cit.

TABLE 3.2.3-1Estimates of photoreceptor density by spectral type

McCrane, 83 Ahnelt, 87 Curcio, 91 Roorda, 99 This TheoryWithout UV With UV

L–channel 50.6- -75.8 6% 5%M–channel 44.2- -20.0 88% 50%S–channel 7% 3- -5% 4.2- -7.6 5.2- - 4.2 6% 5%UV–channel --- --- 40%

Ahneldt gave a range of densities for likely blue-sensitive photoreceptors based on light microscopy. Their numbersvaried from 3-5% in the foveolar center to 15% near the foveolar slope to 7-10% in the peripheral retina. They didnote “The cones differ in having different photopigments and different neural connectivity, but no morphologicaldifferences with which to distinguish the three different spectral types have been reported [prior to their paper].” Their criteria was unusual, the defined big-cones and regular-cones and inferred the big-cones were S-channelsensitive. They did not describe an absence of big-cones from the center of the foveola.

The Roorda data is for two separate subjects and was collected under the trichromatic assumption. No explanationwas given for the very high percentage of L–channel photoreceptors although they noted “The proportion of L to Mcones is strikingly different in two male subjects, each of whom has normal color vision. The mosaics of bothsubjects have large patches in which either M or L cones are missing.” These subjects did not report any loss invision over significant (though small) portions of their retinas. Their data was collected by microspectrometry in-vivo. It is possible that the empty areas are those associated with the UV–channel photoreceptors (See below). It isalso possible that some of the cells listed as L–channel might in fact be transparent UV–channel cells reflecting lightfrom the RPE. In a larger study, they found the ratio of L– to M– channel cells varied between 0.6:1 and 10:1181. This is an astounding variation that suggests more exploration is necessary using this technique. The technique theyused to isolate the L– and M–photoreceptors may lead to the problem.

The estimate under the “This work” column is shown based on two databases. Without considering the UVabsorption demonstrated by Tan and by Stark, the ratios can be determined from the best theoretical fit to the CIE(1924) luminosity function as corrected by Judd & others. Elsewhere in this work, these proportions have been usedon the assumption that the signals at the output of the photoreceptor array were all of equal amplitude and a gainfactor was introduced that weighted the different spectral signals used to form the luminous efficiency function. Theweighting could be performed by adding in different numbers of signals from different groups of spectrally specificphotoreceptors or the weighting could be achieved by adding the signals from all photreceptors and depending ontheir relative abundance as a function of spectral absorption to create the luminosity function. It is this latterapproach that will be explored in th remainder of this section.

When the data of Tan and of Stark is considered, the array factors must be adjusted to show a UV sensitivity greaterthan that of either the S– or L–channel and approaching (if not surpassing, see Section 17.2.5) the sensitivity of theM–channel.

3.2.3.4 Candidate photoreceptor groupinng based on this work

Page 73: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

72 Processes in Biological Vision

182De Monasterio, xxxx (1985) in Wandel, pg 52.

Figure 3.2.3-2 Potential mosaic of photoreceptors based ona trichromatic approach to vision. The M–channelphotoreceptors are not colored. See text for the ratios. Theblue cells represent S-channel photoreceptors. The red cellsrepresent the L-channel receptors. When reproduced inblack and white, every other dark cell is a L-channelphotoreceptor. The alternates are S-channel photoreceptors.

Figure 3.2.3-2 shows a candidate arrangement lacking any UV–channel receptors. This array provides sevenpercent S–channel and seven percent L–channel photoreceptors, with 86% M–channel cells. The distribution of theS–channel receptors in this map of the outer segments, in both density and distribution, is not grossly different fromthe map of the inner segments presented by De Monasterio, et. al182. However, it begs the question if there are infact UV–channel photoreceptors in the human retina (whether used effectively or not, see Section 17.2.2).

Section 16.3 and Sections 17.2 & 17.3 of this workdevelop the equations of luminance and chrominanceperformance for the human visual system. Thesesections establish that the luminance response isdescribed by a simple equation. The equation sums thelogarithms of the photocurrent sensed by each spectralclass of photoreceptors (with an appropriate scalingcoefficient in front of each logarithm).

These sections also establish that there are multipleparallel chrominance channels in vision. Each isdescribed by a simple equation. The equation takes thedifference between the logarithms of the photocurrentfor pairs of spectrally different photoreceptors (with anappropriate scaling coefficient in front of eachlogarithm).

The above equations accept and account for the limiteddynamic range of the neural system. In their completeform, they describe the luminance and chrominanceperformance of the human visual system with aprecision of better than a factor of two at allwavelengths.

The physical arrangements shown in this and thefollowing figure suggest the simplicity of the signal processing associated with this arrangement. Summing(logarithmically as described above) the signals from all of the cells in two adjacent S– and L–centered capsuleswould provide a theoretical luminous efficiency function well matched to the observed function. Differencing(logarithmically as described above) the signal from a center cell of a capsule and the signal from anyone of itssurrounding cells would provide the correct chrominance channel information. This arrangement would remove anyneed for complex interconnections between the photoreceptor array and the subsequent signal processing of stage 2.

Page 74: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 73

183Labin, A. & Ribak, E. (2012) Retinal glial cells enhance human vision acuity.http://physics.technion.ac.il/~eribak/LabinRibakGlialCells.pdf

1 8 4 S t r a s b u r g e r , H . ( 2 0 0 8 ) A m o d e l o f M u l l e r c e l l s i n a r e t i n awww.hans.strasburger.de/materials/muellerzellen.ppt

Figure 3.2.3-3 Potential photoreceptor array based on atetrachromatic human retina. The M–channels areuncolored. The UV–channels are hatched. Whenreproduced in black and white, every other dark cell is a L-channel photoreceptor. The alternates are S-channelphotoreceptors. See text.

Figure 3.2.3-3 shows an alternate candidate arrangement that includes UV–channel photoreceptors in roughly theproportions required to meet the extended luminous efficiency function. The UV–channel cells make up 28% of thearray compared to the M–channel cells 57%. The S– and L–channels each remain at 7.1%. This configuration isstill compatible with that of De Monasterio. It is also compatible with the images of Roorda, et. al. (recognizing thatthe UV-channel photoreceptors are photographically transparent and appear as “blood” red in their figures).

As above, summing (logarithmically) the signals from all of the cells in two adjacent S– and L–centered capsuleswould provide a theoretical luminous efficiency function well matched to the observed function, even whenextended into the ultraviolet (see Section 17.3.XXX). Differencing the signal from a center cell of a capsule and thesignal from anyone of its surrounding M–channel cells would provide the correct chrominance channel information. There is no current information concerning the composition of signals related to the chrominance channel(s) definedby either UV– minus M– or UV– minus S–. But, differencing the signals from either one or two of the UV–channelcells and one, two or all of the associated M–channel cells in a capsule would provide an appropriate chrominancesignal. As above, this arrangement would remove any need for complex interconnections between the photoreceptorarray and the subsequent signal processing of stage 2.

The placement of the UV–cells in opposition withinthe capsule may cause a directional aspect to the spatialfrequency resolution of the eye. A solution would beto have the UV–cells at the 1 & 3 positions within thecapsule instead of the 1 & 4 positions. They could alsobe randomized further, either within the capsule or byrandomizing the capsule orientations.

Both of the above arrays of capsules (hexagonal groupsof seven photoreceptors) are probably too regular tomatch the available experimental data. However, if thecapsules were to be rotated randomly, or even arrangedaccording to a higher set of rules, groupings similar tothose of Roorda & Williams could be achieved. Thiswould be particularly true if the lack of absorption bythe UV–sensitive cells resulted in their locationappearing red due to the RPE behind the retina. Theserandomizing steps would also insure that the spatialfrequency response of the eye was not seriouslyimpacted by the presence of different spectrallysensitive photoreceptors in an orderly arrangement.

Both of the above arrays appear indistinguishable fromthe arrays, with the blue cones highlighted, of Curcio, et. al.

3.2.3.5 Putative arrangement of glia cells acting as light pipes

Labin & Rabik have recently modeled the potential for glia to operate as light pipes through the neural layers of theretina outside the fovea (in the parafovea)183. This area of the retina exhibits substantially less resolution than thefoveola and fovea. There is considerable evidence that the resolution in this area is limited substantially by thesummation of signals from multiple sensory neurons by the stage 2 circuits in order to obtain change sensitivity atthe expense of resolution. The authors did not provide a detailed physiological model to justify their assumptionsconcerning the need to preserve resolution in the parafovea in the face of the neural layer between the pupil and thesensory receptors possibly degrading the resolution of the imaged light. Their cartoons of figure 1 do not explainhow the broadly spaced Muller cells contribute to the resolution performance of the much finer array of rods andcones shown. Strasburger provided a physiological model of a retina in 2007-08 but it does not show the foveolasituation, only the parafovea184. This research activity does not appear to be based on a solid physiological model, or

Page 75: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

74 Processes in Biological Vision

185Sabesan, R. Schmidt, B. Tuten, W. & Roorda, A. (2016) The elementary representation of spatial and colorvision in the human retina Sci Adv vol 2:e160079186Sabesan, R. Schmidt, B. Tuten, W. & Roorda, A. (2016) The elementary representation of spatial and colorvision in the human retina Sci Adv vol 2:e1600797187Saey, T. (2016) Color vision strategy defies textbooks Science News 15 October, pg 10

more importantly, a solid photomicrograph of a real human retina.

3.2.3.6 The “text rewriting” work of the Roorda team with optimized AOSLO

The subject matter of this subsection involves the intimate interaction of the human retina, the optics of theeye, a very state-of-the-art test set and the changing of a variety of long held intellectual concepts. As aresult, it may be appropriate to move part or all of the following discussion to a later section of this work at alater date.

Many of the most subtle mechanisms associated with the visual modality developed within the context of this theoryhave been confirmed during 2016. The Roorda team, based at the University of California, Berkeley has expandedduring the 21st Century. Their work has introduced and qualified a new AOSLO with a form of real time tremorcompensation to provide a totally new degree of precision in in-vivo human retinal topographic analysis. Sabesan etal185. reported on this very sophisticated test set augmented with adaptive (active) optics, tremor cancelling featuresand additional features associated with optical coherent tomography (OCT) . This test set allowed them to stimulateindividual sensory neuron photoreceptors of the human retina in-vivo for extended periods of time (up to 500 ms.). With this capability, they were able to demonstrate that the M – and L–channel photoreceptors mediated theintensity signaling R–channel of the visual modality as well as mediated their individual M – and L–signalingchannels (Sections 1.2.1.2.2 & 1.3.3 & 1.6.1) . They accomplished this psychophysical experiment by stimulatingindividual photoreceptors and interrogating the fully aware subject as to their perceived response. The major paper isthat of Sabesan et al186. but consists of a set of papers including the following major components.

Lead author Date Focus

Arathorn 2007 Method and performance of a retinally stabilized stimulus directed at conesBenson** 2014 Algorithmic simulations of unsupervised learning of cone spectral classesBrainard 2008 Reconstruction of a static SML mosaic using a Bayesian model (& no tremor)Curcio 1990 Human photoreceptor topography based on in-vitro cell geometry, not

absorption–Archaic*Field 2010 Functional connectivity of macaque SML photoreceptors using multielectrodesHarmening 2014 Physiological features of human retina observed photopically through the lensRoorda 2002 Detailed optical description & performance of one of the AOSLO ca., 20002sabesan 2015 Mosaic characterization via through-lens dynamic photopigment densitometryYang 2014 Full time closed-loop optical stabilization reduced residual motion by 10-15x* Other than the Curcio paper examining inner segments of photoreceptors in-vitro, rods were not found to bepresent in the imagery of the photoreceptors of the retina.** Paper not critical to the operation of the tremor compensated AOSLO

Saey187 reported on the Sabesan paper which she described as “The textbook-rewriting discovery could changescientists’ thinking about how color vision works.” However, this assertion requires tempering. The suggestion bySaey that, “Red and green cones each come in two types: One type signals “white” and another signals color, visionresearcher Sabesan and colleagues at the University of California, Berkeley discovered.” is not supported by anyphysiological model. By merging the theoretical models of this work (drawn from analyses of empirical data in theliterature) with the empirical findings of the Roorda team, a clear case for “textbook-rewriting can easily be made, especially after addressing the challenges in the Roorda team’s reporting in a number of areas enumerated below.

- - - -

Sabesan et al, 2016 asserted in their Abstract, “We unraveled behavior at the elementary level of single inputunits—the visual sensation generated by stimulating individual long (L), middle (M), and short (S)wavelength–sensitive cones with light. Spectrally identified cones near the fovea of human observers were targeted

Page 76: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 75

188Yang, Q. Arathorn, D. Tiruveedhula, P. et al. (2010) Design of an integrated hardware interface for AOSLOimage capture and cone-targeted stimulus delivery Opt Express vol 18(17), pp 17841-17858

with small spots of light, and the type, proportion, and repeatability of the elicited sensations were recorded.” Theirstimulation was through the human lens and did not attempt to stimulate the UV-wavelength sensitive photoreceptorsfound in the retina of all chordates, most insects and many molluscs. Their elicited sensations constitutedperceptions reported following cognition by the stage 5 engines of the CNS. They reported, Two distinctpopulations of “cones” were observed: a smaller group predominantly associated with signaling chromaticsensations and a second, more numerous population linked to achromatic percepts.” This wording should bereinterpreted to indicate the experimenters received two different classes of reports from their subjects: a smallergroup predominantly associated with signaling chromatic sensations and a second, more numerous population linkedto achromatic percepts. This wording better supports their conclusion in the abstract, “Overall, the results areconsistent with the idea that the nervous system encodes high-resolution achromatic information and lower-resolution color signals in separate pathways that emerge as early as the first synapse.” The first synapse occurs atthe output of every stage 1 sensory receptor and prior to the stage 2 signal processing within the retina. This signalprocessing is more complex than suggested by their closing remarks as identified in this work by the high-resolutionPGN-pulvinar pathway and the low-resolution LGN-occipital pathways, both of which process chromatic andachromatic information (generically in Section 1.6.1 and more explicitly in Section 1.7.5).

The false-color fundus photographs show a preponderance of L–channel photoreceptors apparently due to themethod of identification employed. They made a specific effort to identify M –channel receptors and a lessor effortto identify S–channel receptors. They made no effort to identify UV–channel receptors and did not encounter anyachromatic (rod) receptors in the fields of view provided. It is likely that any UV–channel receptors and in manycases, the S–channel receptors were lumped in with the L–channel receptors and shown in bright red. This positionis supported by their Table 1 which demonstrates that a majority of their L–cones did not report a “red” response butinstead reported a “white” response. The white response is indicative of a photoreceptor contributing to anR–channel (brightness) response without contributing to the expected Q–channel response associated with thechromatic signaling channels.

Figure 3.2.3-4 reproduces figure 5 from Yang et al., 2014. It shows the excellent tremor cancellation achieved bytheir frame-to-frame matching system operating at 30 frames per second on the Rochester AOSLO. The inset showsthe fine “hunting” associated with the 30 frames per second cycle time. Yang et al188, 2010 described the design ofthe hardware and software added to the basic AOSLO, Table 3 of the Yang paper compares the performance of thistechnique with a variety of previous eye trackers.

Page 77: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

76 Processes in Biological Vision

189Saey, T. (2016) Color vision strategy defies textbooks Science News 15 October, pg 10

- - - -

[xxx extract and edit into other paragraphs of this work ]The experiments involved stimulating 273 individual photoreceptors in two male subjects. When stimulating an M –channel photoreceptor, the subject responded 21percent (21 samples) of the time that heperceived a greenish response indicative of a color sensing channel. The other 77 percent of the time (77 samples) ,he perceived a whitish response indicative of brightness rather than color. When stimulating an L –channelphotoreceptor, the subject responded 29 percent (48 samples) of the time that he perceived a reddish responseindicative of a color sensing channel. The other 71 percent of the time (119 samples) , he perceived a whitishresponse indicative of brightness rather than color. ; the investigators did not investigate any S–channelphotoreceptors. Not having an aphakic subject available, and probably not being aware of the significance of such asubject, they did not investigate the putative UV–channel of biological vision either.

Saey189 reported on these experiments which she described as “The textbook-rewriting discovery could changescientists’ thinking about how color vision works.” The inset in the figure of her article should be understood to bebased on false-color for purposes of illustration. The photoreceptors do not reflect significant light indicative of

Figure 3.2.3-4 Eye motion trace computed from an image sequence showing vertical (y) component of eye motion before(red) and after optical stabilization alone (blue) and optical stabilization combined with digital registration (green)providing tremor cancellation. Inset is an expansion of the 11 to 15 second interval. Asterisks denote spurious motionmeasurements during blinks or large saccades. From Yang, 2014.

Page 78: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 77

190Nickerson, D. & Newhall, S. (1943) A psychological color solid J Opt Soc Am vol 33(7), pp 419-421191Kelly, K. & Judd, D. (1955) The ISCC-NBS method of designating colors and a dictionary of color names;National Bureau of Standards Circular 553. Washington, DC:US Government Printing Office.

their spectral absorption performance. From the Sabesan et al. article, “Each cone tested psychophysically wasidentified with a specific spectral type by aligning the 842-nm reflectance image and the corresponding trichromaticcone mosaic map obtained from densitometry. The alignment was guided by the pattern of blood vessels in thesubjects’ retina, and individual cone locations remained stable across days.” The suggestion by Saey that, “Red and green cones each come in two types: One type signals “white” and anothersignals color, vision researcher Sabesan and colleagues at the University of California, Berkeley discovered.” is notsupported by any physiological model. The situation can be interpreted quite differently in the presence of a viablemodel of the visual modality that processes the sensory neuron signals into both chromatic and achromatic channelsfollowing the first synapse (within stage 2) as suggested by the investigators in their Abstract.

To achieve the remarkable stability required with this test apparatus, the two subjects used dental impressions toassure constant head position and pointing. Their abstract includes several important statements. They noted,“Spectrally identified cones near the fovea of human observers were targeted with small spots of light, and the type,proportion, and repeatability of the elicited sensations were recorded.” “Sensations generated by cones were rarelystochastic; rather, they were consistent over many months and were dominated by one specific perceptual category.” “ Overall, the results are consistent with the idea that the nervous system encodes high-resolution achromaticinformation and lower-resolution color signals in separate pathways that emerge as early as the first synapse.”

Their introduction adds more detail to their experimental goal. “There are two fundamental impediments to linkingvisual perception to the activity of individual retinal neurons. First, the retina is situated inside the eyeball and thuscan be neither visualized nor stimulated at cellular resolution due to the eye’s aberrated optics. Second, even whilesteadily fixating, the retina moves over spatial scales far greater than the size of a single cone, impeding the repeatedand reliable stimulation of the same cell.”

The section labeled “Brief Methods” adds additional protocol information. “ Cone-sized spots (0.45 arc min;543-nm wavelength; 25-nm bandwidth) were targeted on a mosaic of spectrally identified long (L), medium (M), andshort (S) wavelength-sensitive cones in [only] two male, color-normal subjects — S10001R and S20076R. Thesubjects reported the color of each flash. The stimuli appeared for 500 ms on a dim background subjectivelyadjusted to appear approximately achromatic at the start of each session. A gray background was chosen to roughlyequate the resting-state activities of L- and M-cones to minimize biases in downstream neurons. In the case of alarge stimulus, chromatic contrast from a gray background that increases only L-cone activity produces a reddishsensation in most subjects, whereas excursions from gray that increase only M-cone activity appear bluish green.” These perceptions are totally in accordance with the perceptions expected from the theoretical analyses of this work. The “reddish” sensation is compatible with the L–channels peak response at 610 nm along the spectral locus of theNew Chromaticity Diagram of this work. The “bluish green” at threshold is compatible with the M –channels peakresponse at 532 nm along the spectral locus. Some of their tests were repeated using a 511 nm stimulus according totheir supplemental material. However, the difference between 543 and 511 nm is relatively small and not likely tochange their results significantly according to this work. They utilized a narrow range of intensities in theseexperiements (described as 0.5, 0.75 and 1.0 a.u.). They did note with regard to their figure S2, “Frequency ofseeing decreased with decreasing intensity.”

It is also important to note that the perceived response of humans is a function of the exposure time to the specificstimulus and to the precise state of adaptation of the individual eye at the time of stimulation. This is particularlytrue in the case of the Sabesan et al. paper where no data was provided to demonstrate their 543 nm stimulant did notspill over and cause the stimulation of nearby photoreceptors. They relied upon predicting the location of theirstimulation on the surface of the retina several milliseconds in the future based on their calculation of the dynamicproperties of the tremor encountered and the ability of their tremor cancellation technique.

It is important to note the necessity of providing continuity in their discussion where the names of colors areconcerned. “Reddish” does not equate to “red” and “Greenish blue” does not equate to “green” in the pantheon ofcolor names associated with either the Munsell Color Space190 or the “color names dictionary” propounded by theUnited States NBS (now NIST)191.

The precise significance of the regression analyses shown in their figure S4 is difficult to ascertain in the absence ofa graphic model of their visual modality, and the specific wording in the caption. They noted, “These regressions,while statistically significant, do not fully capture the variance in our data.”

Page 79: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

78 Processes in Biological Vision

Sabesan et al. also explore in concept the ganglion arrangement within the retina but without any graphic model ofthe signaling involved. “There is little doubt that the long-duration suprathreshold stimulation of individual coneshere influences the firing of a number of different ganglion cell types. In particular, a multielectrode studydemonstrated that the activation of a single cone simultaneously evoked responses in both midget (parvocellular) andparasol (magnocellular) ganglion cells. The latter has been posited to contribute to achromatic percepts. However, atthe eccentricity studied here (1.5/ temporal retina), midgets make up more than 90% of the ganglion cells samplingthe cone mosaic. Moreover, the midget ganglion cells respond best to high-contrast, low-temporal frequency stimuli. A 500-ms suprathreshold illumination of the center cone of a midget ganglion cell receptive field, with minimal lightfrom the spot falling on surrounding cones, represents an ideal stimulus for these cells. Therefore, our results maybe particularly informative in differentiating proposals about the role of parvocellular neurons in achromatic spatialand color vision.” They briefly discuss the common wisdom and then conclude, “. . .but our results do not align withthis expectation (Fig. 4 and fig. S4). They go on to speculate about the prior common wisdom but without anyschematic diagram of the visual modality.

- - - -

Tentative conclusions relative to the Roorda team approach/results.

1. They did not investigate the UV–channel photoreceptors of the human eye (Section 17.2.2). The result is theiruse of a simplified schematic (Figure 1.7.5-3 rather than that of Figure 1.6.1-1 of this work) and topography of thehuman retina (Figure 3.2.3-2 rather than Figure 3.2.3-3 of this work).2. They intentionally, omitted investigations related to the S–channel photoreceptors but did note the presence of S-channel photoreceptors in the fovea and foveola of the human retina.3. They described the fine motion of the eyes below the level of saccades but did not relate it to the tremor welldocumented by Yarbus and other contemporaries during the 1960's (Section 7.3.3).4. They recognize the waveguide character of the outer segments of the photoreceptors, but at one point assign theresponsibility for this condition to the inner segments (their xxx). This operation of the outer segments aswaveguides places their effective absorption area to a fraction of their physical cross section. See Stiles-CrawfordEffect of the 1st kind, Section xxx.5. They continue to implicitly assume the peak sensitivity of the L–channel is near 564 nm based on their assumptionthat a 543 nm laser stimulates both the M –channel and L–channel photoreceptors nearly equally based on individual normalization of their sensitivity responses and based on linear arithmetic to establish their crossover point. The 543nm value was promulgated in the 1940's based on a conceptual error in a mathematic analysis. The correct peaksensitivity of the L–channel is near 610 nm based on Thornton, and universally accepted within the lighting anddisplay technology areas.6. They correctly note the L–channel response is perceived as a “reddish” color and the M–channel response isperceived as a “bluish green.” These perceptions are totally in accordance with the perceptions expected from thetheoretical analyses of this work. The “reddish” sensation is compatible with the L–channels peak response at 610nm along the spectral locus of the New Chromaticity Diagram of this work. It corresponds to 8YR in Munsell ColorSpace. The “bluish green” at threshold is compatible with the M –channels peak response at 532 nm along thespectral locus. It corresponds to 2.5G in Munsell Color Space.7. Following identification of the principle perceptions of the M –channel and L–channel as “bluish green” and“reddish,” they revert to the colloquial forms green and red in their discussion. Red and green are defined asexplicitly different colors than reddish and bluish green in the technical literature (Section 17.3.8). As noted in thesection cited, red is a non-spectral color in perceptual space best defined as either 494c or 494,610 nm in the New(Perceptual) Chromaticity Diagram (Section 17.3.8) underlying the CIE 1976 UCS Color Spaces. There is aconcern whether the two (experienced) subjects used by the Roorda team should have identified the perception of theM –channel as yellowish-green rather than bluish-green based on Section 17.3.9 of this work. A larger group ofnaive subjects would clarify this question.8. On multiple occasions, the authors of the various reports of the Roorda team limit their investigations to thephotopic irradiance regime, thereby avoiding direct consideration of the putative “rod” photoreceptors. However, onoccasions, they report activities under threshold conditions that imply conditions requiring rod participation(xxx citepaper). More broadly, their reports do not identify the presence of any rods within the field of theirophthalmological images of the retina. [xxx cite at least one paper ]

9.

- - - - -

Page 80: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 79

192Garrett, L. (1970) Integrated circuit digital logic families. IEEE Spectrum, Vol. 7, Oct. pp. 46-58, Nov. pp.63-72, & Dec. pp. 30-42193Rushmer, R. (1970) Cardiovascular dynamics. 3rd. Ed. Philadelphia PA: W. B. Saunders pg. 2.

Figure 3.3.1-1 Operation of the typical living cell with theinput and output signals added that are peculiar to a neuron.Pull on the lariat to squeeze this morphological bipolar cellin to a monopolar cell. The cell may contain one moreintrinsic electrostenolytic site that polarizes the soma of allcells. Compare to Rushmer (1970).

3.2.4 Electronic architectural level

The volume of signal processing carried out within the retina places significant constraints on the electrical topologyand topography of the retina. These architectural constraints are familiar to an engineer experienced in the design ofelectronic microcircuits. For those interested at the very detailed level, many of the comments by Garrett, althoughfocused on phasic applications in man-made circuits, are worth reviewing192. From his figures, one can easilyrecognize the similarity of the visual circuits to those known by the names DTL (Diode-Transistor Logic) and TTL(Transistor-Transistor Logic). In the retina, the synapses are generally employed as highly efficient diodes. Thesimple neurons of the bipolar and lateral types employ individual Activas. The resulting circuits are virtuallyidentical, architecturally, to DTL circuits used in an analog mode. The projection neurons consist of a series ofActivas easily compared to the architecture of TTL circuits. The complexity of the photoreceptor cells is moreclosely related to analog operational amplifiers than to standardized TTL circuits. The constraints on the diameter,length and placement of the electrical conduits connecting these circuit elements are discussed in Garrett. Thehigher impedance level of the neurons must be considered when interpreting these constraints and guides. Whereasa TTL circuit operates near the 10,000 Ohm level, neurons operate at levels 103-104 higher. Where a TTL circuitmight support output conduits 3-5 cm, long, a signal sensing or manipulation neuron can only support output conduitlengths of a few microns. The specialized, higher capability, signal projection neurons operate like TTL line drivers. They can support conduit lengths up to 2 mm.

3.3 Metabolism of the chordate retina

3.3.1 Static considerations related to a cell

Before discussing the metabolic system serving the eye, reviewing the elements and constituents involved in theprocess for a single cell is useful. The conventional wisdom193 is shown in the upper portion of Figure 3.3.1-1. Most introductory material develops the analogy between a living cell and a manufacturing plant. It presents theidea that metabolic fuel, oxygen and various chemicals are taken up by the cell and the output consists of work,carbon dioxide, heat and various secretions and excretions. This is adequate for the retinal cells of the thirdlaminate, the RPE. It is not an adequate depiction of the neurological cells of the photoreceptor and neural laminates. The neuron also involves the output of a signal currentand the input of a signal flux that will be defined moreprecisely below.

3.3.1.1 The Neuron

Neurons exhibit an additional set of functions. Theseare shown at the bottom of the figure. They accept aninput electrical signal, channel that signal to anelectrical amplifier and distribute the output of thatamplifier to subsequent neurons. The channels areformed by additional bilayer membranes formed withinthe cell. These membranes act as insulating partitionswhen the electrical potential across them isappropriate. In accomplishing the channeling functionand establishing the insulating properties of theadditional membranes, the neuron requires the supportof several electrostenolytic sites to convert metabolicenergy sources into electrical potential. Thesespecialized sites appear to be functionally identical to asimilar site that polarizes the prototypical biologicalcell, whether it is a neuron or not.

Neurological cells accept an afferent flux anddischarge an efferent flux. The efferent flux of asensory neuron (combined electrically with a synapse)is a current. In most neurological cells, the input flux

Page 81: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

80 Processes in Biological Vision

194Oyster, C. (1999) The human eye. Sunderland, MA: Sinauer Associates pp 258-260 & 274195Rodieck, R. (1998) The First Steps in Seeing. Sunderland, MA: Sinauer Associates pg 29

Figure 3.3.1-2 EMPTY Operation of the typical RPE cell.The functions shown above the blood/brain barrier areprimarily in support of the functions performed below thebarrier. These functions involve the salvage (phagocytosis)of the disks of the Outer Segments. Serviceablechromophore material is recovered, stored and eventuallysecreted back into the IPM. Damaged chromophore and theprotein materials are excreted to the blood stream. Theblood/brain barrier is sometimes called Verhoeff’smembrane or described as built of terminal bars by thehistologist.

also consists of a current. For a sensory neuron, it may consist of any flux that can cause a current to be generatedwithin the cell. In vision, the photoreceptor cell receives an exciton flux created by the photo/piezo effect occurringin the transduction material of the Outer Segment.

In neurons, work is not a significant constituent of the output. Surprisingly, heat is not a significant output productof a sensory neuron either. This is because of the unique reversible thermodynamic process used to operate theneuron. Secretions are significant output constituents for the photoreceptor cells. The Inner Segment of these cellssecretes the protein material used to form the disks of the Outer Segment.

The figure illustrates a problem with the terminology of morphology. Neural cells have traditionally been namedbased on the assumption that the cell nucleus, and the elements surrounding it, were the most important elements ofthe cell. The cell shown would normally be described as a bipolar cell if the neural channels are significantly longerthan the diameter of the cell body. The designation is associated with the two long channels ostensibly emanatingfrom the cell body. However, if the lariat shown by the dotted line is pulled and the cell body becomes moreisolated from the neural components, the cell becomes known as a monopolar cell because under this condition,there is only one channel emanating from the cell body. In neurons, the Activa or electronic amplifier is actually themost important part of the cell. The nucleus and other elements within the cell body play only a supporting role.

3.3.1.2 The RPE cell

The upper portion of the above figure is nominally adequate to describe the functions of the retinal pigmentedepithelium cells. However, here again, the soma and nucleus of the cell only perform a supporting role. The majoractivity of the RPE cells is the re-manufacturing of the non-protein parts of the disks of the Outer Segments asshown in Figure 3.3.1-2.

The RPE cells obtain the necessary chemicalcomponents from two sources, the blood stream andphagocytosis of the Outer Segments originallyproduced by the photoreceptor cells and coated withchromophoric material from within the IPM. As areminder, phagocytosis can involve the ingestion ofboth cellular and extracellular material by phagocytes. Phagocytosis does not refer to the nature of thematerial ingested. The cells of the RPE re-manufacture, to a large extent, the chromophoricmaterial required by new disks formed by thephotoreceptor cells (Section 7.1.3). They also breakdown the salvaged protein material and return it to theblood stream as an excretion. Any damagedchromophoric material is also returned to the bloodstream for disposal.

3.3.2 The vascular supply to the retina

The blood supply to the chordate eye is quite limiteddue to many operational constraints. Oyster provides adescription of the vascular supply at several levels ofdetail194. When combined, the figures can be used todescribe the hydraulic capacity of the visual system. Rodieck shows a simplified view195, similar to Oyster. All of the vascular flow must enter and leave the globeas near the optic nerve as feasible. This is necessary toachieve the desired ability to rotate the eye inChordata. Figure 3.3.1-3 diagrams the vascularnetwork of the eye. At least part of the blood flow

Page 82: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 81

196Hogan, M. Alvarado, J. & Weddell, J. (1971) Histology of the Human Eye. Phila. PA: W. B. Saunders indetail pg 403 & 508-522

Figure 3.3.1-3 Vascular circulation within the retina.Rectangular boxes within the INM and optic nerve representneurons. RPE cells exhibit notches to receive OuterSegments of photoreceptor cells. The flow from and to theoptic nerve is idealized. It divides approximately as shown. The hydraulic path length within the INM diffusion bed istypically 0.1 mm except for the foveola where it reaches0.25 mm. The path length within the choroid space is muchshorter. The RPE layer/Bruch’s membrane and the OLMeffectively isolate the IPM chemically from the bloodstream. They also may isolate most of the electricalpotentials of the photoreceptors from the INM.

must occur on the surface of the retina, in the optical path of the incoming photons. The artery enters the eyewithin the optic nerve. It branches into two main portions, the portion supporting the sclera and choroid, and theportion supporting the retina. The choroid circulation also supports the muscles and tissue at the front of the ocularas well. The retinal portion further divides into a portion supporting the RPE laminate, including the Inner and OuterSegments of the photoreceptor cells, and a portion serving the neural laminate.

The vascular circulation of the eye begins in the cranium and branches at the optic disk of the eye. One portion ofthe circulation serves the RPE and other tissue on one side of the Outer Limiting Membrane of the retina and theother portion serves the bulk of the neural tissue within the retina on the other side of the OLM. This circulation isvolume limited. It is also required to provide a variety of functions. Its ability to satisfy these functionalrequirements can be described using the conventional nomenclature of hydraulics. As each branch furthersubdivides, until it terminates in a vascular or diffusion bed, the individual hydraulic paths becomes less capable ofsupporting significant hydraulic flow. However, the total capability remains significant.

The vascular network related to the sclera/choroidlaminate will not be discussed further. Hogan hasprovided an overall discussion of retinal circulation196.

The retina is a portion of the brain. It is protected fromthe bulk vascular system by a so-called blood/brainbarrier. The vascular network related to the retinamust pass the necessary nutrients, energy sources andconstruction materials across this barrier. How this isaccomplished varies between the two sides of theretina.

Note the role of the Outer Limiting Membrane in thefigure. It plays a major chemical role and a majorelectrical role. It chemically isolates the IPM from theINM. It also effectively isolates the signaling portionsof the retina from the photo-sensing portions.

3.3.2.1 Blood flow to the INM and mostneural laminates

The vitreal side of the retina is protected by the InnerLimiting Membrane from the vitreous humor of theocular. Whether this membrane also forms theblood/brain barrier is less clear. The blood/brain barrier may be more ephemeral and be formed by thesurface of the vascular walls, or an additionalmembrane associated with these walls. By whatevermeans, the necessary nutrients are passed into thematrix of the INM and they diffuse to the surface of theindividual neurons. At the same time, any materialsdeleterious to the neural system are excluded.

The neural diffusion bed is approximately 310 micronsthick as shown on the right. It serves all of the neuronsof the retina, including the somas of the photoreceptorcells which are included between the Outer LimitingMembrane and the Inner Limiting Membrane. Most of the capillaries of this laminate are found close to the InnerLimiting Membrane. They cause less degradation to the optical image when placed there. However, the diffusionpath to the soma of the photoreceptor cells is lengthened.

The vascular network of the neural laminate is seen to branch profusely in the above figure, eventually reaching thelevel of the capillaries. The capillary walls are porous and allow the diffusion of materials into and out of the

Page 83: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

82 Processes in Biological Vision

197Dreher, B & Robinson, S. (1991) Neuroanatomy of the Visual Pathways and their Development. In Visionand visual dysfunction, Vol. 3. Boston: CRC Press Fig. 1.2 Also in Rodieck, R. (1988)

diffusion bed with little difficulty. Similarly, the cells of the visual system can exchange material with the diffusionbed. As indicated in the figure, the typical distance between a visual cell and a capillary is about 0.1 mm. However,the distance between the nearest capillary and a cell at the center of the foveola in human is 0.25 mm. As will beseen below, 0.25 mm. is approaching a “lethal corner” in terms of the distance between an active cell and its supplycapillary. This longer path may play an important role in the after images perceived after looking from a bright lightinto a dim area.

3.3.2.2 Blood flow to the IPM, RPE and photoreceptor cells

The vascular matrix serving the RPE laminate is shown at the top of the figure. This laminate is thin, about 20microns thick in humans. It is able to support the entire surface of the retina equally well because it is not in theoptical path of vision. There is no degradation in hydraulic performance related to the foveal area. The blood/brainbarrier between the vascular matrix and the IPM is formed by a combination of Bruch’s membrane, the closepacking of the RPE cells, and (in some works) Verhoeff’s membrane197. This structure plays a complex role in that itmust not only allow the nutrients required by any neural cell to pass, it must also extract the chromogenic materialfrom the blood stream. The blood stream itself is hostile to the chromogenic material. Therefore special steps arerequired. These steps will be discussed in detail in Chapter 6.

The photoreceptor cells are seen to be in a unique position with regard to the Outer Limiting Membrane, the IPMand the INM. While the Outer Segment and much of the Inner Segment are within the IPM, part of the InnerSegment and the soma are located within the INM. This is important. As shown by the electrical components andelectrical paths associated with the Inner Segment, the electrostenolytic supplies to both the Outer Segment and theInner Segment are generally supplied from the RPE vascular matrix. However, some materials required by the InnerSegment are hostile to the chromophoric material within the IPM. These materials are provided to the InnerSegment via the soma from the INM.

3.3.2.3 Block Diagram of the Metabolic Components

The hydraulic block diagram of the retina is a restatement of the morphology of the vascular network in hydraulic engineering terms. The diagram begins by recognizing that the arterial and venous systems form a balanced systemwith the various capillary systems shunted between them. Since working with unbalanced systems is mathematicallyeasier, the effective impedance of each section of the balanced network is usually replaced by an equivalentimpedance in an unbalanced network. Figure 3.3.1-4 provides a schematic network of the metabolic system of theeye using this unbalanced notation. It is shown using electrical instead of hydraulic symbols on the assumption thatmost readers are more familiar with this notation. Specific labels and values will be assigned to the various circuitelements in later chapters. The diagram can be used to describe both the RPE or the neural vascular networks. Fivesections are shown along the bottom of the figure beginning with the flow through the optic nerve. It is assumed thatthe optic nerve section is supplied by an infinite capacity source within the head as shown.

The optic nerve section divides into three sections at the optic disk, the choroid/sclera section, the RPE section andthe neural section. Each of these sections continues to ramify until it develops a diffusion bed in the immediatevicinity of its end user cells. Only the RPE laminate is shown in full detail. The vascula of the optic nervesubdivides into a series of vascuolas, then into a larger number of capillaries, and then into a diffusion bed of verygreat area. The individual cell can be considered a terminating section of this network. The hydraulic characteristicsof each of these sections, except for the terminating section, can be described by a serial resistance and a shuntcapacitance. Each of these resistances will be considered fixed although each of the vascular sections involves avariable impedance due to the muscular capabilities of the conduit wall. Each terminating section consists of a shuntresistance and capacitance. The shunt resistance is a variable reflecting the energy requirements of the cell whentransmitting a signal. Fortunately, the adaptation amplifier of the photoreceptor cell stabilizes the signal levelswithin a majority of the signaling channels of the eye so that these impedances are also fixed. It is a unique propertyof the adaptation amplifiers that they also can be represented by a fixed resistance over a majority of the illuminationrange of the eye.

Page 84: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 83

Figure 3.3.1-4 The equivalent circuit of the vascular systemof the eye. The optic nerve artery divides into threebranches, arteriola, serving different laminates of the eye.These further divide into capillaries serving the variousdiffusion beds. Individual neural cells draw resources fromthese beds. Only the RPE circuit is shown in detail.

All of the vascular sections involve conductive flow ofconstituents of the blood and are represented bygenerally low serial impedances and high shuntcapacitances compared with the diffusion section. Thus, the diffusion beds represent the majorconstriction in the blood supply to a neuron of theretina, and particularly to a neuron in the foveola. Thisis not always an adequate representation of thesituation as will be seen in Chapter 7. Particularlyunder laboratory conditions, it is necessary to considerthe hydraulic system a second order hydraulic system. This requirement is particularly evident in both thedark and light “adaptation characteristic” of vision.

In a worst case, adjacent diffusion beds are depletedand they begin to draw down the level of resources inthe capillaries. These can, in turn, draw down thevascuolas, etc. Each of the circuits contributes anadditional time constant in the above scenario. Theappropriate mathematics for computing the transientresponse of the system is messy but manageable. It isalso presented in Chapter 7.

An additional complication is that many adjacentneural cells draw upon the same region of the diffusionbed for nutrients and excretion. These are shown asparallel sections in the above network. The effect of their drawing on a common diffusion bed is to cause many ofthe edge effects associated with vision under high contrast conditions.

3.3.3 Dynamic considerations

The retina is clearly an organ with strict limitations on its ability to draw energy and building materials from the restof the body. With only one principal artery feeding the entire eye, the different regions of the eye are subject torelatively severe rationing. This rationing concerns what the region can obtain from the system, and what is moreimportant, the rate at which it can obtain it. This becomes even more serious in the central areas of the retina wherea high rate of metabolism is needed to process the detected visual signals. The nutrient flow within the actual retinais by diffusion from the nearest capillary. This results in a given signal channel drawing its needs from a pool sharedwith its neighbors. If one channel is called upon to draw down an excessive amount of nutrients, this will lower thelevel of nutrients available to its neighbors. In the normal operation of the eye observing natural scenes, this onlyhappens rarely, when looking at the sun for instance. However, in the psychophysical laboratory, it frequentlyhappens when the experimentalist introduces a bright, and/or pulsating light source. It also occurs when the lightintroduced is of sharply defined geometry. Many “halo” effects have been assigned a cause involving signalmatrixing when in fact they have been due to depletion of the metabolic reserves of the photoreceptors and signalingcircuitry in a given localized zone of the retina. Furthermore, it will be shown that this depletion of reserves is aprincipal factor in the so-called dark adaptation of the eye. The process of adaptation is a continuous one, involvingboth light and dark adaptation. It also involves a number of state-variables. This makes any measured effects highlydependent on both the test conditions and the coordinates of the retinal location used. These coordinates can aid indetermining the level of nutrient reserves available in that region.

3.3.3.1 Bulk characteristics

Many approaches have been tried to quantify the consumption of energy by the brain and/or the visual system of thebrain. These have been less than totally adequate for a number of reasons. The primary reason has again been alack of an adequate model to describe forms of energy are involved. The conventional wisdom has been that theprimary gauges of energy consumption are the consumption of oxygen and the release of heat. However, to a largeextent, the first order operation of the visual system is independent of these two parameters. In the second order,oxygen does play a major role in forming the primary metabolic reactants. The release of heat is not a major firstorder factor in the operation of the neurons of the visual system. The primary gauge of the energy consumed by theneural system is the consumption of chemical energy associated with the components of the glutamate nutritioncycle used in the electrostenolytic mechanism supporting vision. It appears that a better approach to determining theenergy consumption of the visual system, the neural system, the retina or the brain would be to divide the task intoan aerobic and an anaerobic components. These could then be evaluated separately and summed.

Page 85: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

84 Processes in Biological Vision

198Habib, M. & Bockris, J. (1986) in Gutmann, F. & Keyzer, H. Modern Bioelectrochemistry. NY: PlenumPress pg 78199Kageyama, G. & Wong-Riley, M. (1984) The histochemical localization of cytochrome oxidase...withparticular reference to retinal mosaics. . . . Jour. Neurosci. Vol. 4, no. 10, pp. 2445-2459200 Guyton, A. (1976) Textbook of medical physiology. Philadelphia, PA: W. B. Saunders pg. 251201White, M. (1973) Animal cytology and evolution, 3d ed. Cambridge, [Eng.] University Press, pg. 1009202Miller, D. ed. (1987) Clinical light damage to the eye. NY: Springer-Verlag pg. 83

There is virtually no heat released as part of the glutamate cycle. Thus a primary measure of the energyconsumption of the visual system, and the neural system in general, is a measure of the difference in chemicalenergy between the materials entering the blood stream of the system and the materials leaving it.

Much of the material consumed directly in the electrostenolytic process is reconstituted within the environs of theneural system, a probable role for many of the glial cells closely associated with the neural system. Thisreconstitution appears to be the mechanism consuming glycogen within the system. Glycogen consumption isprobably a good measure of the anaerobic energy consumed within the major elements of the neural system.

The literature contains a number of references to the consumption of power by the human body as approximately 20Watts198. This number is most likely based on calorimetry.

3.3.3.1.1 Studies in heat generation within the retina

The remarkable fact about the energy consumption of the retina is that it is not converted into heat. Kageyama &Wong-Riley199 attempted to measure the heat generated in the retina as an indicator of which neurons were thehardest working. Much to their surprise, little heat could be detected. Energy is frequently juggled within a group ofmolecules in chemistry without any significant generation of heat. The chemical energy consumed in the retina wasused to cause reversible chemical reactions and to support the recycling of photoreceptor materials. Little heat wasgenerated by neural signaling activity. Thus, while energy consumption is high in the retina, heat generation isremarkably low!

3.3.3.1.2 Studies in Oxygen consumption

The following material is paraphrased from the original sources without conforming the language to the terminologyof this work. This is to avoid mis-interpretation and confusion.

Guyton200 says 14% of the entire vascular flow goes to the brain, approximately 700 ml./min. This 700 ml. carriesabout 20 per cent by volume of oxygen. On the average, only about 25% of the available oxygen is extracted duringone circuit of the vascular blood supply, i.e., the utilization coefficient is one-fourth. During heavy exercise, theutilization coefficient can reach 75-85% in muscle. However, in local tissue areas where the blood flow is very slowor the metabolic rate very high, utilization coefficients approaching 100 per cent have been recorded.

White201 said: “The retina has the highest known rate of oxygen consumption of any tissue in the body, per unit ofweight, with an active phosphogluconate oxidative pathway, a high rate of aerobic glycolysis, and an RQ of one. Lactate is the major endogenous substrate contributing to retinal respiration.” This terminology needs to becompared to that of the present day in the same field.

Miller202 reports the blood flow in the human retina to be in the range of 75 micro-liters per minute. This flow isshared between the choroid artery supporting the outer retina, and the retinal vasculature, supplying the inner retina. He indicates the choroid is controlled by the autonomic nervous system and has “leaky” capillaries. The retinalcirculation has no autonomic nervous control, shows auto-regulation of blood flow, and its capillaries have tightjunctional complexes. In the cat, the choroid blood flow, supporting the RPE complex is approximately 20 timesgreater than the retinal blood flow.

The last statement is interesting because it hints at the relative consumption rates for both energy and the materialsconsumed in disk fabrication and coating. It would suggest the process of final chromophore fabrication is muchmore energy consuming than the operation of the signaling system and the fabrication of the opsin substrates

Page 86: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 85

203 Burton, A. (1972) Physiology and biophysics of the circulation. 2nd ed. Chicago, IL: Year Book MedicalPublishers

combined.

3.3.3.2 Detailed characteristics

Capillaries are typically one millimeter long in humans203. At the level of the capillaries, the velocity of the blood is0.5-0.7 mm./sec. These numbers lead to a time constant of the capillary section not exceeding two seconds. Although this calculation is crude, it does generate a flag. The investigator should anticipate a power supply timeconstant in this region when evaluating transient effects.

The nominal impedance characteristics of the diffusion bed are not known to this author. They can only be inferredfrom the perceived effects of tests performed at different locations in the retina. These tests can be of manydifferent types; flash (or flicker) tests, grating tests, etc. There is considerable data in the literature that can be usedto determine the diffusion bed characteristics. However, the data was not collected with that goal in mind. Itfrequently involves many other parameters as variables. Only care and correlation can lead to unambiguousparametric values in the absence of specifically designed tests.

3.3.3.3 Steady state characteristics

By virtue of the accommodation amplifier in the photoreceptor cell, the majority of the circuitry in the retinaoperates over a quite limited dynamic range. The vascular system of the eye and the properties of the RPE andneural diffusion beds are designed to satisfy the requirements of the neural circuitry in their respective regions. Under normal conditions, these requirements are satisfied very well. The animal perceives a uniform sensitivity overits field of view and does not perceive any unusual shading or hue changes that relate to a specific area of the retinathat is undernourished.

There are exceptions to this situation, mostly related to man-made objects in the field of view and/or man madelaboratory test conditions. Most of these special situations involve edge effects in high contrast images.

A high contrast edge imaged upon the retina places significantly different requirements on the vascular supply thannormal. Those photoreceptors associated with the high illuminance scene must accept and process a great manymore photons per unit area than the similar elements in the low illuminance portion of the scene. For those elementssharing a common diffusion bed and/or capillary, competition for nutrients and oxygen will cause a distortion in theperceived image. This distortion will generally be perceived as a sharpening of the edge. The cells in the areaslightly to the higher luminance side of the image transition will find it easier to obtain supplies from the diffusionbed than neurons in the center of the high luminance area. These neurons will tend to generate a slightly highersignal level than their colleagues. The neurons in the area slightly to the lower luminance side of the image willexperience difficulty in obtaining adequate supplies compared to neurons farther from the transition and will report aslightly lower illuminance than their colleagues in the center of the lower illuminance area. The result is an apparentedge sharpening as illustrated in Figure 3.3.2-1. This effect is achieved without any temporal filtering in thesignal channels, of the inhibition or any other type. Although this effect looks to the casual observer like a Gibbs(electrical) phenomena, or other high frequency emphasis in the signal channels, it is merely the artifact of aninadequate power supply under high signal conditions. A similar artifact, due to secondary electron redistribution,was widely encountered in early television camera tubes.

Page 87: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

86 Processes in Biological Vision

204Rushton, W. & Henry, G. (1968) Bleaching and regeneration of cone pigments in man. Vis. Res. vol. 8, pp.617-631

Figure 3.3.2-1 Perceived brightness in response to a highcontrast image transition. The effect is due to a competitionfor metabolic supplies by elements of the Photoreceptorlaminate.

3.3.3.4 Transient characteristics

There are two distinct categories of transients related tovision. The first, the slower transients are directlyrelated to the hydraulic environment of the retina. Thefamiliar adaptation curves of the human eye areactually slow transient response characteristics. Theyare only part of a larger characteristic to be presentedin Chapter 12.

The second and faster class of transients (samplingintervals of less than 0.1 seconds, are actually due tothe method of signal encoding by the signal projectionneurons of vision. These will be discussed inChapters 14 and 18.

3.3.3.4.1 Slow transients

A particular problem relates to the accommodationamplifier in each photoreceptor cell. This amplifierappears to obtain its operating biases from the IPM ofthe photoreceptor laminate which is probably suppliedfrom the RPE laminate. This amplifier exhibits verylarge changes in amplifier gain with respect to bothprior and present signal level. These changes arerelated to very large changes in input signal flux andresulting significant changes in electrical voltage and current at the collector terminal of the Activa forming theaccommodation amplifier.

The properties of both the RPE and neural diffusion beds clearly affect the slow transient effects known asafterimages. Afterimages are fundamentally a transient response to abnormal signal conditions applied to the eye. Unfortunately they can include individual transient responses related to each stage in the signal chain. Thesetransient responses may be related to the inner and outer segments of the photoreceptor cells. In this case, thetransient is associated with the RPE diffusion bed and the IPM. For the other neurons of vision, the transient isassociated with the neural laminate diffusion bed. Afterimages frequently display color effects related to thedifferencing amplifiers of the signal chains. Overall, afterimages show very complex transient characteristics whichare difficult to delineate. Careful test design focused on the use of S-, M- and L-channel light sources, instead of thecommon red, blue, green, yellow and “white,” can aid in the separation of these transient effects and the properidentification of their source.

It is difficult to separate the effect of the neural diffusion bed on each type and subtype of neuron. It is also likelythat the dominant transient effect under large signal conditions will be due to the elements supplied from the RPEdiffusion bed. Both the transducers and the accommodation amplifiers are located within the photoreceptorlaminate. The transducers formed by the disks of the Outer Segment are not active tissue and do not require powerfor their operation. Thus, the transient responses related to the RPE are dominated by the demands of theaccommodation amplifiers.

As a point of departure for later analysis, Rushton204 has co-authored several papers describing the transient responseof what are described as in--vivo human photoreceptors. Measurements were taken both during illumination andduring recovery. A more appropriate description for the recovery data would be the transient response of the in-vivophotoreceptor/RPE system. He describes the time constant of the recovery transient as 120 seconds for the normalfovea (cone), 130 seconds for two color defective fovea and 360 seconds for a normal peripheral photoreceptor(rod). These values were reported for various levels of illumination and are based on reflex densitometry.

3.4 Functions of the Chordate Retina

Page 88: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 87

The previous sections have discussed the morphological characteristics of the retina. It is equally important todiscuss its physiological characteristics. The retina functions in a highly integrated mode that has not beenaddressed in the literature. This has been a major impediment to the understanding of subtleties of vision. There is little comprehensive discussion of the primary function of the retina as an integral unit in the literature. It isusually discussed in very global terms or at the level of individual elements. Most texts limit their discussion of thefunction of the retina and other neural subsystems to a metabolic perspective. The primary function of the retina isto support the acquisition and efficient processing in an optimal manner, of electrical signals resulting from theimaging of light.

The retina is an integral and major portion of the Central Nervous System (CNS) of the animal. It has the veryimportant function of both sensing and processing the visual information available to the animal. The signalprocessing function involves a great amount of signal data convergence, and reduction, without significant loss ofuseful information. It is important to define useful information carefully, as opposed to all information. The visualsystem does not face the same requirements as a television system or film camera. The eye of the system is not animaging device. It is a change detector operating in close conjunction with a full frame memory in the brain. However, the full frame memory is in saliency (vector) space and does not represent a spatial image of the outsideworld. An investigator must determine what information is important enough to include in this saliency space. Thatdetermination is required before one can speculate as to whether the visual system is an efficient system or not.

Later in this work, the retina will be discussed in terms of an information extraction engine of the CNS. Within theretina, all signal processing is in analog signal space involving electrotonic signals. Because of the distancesnormally involved, larger than a mm, the output signals from the retina are encoded for transmission to otherextraction engines of the CNS. This is the same situation found in the rest of the brain. Information is processed inanalog form and transmitted to other centers in pulse coded form.

3.4.1 Functional levels

The primary, or first order, function of the retina can be described entirely in electrical terms. This function is toperform a series of manipulations with electrical signals. These manipulations can be most easily grasped if themanipulations are subdivided into three distinct categories, signal generation, signal processing and signalprojection. These signal manipulations are all performed in an electrical environment which includes all of thefunctions carried out within the individual morphologically defined neurons as well as the all of the manipulationsperformed in transferring the electrical signals between neurons. Chemical mechanisms are only employed in visionin secondary roles supporting the electrical functions. As will be discussed later, pharmacology has little directimpact on the primary functional role of the retina. However, it can impact this function role by impacting thesecondary processes supporting this role.

3.4.1.1 The morphological level

There will always be attempts to understand the function of the retina based on morphological studies. It is better tocircumscribe than to denigrate these studies. From the morphological perspective, the function of the retina can beexamined from the gross level, the inter-neuron connection level and the cellular or intra-neuron level. The grosslevel usually involves visual microscopy and current levels of Magnetic Resonance Imaging, MRI. To achieve agreater degree of detailed understanding, the electron microscope is required at magnifications up to about x100,000. This is true at both the inter-neuron and intra-neuron level. For truly detailed understanding of the functionalmechanisms of the retina, electron microscopy is needed at magnifications of about x250,000 and above.

Contrary to a few scattered comments in the literature, form always follows function and the available topographywithin the visual system. In the case of the difference between rods and cones, the difference is entirely the result ofthe available topography. This difference has no specular or functional cause.

3.4.1.2 The physiological or signal function level

The signaling function hierarchy can be divided into three major stages; signal detection, signal processing andsignal transmission. The signal transmission function is actually shared between the transmitting portion in theretina and the receiving portion in the brain.

The initial function of the signal detection stage is the sensing of the incident photons passing through and imagedby the outer optical system of the eye. The details of this process will be developed in Chapters 4 and 5. Thisprocess is an entirely passive one involving a photon/phonon transducer formed of a coated proteinaceous materialsimilar to a finger nail. After this initial photon sensing, the resulting exciton-based signal is transferred to theneural system and converted into a conventional electrical signal. This occurs in the outer reaches of the

Page 89: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

88 Processes in Biological Vision

205Hageman, G. & Kuehn, M. (1998) Biology of the IPM-RPE-retina interface In Marmor, M. & Wolfensberger,T. eds. The Retinal Pigment Epithelium NY: Oxford Press pg 363

microtubules emanating from the Inner Segment of each photoreceptor cell and in intimate contact with theextracellular Outer Segment associated with that cell.

As noted above, the Outer Segments are supported chemically by the RPE/IPM environment and provide signal datato the photoreceptor cells in a very highly integrated process. The IPM is a very complex chemical environment205. To protect the chromophores, it is intrinsically oxygen and alkali free. The microtubules associated with the InnerSegments, and the Inner Segments are also supported electrostenolytically by the RPE/IPM environment. However,the Inner Segments being integral parts of the photoreceptor cells must also be supported metabolically. Thissupport is provided from the INM as discussed above.

The Outer Segments are capable of operating over a very large dynamic range of photon input flux as will bedemonstrated in Chapter 4. It is the initial amplifiers in the Inner Segment of the photoreceptor cell that provide thecritical ability to vary their amplification factor in order to stabilize the electrical signal within the more limiteddynamic range of the electrical signaling system. They receive an electrical signal that can vary over a range of over1015. Over a narrower range of approximately 106 in amplitude, they are able to deliver an output signal that variesover an amplitude of less than 100:1 in amplitude. This reduction in signal variation greatly simplifies therequirements placed on the subsequent signal processing function.

A majority of the signal processing within the retina occurs between the output terminals, the pedicles, of thephotoreceptor cells, and the input terminals of the ganglion cells. In some animals, a small amount of signalprocessing occurs between the input terminals of the ganglion cells and the actual input terminal of the Activaforming the amplifier within that cell. The area between the pedicles and the ganglion input terminals has becomeknown as the S-Plane due to the pioneering electrophysiological work of Svaetichin in the early 1950's. Theexploratory data obtained at that time was very good. However, it was difficult to specify the exact morphologicalor electrical source of the signals. They were usually composite signals obtained with a probe that was an order ofmagnitude, or more, larger than the circuits under examination. The result was a group of recorded waveformsexhibiting a variety of averaged signals. Some of the more graphically pleasing waveforms were published. Thetotal thickness of the S-Plane in human is approximately 300 microns, 0.3 mm. and it contains on the order of 100million neurons. Using a probe with a diameter of even 0.1 mm. diameter does not lead to discrete waveformsexcept on a statistically infrequent basis.

Within the S-Plane, there are at least three recognizable areas of significant signal processing activity, the plane ofhorizontal cells, the plane of amercine cells and the plane of ganglion input structures. The exact function of each ofthese planes is difficult, if not impossible, to specify precisely at this time. The individual signal processingfunctions may be spatially commingled and there is no requirement that the topology of the circuits exhibit a uniformdirection of signal flow. However, certain assumptions from signaling theory can be relied upon. It is highly likelythat the horizontal cells are heavily involved in extracting chromatic signal information, from the output of theindividual chromatic photodetection channels, and forming the chromatic signals transmitted to the brain. Thisfunction should be performed prior to, or simultaneous with, the formation of the luminance signal. This is thefunction of the bipolar cells. It is likely that one of the major tasks of the amercine cells is the extraction of firstorder spatial relationships between signals from adjacent areas in object space. It is also likely that the inputstructures of some classes of ganglion cells are involved in more complex spatial signal processing involving largerareas in object space.

In this work, the various signal function levels are designated as follows:

Stage 1 The photodetection stage, includes Stage 1a, transduction in the Outer Segments and Stage 1b, translation and amplification in the photoreceptor cells.

Stage 2 The signal processing stage, includes Stage 2a, the 1st lateral matrix involving primarily horizontal cells, Stage 2b, the “straight through” matrix involving primarily bipolar cells, Stage 2c, the 2nd lateral matrix involving primarily amercine cells, and Stage 2d involving the input structures of the ganglion cells.

Page 90: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 89

Stage 3 The signal transmission, or signal projection stage. Stage 3a involves the output of the ganglion cells.Stage 3b is physically located within the Cerebellum.

Although these stages are not marked explicitly, they can be recognized, along with the hydraulic mechanisms of theretina in the “Overall Schematic Diagram of the Processes in Animal Vision” shown in Figure 3.4.1-1. This figurewill be subdivided and examined in detail in subsequent Chapters of this work.

3.4.1.2.1 The spectral signal level

There is wide support in the electrophysiological and microspectroradiometry literature for the spectral absorptioncharacteristics of the animal visual process proposed in this work. Unfortunately, a large part of the psychophysicalliterature conflicts with this work because of an undocumented artifact in those experiments. This artifact, related toone of the Purkinje Effects and the Brezold-Bruche Effect, will be discussed in detail in Chapter 17. There are fourspecific chromophores of vision used throughout the animal kingdom. These chromophores are found in individualstructures related to the photoreceptor cells that vary in morphology greatly within the animal kingdom. Thesechromophores are classed as the Rhodonines and are derived from retinol in the transition from the blood stream tothe IPM via the RPE of the retina. They exhibit equally spaced peak absorptions, based on their stereo-chemistry, at342, 437, 532 and 625 nm. Some animals only exhibit sensitivity to a subset of these chromophores for a variety ofreasons. Humans, and most large terrestrial chordates, cannot sense 342 nm radiation effectively because of thethickness of the outer lens group, consisting of the cornea and “lens.”

The dispersion of these chromophores and their host Outer Segments throughout the mosaic of the retina has beendiscussed in [Section 3.2.2.1.3] above. They are shown in the upper left of the overall schematic diagram wherethey are labeled, UV, S, M and L. It is the ensemble of signals from these Outer Segments, grouped according tospectral response, that are described as the spectral channels in this work.

3.4.1.2.2 The signal channel level

These signals emanate from the signal detection stage and are seen to proceed to the signal manipulation stage. Following processing in the three matrices of this stage, the signals are now grouped, according to their informationcontent, into a luminance, a chrominance and an appearance channel.

The luminance information is shown arriving at the magnocellular area of the LGN marked [S] to label the summingpath. This information is projected as a monophase signal. There is a sub-group of the spectral signals that appearto be treated differently in the signal manipulation stage. The chrominance information is shown arriving at theparvocellular area of the LGN marked [D] to label the differencing path. The signals along this path are biphase incharacter.

There appears to be a separate “straight through signal path” for the spectral signals from the foveola that is passedthrough the signal processing stage directly to the signal projection stage without alteration. This path is shownarriving at the brain marked [S’]. Although neither summing or differencing is involved in this path, the signals aremonophase. In some chordates, there is a significant appearance path from the 2nd lateral matrix of the signalprocessing stage to the Pretectum of the brain and possibly other areas of the brain. As a minimum, this path existsin all chordates to support the control of the line of fixation of the eyes.

Although there is a large convergence in actual neuron count in the signal paths of the retina, there is no similarconvergence in the channels of the signal path. The photodetection stage includes no more than four individualchromatic signal channels. The output of the signal processing stage may involve a different number of channelsdepending on the evolutionary development of the animal. For humans, it typically includes five signal channels, theluminance channel, three chrominance channels and at least one appearance channel. In other animals, particularlyhunters, there may be additional highly-developed appearance channels. The signal transmission channelsessentially replicate the channels of the signal processing stage, although it is possible that there are appearancechannels that originate in the ganglion neurites.

Page 91: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

90 Processes in Biological Vision

206Dreher, B & Robinson, S. (1991) Neuroanatomy of the Visual Pathways and their Development. In Visionand visual dysfunction, Vol. 3. Boston: CRC Press

Figure 3.4.1-1 Overall Schematic Diagram of the Processes in Animal Vision.

A caricature presented as figure 1.2 in Dreher & Robinson206, based on the work of Rodieck in 1988, provides acomprehensive view of the general layout and the complexity of the retina. Unfortunately, it is only a caricature. Interestingly, it defines a blue-cone bipolar, but no red or green cone bipolar. It also seems to define artisticallycells with fat inner segments and correspondingly short and chubby outer segments as opposed to otherphotoreceptors with slim inner segments and slim outer segments. In both cases, the inner ends of all of the outersegments do seem to lie in the focal plane of the optical system (taken as a straight line at this scale). Thechubbiness of the Outer Segments of cones, noted above, is not displayed in figure 1.8, taken from the same paperand attributed to Borwein, although the lengthened microvilli of each RPE cell is shown in both. In this figure, theinner ends of the photoreceptors are not shown in the same focal plane, one of the cells is surely out of focus.

Figure 3.4.1-2 illustrates the signaling aspects of the fundamental summing and differencing path of the retina inblock diagram form. The actual paths associated with any particular cell are much more complicated. The OS andIS of a single photoreceptor always work as a pair but the output of the pair may be shared between any number ofsumming/matrixing circuits. Similarly, the ganglion cells act as signal modulators for individual nerve fibersemanating from the retina but they may receive their input signal from one or more summing/matrixing circuits.

Page 92: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 91

207Lee, B. (1996) Op. Cit.

Figure 3.4.1-2 Block diagram of principal signal paths ofthe eye. The OS of each photoreceptor cell may contain oneof four chromophores. The straight through signal pathsmaintain monophasic signals and connect to parasol typeganglion cells. Initially, these paths only transmitmonophase signals from the foveola . At the next level ofcomplexity, the signal processing neurons of the straightthrough signal paths may collect signals from multiplespectral paths in order to form luminance channel signals(4,5,6). The differencing paths create biphase signals thatconnect to midget type ganglion cells (1,2,3).

Note the difference signal formed from photoreceptor1 & 2. This difference is typically associated withhorizontal cells and represents chrominanceinformation. However, the difference signal between 2& 3 is typically associated with amercine cells andrepresents appearance information. Both of thesesignals are biphase and their encoded action potentialstreams are marked [D]. However, it is very unlikelythat they both go to the same area of the LGN. Similarly, the straight through signal from 2, thesummed signal from 4,5 & 6, and the signal from thefoveola all create monophase signals. However, theydo not all go to the same area of the LGN. Chapter 13will provide complete details concerning the actualcircuits of the signal processing stage and theirproperties.

In the more highly developed eyes, particularly thosehaving a fovea, the size of the photodetectors, thepresence of certain types of cells and the degree ofmatrixing in a given zone of the retina are functions ofthe radius of the zone from the fovea. Thus, in anyexperimental work, the zone in which the work isbeing done should be specified by its coordinatesrelative to the fovea if present. Otherwise, the datacannot be related properly to the overall operation ofthe eye or to the work of other experimentallists. Thisleads to many unneeded contradictions and occasionalarguments in the literature. In a few animals, morethan one fovea is found in a single eye and it becomesdoubly important to describe by specific coordinatesthe zone being investigated.

By using the above two figures as guides, andincorporating the concepts of time-diversity andspatial-diversity encoding into the signaling plan of thevisual system, the material provided by Lee in a recentmini-review is brought into better perspective207,including the conflict between the fourth paragraphand first sentence of the fifth on page 637. The result may provide answers to some of the six open questions hepresented in his conclusions.

3.4.1.2.3 The signal projection level

A feature of the retina that has caused considerable confusion in the understanding of the retina involves the natureof the signals projected along the optic nerve. While all of these signals involve pulse waveforms, they carryinformation by two fundamentally different modes. While the signals passed to the brain along the [S] and [S’]paths involve pulses that appear to be proportional to the monophase luminance of the initial scene, the remainingsignals passed to the brain along paths such as that labeled [D] do not exhibit this property. These signals employ adifferent form of encoding that represents the biphase character of the information transmitted. There are twoclasses of these signals. Those signals representing the chrominance of the scene and those signals representing the(nominally monochromatic) appearance of the scene. It is likely that some of these biphase signals do not go to theparvocellular area of the LGN but to the Pretectum or other areas of the midbrain. As seen above, the [S’] signals donot go to the magnocellular portion of the LGN.

Because of the above variation in signal projection paths between the retina and the brain, the designationsparvocellular and magnocellular pathways between the retina and the non-cortical portions of the brain should beavoided.

In the above reference, Lee has also provided a tabulation of both the proportion of different types of ganglion cells

Page 93: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

92 Processes in Biological Vision

208Boynton, R. (1979) Human color vision. NY: Holt, Rinehart & Winston pp. 238-250209DeValois, R. & DeValois, K. (1975) Neural coding of color. In Carterette, E. & Friedman, M. eds.Handbook of perception, vol. 5. NY: Academic Press210Dacey, D. (1996) Circuitry for color coding in the primate retina. Proc. Natl. Acad. Sci. USA vol. 93, ppp.582-588211Wassle, H. & Boycott, B. (1991) Op. Cit.

and their apparent physiological connections. The tabulation contains considerable material that is conceptuallyrelated to the conventional wisdom concerning the magnocellular and parvocellular pathways and the center-surround phenomenon which is discussed in the next section. It also defines a broader range of ganglion cell typesthan outlined above based on morphology. The test methodology used did not separate difference signals related tochrominance information from that related to appearance information.

3.4.2 The center-surround phenomenon (temporary home)

There is a large amount of data related to the response of the visual system to stimuli of a uniform color. This hasbeen extended to concentric stimuli of various colors. However, there is very little theoretical information to explainthe exploratory results obtained. Boynton has provided an introductory discussion of these phenomena208. The firstelectrophysiological experiments related to the receptive fields of vision are attributed to Hartline in 1938.DeValois subsequently studied this area intensely209. More recently, Hubel has provided additional experimentalresults. Recently, Dacey has presented a variety of papers on this subject. In 1996, he provided a discussionhighlighting some of the difficulties and stating: “Although this spectral opponency has been studied for more than30 years, the underlying retinal circuitry remains unclear210.” This paper did highlight the bistratification of ganglioncells as well as that of the more commonly reported bistratification of lateral cells. The bistratification of thearborization between the dendritic and poditic terminals of the neuron is a feature of the theoretical foundation ofthis work.

Major difficulties in all of these studies have included;

+ the lack of an adequate theoretical model of the visual system, + failure to correlate the spectrum of the stimuli with that of the chromophores of vision (resulting in cross productsin the matrix algebra of the system), and + failure to recognize the limited orthogonality of the chrominance channels of vision.

These problems will be addressed in later chapters. The subject of interest here is the relationship between thearborization of individual neurons of the signal processing stage of the retina and the center-surround phenomenon.

Individual neurons are known to have extensive neuritic arborization supporting interconnections with numerousother neurons. The number of these interconnections mentioned in the literature usually start at around 80-100 andrise from there. Even this number would suggest that a single signal processing neuron could easily exhibit a span ofstimulation much larger than the cross section of soma of the neuron. Spans of 250-500 microns are common in theliterature211.

3.4.2.1 Types of experiments

Center-surround experiments have been performed based on non-invasive visual evoked potentials and electroretinograms. However, these have given very coarse data. Finer data has been obtained through invasiveelectrophysiology probing the cortex, the LGN and the retina. A major problem with the probing techniques is thelimited scope of the cell sampling that can be achieved in a given experiment. Frequently, about one hundred cellswill be probed but only ten to twenty will be found that display the characteristic that was sought. The performanceof these cells will be reported on in detail but the others will be ignored. Most of the experiments have involved thelower chordates. Only since the mid-1970's have significant invasive experiments been performed on primates.

As the studies evolved, a wide range of spot diameters have been used to evaluate the center-surround phenomenon. Unfortunately, most of these experiments have employed large, generally circular spots of stimulation. When notcentered on the fovea, their definition relative to location on the retina has been poor.

Page 94: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 93

212Kaneko, A. & Tachibana, M. (1983b) Op. Cit. 213Yang, X-L. Tauchi, M. & Kaneko, A. (1983) Convergence of signals from red-sensitive and green-sensitivecones onto L-type external horizontal cells of the goldfish retina. Vision Res. vol. 23, no. 4, pp. 371-380214Dacey, D. (1996) Circuitry for color coding in the primate retina. Proc. Natl. Acad. Sci. USA, vol. 93,pp.582-588215Lee, B. (1996) Receptive filed structure in the primate retina: A minireview. Vision Res. vol. 36, no. 5, pp.631-644

3.4.2.2 Span of stimuli versus span of neurons

Most of the center-surround data available is based on the use of a few arbitrary size circular stimuli. Only a fewexperiments involved a space between the outside of the inner stimulus and the inner diameter of the outer stimulus. This type of data only provides an integral of the response from the underlying mechanism as a function of thediameter of the circle. Analyzing this type of data requires the assumption that the phenomenon is symmetrical withrespect to the center point of the field of the stimuli. This assumption is a poor one. By reviewing a large amount ofdata, some involving long edges, some defendable conclusions can be drawn but a more extensive mapping, usingsmall diameter stimuli at large radii from the center of stimulation, would provide better understanding.

Kaneko & Tachibana have provided some data on the response of a cell to various size circular stimuli for in-vitrocarp retina212. A related paper provides additional valuable data in other areas213. The original data was providedusing a logarithmic abscissa. Although compacting the data, this display does not highlight the underlyingrelationships. Figure 3.4.2-1 presents their data in both the original and linear form. The two dashed lines on thelogarithmic graphs suggest a common functional relationship for the data. Their paper did not present a model ofthe process they were exploring and the experiment design involved a large number of uncontrolled and/orunmeasured variables. This work would also suggest that 600 and 500 nm were particularly poor choices ofwavelength for this type of experiment. Because of these matters, attention will only be called to the spatial aspectsof the waveforms. The graphs on the right plot the same data points as on the left. If the data points are assumed toresult from an integrative process as a function of stimulus diameter, one could differentiate the smooth curveconnecting these points and, ignoring several details, plot the probable signal input from each photoreceptorultimately connected to this cell. This process suggests that there was little input to the cell from a distance beyondabout 500 microns from the center of the stimulus irradiance. In the upper right frame, most of the input appears tooccur within 350 microns as suggested by the authors and all inputs lead to an increase in signal amplitude. In thelower right frame, a different result is obtained. A majority of the photoreceptors within a 100-micron diameter ofthe center of the cell arborization lead to an increase in signal amplitude while a majority of the photoreceptor inputfrom beyond the 100micron diameter appears to cause a decrease in signal amplitude. The net signal amplitudeapproaches an average of zero in this case after a possible third order input or some transient overshoot due to theprocedure employed.

Much of the recent data regarding the dimensions of the arborization of neurons is obtained using very sophisticateddye techniques at the cytological level. A considerable volume of this work is associated with Dacey and hisassociates214,215. Of particular interest is what they describe as bifurcated dendritic trees. In this work, these trees arefunctionally separate. One is associated with the dendrite and one is associated with the podite of the neuron. Thesignal input through the poditic tree is inverted at the axon of the neuron.

Unfortunately, most of the center-surround and arborization data has been interpreted under the assumption that a setof orthogonal Hering axes provide the appropriate foundation. Later Chapters of this work will address thesematters. Here it is remains appropriate to look only at the spatial dimensions involved. As Kaneko & Tachibanasuggest, the nominal diameter of carp dendritic arbors is usually 60-120 microns with a maximum reported value of184 microns. Taking 60 microns as the standard deviation of the carp arborization, a scale has been providedbetween the two right frames calibrated in standard deviations.

Page 95: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

94 Processes in Biological Vision

Figure 3.4.2-1 Amplitude versus stimulus size of double opponent cell of carp. Left data using logarithmic abscissa.Dashed lines added for discussion. Right, same data using linear abscissa. Shaded areas suggesting input density fromindividual photoreceptors. Central scale on right suggesting distance from center of cell to individual photoreceptorsin standard deviations. Data points from Kaneko & Tahibana, 1983.

3.4.2.3 Interpretation of experiments

Kaneko & Tachibana indicate in their paper that of 85 “bipolar cells” they examined, only one-quarter (15 on-centerand 3 off-center cells) exhibited the double opponent receptive fields they sought. Since their work was in-vitro on alower chordate, there was no way to determine if the signals from the cells they examined were actually used within

Page 96: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 95

216Messenger, J. (1991) Photoreception and vision in Molluscs. In Evolution of the eye and visual system.Cronly-Dillon, J. & Gregory, R. ed. Vol. 2 of Vision and Visual Dysfunction. Boca Raton, FL: CRC Press,pp. 364-397

the chrominance channels of the visual process. It also becomes difficult to insure the irradiance was projected ontothe retina in a sufficiently precise manner.

One of the hallmarks of this work is the fact that there are no achromatic photoreceptors in vision. Therefore, any spatial signal processing must process chromatic signals from photoreceptor cells. This fact complicates the analysis and interpretation of these types of center-surround recordings.

They did not indicate how they determined all of the above cells were “bipolar cells.” They did not indicate howthey insured the cells were from the bipolar sublayer of the inner nuclear layer and not from the 1st or 2nd lateralsublayers which consist primarily of horizontal and amercine cells.

A review of the above statistics, the data of Kaneko & Tachibana and the many arborizations provided by Dacey et.al., suggests that much of the signal information recorded for these cells did not originate at the junction of thesecells with an immediately adjacent photoreceptor cell. There is strong reason to believe these waveforms wereobtained from a cell at the apex of a much wider signal collection environment. Considering a simple two-dimensional pyramid with one output cell and two input cells, it would be expected that Kaneko & Tachibana wouldhave found about 30% of the cells exhibiting the waveforms they sought. If the pyramid consisted of one output andtwo layers in the pyramid with each node accepting signals from two other signal processing nodes, one wouldexpect 14% of the neurons examined to exhibit the sought after waveform. However, even this number of stages inthe pyramid would not span more than about 240 microns (one sigma). This is about the value Kaneko & Tachibanareported (a peak in their graphs at about 350 microns with no stated tolerance).

The waveforms in the upper frames are basically monopolar. However, this may not be the true nature of the signalsprocessed by this cell since it was acquired at a wavelength well beyond the nominal differencing range of this typeof cell. The tail also could be the result of the summing of signals from a broad range of distal cells. Alternately, thesmall negative component of the derivative shown in gray could be caused by a variety of instrumental problems. On the other hand, the waveforms in the lower frames are clearly bipolar. Whereas the upper waveforms could begenerated by a bipolar cell that normally only processes monopolar electrical signals, the lower waveforms areelectrically bipolar. This type of electrical signal is normally associated with lateral cells within the 1st or 2nd lateralmatrices.

Looking at the problem in two dimensions changes the percentages slightly but several reasonable conclusions canbe drawn. First, the recorded waveforms were generated as a result of a pyramiding process probably occurringwithin the 1st lateral matrix of the retina. Second, the pyramiding probably involved a three level pyramid. Third,the recorded signals were probably from the axons of some horizontal cells. Fourth, there is no way to determinewhether the recorded waveforms were ultimately used to extract chrominance or appearance information within thebrain of the host animal.

Since there is no way to determine the ultimate purpose of the recorded waveforms (their ultimate use could beeither for chromatic or appearance purposes), it appears inappropriate to label these waveforms and the presumedreceptive fields of the signals from which they were processed to be color related.

3.5 Electrophysiology, morphology & function of the eyes of Mollusca

The eyes and retina of the molluscs are extremely diverse and many show specific ecological adaptations. Messenger reviewed the available data in 1991216 and noted the immense morphological diversity in thephotosensitive elements in the phylum. This varies from a variety of simple photosensitive cells, through simpleeyes, to the highly-developed complex eyes of the cephalopods.

The terminology of the histologist and anatomist who are focused on Mollusca continues to differ significantly fromthat of those investigating Chordata. The same assertion applies generally to those investigating Insecta (Section3.6).

3.5.1 The compound eye of Mollusca

As noted in Section 1.7.2.2, the eyes of Mollusca are unbelievably diverse, extending from photo-sensitive spots, tosimple eyes to compound eyes. Even the compound eyes exhibit a wide range of specializations adapted to their

Page 97: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

96 Processes in Biological Vision

217Cronly-Dillon, J. (1966) Spectral sensitivity of the scallop Pecten maximus, Science, vol. 151, pg. 345-346218Hamasaki, D. (1968) The electroretinogram of the intact anaesthetized octopus. Vision Res. Vol. 8, pp. 247-258219Young, J. (1971) The anatomy of the nervous system of Octopus Vulgaris London: Oxford University PressChap. 16

Figure 3.5.1-1 Diagram of the retina of Octopus; above asseen in tangential section and below in radial section. Seetext for interpretation and abbreviations From Young (1971)

environment.

3.5.1.1 Multispectral mollusc retina

As a phylum, it contains all of the expected chromophoric spectral bands. The literature specifically reports thepresence of the UV-, S-, & M-channels as expected due to anisotropic absorption by the rhabdom. It also reports afrequently recorded isotropic peak at 500 nm. The instrumentation used to measure this peak must be examined ineach case. The arrangement of the chromophoric material in the retina of the higher Mollusca seems to facilitate themeasurement of an isotropic (nonfunctional) peak at 500 nm.

Limited detailed information is available on the retinaof the more advanced members of Mollusca. Itobviously consists of multiple individual photoreceptorcells in a two-dimensional array lining the inside of anenclosure opposite the aperture. The retina is of thedirect type. The photoreceptors are illuminated at their distal end. The cartoons of Eaken, Wolken and othersare relatively simple and difficult to correlate to a two-dimensional array. Both spectral and behavioral datashow that the retina is sensitive to at least the S- andM-chromophores217. The spectral response recordedby Hamasaki,218 using electroretinographic techniques,also suggests sensitivity in the ultraviolet spectrum. The animal is sensitive to the polarization of light in atleast one spectral region. Young has provided themost details on a retina of Mollusca219 in Figure 3.5.1-1. This hand drawn caricature is apparently based onvisual microscopy. Many finer details expected froman electron micrograph are missing. Theidentification of the views is also questionable from adraftsman’s perspective. The lower view was notoriginally aligned to the upper view. It has beenrealigned in this figure. The lower view appears to bea side view along a fractured surface rather than a truesection view. The supporting cells are not asprominent in the upper view as in the lower view. The abbreviations are: bas., basement, ce., cell, co.,collateral, eff., efferent, ep., epithelium, f., fiber, in.,inner, lim., limiting, mem., membrane, nuc., nucleus,out., outer, pig., pigment, pl., plexus, ret., retina, rh.,rhabdom of retina, and su., supporting.

There are two important aspects of the tangentialsection. First, the individual cells In the tangentialsection are seen to be symmetrical with the retinula inthe center and rhabdomere extending from oppositesides. The heavy black lines, representing fouradjacent rhabdomins, appear to form a box like unitsimilar to that of the crustacean eye. This group couldbe called a rhabdom if one ignores the rhabdomereextending outward from the cells into adjacentrhabdoms. On the other hand, a diamond shaped

Page 98: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 97

220Saibil, H. & Hewat, E. (1987) Ordered transmembrane and extracellular structure in squid photoreceptormicrovilli J. Cell Biol. vol. 105, pp 19-28221Young, J. (1971) Op. Cit. Chap 17222Young, J. *1971) Op. Cit. pg 420

rhabdom can be defined by four rhabdomin arranged in a cross. This rhabdom consists of four completerhabdomins, including both rhabdomere of each cell. This assumes the rhabdomeres are not interdigitated. Assuming this latter arrangement forms a rhabdom, it is interesting to note the presence of two pairs of cellsarranged similarly to that of the crustacean eye shown in the previous figure. The caricature of Octopus does notshow the interdigitating of rhabdomere although higher resolution work might. It does show rhabdomere arrangedorthogonally which is usually associated with sensitivity to light polarization. If this assumption is correct, eachrhabdom would include two pairs of rhabdomins. It is quite possible that one pair incorporates the S- and one pairthe M- chromophore. The result is an individual rhabdom that is both polarization and color sensitive at twowavelengths. If this interpretation is reasonable, the locations at the center of the rhabdomeres of a given rhabdom,and marked pig. in the upper figure, would be the equivalent of the IPM of the chordate eye. An inset has beenadded to the figure focusing on the proposed unit rhabdom [xxx ?]. It is consistent with one of the supporting cellsproviding structural support to, and with pigment material placed around the periphery of, each of these groups. Thematerial surrounding each of these rhabdoms could also act as the wall of a light pipe. Saibil & Hewat220 haveprovided an alternate configuration and excellent electron micrographs. It shows the OS of the retinula to be 200-300microns long with the orthogonal microvilli (coated dendrites) about one micron long and 60 nm in diameter. Thediameter is consistent with the diameter of the microtubules along the disk stack in Chordata and with thedimensions required to form a distributed Activa. Their characterization of the detailed microvilli arrangement isbased on a single retinula and varies slightly from the above interpretation because of their attempt to maintain a cellconfiguration similar to the Young template for Chordata.

Second, the orientation of this figure is unknown with respect to the axis of symmetry found in the Octopus eye. Asshown above, the Octopus retina exhibits an axis of symmetry formed by the projection onto the retina of the planeformed by the pivotal axis of the eye and the center of its aperture. The angle between the retinal array and the axisof symmetry could be important when discussing the effect of tremor on the performance of the total visual system.

Valuable insight is also available from the side view of the fractured retina (radial section). The most importantfeature is that there is essentially no signal processing performed within the retina of Mollusca221. The axons of thephotoreceptor cells go directly to the adjacent optic lobe of the brain. There are no lateral processing and noencoding by projection neurons. The figure shows a few efferent fibers entering the retina and contacting thephotoreceptor cells through collateral fibers of unspecified function.

Under the above interpretation, the rhabdom of Mollusca is less well defined than the rhabdom in Arthropoda butbetter defined than the rhabdom (if any) in Chordata. The output of the photoreceptor cells goes directly to the brainwithout using projection neurons as in Chordata. This is the same configuration as in Arthropoda.

3.5.1.2 Details of the rhabdom

Having electron microscopy of the Octopus retina would be useful. Lacking that data, Figure 3.5.1-2 illustrates thebasic geometry of a single mollusc photoreceptor based on Young and using his hierarchal notation222. Although theYoung figure showed the rhabdomere of each cell forming triangular collection surfaces, it is proposed that they aremore likely to form rectangular surfaces interdigitated with adjacent photoreceptors both across from andperpendicular to this cell. It is also proposed that the features he labeled as “pig” are the Golgi apparatus ormitochondria. In this interpretation, the chromophoric material would be provided from another cell nearby.

A more advanced caricature of a generic individual ommatidium is shown in Section 1.7.2.1.2.

Page 99: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

98 Processes in Biological Vision

223Land, M. (1965) Image formation by a concave reflector in the eye of the scallop, Pecten maximus. J.Physiol. (London) vol. 179, pp. 138-153

Figure 3.5.1-2 Early prototypical photoreceptor ofMollusca. The form caricaturized by Young in 1971 isshown on the left and upper right (in cross section). Thelower right shows an alternate cross section providinghigher sensitivity and polarization sensitivity through inter-digitization.

It is proposed that: the photoreceptor cell consists of anucleus placed proximal to the basal membrane of theretina (as shown) but with an Inner Segment locateddistal to that membrane. The primary purpose of theInner Segment is to secrete and extrude multiple ciliaof the protein material, opsin, in a directionperpendicular to the length of the Inner Segment. These cilia (rods of protein) are similar to those inArthropoda and the equivalent of the disks formed inthe eyes of Chordata. Upon coating with a liquidcrystalline chromophore and contacting by a dendriteof the Inner Segment, these structures become thephotosensitive rhabdomere of the cell. The dendriteswould be expected to be approximately 250 nm indiameter and can only be identified through electronmicroscopy. The primary role of the supporting cellsof the retina is to produce the chromophoric materialand transfer it to the IPM. In this role, they areanalogous to the RPE cells of Chordata.

3.5.1.2.xxx The retina of Pecten maximus

More data is available, at a gross level, for the unusualcase of Pecten maximus223. Figure 3.5.1-3 shows thatPecten has two separate retina that appear to bearranged back to back. This would imply that oneretina must be of the reverse type. However, as seen inthe section on mollusc optical systems above, thisconclusion is incorrect. Although the two retina areback to back in a physical sense, they are not in theoptical path sense. If this cartoon of Land is correctand if the generic eye of Mollusca contains a singledirect retina, Pecten took advantage of an opportunity. It pushed its original retina away from the aperture,and backfilled with a new retina near the argentea. The result is a mollusc with two reverse retinas in each eye. More careful measurements might locate the two focal surfaces differently. This result would be consistent with twodirect retinas in each eye. As indicated earlier, the transmission efficiency of this eye is poor because the unfocussedlight must pass through one retina before imaging at the proper image surface. A probable 50% of the light is lost inthis process.

Page 100: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 99

224Menzel, R. (1979) Spectral sensitivity and color vision in invertebrates. In Comparative Physiology andEvolution of vision in invertebrates. Autrum, A. ed. NY: Springer-Verlag pp. 537-540225Cronly-Dillon, J. (1966) Spectral sensitivity of the scallop Pecten maximus. Science vol. 151, pg. 345226McReynolds, J. & Gorman, A. (1970) Photoreceptor potentials of opposite polarity in the eye of scallop,Pecten irradians. J. Gen. Physiol. vol. 56, pp. 376-406 (Two papers)

Figure 3.5.1-3 Structure of the dual retina of Pectenmaximus and the location of the two focal surfaces. Thescale is of axial distance from the argentea. (Modified fromLand, 1965)

The individual photoreceptors are similar in structureto those of Arthropoda, i.e., the chromophoric materialis found in rods exuded from the sides of thephotoreceptor cells. This similarity leads some authorsto use the same element names as in Arthropoda. However, this is not done consistently and confusion isthe result. Major differences appear at the next higherlevel of organization. Whereas the rhabdom ofArthropoda exhibits a circular symmetry withresp10ect to the centerline of the assembly, this ismuch less evident or nonexistent in Mollusca. Thelimited data available indicates an orthogonal groupingof photoreceptor cells to achieve a higher sensitivity tothe polarization of the incident light.

It is likely that these groupings are repeated across theretina with different groups employing differentchromophores to achieve spectral diversity. Asindicated in the figure, it is possible that a single groupof orthogonal cells could employ more than onechromophore. Further research will be needed to learnthe true organization of the retina.

Menzel224 opened a discussion in 1979 with thestatement “A mollusc eye containing more than onephotopigment has yet to be found.” He then goes on tomention the meager amount of available data, the factthat virtually no intracellular recordings had beenpublished, and the ERG data available tended to emphasize the dominant photopigment present. He then reviews thedata, pointing out that no measurements in the ultraviolet were available and that two peaks were frequentlymeasured at 475 nm and 540 nm, including those by Cronly-Dillon in 1966 225. Based on the model used here andthe location of these two peaks, the data strongly suggests the presence of at least an M-channel and an S-channelchromophore in the mollusc eye. The peak at 540 nm. caused by the M-channel chromophore with a theoreticalpeak at 532 nm. The 475 nm. peak is due to the Bezold-Brucke Effect in the presence of both an S-channelchromophore with a peak at 437 nm. and the M-channel chromophore. The Bezold-Brucke Effect is normallyreported at the psychophysical level. It is observable at the electro-physical level, especially when using a very highimpedance (current) probe. A current probe introduced into the IPM is a low impedance device relative to itssurroundings. As a result, it will sample and sum the currents from a group of nearby photoreceptors in anuncontrolled manner.

McReynolds & Gorman provide a comprehensive study of the signals emanating from the two retinas of Pectenirradians226. Lacking a credible model, they were unaware of how the eye actually operated. They suggest a 2-3log unit difference in sensitivity between the two retinas while their imaging light was focused at the entranceaperture of only one of them. They presented considerable electrophysiological data but it is not at all clear whichpart of the various photoreceptor cells were probed. This leaves much of their data in conflict with other literatureand the model presented here. The data in their papers are worth re-analyzing. However, the use of a highimpedance probe must be accounted for and their imprecise specification of the probe location recognized.

3.6 Electrophysiology, morphology & function of visual modality of Insecta

Page 101: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

100 Processes in Biological Vision

227Invertebrate Brain Platform (as of 2016) https://invbrain.neuroinf.jp/modules/htmldocs/IVBPF/Top/index.html A more specific although not totallyedited page is https://invbrain.neuroinf.jp/modules/htmldocs/test/General/optic_lobe.html

The material in this section is broader than just the retina of Insecta. It is a larger representation of the visualmodality of Insecta to show how this modality is a dual of the visual modality of Chordata but employs significantlydifferent anatomical morphological and some histological features. However, except for stage 3, defined below, theelectrophysiology of the insect eye and the chordate eye are analogous. The material below is reproduced fromAppendix E of this work with the same title as above.

The term “process” is used by the authors of the papers in this section as a noun and is used to describe avariety of physical elements with separate functions but not further identified. This is a difficulty thatappears to live on to this day among experimentalists. An alternate for process found in some papers is“collateral.” It is meant to include axons and various neurites. In this work, the term process will only beused as a label describing a higher order or broader mechanism. When appropriate, the term element, or amore specific name for a functional element will be used to replace “process” in this work.

To orient the reader, Ball has provided an excellent view of the head of a member of Insecta, Figure 3.6.1-1, showing the dominant character of its two compound eyes. The individual hexagonal cornea of each ommatidium issmall enough to allow a very orderly array of hexagonal cornea on the otherwise curved surface of the eye.

The terminology of the histologist and anatomist whoare focused on Insecta continues to differ significantlyfrom that of those investigating Chordata. The sameassertion applies generally to those investigatingMollusca (Section 3.5). This section will review andcompare this terminology problem. For a good sourceof data in this area (although not always professionallyedited) appears to be the “Invertebrate BrainPlatform227.

Two putative types of compound eyes, consistingof fused ommatidia are found in the literature; theapposition eye (sometimes labeled the photopiceye) employs a one-to-one relationship between asingle cornea and a single retinula. A distinctlydifferent superposition eye (sometimes labeled thescotopic eye) is described where the light from asingle source is gathered by multiple independentcornea and directed to a single retinula. Thesuperposition eye, originally described in 1891 byExner, encounters many difficulties when pursuedbeyond the conceptual stage. Goldsmith & Bernard(pp 221-229) have summarized these discussions. A detailed optical ray tracing activity would beneeded to draw firm conclusions regarding this eye. If Nilsson et al. are correct that the cone forms anafocal telescope, the discussions of a superpositioneye are seriously undermined. Only the appositioneye will be considered below.

There appear to also be two major forms of theommatidium supporting both the simple andcompound eyes of Insecta. The fundamentalommatidium includes a set of retinula that generallyextend the entire length of the ommatidium. Thereis also a complex ommatidium involving two sets ofretinula extending only about one-half the length ofthe ommatidium.

Figure 3.6.1-1 Anatomy of the head of an insect. Note thelarge number of individual ommatidium fused together toform each compound eye. A hair is frequently foundemanating from the junction between the hexagonal corneaof adjacent ommatidia. In many species, each corneaexhibits a very low reflective component due to a dense, butlow average volumetric density, array of nipples protrudingfrom the bulk structure of the cornea. This array acts as ananti-reflection coating. From Ball, 2016.

Page 102: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 101

228Rockstein, M. ed. (1974) The Physiology of Insecta, 2nd Ed. Vol II. NY: Academic Press 229Zettler, F. & Weiler, R. (1976) Neural Principles in Vision. NY: Springer-Verlag230Land, M. Laughlin, S. et al. (1981) handbook of Sensory Physiology, Volume VII/6B. edited by Autrum, H.et al. NY: Springer-Verlag231Goldsmith, T. & Bernard, G (1974) The visual system of insects in Rockstein, M. ed. The Physiology ofInsecta, 2nd Ed. Vol II. NY: Academic Press page 166-272232Miller, W. (1979) Chapter 3, Ocular Optical Filtering In Comparative Physiology and Evolution of Visionin Invertebrates: Volume 7 / 6 / 6 A of the series Handbook of Sensory Physiology pp 69-143233Snyder, A. Menzel, R. & Laughlin, S. (1973) Structure & function of the fused rhabdom J Comp Physiol vol87, pp 99-135234Chittka, L. & Menzel, R. (1992) The evolutionary adaptation of flower colours and the insect pollinators’colour vision J Comp Physiol A vol 171, pp 171-181

3.6.1 Background

Much work was done concerning the electrophysiology and histology of the visual modality of Insecta in the middleof the 20th Century. Rockstein in 1974228, Zettler & Weiler229 in 1976 and Land et al230 in 1981 published significantvolumes on these subjects. Both the Rockstein and Land et al. volumes were parts of much larger sets of bookscovering a wide field of vision among many species. Although dated, and in many areas archaic, these volumesremain excellent reference works today. Goldsmith & Bernard presented the broadest summation of the literature(105 pages) within Rockstein231. While they touched on a great many mechanisms, they focused on histology andprovided very little electrophysiological information. More recent work has been less than consistent from thefunctional perspective due to the wide variety of species being explored. The result has been limited new functionaldata to support the large scale anatomical and histological investigations described in the above volumes. Miller hasprovided a now dated but broad discussion of ocular optical filtering as it applies to a variety of animals232.

Form does follow function, and relies upon function to explain the potentials of different forms to support anecological niche for the parent organism. Snyder and colleagues produced a large number of papers during the1970's attempting to model the visual modality of Insecta. However, the modeling was very limited in scope andattempted to begin with a series of assumptions that appeared logical at the time but which now seem quaint233. Thepaper entitled “Structure & Function of the fused Rhabdom” is cited particularly in this regard. Their firmestfoundation was a set of templates originally “from Dartnall's (1953) nomogram which was extended into the UV.” Dartnall did not claim any theoretical or even extensive measurements to support his original nomogram. The peaksensitivities of Snyder’s figure 3 are given as 340 nm, 430 nm and 530 nm for his trichromatic nomogram. Afterproducing at least five papers in 1973, the broadest paper has only been cited about 160 times in the subsequent 40years. One useful paper from the psychology community was from Chittka & Menzel234. The majority of the papershave provides histograms suggesting peak sensitivity at wavelength within 300-350 nm (UV), 430-450 (S), 520-540nm (M –) and 600-650 nm (L–). A complicating factor is noted by Chittka & Menzel, “A further accumulation ofslopes (of spectral sensitivity functions) can not be exploited for discrimination by animals without red-receptors. Itthus appears that pollinators with tetrachromatic systems (such as beetles and butterflies and very few species ofHymenoptera) that possess such receptors. . . .” The variation in the spectral, and polarization, performance, andlogically in the histological form of the related sensory neuron receptors of Insecta is very great. It is noteworthythat equation (1) in Chittka & Menzel is labeled the “relative quantum flux P but the equation is in conventionalphotometry units and does not recognize the 2:1 difference in quantum flux associated with a given narrowbandintensity signal with a 2:1 change in wavelength (such as between 300 and 600 nm). This factor is significant whencalculating the broad overall spectrum associated with both photopic and scotopic vision.

Page 103: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

102 Processes in Biological Vision

The phylogeny of the butterflies is exceedingly complex with many groups within the various groupssubordinate to Lepidoptera. As a convenience, this phologeny is reproduced here;

Kingdom: AnimaliaPhylum: ArthropodaClass: InsectaOrder: Lepidoptera (Butterflies and moths)Suborder: RhopaloceraSuperfamily (at least three)

There are multiple suborders and many families within superfamilies below the above descriptors dating fromthe Palaeocene era, about 56 million years ago. The current Wikipedia is a useful guide in this area as thesedescriptors change frequently. The Url www.itis.gov is a much more authoritive, but large, taxonomy.

Chittka & Menzel focus on the color discrimination between flowers based on “slope detection” associated with asingle spectral channel (within stage 1, see below) rather than differential signal discrimination based on channeldifferencing in stage 2 and subsequent information extraction in Stage 4. Both techniques are used in the colorvision modalities of all species known to have such visual capabilities. The slope detection technique is usedspecifically when performing differential color discrimination between small color differences.

Snyder described potential optical coupling and electrical coupling between sensory channels emanating from theommatidia of Insecta based on his estimates of the detailed histology of various species, and his extended nomogramfrom Dartnall. The importance of this coupling is less severe based on the theoretical spectra presented in this workand used in Section 3.6.3.1. However, it may be significant in some species. His paper cited above includes twospectral responses that exhibit two relative peaks. He goes to some length to demonstrate it is not due to electricalcross talk due to imprecise probing that resulted in intercellular recording from two separate sensory receptors.

In general, the block diagram and stage designations used in this work are compatible with the sensory modalities ofInsecta as described in Section 1.5.1 and further annotated in Figure 3.6.1-2. The visual modality of Insecta ismuch simpler than that of Chordata, partly because the entire nervous system of the phyla is much simpler. As ageneral rule, no stage 3 neural circuits are found in Insecta (except possibly in very large or extinct species) thatemploy pulse signaling and generating what are labeled “action potentials.” The distances between the outputcircuits of one stage or ganglia and the input circuits of another are generally less than 2 mm, the nominal criteria forintroducing stage 3 signal projection techniques.

Page 104: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 103

3.6.1.1 Diversity among eyes of Insecta

As suggested in the previous paragraphs, the diversity among the eyes of Insecta is an order of magnitude greaterthan among the eyes of Chordata. The visual modality of Insecta,

C can be explored using the same block diagram as used within Chordata,C can be described completely and concisely using the Electrolytic Theory of the Neuron,C employs the same photodetection/de-excitation mechanism as in the eyes of Chordata, C employs the same four spectral sensitivity bands as in chordates, UV–, S–, M –, & L–, C involves animals small enough that they do not encounter the limitation on UV performance, due to absorption by

the lens behind the cornea, of large chordates,C employs stage 2 signal processing circuits that appear analogous to those of Chordata (except for the assigned

names),C involves stage 4 and stage 5 circuit topologies that are so simple they can frequently be defined at the individual

functional engine level,

Figure 3.6.1-2 Annotated visual modality block diagram applicable to Insecta. The diagram is modified from andcompatible with that used throughout Chordata. Retina us used to name a group of fused retinula. The nomenclaturefor axon names have been added for convenience. The subscript x is replaced by the letters UV, S, M or L for the axonsof sensory receptor and by the letters O, P, Q or R for the axons of lamina output neurons to indicate the spectralsensitivity of the signals they carry. These axons may also carry spatial information signals. See text.

Page 105: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

104 Processes in Biological Vision

On the other hand the visual modality of Insecta,

C employs multiple structural and functional forms of “eyes”C does not employ stage 3 signal projection signals of the pulse (action potential) type,C reorients the sensory receptor active material relative to the axis of stimulation to allow the detection of polarized

light using techniques not found in Chordata, C employs a nearly infinite variety of combinations of spectrally sensitive and polarization sensitive receptor

configurations within a single sensory receptor grouping (rhabdom),C occasionally uses sensory receptor groups in rhabdom that are so small, they frequently limit the L–channel

spectral sensitivity,

A fundamental problem in research among the very large numbers of Insecta within an Order, Family and Genus isthat members of these groups frequently exhibit different visual characteristics based on their environmental niche orother criteria. These differences make it very difficult to describe the visual modalilty of Insecta without referenceto a taxonomy based on the visual capabilities of various species and their grouping into Families, Suborders, Ordersand potentially higher groupings. This work will only attempt to highlight a few of these, sometimes subtle,differences among groups (Section 3.xxx). It is extremely difficult to present a “model” eye of even the butterfliesof the Class Insecta as Nilsson et al. attempted to do in 1988.

3.6.1.2 Simple versus compound eyes–apposition versus superposition etc.

The eyes of Insecta consist of the unitary ocular or simple eye, and/or a group of fused ocular described as acompound eye. The simple ocular consists of a single optical aperture, the cornea. It is generally followed by a“lightpipe” described as the crystalline lens. The cornea is a simple lens generally characterized as a thin lens byopticians. Conversely, the crystalline lens is generally described as a thick lens. It may exhibit both an entranceaperture with optical power and an exit aperture with optical power. In between these two apertures is a solidmaterial with a graded index of refraction. In the general case, it appears the optics of the simple eye constitutes anon-inverting terrestrial telescope that projects an image of the external environment into the light sensing elementof the eye known as an ommatidium in the world of Insecta. The ommatidium contains a group of visual sensoryneurons labeled retinula surrounding a central light pipe called the rhabdom. The sensing portion of the sensoryneurons, called the rhabdomere are inserted into the rhabdom in a variety of configurations depending on the Orderand Species of the particular specimen. Some of these configurations support the sensing of the polarization of theincident light; some do not. Virtually all of the rhabdome of Insecta (with some putative exceptions) sense utraviolet(UV), short (S–), medium (M–) and long (L–) wavelength light.

The compound eye of Insecta consists of a large number of simple eyes fused together to form a large collectionaperture eye (generally approaching a hemisphere in form) supporting anywhere from a few thousand to a hundredthousand ommatidia. In the compound eye, each cornea is constrained to a hexagonal shaped aperture to support theoverall package as shown above.

The degree of fusion may vary significantly between the apposition form and the superposition form. In theapposition form, each external facet is directly associated with a single detector assembly. The resulting assembly isdescribed as an ommatidium. In the superposition form, the facets form a geodesic dome well separated from thedetector assemblies. The external surface of the superposition eye is necessarily spherical. A group of facets directlight from a single source to a single detector assembly. A discussion of the superposition eye will be ignored untilSection 3.6.2.4.

3.6.1.2.1 Potentially more complex simple eyes

The literature includes a number of references to an ocellus containing up to 100 retinula, grouped into severalommatidia behind a single cornea (and presumably a single crystalline cone). Until these potential configurationsare more fully documented, they will be ignored in this discussion.

3.6.1.3 Fundamental versus complex ommatidia

Differentiating between the fundamental ommatidia, consisting of retinula that extend the length of the ommatidium,and the complex ommatidia, where the retinula are divided into ranks arranged serially along the length of theommaticium must currently be done primarily using caricatures. Laughlin (1976) describes a fundamentalommatidium in his figure 2 that will be discussed in detail in Section 3.6.3.3. His text suggests the fundamentalommatidium of the dragonfly, Hemicordulia tau, contains only six retinula that extend the length of theommatidium. Smola (1976) provides cross sections of the worker bee, Apis mellifica, ommatidium that shows eight

Page 106: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 105

235Mote, M. & Goldsmith, T. (1970) Spectral sensitivities of the color receptors in the compound eye of thecockroach, Periplaneta J Exp Zool vol 173, pp 137-146236Smola,U. & Meffert. P. (1975) A single-peaked UV-receptor in the eye of Calliphora erythrocephala J.Comp Physiol vol 103, pp 353-357 237Sauman, L. Briscoe, I. Zhu, A. et al. (2005) Connecting the navigational clock to sun compass input inmonarch butterfly brain. Neuron vol 46, pp 457– 467.

retinula of nominal equal cross section within a fundamental ommatidium at a distance of 130 microns below thecrystal cone. Smola & Meffert (1975 presented similar information for the cockroach, Periplaneta americana235, andthe blowfly, Calliphora erythrocephala236. Their investigations of the blowfly defined six retinula sensitive to theconventional visual spectrum of humans, and two retinula, R7 & R8 with sensitivities in the UV peaking at 344 nmwith no sensitivity in the conventional visual spectrum. [xxx get both citation ] Jarvilehto & Moring (1976)indicated a total of eight retinula of similar size in the cross section of the ommatidium of blowfly with each showingsignificant polarization sensitivity.

Zettler & Weiler (1976) provided Figure 3.6.1-3 describing a fundamental ommatidium organization in the fly,Diptera, along with additional information regarding the projection of the sensory neuron axons to the lamina (andR7 & R8 projecting lvf axons bypassing the lamina). The caption and notation supporting this figure suggest thecartridge is summing signals from adjacent ommatidia in order to create a type R–, broadband luminance signal,emanating from the lamina as part of a color vision capability. It is questionable whether all of the ommatidia are“looking” in the same identical direction.

Stavenga & Arikawa (2006) describe a complexommatidium containing two distinct groups of retinula,R1 to R4 in the distal ommatidium and R5 to R8 in theproximal ommatidium, in the following figure. Theyalso defined three types of complex ommatidium basedon a different criteria; the number and variety ofrhabdomere sensitive to UV and Short wavelengthlight. However, they encountered a paradox, “A mostcurious property of insect ommatidia has thus emerged,namely that the visual pigment expression patterns arenot identical in different ommatidia, even though theanatomy of the ommatidia seems virtuallyindistinguishable.” Rewording slightly, the putativeDNA patterns and their proposed expression does notconform to the measured results in the laboratory! Based on this work, the spectral sensitivity of arhabdom is not dependent on its anatomicalcharacteristics or its DNA coding related to proteins. They were investigating the butterflies of the OrderLepidoptera and further analysis is undoubtedly underway. Sauman et al. (2005) encountered a similarparadox, “Strikingly, an area at the dorsal-most marginof the monarch eye stands out from the rest of theretina: cross-sections of this area showed that the R1through R8 photoreceptor cells express only DpUVRhmRNA (Figure 3A).”

Stavenga & Arikawa (2006) cite Sauman et al237. asdescribing a more graded arrangement of sensoryneurons, from retinula extending the length of theommatidium to arrangements that clearly exhibit twodistinct ranks. “The monarch photoreceptor cells arearranged in a semi-tiered fashion (Figure 1B), in whichthe cell bodies of the R1 and R2 cells are widest near the crystalline cone and then taper considerably as theyapproach the basement membrane. The tiny R9 cell sits just above the basement membrane.” Their figure 1B is acharacture rather than a micrograph.

Figure 3.6.1-3 Schematic diagram of the anatomicalorganization of the fly, Diptera. “Solid black dots representtype R1-R6 receptors of fundamental-type ommatidia “alllooking in the same direction” and converging on onecartridge of the stage 2 lamina.” See text. From Zettler &Weiler, 1976.

Page 107: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

106 Processes in Biological Vision

238http://www.insectidentification.org/orders_insect.asp

Sauman et al. also noted an additional variation among monarch butterlies, Danaus plexippus, “Like many insects(Labhart and Meyer, 1999), monarchs possess photoreceptors in the dorsal rim area (DRA) of the compound eye thatare anatomically specialized for polarized-light detection (Reppert et al., 2004).”

It appears that the order Lepidoptera (butterflies and moths) includes many insects exhibiting complex ommatidia. Simultaneously, the order Odonata (dragonflies & damselflies) includes many insects exhibiting fundamentalommatidia. However, there are 34 orders subordinate to the class Insecta238. Where the differentiation in formoccurs in the taxonomy of Insecta is not clear based on this investigation. However, it is an important differentiationwhen reviewing the literature of the visual modality of Insecta. The URL www.itis.gov provides an excellent entrypoint to the taxonomy of insects. http://www.itis.gov/ItisDataTools/jsp/hierarchy.jsp provides a list of all Orderswithin Animalia. It includes at least 28 Orders under the Class Insecta. Each Order varies from a few dozen to one-half million species. [xxx See Excel file Insecta taxonomy under Insecta ]

3.6.1.4 Reconciling the definition of the “pigments” of Insecta vs other animals

As noted by Goldsmith & Bernard on page 166 of the second edition of “The Physiology of Insects, ”As inthe first edition, the problem of functional units continues as a recurrent theme.” This quotation from 1974can be paraphrased in the present. This section will only address the clarification of the terms pigments andchromophores related to vision.

Several distinctions must be made between the common and scientific descriptions related to “pigments.”

These include;

1. the common descriptions used for humanly perceived colors and the standardized descriptions of color science.

2. The distinction between material exhibiting color used for optical shielding purposes, that stored as proto-chromophores, and the actual chromophores of vision.

As noted below in Section 3.6.2.1, the term pigment is almost universally used to describe a material exhibiting adistinctive color as perceived by the human visual system. It is conventionally a bulk material that is observed inartistry and printing by reflected light. A commercially viable pigment has a well characterized spectral response toreflected light as a function of wavelength.

A chromophore is a material that is sensitive to stimulation by light. It may participate in a conventional chemicalprocess when stimulated by light or it may participate in a quantum-mechanical process that leaves the chromophoreunchanged in a chemical sense. Only true chromophores are employed in the quantum-mechanical transductionprocess of animal vision. They are usually found in monolayers of the liquid-crystalline form coating substrates indirect contact with the dendritic elements of sensory neurons (or the dendrites themselves). As used in vision,chromophores are very efficient absorbers of light and appear black by reflected light (they do not reflect asignificant amount of light). When stimulated, they assume an electronically excited (quantum-mechanical) stateand appear transparent. In the case of the retina of Chordata, the ophthalmologist observes the material behind thechromophore when it is in the transparent excited state. This may be a highly reflective tapetum or some otherbiological tissue.

Closely associated with the chromophores, from both the chemical structure and physical distance perspectives, arethe proto-chromophores (sometimes labeled visual pigments). These materials are usually stored in bulk form,typically described as granules within a nearby cell associated with the sensory neurons. In the case of chordates,these cells are labeled retinal photoreceptor epithelium (RPE). No explicit name for these types of cells have beenfound in the literature of Insecta. The proto-chromophores are typically retinenes or retinines derived from vitaminA.

As developed by Goldsmith & Bernard, most of the pigments associated with the optics of vision are complex,multi-ringed molecules related to the xanthone family. They are readily oxidized or reduced. As noted in the MerckIndex, the perceived colors of members of these families, by reflected light, varies widely depending on theirchemical environment. The perceived colors of yellow, orange and red are frequently associated with this family. Goldsmith & Bernard continued using the name “ommochromes,” but this term is now archaic. They also describeone of many ways these molecules can be created. They also describe a second family of pteridines, that are

Page 108: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 107predominantly red, yellow or colorless. They describe a variety of properties of these materials observed in-vivo.

Several investigators have discussed the physical movement of the pigments associated with the optics. It may bethe pigments do not move but are either oxidized or reduced in a given region, thereby changing their appearance inthat region dramatically (generally causing them to become transparent).

3.6.1.5 Color vision potential of Insecta

As of 2016, there is negligible doubt about the ability of Insecta to demonstrate color vision via electrophysiologicalmeans as well as behavioural means. The subject of color vision within Insecta appears to be as controversial in the recent literature as it was among theliterature of Chordata for many years. The discussions have been more complicated because the human investigatorcan not sense the ultraviolet spectrum, or the polarization parameter associated with each spectrum that play suchimportant roles in insect vision. However, using the multiple stage description of the visual modality of insects, itappears quite definitive that many if not most insects have the capability of sensing a chromatic environment notunlike other animals. The stage 2 signals emanating from the lamina show all of the characteristics required to forma complete color sensitive visual modality. In some cases, it may be based on a trichromatic color spectrum basedon UV–, S– and M – light. In the more general case, it is tetrachromatic, including the L– spectrum, plus apolarization component. As an example Sauman et al. (2005) asserted with regard to the Monarch butterfly, Danausplexippus, “It is therefore likely that monarchs have tetrachromatic vision based primarily on three opsins and alateral filtering pigment.” Sauman et al. offered no block diagram or circuit schematic to support their assertion. Asnoted in the previous section of this work, the protein opsin plays no functional role in vision. The primaryfunctional discriminator is the specific member of the Rhodonine family of chromophores used to coat the microvilligrid of the rhabdomere of a particular sensory neuron. The character of this coating cannot be determined byanatomical or histological experiments.

The differential signaling neurons within the “lamina” of stage 2, known histologically as large monopolar cells(LMC) are analogous to the horizontal cells of chordate vision. As in the horizontal cells, the soma plays nofunctional role in the visual modality. The electrolytic output signals from LMC are functionally bipolar. Theirusage has been extended to not only form differential O–, P– and Q–channel signals but also e-vector axisdiscrimination among polarization channel signals. The three differential signals are the basis of color vision inhumans (Section 17.3.3 and specifically Section 17.3.3.6). An extension of the New Chromaticity Diagram to 300nm would display the complete spectral capability of insects with full spectrum vision. See also Section 3.6.6.

3.6.1.6 A generic eye of Insecta

Figure 3.6.1-4 shows an elaborated simple eye consisting of most of the features to be found in any eye of Insecta. This eye includes the stage 0 optical elements, the stage 1 sensory neural elements, and potentially a ganglia ofsynapses supporting connection to the stage 2 signal processing neurons located outside the ommatidia of stage 1. Stage 0 includes the light waveguide formed by the outer envelope of the rhabdom. The rhabdom consists of thereceptor portions of each sensory neuron, labeled its rhabdomere. These rhabdomere and their associated sensoryneurons are frequently stratified into a distal group and a proximal group. In the illustrated complex ommatidium,the rhabdomere are stratified into two assemblies labeled the distal part of the rhabdom (D) and the proximal part ofthe rhabdom (P). The arrangement of the complete sensory neurons, housed within the distal and proximal parts ofthe retinula of stage 1 are shown in cross-sections below the distal (D) and proximal (P) portions of the retinula. Each cross-section includes four numbered sensory neurons as shown. The un-labeled lobes are the projectedimages of the sensory neurons of the other part. The central shaded portion of these representations represent therhabdomere (receptor portion) of each sensory neuron. The shape of the shaded area varies widely between circularand highly rectangular regions as discussed with regard to their waveguide properties in Section 3.6.2.3.3.

Page 109: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

108 Processes in Biological Vision

239Altner, I. & Burkhardt, D. (1981) Fine structure of the ommatidia and the occurrence of rhabdomeric twistin the dorsal eye of male Bibio marci (Diptera, Nematocera, Bibionidae) Cell Tis Res 607-623

The precise configuration of the crystalline cone–rhabdom interface remains controversial. Nilsson has providedconsiderable material on this subject, and included an electron–micrograph of the region for Argynnis paphia. Itmay differ significantly from that of other members of Lepidoptera.

The rhabdom is shown as a square waveguide with a possible twist of 45 degrees in the area of transition betweenthe distal and proximal parts of the stratified, or tiered, rhabdom. This is different than most depictions, such as thatof Stavenga & Arikawa to emphasize a point. A circular cross-section guide is ideal for circularly polarizedradiation. However, a rectangular cross-section guide is preferred for linearly polarized radiation (Section3.6.2.3.3). The grid polarizers of the microvilli (containing conductive fluid rather than conductive wires) clearlyselect, or convert the randomly polarized stimulus to, linearly polarized light within the ommatidium of most species. Altner & Burkhardt have investigated the twist in the rhabdom of the dorsal eye of the male March fly, Diptera,Nematocera, Bibionidae Bibio marci 239 A twist has been observed in many species of Insecta. However, Altner &Burkhardt acknowledge the continuing controversy and provide several citations. If polarized light is not animportant functional element in the proximal portion of the ommatidium, the rotation suggested by the circular arrowbetween the two parts may not be present.

Goldsmith & Bernard (page 219) presented the theoretical modal patterns of light in a circular waveguide. However, no empirical evidence was provided that the rhabdome of all members of Insecta conformed to thisassumption. The alternative is the well-known rectangular waveguide. It requires a trained eye to differentiatebetween the modal patterns of the circular and rectangular waveguides. See Section 3.6.2.3.3.

Goldsmith & Bernard did not describe or even speculate on the mechanisms related to polarization within therhabdom. It is doubtful they were aware of the grid-type polarizer described in this work. They reported on

Figure 3.6.1-4 A generic simple eye and component of the compound eye of Insecta REDRAW FOR QUALITY. Arrowpointing to the right represent incident light. Arrows pointing to the left represent light reflected at the tapetum (ifpresent). CL; corneal lens. CC; crystalline cone. CZ; clear zone. D; distal part of retinula in the complex ommatidium.P; proximal part of retinula in the complex ommatidium. B; basal part of retinula and ommatidium. T; tapetum. PP;primary pigment area. SP; secondary pigment. Not to scale, retinula is typically smaller diameter than corneal lens. Arotation in the geometry of the waveguide is assumed to occur between the distal and proximal parts of the tiered, orstratified, rhabdom as shown by the circular arrow and in the cross sections. Lower left; arrangement of sensory neuronsand their rhabdomere. When the e-vector of the incident light is parallel with the microvilli, the light is maximallyeffective at stimulating the chromophore coating the microvilli. See text. Compare to figure 3a of Stavenga & Arikawa,2006.

Page 110: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 109

240Hecht, E. (1990) Optics, 2nd ed., Addison Wesley ISBN 0-201-11609-X. Chapter 8.241Wehner, R. (1976) Structure and function of the peripheral visual pathway in Hymenopterans in Zettler, F.& Weiler, R. eds. Neural Principles in Vision. NY: Springer-Verlag page 292242Marcuse, D. (1982) Light Transmission Optics, 2nd Ed. NY: Van Nostrand Rheinhold243Land, M. Laughlin, S. et al. (1981) handbook of Sensory Physiology, Volume VII/6B. edited by Autrum, H.et al. NY: Springer-Verlag page 313, fig. 27 & 28

experiments using crustaceans where the microvilli were excited by lateral light relative to the axis of the rhabcom. They employed crustaceans because of the larger dimensions of the microvilli structures within a rhabdom. Theydescribed the orientation of the e-vector of polarization relative to the grid of microvilli based on this non- vivoexcitation (page 249). Their description is counter to the more recent text of Hecht quoted below.

“The simplest linear polarizer in concept is the wire-grid polarizer, which consists of a regular array of fineparallel metallic wires, placed in a plane perpendicular to the incident beam. Electromagnetic waves whichhave a component of their electric fields aligned parallel to the wires induce the movement of electrons alongthe length of the wires. Since the electrons are free to move in this direction, the polarizer behaves in asimilar manner to the surface of a metal when reflecting light, and the wave is reflected backwards along theincident beam (minus a small amount of energy lost to joule heating of the wire).

For waves with electric fields perpendicular to the wires, the electrons cannot move very far across the widthof each wire; therefore, little energy is reflected, and the incident wave is able to pass through the grid. Sinceelectric field components parallel to the wires are reflected, the transmitted wave has an electric field purelyin the direction perpendicular to the wires, and is thus linearly polarized. Note that the polarization directionis perpendicular to the wires; the notion that waves "slip through" the gaps between the wires is wrong240.”

Wehner has provided figures 5 & 6 showing both rectangular and circular rhabdoms in a variety of families ofInsecta241. His caption to figure 5 appears to contain a typographical error. The diameter of the microvilli aretypically between 40 and 70 nm rather than μm based on his scale bar. He includes an extensive discussion of thetwisting of the retinula within a given ommatidium. His discussion of E-vector navigation on page 321 is useful butprobably archaic. “At least in the bee, only the UV receptors are involved in E-vector navigation.” His spectralsensitivity of the M –channel of the worker bee in figure 29 provides an excellent match to the theoretical 532 nmspectrum of this work.

Nilsson et al. have provided a major work in the visual modality of butterflies. However, they did not identify thefeature of the ommatidia that acts as a polarization analyzer (the term is used to define either of two analyzers, oneintroducing polarization and a second used with a detector to identify the principle axis of the e-vector of thepolarized light present. They also did not consider the potential for rectangular waveguides even though manyinvestigators have provided data suggestive of this possibility. All of their citations in this area were to closecollaborators. It may be useful to augment their paper with additional information from the fiber opticscommunity242 (Section 3.6.2.3.3). Their paper is reviewed in Section 3.6.2.3.4. The stack of rhabdomere associated with an individual sensory neuron are described at lower left. Each sensoryneuron supports a large number of individual dendrites that in-turn support a large number of microvilli arranged in agrid-like structure that is coated with the appropriate chromophoric material. Each of these structures is labeled awafer in the figure. It is to be demonstrated that the grids of all of the wafers are aligned relative to the incidentlight. This would appear to be the most efficient arrangement. These grids introduce the polarization sensitivefeature of insect vision (Land et al., 1991 fig 27). The spacing between the microvilli of a specific wafer is less thanthe wavelength of the light to which the ommatidium is sensitive. Thus the grid-type polarizer can act as amodulator of the light sensed by the sensory neuron. It can also act as a polarization analyzer for the light passedthrough the distal part of the retinula. Optimally, the portion of the rhabdom beyond the distal portion would berotated 45 degrees to optimize the projection of the now rotated polarization vector associated with the remaininglight, in species still utilizing polarization in the sensing mechanism of R5-R-8. The number of wafers supporting agiven sensory neuron can be compared to the number of discs in the outer segment of the sensory receptors of thechordate retina243. As demonstrated by the re-crystallization experiments reported by Arikawa et al. (1999a), there isno opsin required for the operation of the ommatidium of the insect retinula (Section 3.6.4).

The term cartridge is used variously in the histology literature when speaking of various families ofArthropoda. In some cases, it refers to the above parts of the ommatidium. In other cases, it is used todescribe the individual sensory neurons (R1 through R8 or R9). The most appropriate usage has been todescribe the ganglia associated with a given ommatidium but located behind the basal (or basement)

Page 111: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

110 Processes in Biological Vision

membrane of the eye. These cartridges constitute individual elements in the lamina of stage 2.

In some concepts, the ommatidium also contains some synapses between the axons (generally identified by theirassociated receptor number of the stage 1 sensory neurons (ex., R3) and the neurites of stage 2 signal processingneurons (generally identified by their sensory neuron source but using the letter L (ex., L-3). These synapses mayoccur in knots (or ganglia). The individual neurons of stage 2 are identified by a different nomenclature. In otherconcepts the axons of the sensory neurons extend to beyond the physical boundary of the ommatidium and into thelamina of stage 2..

Each simple eye consists of a complex stage 0 optical system that can vary significantly among families and species. In the generic form shown, the corneal lens (CL) consists of a two-element lens, sometimes described as a corneallens and an associated corneal process (CP). In a variety of moths, the initial corneal lens exhibits an antireflectioncoating (AR) created by a “nipple array” of projections from the material of the cornea (Section 3.6.3.4.1) but withan average, or effective, index of refraction of one half of the difference in index between the cornea and air. Thislens group is associated with the subsequent crystalline cone (CC). The spacing may be negligible or significant in agiven species. The character of CC has not been clearly elucidated in the literature. However, Nilsson et al. havedescribed it as consisting of two optical elements forming a terrestrial (or non-inverting) telescope. In such aconfiguration, the acceptance angle of the proximal aperture of the waveguide formed by the rhabdomere may besignificant. Most histologists have described stage 0 like Ribi, 1987, as consisting of a dioptric apparatus with botha cornea and a crystalline lens (cone). Such descriptions appear in many textbooks and journal articles but will notbe adequate in the following discussions.

While the rhabdom is typically shown as cylindrical, electron microscope images typically demonstrate the envelopeof the rhabdomere is a parallelepiped, more nominally a square. Land et al. have shown this relationship in theirfigure 24 in 1991 for the compound eye of Daphnia. This condition is also demonstrated by the modes of axialenergy transmission through the rhabdom. Furthermore, it is the only form that would accommodate the polarizationintroduced by the grid-polarizers formed by the microvilli of the individual rhabdomere (Section 3.6.3.1).

Each of the sensory neurons introduces its sensory receptor component, the rhabdomere into the central opticalwaveguide created by the rhabdom. The input end of the rhabdom accepts light from the preceding stage 0 opticalelements (within the acceptance angle of the waveguide). Many ommatidia of Insecta exhibit a striated or tieredarrangement of the sensory neurons. Neurons labeled R1–R4 in the distal group (D) and neurons R5-R8 in theproximal group (P). The rhabdomere are able to intercept a designated spectral portion of the light projected into thewaveguide. This light is transduced into an electrical signal within the dendrites (microvilli) of the sensory receptorneuron. This electrical signal is processed by the two Activa (biological transistors) found in each sensory receptorneuron and appears in the axoplasm and at the pedicle of the axon of the neuron. The signal is passed from thepedicle of the axon via a synapse to one or more orthodromic neurons forming the Lamina and/or distal medulla of stage 2. There is an unresolved question of whether the ninth sensory receptor neuron (R9) is actually light sensitive(and belongs to stage 1, or is actually a signal processing neuron of stage 2. Its location and properties appear quitesimilar to the so-called eccentric cell of the retinula of the crustacean Limulus. Ribi (1987) notes, “The ninthproximal retinula cell. . . makes no synaptic contacts in the lamina.” Laughlin (Land et al., 1991, page 173) minceshis words regarding the eccentric cell of Limulus, “The eccentric cell is, judging by the poorly developed microvilli,and its position in the ommatidium, a specialized photoreceptor. Its particular function is to integrate recepor inputand then transient the resulting signal to the brain. . .Simultaneous intracellular recordings from retinual cells and theeccentric cell show that they are linked. . .”

The simple eyes, and particularly the compound eyes, are reported to contain a variety of bulk pigment particles (PP)along the area adjacent to the crystalline cone (CC). Both eyes are generally reported to contain a variety of bulkpigment particles (SP) stored in the area surrounding the ommatidium, but at a distance outside of the waveguidewall that makes their influence on the light within the rhabdom questionable. It is more likely these pigmentparticles are used to form the chromophores deposited on the surface of the microvilli forming the rhabdomere of aparticular sensory neuron. Some of the above particles appear to be quite dynamic in their presence and/orappearance. Consistent data in this area is difficult to obtain. The particles are frequently translucent (andsometimes fluorescent when subjected to ultra-violet stimulation). Whether the particles move or merely changetheir opacity is usually not discussed or demonstrated. Their visual appearance is frequently characterized poorlyusing terminology that does not conform to the rules of additive and subtractive color used in the printing and otherindustries. The precise spectra of these particles is needed to understand their precise roles.

The basal section (B) is associated with a ninth cell that is usually described as the ninth sensory neuron receptor(R9). As noted above, there is considerable question whether this is in fact a sensory neuron of stage 1 or a signalprocessing neuron of stage 2 (similar to the eccentric cell of Limulus). R9 is generally reported to not contain or be

Page 112: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 111

244Goldsmith, T. & Bernard, G (1974) The visual system of insects in Rockstein, M. ed. The Physiology ofInsecta, 2nd Ed. Vol II. NY: Academic Press

associated with any pigment particles, and to not contain any axons (R-9) connecting to any orthodromic lamina viaa neurite (L-9). In other representations, the basal section is associated with a “monopolar cell,” not unlike theeccentric cell of Limulus. The monopolar designation is extraneous, the cell exhibits all the functionalcharacteristics of a normal neuron, with 3-6 definable electrical terminals, including two associated with the axon, anelectrical power connection and a electrical synapse connection at the pedicle.

The tapetum (T) is frequently considered a continuous structure serving more than one ommatidium in the compoundeye. It is generally perforated to allow the routing of axons and/or neurites through its surface.

Ribi also makes an observation likely to be important with respect to color vision in insects, “investigations suggestthat Papilio augues has at least one lamina-cell fibre, L-1, which receives additive inputs from all svf (short visualfibres from each receptor) and (long visual fibres) lvf 1 and lvf 2.” This would suggest P aegues creates abroadband spectral channel similar to the R–channel of this work and found throughout Chordata. Ribi also notes,“Comparable L-neuron types in each cartridge, with inputs from receptor cells displaying different spectrasensitivities are found in other insects, (example, bee & dragonfly).” Ribi goes on to note the presence of L-fibreswith broad spectral sensitivities and other L-fibres with apparently narrow spectral sensitivities commensurate withdifferencing between the UV–, S–, M – & L– to generate relatively narrow spectral sensitivities corresponding to theO–, P– & Q– channels of chordate vision. Ribi discusses the interaction of spectrally different signals interactingantagonistically and notes, “In P. aegeus, narrow peaks are not spread through a spectrum between 500 nm and 550nm (exactly as predicted in this work, and found among Chordata). Ribi’s discussion did not address the question ofsignals of different polarization sensitivities being combined to form alternate (and potentially parallel) O–, P–, Q–or R– channel signals.

The draft cited in the longer URL of the Invertebrate Brain Platform. “Optic Lobe,” asserts the signaling within theinsect eye conforms to the sequence indicated in the above figure. In some discussions, the lamela is described asthe first of two medulla. The lamina, lamina ganglionaris, is described as “second-order visual neurons that receivedirect input from the photoreceptors (= retinula cells).”The lobula is described as the second, or proximal, medulla. The reference also indicates the visuotopic field of each eye is preserved in the topology of the stage 1, stage 2 andinitial stage 4 circuits “similar to arrangements in the mammalian visual system.” Ribi makes a slightly strongerassertion from a geometrical perspective, “Together the photoreceptor axons project to a single neuropil unit called acartridge in the first synaptic region layer, the lamina. . .This retina-lamina projection preserves angular informationexactly.” Ribi also defines a “pseudocartridge” of axons from a given ommatidium before they merge with otheraxons at the cartridge of the lamina. He also notes, “No synaptic contacts or invaginations between the nine retinula-cell axons could be recognized at this level.” Ribi presents considerable information about the course of neuronaxons, labeled L-fibres, within the lamina. He also indicates that the color sensitivity of these fibres had not beendetermined sufficiently to describe a pattern to them. The cited reference also notes the Lobula complex (labeledstage 4 in the figure) includes both a lobula and a lobula plate that perform processing in parallel. The lobula systemis reported to identify objects by their shapes.

As noted in Section 3.3.1, the anatomical and histological designation of neurons as monopolar, bipolar andmultipolar have no meaning in the electrophysiology of these neurons. The terms relate to how the neuronsare arranged for optimum physical packaging. The soma does not play a significant role in electrophysiologyexcept to sometime incorporate the amplifiers known as Activa. At other times the Activa are located at thejunction of the dendritic and axonal elements regardless of the location of the soma. All neurons exhibit threeelectrical contacts with their environment for signaling; an input neurite structure (potentially consisting ofboth dendrites and podites) and a signaling output structure (an axon and its pedicle). There are also threeelectrical contacts related to the power supply that may be closely aligned with the above signaling contacts.

The reference distinguishes between intrinsic neurons as arborizing within stages 2 & 4 while projection neuronsconnect the major engines (knots of neurons) of the optic lobe and the central brain (lobula) Some of these projectionneurons are centripetal and centrifugal as indicated in the above figure. The continuing discussion of the neurons inthat reference is in serious need of a figure like that above to aid interpretation of the text. It is important to note thatthe organization of the chromophore coated microvilli (extensions of a dendritic structure) are aligned with their axisperpendicular to the axis of the incident light. This feature was documented by Waterman (Land et al., 1991, page312) after crediting Goldsmith & Bernard244. The result is the different sensory neurons of an ommatidium aresensitive to the polarization of the incident light as well spectrally selective regarding that light. As a result and asdiscussed further in the next section, the output of the sensory neurons is proportional to the product of the intensityand polarization of the spectrally selected light

Page 113: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

112 Processes in Biological Vision

245Ribi, W. (1987) Anatomical identification of spectral receptor types in the retina and lamina of the Australianorchard butterfly, Papilio aegeus aegeus D. Cell Tissue Res vol 247, pp 393-407246Stavenga, D. & Arikawa, K. (2006) Evolution of color and vision of butterflies Arthropod Structure &Development vol 35, pp 307-318

The cited paper from the Invertebrate Brain Platform provides extensive description of the neurons and neural pathsassociated with the lamina, medulla and lobula, but without describing the wavelength sensitivity of the signals inthese neural paths.

A clear understanding of the ommatidia of a variety of butterflies has emerged during the turn of the 21st Century.Ribi245 has described the morphology of the ommatidia grossly and the neural circuits involved in significantlygreater detail. This Ribi paper cites a considerable number of his papers that will not be cited in detail here, but onlywith respect to their year. As an example, Ribi (1978) focuses on his concept of screening pigments. The papersprovide a large number of light and electron micrographs accompanied by considerable text. However, they lackany circuit diagrams of the neural system. The text is frequently open to various interpretations that a set of circuitdiagrams could avoid.

[3.6.1.6.1 The dimensions of the rhabdom of butterflies EMPTY

The absolute dimensions of the rhabdome of various Orders through Species of Insecta vary widely andsignificantly. Some of the rhabdom appear to be circular, however, most are shown to be rectangular in basic form. The effective form of the particular rhabdom can be determines using the techniques of Nilsson et al. (1988)discussed below.

3.6.2 The morphology of the compound eye of Insecta

The compound eye of Insecta appears to employ an ommatidium very similar in morphology to the simple eye(including individual crystalline lenses) but combines many thousands of ommatidium in a single fused structure. Each ommatidium exhibits a very narrow field of view (on the order of 1.5 degrees). The complete compound eyeprovides a total field of view of nearly a hemisphere in most cases. Ribi describes the compound eye of his butterflyas consisting of 19,000 ommatidia in both sexes and the flattened fused cornea as covering an area of 9.6 mm2.

The compound eye of Insecta is used in an almost endless variety of families within the kingdom. Only a fewfamilies have been investigated in any detail, but even some of these families are highly diverse. Stavenga &Arikawa provided a review in 2006 (now dated but useful) that includes many conceptual details relative to anumber of the species within several families246. As noted in Section xxx, it appears that a few, if not most, of thesefamilies use an unusual form of vitamin A defined as Vitamin A3. This form appears to be utilized by insectsfeeding primarily on carrion where the vitamin is further oxidized following death. The use of Vitamin A3 byDiptera is reported in Section xxx.

3.6.2.1 The compound eye of the butterfly according to Arikawa & colleagues

During the last two decades, Arikawa and colleagues have built on the earlier work of Stavenga and providedextensive information concerning primarily butterflies. At least three papers will be analyzed (at least partially)below; Bandai et al., 1992, Arikawa et al., 1997, and and Chen et al., 2016. Other papers will be introduced whenspecific information is important to the understanding of the subject matter.

There are cautions that need to be reviewed before analyzing these papers.

1. It is important to distinguish between a pigment (in bulk form and generally stored separately froma rhabdom) from an active chromophore (present in liquid crystalline form deposited on a microvilli,or in the case of chordate eyes on the disks of the outer segment. These forms can exhibit significantlydifferent spectra even though they consist of the same chemical molecule. A pigment denotes a material thatis observed visually by reflected light whereas a chromophore is a material that absorbs light passed throughit. The color of a chromophore resulting from observing the color that passes through it is the complement ofits absorption spectrum (Section 17.3.4.3).

Page 114: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 1132. When in the liquid crystalline form, the chromophores may exhibit anisotropic absorption spectra. Generally, the spectrum used in vision is the narrow band spectrum with high sensitivity when excited bylight traveling parallel to the axis of the conjugated atoms within the molecule.

3. Most “color” recording media, such as the human eye, printed material using conventional 3-colorsubtractive printing techniques, and conventional television equipment will not sense and cannot presentinformation present in the samples under examination. As a result, a pigment (observed by reflected light) related to a UV chromophore may appear to reflect little light over a broad spectrum. When observed usingthe above equipment by transmitted light (sometimes described as back-lighted), the UV chromophore mayappear to be transparent. The granules present in the RPE of chordates are examples (Sections 4.5 &4.6.2.2.3). In [Figure 4.5.1-1], the chromophores are described by their absorption spectra. [Figure 4.6.2-6]shows actual imagery of the RPE by transmitted light from Wolken, 1966.

4. When the appearance of a pigment or chromophore is described as whitish visually, it is important torealize that the human eye will report narrow band light between 380 and 420 nm as whitish or milky white(what is defined as Lilac in Section 2.1.1.6.1 of this work). Such light is obviously not related to theperception of white occurring when short wavelength light at a net 494 nm (P = 0.00) and long wavelengthlight at a net 572 nm (Q = 0.00) are mixed (Sections 17.3.3 & 17.3.4). 494 & 572 nm constitute the HeringAxes of human color space.

5. The labels used by the Arikawa investigators (and many others) do not recognize the difference betweenthe names of colors observed by reflected light and those observed by transmitted light. The labels arefrequently significantly different from those adopted by the US National Bureau of Standards and the CIE(Section 2.1) While the difference between purple and magenta may appear semantically trivial, purple is thename of a spectral color (generally at 410 nm and observed by transmission through a filter or spectrometer),while magenta is a non-spectral color (consisting of a mixture of red, ~625 nm, and blue, 470 nm) observedby reflection in the printing industry. Magenta is frequently labeled 532c nm, the complement of the peak ofthe M – channel chromophore at 532 nm.

The problem is highlighted by a statement in Goldsmith & Bernard (page 188), “The red granulescontain chiefly reduced xanthommatin; they absorb maximally at 540 nm, . . .” A material absorbingmaximally at 540 nm has a complement at 540c nm and appears as magenta when the source of theillumination has a color temperature of 6500 K (sunlight) and the human eye is not chromaticallyadapted. If the color temperature of the source is less than 6500 K, the source is deficient in blue andthe observer will label his perceived color as red instead of magenta. In many cases, the investigatoris not sufficiently trained to know when the light source is inadequate or recognize the differencebetween magenta and red.

6. The chromophores of chordate vision do not fluoresce under normal in-vivo conditions. They bleach. That is they absorb the stimulant energy and reconfigure to a long-life transparent electronic state (Sections5.3.5 & 5.5.15).

The distinction between purple and magenta is not observed in the papers cited in this section. Neither is thedistinction between yellow (570 nm) of the CIE and the yellow of these papers (520 nm). Two features offigure 4 in Arikawa et al. (1999) should be noted. The absorption coefficient of both the “yellow” and “red”screening pigments are quite low (they are nearly transparent), and the “yellow” curve, if it showed greaterabsorption at short wavelengths, would be called a “minus blue” filter in the photographic field. A minusblue filter is typically perceived as bronze by the human eye, not yellow.

5. “Lucifer yellow CH, lithium salt is a water-soluble dye with excitation/emission peaks of 428/536 nm. It isa favorite tool for studying neuronal morphology, because it contains a carbohydrazide (CH) group thatallows it to be covalently linked to surrounding biomolecules during aldehyde fixation” according to Thermo-Fisher Scientific. They note this material is “For Research Use Only. Not for use in diagnostic procedures.” The 428 nm emission line is less intense than the 536 nm line. The emission at 536 nm is moreyellowish–green (where yellowish is an adjective modifying the noun green) than yellow using the NBS andCIE nomenclature. A much better yellow “pigment” is strontium chromate, SrCrO4 with a peak reflectance at572.3 nm . The visual modality of chordata does not use any chromophore absorbing near 570 nm. Yellowis a perceived color based on computation within the central nervous system (CNS), Section 17.3.4.2.

6. The use of “pigment “will continue to be used in both situations in this section to avoid confusion betweenquotations from the literature and assertions related to that literature in this work.

Page 115: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

114 Processes in Biological Vision

247Bandai, K. Arikawa, K. & Eguchi, E. (1992) Localization of specral receptors in the ommatidium of butterflycompound eye determined by polarization sensitivity J Comp Physiol A vol 171, pp 289-297

Bandai et al247. provide material on the physical arrangements and additional data on the experimental protocol usedby Chen et al. and Arikawa et al. whose papers are discussed below. Figure 3.6.2-1 shows the orientation ofelements involved in their experiments. The Bandai et al. paper provides a wealth of other polarization and spectraldata. They do not demonstrate that their probe has a high probability of only penetrating one sensory neuron duringa given insertion. They do note that “Because 95% of our data were obtained from R1-4, it appears that ourelectrodes usually tracked in the distal half of the retinula, so that there was only a small chance that R5-8 would beimpaled, and virtually no possibility that R9 would be.”

Figure 3.6.2-2 builds on the layout of the butterfly ommatidium shown earlier and defines the polarizationsensitivity of the receptors in one type of ommatidia. The polarizations of R3 & R4 are at 90 degrees to that of R1 &R2. The polarization of R5 & R7 and R6 & R8 are at 45 degrees to the above pairs and are at 90 degrees to eachother. These values are consistent with the grid-type polarization analyzers generated by the microvilli of individualrhabdomere and the 45 degree rotation of the grids of the rhabdomere of R5-8 required to operate optimally behindthe rhabdomere of R1-4 (Section 3.6.2.3).

Figure 3.6.2-1 Stimulus condition and typical responses for Bandai et al. The intent was for the micro-electrode topenetrate only a single photoreceptor soma in an ommatidium near the center of the left eye. Note the linearity in thegenerator potentials developed in response to the log of stimulus intensity and the strength of the recorded signal. Theseare not action potentials. No absolute voltage is given. From Bandai et al., 1992.

Page 116: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 115

Figure 3.6.2-2 Polarization sensitivity curves predicted fromthe microvillar orientation of the Papilio xuthusphotoreceptors. The labels of figure 12 indicating colorsensitivity have been added to this figure. From Bandai etal., 1992.

Page 117: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

116 Processes in Biological Vision

As noted above, Chen et al. have studied the compound eye of the butterfly, Graphium sarpedon, quite intensely butwithout the benefit of any block diagram or schematic of the visual modality or the sensory receptors. Theirsupplementary material presents their raw findings without significant additional discussion. They provide 19spectra combined with a polarization diagram for each (in pairs lettered A through V). Many of these pairs appear torelate to the same UV-, S-, M- & L- channels of this work. Figure 3.6.2-3 shows the statistical correlation betweenthe Chen et al. channels compared to the theoretical responses of the Rhodonines believed to be used as primaryphotoreceptors throughout the animal kingdom. The single channels indicated by letters appear to be well matchedto the individual theoretical spectra shown. As noted in Section 3.6.3, the potential 2 photon —> 1 excitonmechanism or the alternate 2 exciton —> 1 free electron mechanism may represent a second order mechanismmoving the wavelength of maximum spectral sensitivity into the 610 nm region. Their polarization responses havebeen discussed above. The D and O channels of Chen et al. appear to represent linearly mixed signals from twophotoreceptors of the UV- and S- type or the UV- and M- type respectively.

The spectral sensitivities of the F, N, Q, R & S channelsof Chen et al. exhibit more complex spectra than thosediscussed above. The F, Q & R channels exhibit peaksensitivities near 500 nm that Chen et al. label BG. Spectra with a peak sensitivity of 500 nm havefrequently been associated with a putative “rod”photoreceptor in mammals in the past. Chen et al. havestruggled with these spectra and note;

“The enigmatic BG photoreceptor class is extremelyrare; we only found a few clearly labeled examples,both of which were R3/4; of type I ommatidia. This ispuzzling, because R3/4 in type I ommatidia are usuallydG (dorsal) or O (ventral) receptors.”

And

“The BG receptors, which peak around 500 nm andhave a long tail extending toward the long wavelengthregion, also cannot be explained by the identifiedvisual pigments. However, their spectral sensitivitycan be reproduced if we assume the existence ofanother visual pigment peaking at 480 nm, R480.”

Physical chemistry offers an alternate explanation forthe peak at 500 nm. This peak appears to representspectra obtained from the isotropic sensitivities of allphotoreceptors when stimulated from other than theirpreferred direction (See Section 5.5.9.2). Along theirpreferred axis, the pigments exhibit a different spectralabsorption directly related to their resonant conjugatedstructures. These conjugated structures exhibitdifferent peak sensitivities due to the specific length oftheir conjugated structures (Section 5.5.8.2) anddirectly calculable using the Helmholtz-BoltzmannEquation. These rhodonine pigments and the associatedcomputations are in complete support of the “onephotoreceptor class–one rhodonine” rule. This rule is amore precise rendition of the “one receptor–one visualpigment” rule quoted in two different forms (pages 5 &10) in Chen et al. without citation. This clarification of the rule may aid Chen et al. to simplify their discussion of agreat many opsins (their figure 5)–many more than needed to accommodate even their multiple potential

Figure 3.6.2-3 Correlation between spectra of Chen et al.and theoretical rhodonine spectra of this work. Top frame,the letters relate to the spectra shown in their supplementalmaterial as they align to the lower frame. The lower frameshows the nominal individually normalized (first order)spectra of the mammalian retina based on vitamin A1.Above the spectra are the commonly associated labels forthe individual spectra. See text.

Page 118: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 117

248Arikawa, K. Scholten, D. Kinoshita, M. & Stavenga, D. (1999b) Tuning of Photoreceptor SpectralSensitivities by Red and Yellow Pigments in the Butterfly, Papilio xuthus Zool Sci Of Japan vol 16(1): pp17-24

photoreceptor spectra. In their discussion, they define an opsin, after citing Arikawa et al248., as “L2 (and L1) opsinsare supposedly of the protein part of green-absorbing visual pigment and L3 opsins are of red-absorbing visualpigment.”

In this work, various opsins form the disks of mammalian outer segments of the photoreceptors and themicrovilli of the sensory receptors of non-mammalian species. These opsins are physically coated with theliquid crystalline form of the actual visual pigments, the rhodonines. There is no direct relationship betweenthe opsins (actual proteins) and the rhodonines which are not proteins but conjugated arenes derived fromretinol (vitamin A) through further oxygenation. The rhodonines are not “chemically” attached to proteins. The mammalian rhodonines are produced in the retinal photoreceptor epithelium (RPE) totally separate fromthe extrusion of the opsins by the sensory receptor neurons.

There are three distinct sets of rhodonines based on the three recognized forms of vitamin A as described inSections 1.2.1.1 and 6.3.4.4.2.

It is important to distinguish between a pigment (in bulk form and generally stored separately from arhabdom) from an active chromophore (present in liquid crystalline form deposited on a microvilli, orin the case of chordate eyes on the disks of the outer segment. These forms can exhibit significantlydifferent spectra even though they consist of the same chemical molecule. A pigment denotes a material thatis observed visually by reflected light whereas a chromophore is a material that absorbs light passed throughit. The color of a chromophore resulting from observing the color that passes through it is the complement ofits absorption spectrum (Section 17.3.4.3). When in the liquid crystalline form, the chromophores mayexhibit anisotropic absorption spectra. The use of pigment will continue to be used in both situations in thissection to avoid confusion between quotations from the literature and assertions related to that literature inthis work.

This clarification may also explain the D and O spectra of Chen et al. wherein this analysis asserts these spectra arelinear summations of the spectra from two distinct photoreceptors that each satisfy the “one receptor–one visualpigment” rule. It may also help them simplify their discussion of the expression of various mRNAs within a singlesensory neuron, or sensory neuron photoreceptor.

Spectra N and S of Chen et al. cannot be characterized at this time. N exhibits a peak at 400 nm but consists of onlythree data points. S exhibits a peak near 560 nm with a wide range bar at 540 nm. It is possible that additional testdata would confirm the S spectra corresponds to the theoretical M- channel spectra of this work.

The cited paper by Arikawa et al. is important in further describing the ommatidia of Insecta. However, much of itsempirical calculations and broad speculations need to be reviewed carefully. They note in their introduction, “thenature and composition of spectal receptors in the ommatidia has yet to be elucidated? Their goal was to movecloser to that elucidation. This work does not support the concept of self-screening of a pigment in place of a moreprecise quantum-mechanical calculation using the Helmholtz-Boltzmann Equation and concept. This may be aquestion of semantics. If they mean self-screening within an ommatidium by rhabdomere of different peak spectralsensitivity along the axis of the rhabdom, this work would suggest the term cross-chromophore, inter-chromophoreor inter-rhabdomere optical screening along the axis of the rhabdom. If they mean, self-screening within therhabdomere associated with a single sensory neuron receptor (Section 5.3.5.3.1), it is recommended the Helmholtz-Boltzmann conceptual equation combined with the Pauli Exclusion Principle of quantum physics/chemistry is amore direct means of accounting for the phenomenon. As a matter of fact, the first wafer of rhabdomere are excitedby the appropriate spectral light and turn transparent. As a result this first wafer does not screen the second wafer. The second wafer then absorbs the incident light before becoming transparent. This progressive increase intransparency is the mechanism behind adaptation as well as the observed “bleaching” of the receptors in vision.

The geometry of the disks of Chordata and the microvilli of Insecta (and probably also of Mollusca), cause theabsorption process to operate like a series of independent absorption filters spaced at nominally equal intervals.

This work also questions the utility of their figure 2 in describing the character of the “pigment area.” The areashown exhibits a relatively low density of pigment material in terms of the bulk of the cytoplasm of the enclosingcell. The paper does report the presence of chromatic material stored in the area surrounding the rhabdom. Thismaterial appears to be stored in separate cellular structures from the sensory neurons, similar to the arrangement inthe retinal photoreceptor epithelium (RPE) of mammals. These pigments are used to replace the chromophores lost

Page 119: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

118 Processes in Biological Vision

over time from the microvilli (possibly through phagocytosis as in mammals). While Arikawa et al. assert therhabdom acts as a waveguide for light, they did not review the requirements of such a guide. Their text suggests thesurrounding pigments, rather than the fluid matrix surrounding the rhabdom forms the wall of the guide. This worktakes the opposite view, the stored pigment plays no significant role in projection and absorption of light within theinterface with the surround. In fact, figure 3 of their paper shows the stored pigment is not in direct contact with theperiphery of the rhabdom but immersed in the cytoplasma of a cell. The difference in index of refraction betweenthe two sides of the interface is the critical factor. The pigment is shown stored within the lemma of other cellsinterdigitated with cells R1-4 and R5-8. Figure 3.6.2-4 reproduces figure 3d and 3e of the Arikawa et al. (1999b)paper. In this figure, R-ommatidium refers to a long wavelength sensitive structure with peak sensitivity in the 600nm region. Y-ommatidium refers to a medium wavelength sensitive structure with a peak sensitivity in the ~520 nmregion. The 520-532 nm region is considered to absorb in the olive-green, or yellowish-green in this work ratherthan yellow at 578 nm as defined by the 1976 UCS Chromaticity Diagram (Section 2. 1.1.3). It is not obvious fromthese electron micrographs whether a process of phagocytosis can be accommodated in the plane of these images. Itis also difficult at the scale of these images to determine if the microvilli are extruded by one sensory cell andabsorbed at another cell of different function. As noted in the caption, whether the microvilli are curved or straightshould not be implied from these two frames alone. Furthermore, the structure of R9 is not shown in these frames. The amount of polarization associated with the curved microvilli may be greatly reduced.

As in the case of Mollusca (Section 1.7.2.2), the role of the putative ninth receptor (R9) in Chen et al. and inArikawa et al. is unresolved. It may play an alternate role similar to that of the eccentric cell in the eyes of thehorseshoe crab, Limulus, (Xiphosura polyphemus) . In that species (Appendix D), the analog of R9, the eccentriccell, appears to act as a combined stage 2 (signalprocessing) and stage 3 (encoding) neuron for visualsignals. In Arikawa et al. (page 19 under Results), it isnoted “The soma of the basal photoreceptors, R9, hasno evident pigmentation.” This statement suggests thecell does not perform a sensory function but the choiceof words allows for a variety of interpretations relatedto the situation.

As shown in [Figure 1.7.2-2], Chen et al. number theindividual sensory receptor neurons within anommatidium differently than does Horridge in [Figure1.7.2-4]. The terms rhabdom, rhabdomin andrhabdomere appear to be used differently thant earlierinvestigators cited above. [xxx check this ] The polarization sensitivities of the enumerated singlechannels appear quite varied. The difference betweenthe dorsal and ventral portions of the eyes of thebluebonnet butterfly, Graphium. sarpedon may be dueto the desire to achieve polarization sensitivity inconjunction with stereopsis as in the Arthropoda(Section 1.7.2.1.3). The higher degree of scatter in thedata points of cases D, N & O may also be relevant. InD and O, they may confirm the measurements wereobtained from electrophysiology capturing andsumming the signals from two spectral types of sensoryreceptor neurons simultaneously as discussed above. As noted by Chen et al., the polarization shown by theremaining sensory receptor neurons show severalpatterns;

C The samples A, B, C, D, L, M, N, O & V of thesupplementary data show maximum spectral sensitivityfor e-vector angles of 0 and 180 degrees. Thesesamples are all labeled R1-2 with the additionalnotation that A, B, C, & D are from the dorsal portionof the eye. Samples M, N, O & V are associated withthe ventral portion.

Figure 3.6.2-4 Electron micrographs of transverse sectionsof Papilio retina. d; R-ommatidium, cut through the regionslightly distal to the transitional zone. The pigment clustersappear electron-dense (arrowhead) The rhabdomeralmicrovilli of R1-4 are curved, whereas those of R5-9 arestraight (not shown). E; Y-ommatidium. The pigmentclusters appear electron-lucent (arrowhead). Therhabdomeral microvilli of R1-4 as well as R5-9 (not shown)are straight. 1-4; photoreceptors R1-4. Bars = 1 micron ind and e. See text. From Arikawa, 1999b.

Page 120: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 119

249Chen, P-J. Awata, H. Matsushita, A. Yang, E-C. & Arikawa, K. (2016) Extreme Spectral Richness in the Eyeof the Common Bluebottle Butterfly, Graphium sarpedon Front Ecol Evol vol 4, article 18 |http://dx.doi.org/10.3389/fevo.2016.00018

C Conversely, samples E, F, G, H, P, Q, R & S showmaximum spectral sensitivity for 3-vector angles near90 degrees. These samples are all labeled R3-4. Samples E, F, G & H are labeled dorsal. Samples P, Q,R & S are labeled ventral.

C Samples T, U & V show peaks at 45 degrees andnulls at 135 degrees. They are all labeled R5-8 andventral.

C Sample K is an anomaly, it exhibits a very low degreeof polarization with a maximum near 135 degrees and aminimum near 45 degrees. This sample is labeled R5-8and dorsal.

Recently Chen, Awata, Matsushita, Yang & Arikawahave provided a more advanced cartoon related to thebutterfly ommatidium249 than shown for Molluscaabove and in Section 1.7.1.2. The cartoon wasdiscussed briefly in Sections 1.2.1 & 1.7.2.1.1. Theirpaper is extensive but does not provide any blockdiagram or schematic of the butterfly eye except for arelatively simple cartoon. Their figure 1a is presentedas Figure 3.6.2-5. The unlabeled central hatchedcolumn represents the active absorption column ofommatidia, the rhabdom. A better annotated figure 1for Papilio xuthus is provided in Arikawa et al.(1999b). See Section 3.6.3.4 for a better rendition ofthe stage 0 optics.

They define three distinct types of ommatidia based ontheir fluorescent properties. Type I does not fluoresce,type II fluoresces strongly and type III fluorescesweakly. Types I (42.3%) and III (48.0%) represent90% of their samples. A vast majority of theirommatidia did not fluoresce significantly!. They notein their results, “This fluorescence is only observed inommatidia of the ventral eye region. The dorsal regionhas no fluorescence, . . .” Since all of these eyescontain the appropriate chromophores to supportvision, it can be deduced that fluorescence is not acharacteristic of the chromophores, but relate to someother material. Page 3 of their paper provide moredetails on this subject.

They indicated the presence of a selection offluorescing pigments; a UV fluorescing pigment nearthe top of the ommatidium and a reddish pigmentbelow the letters C, C’ in the figure. Both pigmentssurround the active receptor area defined by the centralcolumn, the rhabdom. These pigments may complicatetheir experimental protocol and results in theirdiscussion. Whether these pigments are excited withinthe natural environment was not discussed in the paper. Based on closer evaluation of their electron-micrographs, it does not appear these pigments play arole in active photoreception. These pigments appearto be stored in cells equivalent to the RPE of the

Figure 3.6.2-5 The ommatidia of Graphium Sarpedon. Two distinct sets of photoreceptors are shown, R1-R4 andR5-R8. The letters C, C’, D & E relate to other inset imagesin the original figure. Receptor R9 is shown near the basalmembrane. See text. From Chen et al., 2016.

Page 121: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

120 Processes in Biological Vision

250Bernard,G. (1979) Red-absorbing visual pigment of butterflies. Science 203, pp 1125–1127.doi:10.1126/science.203.4385.1125

chordate eye and distinctly removed from the matrix surrounding the rhabdom

Their extensive investigation identified photoreceptors with 15 distinct spectral sensitivities. However, they onlyidentify eight or nine distinctive locations within the nominal ommatidium. They appear to focus their attention onthe soma of the individual photoreceptors rather than the active sensory material of each sensory neuron, theindividual rhabdomere, within the central column, the rhabdom. See the next section for a discussion of theommatidia of other members of Arthropoda.

They indicated surprise at their results, based on intracellular electrophysiological evaluation, and their discussionindicated a fundamental group of four photoreceptor types providing a typical tetrachromatic capability (UV, S, M &L). They noted,

“Do G. sarpedon use all 15 spectral receptors for seeing colors, i.e., is their vision pentadecachromatic? Thishas to be checked by behavioral experiments, but it is rather unlikely.”

Some of the treatments of their ommatidium are brutal (microwaved for 30 seconds) considering the delicacy ofchromophores (bandgaps of one to three electron-volts). Depending on the amount of liquid present and the powerlevel, this could reduce the Rhodonines to retinenes. Some of their treatments are similar to the equally brutaltreatments of Seki et al. and Suzuki et al. described in Section 5.5.15.4.

Figure 2 of Bandai et al. provides a more revealing caricature of three adjacent ommatidia from another butterflyfamily, Papilio xuthus, and assert the ommatidia are identical to those in Papilio augeus. It shows four distinctelements leaving each ommatidium through the basement membrane; one from each R9 cell, a group from the distalsensory neurons (R1-4), a separate group from the proximal sensory neurons (R5-8) and a fourth path that cannot bedefined from the figure and is not addressed in their text. No citation is given for the figure although a reference toRibi (1987) is provided.

Chen et al. describe the peak sensitivities of the dorsal type I ommatidia photoreceptors in their figure 5 (shown hereas Figure 3.6.2-6) as 354, 455, 541 & 599 nm respectively. These values are in good statistical agreement with thetheoretical peak sensitivities of this work (342, 437, 532 & 625 nm, where the 625 nm value may be subject tosecond order effects not considered in the theory) for terrestrial mammals. Terrestrial mammals employ rhodoninesbased on vitamin A1. There are suggestions in the literature that Insecta may employ rhodonines based on vitaminA3 during at least part of their lifetime. This may result in differences on the order of a few nanometers between thetwo sets of values given above (Section 6.2.2.4). Chen et al. indicate the long wavelength peak sensitivity in thebutterflies was contentious among investigators. The sample size of four does not support their smoothed curve forthe L–channel, particularly in the absence of range bars on a majority of the points in that spectrum. The old paperof Bernard suggested the peak among nine species of butterflies was at 610 nm250. The right skirt of their longwavelength receptor appears to support a peak value of about 610 nm and conform to the theoretical shapedeveloped in Section 5.5.10 based on the Helmholtz–Boltzmann Equation. The non-Gaussian nature of the spectralsensitivities of the Rhodonines predicted by the Equation indicates it is best to use the half-amplitude points on agiven spectral response to more precisely describe the spectral sensitivity of a particular photoreceptor. However,the width between the half-amplitude points is species dependent and depends on the amount of chromophorepresent of a given type due to Pauli’s Exclusion Principle. Alternately, it appears that most species employ similaramounts of each chromophore to assure a reasonably uniform spectral sensitivity for the complete eye when thesignals from the individual photoreceptors are summed exponentially (Section 15.xxx or 17.xxx). If it is desired touse an mathematically derived peak in the spectral response for a signaling channel, the average value between thetwo half-amplitude values can be used.

The absorption spectrum labeled BG in Chen et al. has a peak absorption near 500 nm. This is normally theisotropic absorption of all photoreceptors when they are stimulated by light perpendicular to their normal axis ofexcitation. This spectrum has frequently been associated with a “broadband” receptor described (inappropriately) asa “rod” in mammalian eyes. The BG channel does not exhibit the broad spectrum required of a “rod.”

Page 122: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 121

They only speculated briefly in their discussionconcerning the reason for the other spectral types. They introduced considerable mRNA information. The mRNA data did not correlate well with theirspectral sensitivities. They provide no block diagramor schematic accounting for their “surprisingly” largenumber of different spectral responses. Theirsupporting material attempts to rationalize some oftheir data. Their supplementary material is accessedfrom the right column of their first on-line web page. This data will be discussed after the following sub-section.

3.6.2.1.1 The high frequency limitations ofbutterfly visual sensory neurons

Bandai et al. noted on page291, that the shorteststimulus pulse they could use without reducing theamplitude of their measured generator potentialscompared to longer pulses was 30 ms. This is animportant measurement describing the high frequencybandwidth limit of the Activa circuit found within thesensory neurons of the butterflies and probably a vastnumber of small members of Insecta. A time constantof about 20 ms can be inferred from their observation. As shown in the universal P/D equation of sensorybiology (Section 3.6.3.2), this time constant (and theassociated delay time) is a function of the stimuluslevel. This time constant should not be associatedwith the time constant of the transduction processwhich is much faster (in the microsecond or shorter range)

3.6.2.1.2 Is R9 an eccentric cell?

A question arises as to the spectral sensitivity and the purpose of the cell labeled R9 in much of the workinvestigating Insecta. Some histological studies do not recognize the cell at this location as a sensory neuron. Many such studies note there are no nearby/associated granules of proto-chromophores.

Similar investigations relating to the crustacean, Limulus, has provided more detailed information showing that thecell encountered in the ommatidia at this location is not a stage 1 sensory neuron at all but is a stage 2/3 signalprocessing/signal projecting neuron. The nomenclature stage 2/3 is used because of the apparent primitive characterof the neural system of Limulus vision. Because of the relatively large size of Limulus, it requires the axons of someneurons to be greater than 2 mm long. Because of this requirement, it appears this single eccentric cell is used likemany stage 3A stage neurons to both perform simple signal processing (summation and differencing) as well asgenerating action potential pulse streams encoding the result of these activities.

Figure 1 of Bandai et al. shows R9 as a single sensory neuron “where the bilobed R9 photoreceptor contributesmicrovilli to the rhabdom.” However, they then note, “R9 is pigment free.” It is not clear if their test equipment wascapable of recording the complete transmitted or reflected spectrum of a UV sensitive material. Humans are notcapable of observing such a spectrum visually. This observational limitation is found throughout the papersanalyzed in this section. It is also virtually impossible for humans to observe the chromophores potentially presentin thin (typically one molecule thick) liquid crystalline layers because of the nearly instantaneous adaptation of suchindividual layers to the transparent condition upon even minimal photon excitation. It is noted that Ribi, 1987,shows only a single lobed R9 located behind R1 in figure 1.but a residual or remnant of a second lobe (also in theaccompanying unstained transmission electron micrograph of figuer 2c.

These questions can not be satisfactorily resolved until more detailed information is gathered concerning R9 of theinsect ommatidia until additional information is gathered from this and various other species. The light used tostimulate the cells in these investigations must be applied along the long axis of the rhabdom hosting the R9 cell.

3.6.2.2 Special features of the polarization mechanism

Figure 3.6.2-6 Spectral sensitivities of photoreceptors oftype 1 ommatidium of Graphium sarpedon retina. Valuesare means plus their associated statisitical ranges. Theirfour specific chromatic channels are described along with aBG channel that may be the intrinsic absorption of thesensory material when excited by light orthogonal to theconventional description. The number of samples for theBG and sR spectra were quite small. See text. From Chenet al., 2016.

Page 123: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

122 Processes in Biological Vision

The next paragraph is specific to the butterflies of the family Papilio. As noted earlier, there appears to beconsiderably different polarization sensitivity among butterflies in general. The principles described wouldapply to other species, but their grid polarizer arrangements might be different.

The microvilli, being water-based liquid filled, act as electrical conductors. In their grid arrangement, with a pitchof approximately 0.1 microns, roughly a fifth of the wavelength of the incident light, they act as a grid type polarizercommonly found in the morphology of many animals (butterflies and hummingbirds as examples). In theommatidia, this grid polarizer is used to detect polarization in the stimulating light. The polarizers of the distalportion of the rhabdom introduce a unique situation. Figure 3.6.2-7 reproduces figure 3(e) of Arikawa et al,(1999). Note that the light passing through the rhabdom (into the paper) encounters either horizontal or verticalpolarizing grids. As a result, all of the light reaching later sections of the rhabdom has its e-vector parallel to eitherthe horizontal or vertical axis. This is true of all of the light, even that not absorbed by the rhabdomere shown. As aresult of these two orientations, the light not absorbed by the first rank of receptors will be either circularly polarizedor polarized at a nominal 45 degrees from the light polarized by the grid polarizers of the first rank since thesecomponents add vectorially. If the resultant light is circularly polarized, fifty percent of the light will be lost wheninterrogated by the grid polarizers of the second rank. On the other hand, if the light is polarized by 45 degreesrelative to the grid polarizers of the first rank, they would be optimally absorbed by the second rank of sensoryreceptors if their grids were oriented at 45 degrees to those of the first rank. This situation explains the observationsof Ribi (1987, figure, 1b) that neurons R5-8 are rotated in their spatial position by 45 degrees relative to R1-4. Italso questions or clarifies the predictions of Bandai et al (1992, page 295). The polarization sensitivity of sensoryneurons R5-R8 will be rotated by 45 degrees relative to R1-4 but this will not be indicative of the polarizationsensitivity of these neurons to the light incident at the rhabdom because this rotation is internal to the rhabdom.

Note how the rhabdomere of this figure defines a nominally square cross-section optical waveguide, with walls ofthe guide parallel to the grid of either the horizontal or vertical grids associated with different rhabdomere.

Some electron microscope images show significant in-plan curvature of the microvilli of a given rhabdomere wafer. No analysis has been carried out to understand or explain this phenomenon. [Xxx see figure 3.6.3-4d ]

3.6.2.3 Detailed specific features of theoptics of the ommatidia

[xxx edit re index of refraction ]There is considerable information in the histologyliterature that is important to understanding theoperation of the ommatidia of Insecta, with emphasison the butterflies. However, specific crucialinformation has not been located, such as the index ofrefraction (preferably as a function of wavelength) forthe fluid within the rhabdom and the index of the fluidsurrounding the rhabdom. Thus, the assumption thatthe rhabdom acts like an optical waveguide cannot besubstantiated at this time. As a result, many of theconclusions in this section must be consideredtentative while awaiting these index values. XXX edit

The difference in index of refraction between thecontents of the rhabdom and its immediate surroundcreates a very effective barrier to the light travelingwithin the rhabdom. This is particularly true where thewaveguide is long and thin. In this case the lightwithin the guide incurs total internal reflectionwhenever it approaches the “wall” of the guide. Anymaterial (pigment or otherwise) more than ¼wavelength (about 0.1 micron) away from the wall of the waveguide will have negligible impact on the light withinthe guide. As noted in figure 3b of Ribi (1987), most of the pigment granules highlighted are more than 0.1 microns(typically >2 microns) from the nominal wall of the waveguide. With these spacings, the waveguide model of

Figure 3.6.2-7 Electron micrograph of the grid polarizerswithin the rhabdom of Papilio. The grid polarizers formedby the R1 and R2 sensory receptors are arranged verticallyand those of R2 and R4 are arranged horizontally. Thesegrids act as polarization analyzers for any light passingthrough them that is not absorbed by these sensoryreceptors. As a result, all of the light leaving this region ofthe rhabdom, must be polarized either horizontally orvertically. The phase angle between the e-vectors due tothese polarizers is not known. Scale bar = one micron. Seetext. From Arikawa et al., 1999.

Page 124: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 123

251Arikawa, K. Scholten, D. Kinoshita, M. & Stavenga, D. (1999b) Tuning of Photoreceptor SpectralSensitivities by Red and Yellow Pigments in the Butterfly Papilio xuthus Zoo Sci (Japan), vol 16(1), pp 17-24252Nilsson, D-E. Land, M. & Howard, J. (1988) Optics of the butterfly eye J Comp Phys vol 162, pp 341-366

Arikawa et al. (1999b) appears oversimplified251. The “pigment area” of their figure 2 should be described as the“surrounding area” with the appropriate index of refraction. The pigment contained therein, and its index ofrefraction, should be shown as separated from the interface with the rhabdom interface by a significant distance.

Nilsson et al. have provided a 25 page analysis of the optical system of the butterfly252. It includes a valuable set oflight and electron micrographs from various perspectives. They initially note, “The fundamentals of waveguideoptics and Fourier optics may be unfamiliar to many biologists. . . .” They also note, “The agreement betweentheory and experiment is now so good that we can safely take the fly compound eye as being of the best understoodoptical systems in the animal kingdom.” Land, a member of the Nilsson team, has also been active in investigatingthe chordate eye. However, the “best understood” assertion can be questioned. The human eye is also very wellunderstood. The paper is very sophisticated and precise from the perspective of an optician. Additional precision inthe indices of refraction can be hoped for in the future, but this is a difficult measurement for biological material(which is frequently subject to shrinkage). Section 3.6.3.2 provides a summary of the available indices for twospecies. The values given are preliminary and not totally consistent. The major conclusions of Nilsson et al. is thatthe stage 0 optics of the simple and compound eyes of Insecta can vary between members of Arthropoda in asystemic manner and many species use different modifications to the basic optical formulation. The so-calledcrystalline cone should actually be shown as consisting of two optical elements (lenses), most often resulting in anafocal (non-inverting or terrestrial) telescope.

Page 125: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

124 Processes in Biological Vision

Figure 3.6.2-8 reproduces their figure 1. It is accompanied by the assertion, “a simple lens-waveguide model ofthe ommatidium, where the cornea provides most or all of the optical power, must now be abandoned.” As shown,the cornea becomes a thick lens with optical power at each surface and the corneal cone is an optically independentelement also of the thick lens configuration. Their figure 2 shows the clearly defined separation between the cornealprocess (CP) and the crystalline cone. Other investigators assign a different shape to the corneal process. Stavenga& Arikawa introduce a clearly defined separation between the crystalline cone and the rhabdom, along with severalcaricatures (not to scale). The separation is labeled the clear zone (CZ). They assert the CZ can be quite wide insome superposition eyes. but do not consider the acceptance angle of the individual rhabdom as a waveguide.

They deduced the index of refraction of the rhabdom tobe 1.36. Section 2.2.2.3, presents a reported value of1.41 for the outer segment in Chordata. Thesurrounding environment was reported to have anindex value of 1.36. The difference between 1.36 and1.41 is sufficient to support a robust optical wall for awaveguide. See Section 3.6.3.5.2.

[xxx copied from SCORE notes. edit into proper textform ]The coloration along the ommatidia external to therhabdom is usually described using casual language. To be more useful, the actual spectrum of thecoloration is needed, particularly since the spectrummay have a significant UV spectral component notvisible to human eyes or most photographic techniques.

The pigment particles near the proximal end of thecrystalline lens are well characterized from an opticiansperspective, they are designed to prevent unwantedlight from entering the waveguide formed by therhabdom and reducing the interfering with theabsorption of the light from the desired field of view ofthe stage 0 optical system. In that role, the pigmentcan be described as a “pupil stop.”

The term epi-illumination is frequently used in thestudy of the rhabdom of insects. It refers to amicroscope employing frontal illumination asopposed to illumination transiting the specimen. Ametallurgical microscope operates by epi-illumination.

Electron microscopy provides a different perspectiveon the neural circuitry within the rhabdom thathistorical images obtained using light microscopy. Thee-m imagery of xxx shows the presence of axonsneurites and synapses at the stage 1/stage 2 interface insuch profusion as to suggest a neuropil, or neural knot,at these locations.

Those studying the compound eye of insects frequentlydescribe a pseudopupil. “In the compound eye ofinvertebrates such as insects and crustaceans, thepseudopupil appears as a dark spot which moves across

Figure 3.6.2-8 The demonstrated stage 0 optical system ofthe simple and compound eye of Insecta ADD OTHERSYMBOLS. The cornea ( C) is shown as a “thick” lens.with a newly defined corneal process (CP) exhibiting opticalpower. The corneal cone (CC) is also shown as a thick lenswith potentially variable properties among species. Theprimary pigment cells (PPC) and the accessory pigment cells(APC) play a much more dynamic role than previouslydescribed. The role of the pigment shown within theretinula (RC) take on a different role when surrounding therhabdom (Rh). From Nilsson et al., 1988.

Page 126: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 125

253Land, M. Gibson, G. Horwood, J. & Zeil, J. (1999) Fundamental differences in the optical structure of theeyes of nocturnal and diurnal mosquitoes. J Comp Phys A vol 185 (1), pp 91–103.doi:10.1007/s003590050369.254Land, M. & Nilsson, D. (2002) The origin of vision In Willmer P & Norman D. ed. Animal Eyes NY: OxfordUniv Press255Bernard, G. & Miller, W. (1970) What does antenna engineering have to do with insect eyesIEEE Student J vol 8, pp 2-8

the eye as the animal is rotated253.”

3.6.2.3.1 The tapetum & secondary pigment found in many butterfly and moth species

Stavenga & Arkikawa have described a tapetum behind the R9 neuron at the end of the rhabdom in a wide variety ofspecies (figures 1 & 3a). They described the character of the tapetum as highly reflective over a wide wavelengthspectrum created by an air-filled tracheole (citing Land & Nilsson254). The tracheole should be described as gas-filled. It is likely that the gas in these voids does not have the composition of “air.” It is more likely CO2, N2 or O2,gases that are readily available individually within animal tissue. Such gas-filled tracheole create “total internalreflection at their surface because of the large difference in index of refraction between the tissue and the gas. Thesame technique is used in the head of dolphins and other members of Odontoceti, the toothed whales, to formtotal internal reflecting surfaces at auditory frequencies.

Their figure 1 also shows a large amount of secondary pigment extending up along the sides of the crystalline coneto the intersection with the cornea or “cornea process.” This pigment appears to significantly limit the performanceof a superposition compound eye described conceptually in figure 5.

Bernard & Miller have provided an electron micrograph providing considerable detail relating to the form oftapetum, Figure 3.6.2-9 in the buckeye butterfly, Precis lavinia. Each of the tapetum shown in the longitudinalcross section of the ommatidia consists of about forty distinct slabs perpendicular to the direction of the incidentlight. The spacing of the slabs, although regular is not equal. The average spacing is about 0.25 microns (250 nm). In a typical Bragg dielectric mirror, layers of high and low index material are interleaved. If the spacing of thesepaired layers is one quarter wavelength, constructive phase interference is obtained for the reflected light (a veryhigh quality mirror). By varying the spacing of the paired layers logarithmically, the spectral band of the reflectioncan be broadened (as in the log-periodic antenna of radio)255. Note the variable spacing of the slabs near the top ofthe electron micrograph. The spacing variation need not be very great to achieve significant spectral broadening. Itis possible the alternate slabs are gas filled (providing a very large difference in index of refraction betweenmembers of the paired layers of the Bragg dielectric mirror.

The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, lightis "forbidden" to propagate through the structure. Increasing the difference in index of refraction between thematerials of the Bragg pairs increases both the reflectivity and the bandwidth. The reflectivity of Bragg dielectricmirrors are also sensitive to the transmission mode of the incident energy. Typically, the TE mode alone is highlyreflected by this stack, while the TM modes are passed through. Thus, an ommatidium employing a Bragg dielectricmirror can act as a polarizer under light adapted conditions. Under dark adapted conditions, it will be less effective. It is important to explore the polarization sensitivity of an ommatidium to various stimulus intensity levels as well asto various spectral wavelengths.

Page 127: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

126 Processes in Biological Vision

256Stavenga, D. (2002) Reflections on colourful ommatidia of butterfly eyes J Exp Biol vol 205, pp 1077-1085

The reflection of light by the tapeum back through theommatidium and into the exterior environmentconstitutes the “eye shine” or “eye glow” described bymany environmentalists when a bright light is used toilluminate the head of an animal at night. It should benoted that the eye becomes light adapted for theindividual spectral channels of the eye stimulated bythe light source. The adaptation is typicallyaccomplished within less than a second in most animals(Section xxx). In light adaptation, the chromophorescoating the rhabdomere become essentially transparent(less than 1% absorption over the length of therhabdomere stack). They are described as “fullybleached” in the vernacular but this term is misleading. A less than one percent absorption is fully effective athigh light levels. This bleaching is an effective part ofthe adaptation mechanism providiing the eye with awide dynamic intensity range. This adaptation causesthe light reflected by the tapetum to appear muchbrighter than the external coat of the animal. Thespectrum of the reflected light is related primarily tothe spectrum of the incident light and the spectrum ofthe dielectric mirror. Stavenga explored the propertiesof eye shine from several species of butterfly256, but didnot address the variability of the reflectance related tothe polarization properties of transmission mode withintheir rhabdome and the potential of a Bragg mirror asthe tapetum. This additional parameter might place adifferent perspective on the the Stavenga text that noted“Eye regionalization suggests that different eye areas have special functions (Bernard and Remington,1991).” Special functions might refer to specialcapabilities whether these were used operationally ornot. Stavenga also noted, “In a comparative study of a number of heliconian species that all lacked a distinct dorsal area, we found that the ratio of the differentlycoloured facets can change markedly across the eyesuggesting that heterogeneity and regionalization existuniversally in butterfly eyes.” He provided severalcitations. The employment of different tapetum withdifferent Bragg mirror properties may change theinterpretation of Stavenga’s observations regardingscreening pigments.

Goldsmith & Bernard have noted the presence of both specular and diffuse reflector types of tapeta. They have alsodiscussed the relevance of such reflector types to the light polarization mechanism (page 254).

Many butterflies and virtually all moth species exhibit what has been described as a nipple array covering the surfaceof the cornea (Figure 6 in Stavenga & Arikawa). By inspection, this array of protrusions at an array spacingconsiderably less than the wavelength of interest in vision, an optician would define this array as an intermediateindex of refraction surface constituting an antireflection coating on the cornea.

The tapetum and antireflection coating of the cornea should be considered at the detail level of the optical analysis ofNilsson et. al., and included tn th overall analyses of the optics in Insecta vision.

3.6.2.3.2 Index of refraction for ommatidia of blowfly and honey bee

Figure 3.6.2-9 A tapetum based on reflection from a stackof dielectric plates in Precis lavinia at X 3875. Two arrowsat upper right indicate two layers of othrogonally orinetedmicrovilli of the rhabdom ( r). The parallel bars representthe tapetum (t) of each rhabdom. P; pigment of the basalpigment cells. See text. Scale bar = 1 micron. FromBernard & Miller, 1970.

Page 128: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 127

257Goldsmith, T. & Bernard, G (1974) The visual system of insects in Rockstein, M. ed. The Physiology ofInsecta, 2nd Ed. Vol II. NY: Academic Press pg 206

The best data on the index of refraction for Insecta is that from Goldsmith & Bernard for the blowfly, Calliphora, honey bee, Apis and the firefly, Photuris257. Figure 3.6.2-10 reproduces their figure 8. Note the values are given tothree decimal values after the decimal point. They do discuss possible errors in these values and note the value of1.311 is questionable because the region contains cytoplasm and should exhibit a value greater than that of water at1.333. Note the scale at the bottom of the left frame. The question of shrinkage between the in-vivo and in-vitrovalues after physical manipulation of the eyes was not discussed. They also provide excellent background on theoptical system of these species as recognized in 1974. Their discussion should be compared to that of Nilsson et al.of 1988. Nilsson et al. (page 360) suggest the index for the rhabdom of Xois arctoa should be near 1.36 if the indexof the surrounding matrix is taken to be 1.34.

The lower frame shows profiles of constant index of refraction are shown as well as ray paths for paraxial rays (1and 2) and parallel, oblique rays (3 and 4). The contours clearly define a graded index optical system. The outercontour also indicates optical power for the end of the cone adjacent to the rhabdom.

Page 129: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

128 Processes in Biological Vision

3.6.2.3.3 The rhabdom as a waveguide

Figure 3.6.2-10 Longitudinal sections of dioptric portions of ommatidia with indices of refraction as determined byinterference microscopy with light of wavelength 546 nm.. Left; the blowfly, Calliphora (Seitz (1968). Right; the honeybee, Apis (Varela & Wiitanen, 1970). Bottom; cornea and exocone of the firefly, Photuris (Seitz, 1969). See text. FromGoldsmith & Bernard, 1974.

Page 130: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 129

258Goldsmith, T. & Bernard, G (1974) The visual system of insects in Rockstein, M. ed. The Physiology ofInsecta, 2nd Ed. Vol II. NY: Academic Press page 219259Wehner, R. (1976) Structure and function of the peripheral visual pathway in Hymenopterans in Zettler, F.& Weiler, R. eds. Neural Principles in Vision. NY: Springer-Verlag page 292260Marcuse, D. (1982) Light Transmission Optics, 2nd Ed. NY: Van Nostrand Rheinhold

Several sensory modalities of animals employ dielectric waveguides, particularly optical waveguide technology invision and acoustic waveguide technology in hearing. The analyses up through the 1980's of the rhabdom operatingas a dielectric waveguide was early and superficial. The only discussions found in the litereature of Insecta involvedrhabdom of circular cross section. During the 1970's, the subject of dielectric waveguides was under very intensestudy in a parallel field, fiber optics. This section will broaden the framework for analyses of the visual waveguidesof Lepidoptera based on this work. It will describe both cylindrical and rectangular dielectric waveguides and therelevant features of each.

Goldsmith & Bernard presented the theoretical modal patterns of light in a circular waveguide258. However, noempirical evidence was provided that the rhabdom of all Insecta conformed to this assumption. The alternative isthe well-known rectangular waveguide. As noted by comparing the next two subsections and the actualphotographic images of rhabdom acquired by Nilsson, it requires a trained eye to differentiate between thetheoretical modal patterns of the circular and rectangular waveguides and the modal patterns observed in thelaboratory. The theoretical modal presentations of Nilsson are only presented in black and white while thepresentations of Goell provide more informative gray scale images. As a general rule, the rhabdom of Insecta, likethe outer segments of Chordata, do not scale with the size of the animal. In general, either component must have aminimum dimension of about 2.6 microns in order to support a robust L–channel (red) sensitivity. Smaller diameterrhabdome will provide less robust L–channel performance. As discussed in detail in Nilsson, it is important that themodal pattern at the entrance to the waveguide accept the energy within the Airy disc of the preceding portion of thestage 0 optical system if maximum energy transfer into the rhabdom is to be achieved. However, it is generallydesired that the modal pattern of the rhabdom sum both the first and second order modal patterns for the longestwavelength required by the animal and its niche. This summary modal pattern should be broader than the Airy diskof the preceding optical train. Nilsson et al. have provided an electron micrograph of the crystalline cone–rhabdominterface for Argynnis paphia. The shape of the cone differs significantly from that of the cone of their figures 6 &7, and the above figure for the index of refraction at various points. The distance between the cone and rhabdommay also differ significantly. The previous sub-section has provided best estimates of the dielectric indices of refraction supporting waveguideoperation. The values are not of the desired precision but are useful for pedagogical purposes.

[xxx copied from section 3.6.1.6 ]Wehner has provided figures 5, 6 & 8 showing both rectangular and circular rhabdome in a variety of families ofInsecta259. His caption to figure 5 appears to contain a typographical error. The diameter of the microvilli aretypically between 40 and 70 nm rather than μm based on his scale bar. The rectangular rhabdom have an aspectratio of at least 2:1 and are not easily represented by an inscribed or circumscribed circle. He includes an extensivediscussion of the twisting of the retinula within a given ommatidium. Images from other investigators providesimilar geometries for many other members of Lepidoptera.

Marcus has provided a comprehensive volume on the properties of both circular and slab dielectric waveguides. It isthe nominal Bible of the fiber optics community260. The book provides additional valuable definitions related towaveguides in general. When the data for two one-dimensional slab waveguides are combined orthogonally, theresults relate to a single rectangular waveguide. Between the circular and rectangular waveguides, virtually allrhabdom found among Insecta can be described. Only the rectangular dielectric waveguide can describe rhabdomwith an aspect ratio on the order of 2:1. By reducing the aspect ratio, the modal patterns of the general rectangulardielectric waveguide reduce to those of the square waveguide and appear virtually identical to the modal pattern ofcircular waveguides. The rectangular dielectric waveguide is clearly the more general description of the rhabdom ofInsecta.

3.6.2.3.4 The circular waveguide modes of Heteronyrnpha merope

Nilsson et al. provided photographic records of the actual standing wave patterns within the rhabdom of theirbutterfly, Heteronyrnpha merope. Figure 3.6.2-11 shows their idealized (calculated) equivalents of the amplitudefunctions and the modal patterns They also labeled the types of standing waves encountered and proceeded todetermine the dimensions of the rhabdom required to support these modes. The figure and its caption contain a largeamount of critical information. In this species, many of the mode patterns are polarized linearly as indicated by the

Page 131: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

130 Processes in Biological Vision

second column. Using the cut-off factors shown, the size of the related waveguide dimension can be calculated withsome precision. The component modes beginning with a TE or TM are linear modes. Those beginning with HE orEH are circular modes. The amplitude functions describe the electronic phase of the various portions of the intensitypatterns shown. Within the scope of their investigations, they note in their caption that where pairs of circularwaveguide modes are shown, they always occurred in pairs that generated the associated linear modes shown.

Nilsson et al. discuss the mathematics of circular waveguides in considerable detail in their section 3, includingdevelopment of the critical cutoff frequency as a function of diameter and the dimensionless cutoff parameter, VC. Similar equations apply to the cutoff frequency (wavelength) rectangular waveguides. To support their analyses,they changed to the rhabdom of a different species of small nymphalid butterfly, Xois arctoa. Their discussiondescribes various cutoff wavelengths using color names from the vernacular rather than those defined scientificallyby the US National Bureau of Standards (now NIST) or the CIE (Section 3.6.2.1). They showed in the laboratorythat the cutoff wavelength of the rhabdom of Xois arcoa is 590 nm for the 2nd mode (LP11) at VC = 2.4. Theyphysically measured the diameter of 10 rhabdom of three animals of this species and obtained a diameter of 1.86 ±0.12 microns (s.d.). With these values they were able to solve their equation (6) for refractive index of the rhabdomof 1.362 based on their assumption that the surrounding tissue had an index of 1.34.

Figure 3.6.2-11 “Terminology and characteristics of the waveguide modes with cut-off values (Vc) below 5.5. The LP(linearly polarized) terminology denotes sets of modes that combine to give a linearly polarized result, The constituentmodes (TE, TM, HE and EH) form groups with the same or nearly the same cut-off. These modes may show complexpatterns of polarization but they are always excited in pairs that produce a linearly polarized LP mode. (The subscriptsdescribe the state of polarization and number of lobes in the pattern.) The intensity patterns (derived as the square of theamplitude) are shown as they appear at the waveguide aperture. Changing angle, position or wavelength of excitation,causes some modes (like the 2nd) to appear in more than one form (Snitzer and Osterberg 1961). Our use of 1st, 2nd,3rd etc. denotes those families of modes that share the same or similar cut-off values.” The amplitude function andappearance (intensity) are calculated (idealized). Figure and caption from Nilsson et al., 1988.

Page 132: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 131

261Stavenga, D. (1992) Eye regionalization and spectral tuning of retinal pigments in insects TINS vol 15(6),pp213-218262Goell, J. (1969) A Circular-Harmonic Computer Analysis of Rectangular Dielectric Waveguides Bell SystemTechnical Journal vol 48(7), pp 2133–2160263Bierwirth, K. Schulz, N. & Arndt, F. (1986) Finite-difference analysis of rectangular dielectric waveguidestructures IEEE Trans Microwave Theory and Tech vol 34(11), pp 1104-1114

Note the technical problem in this last computation, a problem encountered frequently in the biophysicsliterature. A value precise to four digit accuracy is calculated from values only good to 2 and 3 digitaccuracy. They proceed to rationalize their logic ending with the statement, “A rhabdom refractive index of1.36 is thus likely to be close to the true value.” Thus, they revert to a claim of three digit accuracy based ononly two digit accuracy in their measurements of the diameter and VC.

A long wavelength limit of 590 nm for the cutoff wavelength of the rhabdom provides an explanation why noL–channel spectrum (typically peaking in the 600-625 nm region) is recorded from the eyes of many species ofInsecta. The rhabdom would need to have a diameter of 2.828 times the longest wavelength to propagate suchenergy, i.e., a diameter of 1.77 microns to propagate energy with a wavelength of 625 nm (0.625 microns). Nilssonet al. do note the significant range of diameters in the rhabdome of butterflies (section 3e). They assert that mostbutterflies have rhabdom in the 2 micron diameter range and that Melanitis leda is unusual in having a rhabdom of4.5 microns diameter. The rhabdom of Heteronympha merope is given as 2.1 microns. These butterflies would haveno difficulty of propagating the L–channel spectrum through their rhabdome. The diameter of the outer segment ofthe human photoreceptors is usually taken as 2 micron diameter or marginally larger (Appendix L).

In a review, Stavenga discussed a wide range of functional variations among insects261. [xxx combine with otherintroductory material ]

A similar set of equations apply to a rectangular waveguide formed by a high index inside material surrounded by alow index material, as found in fiber optics.

Nilsson et al. note some of the energy propagated along such a dielectric waveguide is present in the space outside ofthe boundary between the two materials. However, the density of this energy falls off exponentially within a veryshort distance of the interface relative to the wavelength of the energy involved (Section 4.4.3 of Processes inBiological Hearing and citation 123 therein). The vast majority of electron micrographs show the pigments, of anytype, outside of this distance.

3.6.2.3.5 The rectangular dielectric waveguide modes

Dielectric waveguides play a major role in the modern quantum optics of thin films and fiber optics. The knowledgegained in these fields is directly applicable to the waveguides of biology. Goell has provided the foundation for andrequired analyses to understand the modal patterns of a rectangular dielectric waveguide262. A more recent andexpansive analyses involving even more complex arrangements of dielectric materials is by Bierwirth et al.263.

Goell proposed to achieve an analysis of the rectangular dielectric waveguide by matching the known character ofthe fields outside a dielectric interface with the similar fields inside the dielectric interface. He recognizedimmediately that the fields would involve both conventional and modified Bessel Functions rather than simpletrigonometric functions. He used a very sophisticated analysis. He also noted;

C “unlike metallic waveguides, the field patterns of dielectric waveguides are sensitive to refractive indexdifferences, wavelength and aspect ratios.”C “Since the rectangular dielectric waveguide modes are neither pure TE nor pure TM, a different scheme must beused. The scheme adopted is based on the fact that in the limit, for large aspect ratio, short wavelength, and smallrefractive index difference, the transverse electric field is primarily parallel to one of the transverse axes. Modes aredesignated as Ey

mn if in the limit their electric field is parallel to the y axis and as Exmn if in the limit their electric

field is parallel to the x axis. The m and n subscript are used to designate the number of maxima in the x and ydirections, respectively.”

Goell’s major conclusions include;C “The results of the computations show that the circular harmonic method for analyzing rectangular dielectricwaveguides gives excellent results for waveguides of moderate aspect ratio.”

Figure 3.6.2-12 shows several computed modal patterns for rectangular waveguides of modest aspect ratio. The

Page 133: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

132 Processes in Biological Vision

equation for the script B, = (2b/λ0)C(nr2 - 1)1/2.

3.6.2.4 The superposition (scotopic) eye of Insecta

Exner (1891) introduced the concept of the eye of Insecta changing optical configuration as a function of theexternal illumination intensity. He based the proposition on his observation of the change in the degree ofpigmentation between the crystalline cones of the compound eye with time. His proposal was that the optics of theeye changed from an all refractive (dioptric) design to a design involving both refractive and reflective (catadioptric)optics. His proposition was based on a number of assumptions;C the receptive field of the rhabdom at its entrance considerably exceeded the cone of light delivered by the corneaand crystalline lens of a single ommatidium to its rhabdom,C in the absence of blocking pigment, light entering a cornea some distance from a specific rhabdom could bereflected by the wall of the crystalline lens in order to arrive at the reference rhabdom,C as a result, the light collection efficiency of a given rhabdom could be increased significantly under low lightconditions.

The technology of the 1890's did not provide adequate answers regarding the feasibility of this approach. However,it was noted the increase in sensitivity would be accompanied by a significant loss in acuity of the overall eye. Between 1890 and 1970, the drudgery of performing an adequate optical ray-tracing was quite high. With theadvent of the desktop digital computer, the drudgery was greatly reduced but only a few people knew how toperform even simple “thin lens” ray tracing, much less the more sophisticated ray-tracing associated with “thick

Figure 3.6.2-12 Modal patterns for rectangular dielectric waveguides of modest aspect ratios. The aspect ratio, a/b =2, the index of refraction difference was 0.01 and the script B is a parameter provided in the text. From Goell, 1969.

Page 134: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 133lens” designs of biology. Goldsmith & Bernard (1976) reviewed the pros and cons of the superposition lens conceptin considerable detail in 1976, citing the inconclusive analyses of Horridge (1969-1972) and the opposing argumentsof Hausen (1973). They introduce the challenge of “semantics and the superposition eye” on page 225 and addressthe question of adequate investigative protocols on page 261. The question of whether an inverted or non-inverted(erect) image is presented to the rhabdom by the cornea and crystalline lens is also debated. They also note theinadequacy of the precision of the indices of refraction up through the 1970's. At least three digit accuracy isrequired to the right of the decimal point, with four digits preferred. As noted in the preceding figure, even threedigit precision is not commonly available in the biological literature for Insecta. There may be significantdifferences between species.

Nilsson et al. reviewed the potential superposition eye of Insecta in 1988 based primarily on the Australiannymphalid, Heteronympha merope, butterfly. They suggest both optical designs may have evolved from the sameproto-design. They also presents a more modern analysis of the performance of both types of eyes based on bothgeometrical ray tracing and refractive ray-tracing applied to thick lens systems. As noted earlier, their conclusionwas the most important eyes employed a non-inverting terrestrial telescope design that was afocal. It did notproduce a real image at the focal plane (entrance to the rhabdoma).

Nilsson et al. summarize their position in two parts, 1. “Originally, we considered focal and afocal systems to be discrete and even opposite solutions toapposition eye design, and no intermediates appeared to make sense. However, when we consideredwave and geometrical optics together we came to realize that under some circumstances alternativeinterpretations of the same optical system could legitimately co-exist. Some of these problems areconsidered in the Discussion.”2. “In retrospect, it seems not surprising to find evidence for afocal apposition optics in butterflies because ithas been known since Exner (1891) that most other lepidopterans possess an afocal system in their refractingsuperposition eyes. In this other major type of compound eye - the superposition eye - an erect superimposedimage from many facets is formed on the retina, and this is most commonly realized by an afocal system ineach facet. Our findings in butterflies thus provide a link between apposition and superposition eyes thatotherwise seemed unrelated.”

The findings of Nilsson et al. depend on their ray-tracings that were not described or shown. They also did notdiscuss the gradient index form of the crystalline lens of their butterflies. Their section 2 does provide thedimensions and indices they used in their analyses. They did note, “The limited resolution of light microscopictechniques is, however, inadequate to provide detailed information about the narrow cone stalk. Electron microscopy(Fig. 3) demonstrates a more complex structure in the cone stalk, and it is clear that the stalk region cannot be optically homogeneous. A detailed analysis of the optical structure of the cone stalk seems to be beyond the reach ofavailable techniques, but we can still conclude that, whatever is in there, it behaves as a lens-cylinder.”

They conclude that the crystalline lens is a complex optical system of its own, “The other, and more important,conclusion is that a simple lens-waveguide model of the ommatidium, where the cornea provides most or all of theoptical power, must now be abandoned. A ro1e has to be found for the powerful lens situated just distal to therhabdom tip.” They then discuss modeling the “thick lens” of the crystalline cone as a series of “thin lenses.” Thisposition alone shows they were not experienced or competent in thick lens optical ray-tracing. They do indicate thelimited utility of their approach, in a highlighted paragraph on page 348, and indicate their limited skill in opticalray-tracing. No current optician would dare propose such a crude modeling approach. The design illustrated in theirfigure 6 suffers from significant coma, a condition not introduced or discussed in their text. The closing paragraphof section 2c (page 351) is instructive.

[xxx The remarks of Nilsson et al. re waveguides in sections 3a and 3b need to be addressed in a different section ofthis work. ]

[xxx The visibility of the standing wave patterns within the rhabdom can be observed exteernally for variousfamilies of butterflies and moths. Note this in section xxx. Excellent numbers for the size of the rhabdom based onwhich modes are suppressed as a function of wavelength. ]

The discussion in the Nilsson et al. paper is extensive and informative. It deserves further study by those performinglaboratory experiments. The discussion ends with, “A natural question to ask is whether a similar evolutionarysequence has led to the superposition eyes in other insect orders such as the Neuroptera, Trichoptera and Coleoptera. As with the Lepidoptera, a resolution of these questions depends on finding isolated instances of apposition eyes ingroups where the superposition design is dominant.” Their figure 18 reverts to caricatures to show the similaritiesand inter-convertibility of apposition and superposition eyes, but in an interesting way.

Page 135: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

134 Processes in Biological Vision

264Strausfeld, N. (1976) Mosaic orgainzations, layers, and visual pathways in the insect brain. In Zettler, F. &Weiler, R. eds. Neural Principles in Vision. NY: Springer-Verlag Sections 2.6 & 2.7

In 2002, Land & Nilsson reexplored the merits and feasibility of biological optical systems of the apposition andsuperposition types at the level of an introductory text. While presenting a large number of caricatures, they did notprovide optical designs resulting from modern ray-tracing that confirmed the validity of many of their caricatures. They did provide a clear cross section of a superposition type compound eye, Figure 3.6.2-13. They also presentedempirical evidence of the quality of the image formed at the individual rhabdom (pages 162 & 169). Theirdiscussion of adaptation in such an eye was quite superficial and did not address the actual mechanism of adaptationassociated with the “bleaching” of the chromophores.

A more modern analysis employing optical ray-tracingsoftware capable of handling thick lenses consisting ofgradient index of refraction materials would be mostwelcome in this area. It does appear the opticalcharacter of the crystalline cone differs considerablybetween the apposition and superpositions eyes.

3.6.2.5 Morphology of stage 4, informationextraction, and stage 5 cognition

Strausfeld has attempted to describe the entire visualmodality of two species of Dipera, Musca domesticaand Calliphora erythrocephala in block diagram andcamera lucida form based on histology264. The materialis extensive but may differ in terminology from thiswork at the detail level.

Strausfeld describes the number of neurons present inthe fly, 3.4 x 105 in total, with 76% of these related tovision, Within the visual modality, “68% invade themedulla, 12% invade the lamina, 18% invade the lobula and 2% are derived from the lobula plate.” He goes on todescribe the visual modality in considerable detail. However, since he did not recognize the stratified neuritesconsisted of the dendritic (non-inverting input) tree and the poditic (inverting input) tree, the material requiresreinterpretation. Many of his figures are suggestive of signal differencing that is generally associated with thehorizontal and amercine neurons of chordate retina. His description of the amercine neurons makes it clear that theyhave a pedicle even though the axon may be of minimal length. He does assert the visuotopic mapping of theexternal environment is maintained at least as far as the medulla (and potentially the early neurons of the lobula. Hesubdivides the global lobula into two portions, the lobula and the lobula plate. He goes on to map many outputsfrom stage 5 leading to the efferent command neurons of stage 6.

Strausfeld introduces two sections related to the electrophysiology of the fly visual modality but does not provideany circuit diagrams, interconnection diagrams, waveforms or detailed descriptions. His discussion is quite detailedbut not necessarily as consistent as it would be if supported by such diagrams and/or waveforms.

Figure 3.6.2-14 from Sauman et al. (2005) describes the plan view of the visual modality of the monarch butterflyduring their investigations of the butterfly’s navigation capabilities. They employed a Cry-staining procedure; “Cryptochrome (CRY) is colocalized with PER and TIM and is a blue-light photoreceptor involved in photicentrainment.”

Figure 3.6.2-13 Superposition eye of the nocturnal dungbeetle, Onitis westermanni. CC; crystalline cone, CZ; clearzone, rh; rhabdom layer. Vertical background striation is anartifact of the copy process. From Land & Nilsson, 2002,attributed to S. Caveney.

Page 136: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 135

3.6.3 The electrophysiology of the compound eye of Insecta

Ribi has focused on the use of electron microscopy to define the neurological circuitry associated with theommatidia of several insect species. In Ribi, 1987, the same numbering system is used for the retinula of anommatidia as in the papers of Section 3.6.2. By combining the text from Ribi and others with that of theInvertebrate Brain Platform, a reasonably clear understanding of the circuitry of the insect eye can be obtained.

The signals derived from the sensory receptor neurons (R1 through R8 or R9) do not individually indicate thepolarization of the spectrally selected light. However, by comparing the amplitude of the signals from twoorthogonally arranged receptors sensitive to the same spectral wavelengths, an indication of the polarization angle ofthe source light can be estimated. Bandai et al. have described the polarization sensitivity of the sensory neuronswithin an ommatidium. They describe the polarization sensitivity of neurons R1 and R2 of the distal capsule asperpendicular to that of R3 and R4 of the same capsule. In the proximal capsule, R6 and R8 are perpendicular to R5and R7 and at 45 degrees to the polarization sensitivity of the sensory neurons of the distal capsule. However, theywere less sure of the spectral performance of these sensory neurons in the butterfly. They note, “5. We conclude thatR1 and R2 are either UV, violet or blue receptors whereas R3 and R4 are green receptors. Some R6 and R8 are redreceptors.” They offered no comment about spectral performance of the R5 and R7 receptors. They provide gooddata in their Table 1 but identify a violet receptor spectrum peaking at 402 nm. Their Standard Error relative to theMean was quite large for R5 with a peak sensitivity at 402 nm. This work predicts their narrow spectrum in figure 6is actually a difference spectrum between UV– sensitive and S– sensitive receptors. Their peak spectral sensitivitiesare systematically shifted to longer wavelengths in the UV– and S– spectra and to shorter wavelengths in the M –and L– spectra relative to the predicted peaks of this work. This situation could be due to the small sample sizesthey used.

Ribi (1987) has described the interconnections of many of the sensory neurons and orthodromic neurons. However,he notes, “Although we have conclusive electrophysiological recordings the structure of the retinula-cell endings andsecond-order neurons is poorly understood.” This work notes that when Ribi discusses the pigments associated withthe various sensory neurons, he is speaking of gross pigment and not the monolayer of liquid-crystallinechromophoric material coating the individual microvilli. Ribi is not definitive as to whether he is describing thepigments as perceived by reflected or transmitted light. His figure 1 is complicated.

In summary, the spectral performance of the insect sensory neurons are reported as follows;

Figure 3.6.2-14 Visual modality of monarch butterfly visual modality. Figure is a composite showing two distinctmappings shown in red. Left of the cut line; Dorsal Rim Axonal Projections and their Relationship toCRY-Positive Fibers. Fiber is best identified as an lvf fiber between a retinula and the medulla. Right of the cut line;Schematic representation of CRY-positive neurons and their axons. RE, retina; LA, lamina; ME, medulla; LO, lobula;PL, pars lateralis; PI, pars centralis; SOG suboesophageal ganglion. See text. From Sauman et al., 2005.

Page 137: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

136 Processes in Biological Vision

Author Ribi Bandai Arikawa ChenYear 1987 1992 1999 2016Species Papilio Papilio Papilio Graphium

Dorsal VentralSample - - 15-20 ~15 nm 20 20spacingReceptorR1 UV UV, v, S UV UVR2 UV UV, v, S UVR3 S M M MR4 S M M M

R5 M [red pig.] - - L/M LR6 M [red pig.] ?L L/M LR7 M [red pig.] - - L/M LR8 M [red pig.] ?L L/M L

R9 - - [no pig.]

UV (342) 380 364 354(8)S (437) 450 460 455(20)M (532) 540 522 520 541(24)L (625) 610 601 600 599(4)

1. The “red pigment” in brackets was not further defined. It should probably be described as magenta pigment(being the complement of the M–channel absorption spectrum of yellowish-green).2. Several of the papers noted that the R9 receptor was not accompanied by any pigment clusters.

Chen has defined three distinct specializations of ommatidia within the compound eye of individual butterflies ofGraphium. In general the sensitivity of the sensory neurons is consistent among the specializations based on theirTable 1 but R5-8 of the dorsal ommatidia were reported as diverse as indicted. In some cases the divergence fromthe nominal peak absorption was large (640 nm versus a mean of 599 nm in one case). They noted in this regard,

“We extensively manipulated the relative proportions of R541, R582 and R599 in our attempt to reproducethe spectral sensitivities” of several of their “receptors in the ventral type II ommatidium, and also triedincorporating various pigment filtering effects. However, we were unable to obtain satisfactory fits to ourdata.”

The numbers in parenthesis at lower right are the numbers of receptors used to determine the average peaksensitivity. The wavelengths in parentheses at lower left are theoretical values for the non-Gaussian chromophoresrounded to three digit accuracy. The value of 625 nm is a first order prediction that may actually occur at shorterwavelengths due to second order mechanisms. In all of the experiments in the above Table, the monochromatorsample spacing was too coarse to justify three digit precision. In the Chen paper, a set of 22 “narrow band” filterswere used without specifying the actual spectral profiles of the filters.

The spectral sensitivity of R1 through R8 of Ribi (1987) are based on Horridge (1983). The color of the pigmentassociated with R5 through R8 by Ribi is appropriate for these pigments if observed by reflected light. The lowercase v associated with the values of Bandai (1992) is used to indicate a reported value not supported by theoreticalconsiderations. Many of the pigment labels used by the authors in the table are casual and do not differentiatebetween their color by transmitted light (UV, violet/blue, green or red) from their color by reflected light (colorless,cyan, yellow or magenta). The colors observed by transmitted light are associated with the descriptor, additivecolor, whereas the colors observed by reflected light are associated with the descriptor, subtractive color.

The energy threshold for exciting a sensory neuron in Insecta has not been found in the literature. In humans, andprobably a wide variety of species within Chordata, the energy required is approximately 2.0 electron volts (46kCal/mole) based on Sliney (Section 12.5.2.4). This value suggests excitation by photons at wavelengths in the redmay require a 2 photon —> 1 effective exciton mechanism (Section 12.5.2.4), or an alternate 2 exciton —> 1 freeelectron mechanism, in order to excite the sensory neuron (thereby generating a free electron within thedendroplasm). Either form of this second order mechanism may cause a shift in the peak L–channel spectralsensitivity to the region of 610 nm.

Page 138: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 1373.6.3.1 The neural circuitry of stage 1 of the eye of Insecta

Figure 3.6.3-1 is the proposed neural circuit of the sensory neuron of Insecta adapted from the similar circuit of theeye of Chordata. A complete development of this circuit topology is provided in Section xxx. The major differenceis in the implementation of the microvilli of the sensory neuron. In the eye of Chordata, the microvilli (microtubulesin that technical field) are arranged in a circular chalice (or goblet, Section 4.3.2) that contains the disk stack of theouter segment of each photoreceptor neuron. In Insecta, the microvilli extend from the dendrite to form a grid that isperpendicular to the long axis of the rhabdom (top frame) This grid (labeled wafer 1) is followed by wafer 2 andsubsequent wafers for the length of the rhabdomere. Each wafer consists of a group of microvilli connected to oneor a series of dendrites all associated with a single sensory neuron. Each wafer is coated with a layer of one ofseveral Rhodonine chromophores (probably on both sides). The grid of each wafer is a functional structure by itself;the microvilli of the grid are spaced at about 0.2 microns and together they form the polarizer associated with thatrhabdomere. When the e-vector of the incident light is aligned with the direction of the microvilli of the grid, it ismaximally effective at stimulating the chromophores coating the microvilli.

The middle frame shows the electronic components within the sensory neuron that are formed by various internallemma (not shown) and specifically modified sections of external lemma (shown as a jagged line). The lower frameshows the same electronic components configured to be more recognizable to an electrical engineer. The circuit isgenerally known as an unbalanced differential pair of Activa (transistors), or alternately a cascode circuit. The baseterminal of the left Activa is shown open (not connected to any other element) while the base of the right Activa isshown connected to a battery (labeled (3) for electrical bias. The delay element represents the slow ionic conductionof electrical charge along the length of the axoplasm within the axolemma leading to the pedicle of the axon.

The left Activa forms the adaptation amplifier and theright Activa forms the distribution amplifier since itdrives the axon which may distribute a voltage signalthrough synapses with a variety of orthodromicneurites. The “base” of the left Activa is stimulatedquantum-mechanically by the excitons created in thechromophoric liquid crystal due to absorption of aphoton. For each exciton created, the absorptioncoefficient of the liquid crystal is reduced (making itmore transparent) until the exciton is transferred to theleft Activa. The entire sensory neuron is a logarithmicdevice that can accommodate a wide range of stimulusconditions..Under daylight conditions, thechromophores of most eyes are reduced to about onepercent of their dark adapted condition (they aresignificantly bleached).

Goldsmith & Bernard (1974, page 175) reported theunusual charge distribution associated with themicrovilli based on electron-microscopic examination;“the walls of the microvilli are more electron opaquethat the interiors.” The walls (exterior lemma) formthe first (left) Activa. The opaque regions representthe site of the electrostenolytic process convertingglutamic acid to GABA and CO2 while generating anegative electrical potential in the dendroplasm relativeto the external milieu (Section 8.6). Theelectrostenolytic mechanism introduces a componentof the impedance labeled (1) in the figure. On thesame page, Goldsmith & Bernard provide additionalinformation about the dimensional character of themicrovilli grid. These features vary widely amongspecies as indicated by the taxonomy in their Table I(page176). The potential significance of thesestructures to the polarization sensitivity of these insecteyes is noted (page 177 & Section V,C).

The labels IPM (inter-photoreceptor matrix), INM (inter-neuron matrix) and OLM (outer limiting membrane) are left

Figure 3.6.3-1 Proposed morphology and circuit diagram ofthe sensory neurons of Insecta ADD.

Page 139: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

138 Processes in Biological Vision

in this representation because it is important to provide an oxygen free IPM to prevent premature chromophoreoxidation. On the other hand, the INM must accommodate chemicals rich in oxygen moieties/legands. The labelsOS (outer segment) and IS (inner segment) have a less defined meaning in the rhabdom of Insecta than in thechordate eye.

3.6.3.2 The Photo excitation/De-excitation equation of the eye of Insecta

It is proposed that the photo excitation/de-excitation (P/D) mechanism of Insecta is the same as for the rest of theanimal kingdom. Photons are absorbed, generating excitons and the excitons stimulate the specialized lemma of themicrovilli while simultaneously being de-excited. The stimulation of the microvilli lemma allows an electron toenter the dendroplasm shared with the rest of the microvilli of that dendrite as a “free electron.” This overallmechanism is given the name transduction. Note, it does not involve any atoms, ions or molecules moving throughthe lemma of the neuron.

The (P/D) mechanism associated with the transduction of photons into an electrical current within the sensoryneuron is complex. However, the quantum-mechanical differential equations describing this mechanism have beendeveloped and solved (See Section 7.2). The response is significantly different for stimulation by an impulsefunction versus a much longer duration square pulse.

Except for the flash associated with lightning, it is unusual in the natural environment to encounter animpulse or square pulse where the leading edge of the pulse occurs in less than a few milliseconds. The lightadaptation function is dependent on the amplitude of the stimulus. It can be only a few microseconds.

Through the 1970's, it was difficult to obtain a commercial optical shutter mechanism with a fast enough risetime to avoid impacting the recorded rise time of the test set combined with the sensory receptor of thespecimen. An electro-optical shutter was generally required. Pulsed light emitting diodes (LED’s) had notentered the commercial marketplace. Currently, it is important to demonstrate the shape of an impulse or asquare pulse is adequate for use in P/D mechanism evaluations.

[xxx The complete equation for the P/D mechanism in response to an impulse function is shown here in the stand-alone version of this section. The definition of all of the parameters appears in Section 7.2

Eq. 7.2.4-1

under the condition that σ •F•τnot equal 1.00

Note the impulse function “i” includes three distinct terms, a scale factor, a delay term and an amplitude term. Several auxiliary terms are used to quantify the complete function as a function of temperature and stimulusintensity. The P/D response to a square pulse is even more complex and will not be displayed here. xxx]

It should be noted that the amplitude term includes two separate exponential terms where the multiplier to each “t” isdefined as the reciprocal of a time constant. However, the two time constants can not be summed or differenced togive a net time constant applicable to the settling phase of a P/D response.

Page 140: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 139The amplitude term, with the time constant in the first exponential being a function of F accounts for the shape of thecomplete monopolar description of the P/D response.

The delay term introduces an absolute delay in the time scale before the amplitude term departs from the horizontalbase line.

Each term in the total response, i(F,t,j,σ,τ) is a function of the stimulus intensity in photons/sec/unit area, F.

3.6.3.2.1 The stage 1 P/D response to an impulse function

Figure 3.6.3-2 reproduces the nominal response of the P/D mechanism to excitation by an impulse from Section 7.2based on the human eye of Chordata. The P/D Equation involves two separate time constants, an attack timeconstant controls the rise time of the leading edge of responses through an important parameter, the product of thetime constant and the quantum flux absorbed by the chromophores coating the disks of the outer segment of thesensory neuron receptor in Chordata or the microvilli of the rhabdomere of the sensory neuron receptor in Insecta(and Mollusca). The second (decay) time constant controls the relaxation of the response back toward a steady statelevel during long stimulation periods. Once stimulation has ceased after a long period of stimulation, where a steadysignal level has been reached, the decay time constant is the only parameter controlling the decay of the responseback to baseline.

Hodgkin attempted unsuccessfully to develop and solve the P/D Equation during the 1950's, and then tried torepresent this mechanism using the Poisson Equation with poor results. The P/D equation exhibits a uniquecondition (when σCFCτ = 1.000) where the two time constants give way to a single time constant. The responseunder this condition takes the form of the Poisson Equation and has been named the Hodgkin Solution in his honor.

The value of sigma, σ, for the eye(s) of Insecta will be different because of the different configuration andorientation of the rhabdomere relative to the incident light compared to the disks of Chordata. This configurationand orientation may even vary within Insecta because of the desire to achieve different sensitivities to polarizedlight.

Figure 3.6.3-2 Theoretical impulse responses based on the P/D Equation. The latency of the mechanism is shown bythe delayed departure of the waveforms from the baseline, as a function of the peak flux density, F, in photons/micron2-sec. For other temperatures, the time scale can be multiplied by the appropriate value of KT. The value of sigma, σ, isappropriate for perpendicular illumination or a stack of individual disks. The Hodgkin Solution (σCFCτ = 1.000) occursat F =12.

Page 141: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

140 Processes in Biological Vision

Note the change in the apparent rise time constant with different values of the photon flux density in quantum units,F. When quoting an apparent rise time constant the investigator must associate it with a specific flux density orillumination intensity to be meaningful.

Page 142: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 141

3.6.3.2.2 The stage 1 P/D response to square pulse stimulation ADD

There are a wide variety of responses by insects to square pulse light simuli. For reasons not entirely clear to thistheoretician, the investigators tended to use very long square pulses, frequently 500 ms. The effective time constantsof the relevant eyes tend to be much shorter than 500 ms. As a result, the features of the recorded responses tend tobe poorly displayed. Laughlin (Land et al 1981, chapter 2) provided a variety of responses with long stimuliduration for a variety of Arthropoda (dragonflies, house flies & Limulus). Laughlin (1976 in Zettler & Weiler)provided a set of waveforms for both the retinula and “large monopolar cell,” LMC, of the dragonfly, Hemicorduliatau. While not all recorded at the same stimulus intensity, they show a variety of conditions that can be addressedbased on knowledge of the theoretical P/D mechanism that he did not have available, and the subsequentmechanisms within the retinula and the lamina of various eyes of Insecta. Electrical signal saturation at the outputaxoplasm of the sensory neuron is particularly evident in his figures and in Figure 7.2.6-4 of this work. It shows themeasured impulse response waveforms of Baylor (1979) overlayed with the theoretical equation for various stimuliintensity (without significant discussion of the variation in delay with stimulus intensity). The specimen was a toad. Laughlin also ignored or was unaware of the variable delay in the response functions as a result of different stimuliintensities (Section 7.2). This delay was illustrated in the previous figure showing the response of the endothermicanimal eye (at 37 C) to an impulse function. These delays can be significantly longer for the eyes of exothermicanimals.

Figure 3.6.3-3 shows the P/D response of a sensory neuron to square pulse excitation of constant intensity for itsduration.

Figure 3.6.3-3 Theoretical P/D response to a square pulse stimulus. For illustration only, use equation to establish actualvalues. Stimulus 200 ms long beginning at time zero. Responses are each of 200 ms duration following a delayinversely proportional to the stimulus intensity. Decay time constants after cessation of stimulus are all 12.5 ms. F isthe stimulus intensity. Note the potential saturation level associated with the maximum axon current of the first Activaof the sensory neuron. Parameters are those of the previous figure. At F = 10, the response is below the HodgkinCondition and of slightly different form than that at F = 17.5. See text.

Page 143: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

142 Processes in Biological Vision

It should be noted that the P/D response varies with the intensity of the excitation but remains a continuous function. The overshoot should not be described as a phasic portion with the rest considered tonal. The terminology isinappropriate. The overshoot “settles” to a steady state value determined by the intensity of the stimulation. Theovershoot is approximately 45% for F = 140; 30% for F = 70; 15% for F = 35; 10% for F = 17.5; and 0% for F = 10.The initial rise for each waveform is proportional to the product of the stimulus intensity and the first time constantof the P/D Equation. These slopes are not related to any simple time constant in the P/D equation, and are not asimple exponentials. The apparent time constants shown between 50 and 250 ms are not real. The shape of thesettling, after the initial peak involves a combination of the two time constants within the P/D equation. When thestimulation ceases, each response returns to the baseline according to the real second time constant of the P/Dequation only.

3.6.3.2.3 Inversion of the P/D response by stage 2 neurons

Land et al. (1991) presented an extended study of the noise associated with signals measured in the retinula ofvarious members of Insecta. Their page 247 used patch clamp techniques to provid excellent electrophysiologicaldata confirming the P/D mechanism is utilized in the dragonfly at several specific locations in the signaling chain. They used 500 ms stimulation pulses that result in more complex responses than illustrated in the above figure. They show the waveforms measured at the axon of their large “monopolar” cell (LMC) are clearly inverted relativeto the waveforms in the axoplasm of the preceding retinula axon. This inversion suggests the LMC acts like ahorizontal cell of stage 2 in the chordate retina, as detailed in Figure 3.6.3-4. The insert at lower right shows theelectrical schematic of the Activa. The schematic is the same as that for the horizontal cell of the chordate retinaused elsewhere in this work. This figure is a simplification of one from Boscheck shown below.

The label large monopolar cell of histology is highly inappropriate when describing the electrophysiological(functional) performance of this neuron type. Functionally, it is a horizontal neuron with a bipolar electricalsignal at its pedicle.

Page 144: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 143

265Boschek, C. (1971) On the Fine Structure of the Peripheral Retina and Lamina ganglionaris of the Fly, Muscadomestica Z Zellforsch vol 118, pp 369-409

If all of the svf signals applied to this stage 2 neuronsynapsed with only the dendritic (non-inverting) tree,the output signal would constitute a summation ofspectrally (and possibly polarization) distinct signals. The output signal would represent a broad spectralsignal not associated with any sensory neuron of stage1. It would represent a R–type signal of this work. Ifthe svf signals applied to this stage 2 neuron synapsedwith both the dendritic tree and the poditic (inverting)tree, the resulting stage 2 output signal wouldcorrespond to O–, P– or Q– type of signalsrepresenting components of the chrominance channelsof vision. Alternately, for input signals representingsignals of different polarization from stage 1, theycould represent a new group of polarization signals notdefined among the eyes of Chordata.

The above figure was modified from Wehner based onthe availability of considerably moreelectrophysiological data. The Wehner figure,attributed to the worker bee, may be a significantsimplification of an earlier figure by Boschek (1971)based entirely on histological research265. The Boschekfigure also appears in Goldsmith & Bernard (1974,page 196) where it is attributed to the common fly,Musca domestica.

3.6.3.2.4 Generator potentials versus actionpotentials in Insecta

The stage 1 sensory neurons of Insecta produce whatare defined as generator potentials via the P/Dmechanism. These waveforms are entirely tonic(analog) in character. They are not action potentials orany other kind of phasic waveform.

As a general rule, Insecta does not employ actionpotentials, specifically phasic signals generated asmonopolar pulses by stage 3A analog to pulseconverting neurons. There may be an exception in thecase of very large (and possibly extinct) members ofInsecta. Pulse type neural signals (action potentials)are typically required when neural signals must bepropagated for more than two millimeters beforereaching a synapse (or Node of Ranvier). In Chordata, less than 5% of the signals carried within the neural systeminvolve phasic (pulse mode) signals. 95% of all signals within Chordata are tonic (analog mode) signals (Section10.1.1.2).

The eccentric cells of Limulus, and potentially the neuron labeled R9 in ommatidia, can be modified throughevolution to generate stage 3 pulse output signals. This modification typically involves wrapping the long axons inmyelin. As an alternative, the axon may be closely packed within a group of shorter neurons as found in somemembers of Mollusca, such as the locomotion neurons of squid. The locomotion neurons are not properly classifiedas stage 3 neurons.

The long visual fiber (lvf) neurons may carry signals, resembling action potentials, that may be generated byeccentric neurons (or neurons labeled R9). Many of these lvf appear to originate in the stage 1 ommatidia and

Figure 3.6.3-4 Synaptic connections in the cartridge of aworker bee based on data. . . . Black outline unshaded; theLMC (type L1) and its ramifications. Black shaded andhatched; short visual fibers, (svf from retinula cells). Blackoutline & shaded; centrifugal fibers. A, B & C;stratification of laminar neuropil. Activeelectrophysiological elements; Activa shown at the junctionbetween the dendritic tree on the left, the poditic tree on theright hand, and axon of the Activa extending below theActiva. A membrane (internal lemma) is shown risingvertically from the Activa to separate the dendroplasm fromthe podoplasm of the neuron. The termination of this lemmais undefined, and unimportant, at this time. Right;representation of the electrical schematic of the Activa. Seetext. Modified from Wehner, 1976.

Page 145: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

144 Processes in Biological Vision

bypass the stage 2 lamina on the way to the more distant stage 3 medulla. 3.6.3.3 Empirical signals associated with the ommatidium and stage 2 lamina

Figure 3.6.3-5 provides a set of analog waveforms related to the elements of both the ommatidium and lamina of thedragonfly, Hemicordulia tau, from Laughlin (1976). As noted most clearly in the 1976 caption to the figure, “theflash intensities are not equal between records.” The vertical calibration bars are all 10 mV. Note the much tallerbars at the lowest stimulation levels. The flash duration was always 500 ms and its intensity increases from top tobottom in each column.

Laughlin did not describe the spectral or polarization characteristics of his light source, except to say it was a pointsource located at a distance. He referred the reader to his earlier papers.

As noted in earlier discussions, the soma of the large monopolar cell plays no active role within the neural system. Itis responsible for the homeostasis of the neuron and not signaling. The single Activa (active semiconductor device)within the LMC is found at the junction of the neurites and the axon. Based on the inversion illustrated between theLMC and retinula axon waveforms, the LMC probe was inserted into the axolemma of the neuron. This inversionbetween the waveforms indicates clearly that the recordings from the retinula axon synapsed with the poditic

Figure 3.6.3-5 Diagrammatic representation of two retinula neurons and one LMC. The axons from the ommatidiumwere described as short visual fibers. Responses were obtained at different locations, both intracellularly andextracellularly (lamina depolarization column). Interruptions in the flash traces of the lamina depolarization and twoassociated with the retinula axon were not explained in the original paper. See text. From Laughlin, 1976.

Page 146: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 145

266Zettler, F. & Weiler, R. (1976) Neural Principles in Vision. NY: Springer-Verlag, Section 2.4 page 259

terminal of the LMC. [xxx ADD comments about saturation in the retinula axon waveform and in the LMCwaveform at maximum stimulation.

Many of the waveforms shown can be related to the theoretical waveforms shown above resulting from the squarepulse stimulation of a sensory neuron. Those waveforms in row A show negligible overshoot associated with theleading edge of the pulse response and conform th F = 10 in the earlier figure. Waveforms B1 and B3 can beassociated with F = 35 to F = 140 of the earlier figure. C1 can be related to F = 140 or higher and suggests somedifferences in the characteristics of the test set (some test set overshoot and some loss in low frequency bandwidth inthe test set). B2 and B3 show clear examples of saturation in the axonal circuit of the Activa within the neuronsimilar to the truncated form for F = 140 shown in the earlier figure. The truncation is typically associated with anon-exponential decay after cessation of the stimulus, as the Activa within the neuron regains its normal operatingcondition. C3 and its associated stimulus trace are a bit confusing. The waveform suggests the presence of a two-step square pulse. The waveform illustrates a settling to an intermediate plateau before the stimulus is reduced. Thewaveform then settles toward a different plateau before the cessation of all stimulation. The waveform then decaysexponentially as expected. The waveforms of column 4 can be associated with inverted waveforms from stage 2neurons where the driving svf signal was applied to the poditic (inverting) tree as discussed earlier. A4 shows someovershoot similar to and inverted A3. B4 and C4 show similarities to F = 140 or higher but may also show excessivepeaking in the test set response. The waveform at lower right appears to show excessive peaking and inadequate lowfrequency response related to either the neuron or the neuron-test set combination.

A trained and experienced oscillograph operator frequently observes what appear to be waveforms obtainedwithout a properly compensated probe. In biological investigations, this is a frequent problem where a “homemade” probe is prepared from glass tubing and no frequency compensation is provided as part of the probeassembly.

Wehner (1976, page 280) has provided extensive discussion of the histology of the stage 2 lamina (with somematerial related to the stage 4 medulla) of bees, Apis. He notes significant differences between the retinula withinthe ommatidia of bees and the members of Lepidoptera. He notes the axons leaving a given ommatidium arefrequently twisted in the opposite direction to the twisting of the retinula within an ommatidium. He also providesmore background on the ninth neuron frequently described as either R9 or an eccentric cell (page 289) but draws noclear conclusion.

Figure 2 of Zettler & Weiler266 (1976) shows the P/D responses for a sensory neuron, “a marked receptor of type R1-R6,” over an intensity range of 104. They follow the pattern predicted by the P/D Equation. The output signal variesin amplitude by less than a factor of three. Unfortunately, they do not plot the absolute delay associated with eachintensity level and they do not specify whether the neuron was dark adapted between stimuli or the order of stimulus(in increasing or decreasing intensity). There appears to be some overshoot associated with their testinstrumentation. Much of the discussion provided by Zettler & Weiler appears speculative based on currentknowledge if the electrophysiology of vision. See also Section 3.6.1.3 for additional data from Zettler & Weilerregarding stage 2 signal processing. Their figure 6 does quantify the receptive fields of three individual L-typeneurons from a lamina. For three different wavelengths, the UV–, S– and M – receptive fields remain identical. This observation leads to the conclusion that the receptive fields of the adjacent ommatidia are very similar and donot vary significantly with wavelength. The observation also contributes some data relative to whether thecrystalline cone is a terrestrial telescope or not, and whether the receptive field of adjacent rhabdom are equal andindependent of spectral wavelength. Two of the three receptive fields are 4.0 degrees wide at 50% and the thirdreceptive field equals 5.4 degrees regardless of wavelength. Conceptually, such performance is much morecompatible with a terrestrial telescope limiting the receptive field than with a fixed size rhabdom waveguide whereits receptive field is inherently sensitive to wavelength.

3.6.3.3.1 A cartridge of stage 2 of the domestic fly

Boschek provided significant information about stages 1 and 2 of the house fly acquired with high magnificationelectron microscopy. His descriptions of the photoreceptors as well as the synapses involved in these stages isextraordinary. He defines ten configurations of synapses based strictly on histology. Unfortunately, Boschekappears to have fallen into a Bayesian trap. He assumed that every region of high electrical opacity along the lemmaof a neuron was associated with a synapse. In fact, such an area can be associated with either a synapse, a Node ofRanvier (among chordates) or an electrostenolytic area constituting the electrical power source for a neuron. Manydimensions are provided. Figure 3.6.3-6 shows a partial schematic of a single capsule of Musca domestica with awide variety of his histologically defined synapses. Two separate LMC, L1and L2 are shown to the same level ofdetail as the LMC in the figure of Section 3.6.3.2.3. LMC L3 and L4 are shown less completely. The numbers refer

Page 147: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

146 Processes in Biological Vision

to his classification of synapse types.

Page 148: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 147

Figure 3.6.3-6 A semischematic summary drawing of the structure and synapses interconnecting stage 1 and stage 2circuits of Musca domestica. The numerics and arrows define synapses according to Boschek. This work provides analternate description of many of the putative synapses. L1-4; somata of LMC. EC; epithelial glial cells. R1-6; short visualfibers (SVF) are axons from retinula cells axon terminals. α,β; two of six paired centrifugal fibers. R7, R8, Long visualfibers (LVF) from retinula cells. U; unidentified fiber fragments associated with type 8 synapses. “Broken lines havebeen used to indicate areas of inadequate evidence.” Annotation added. See text. Modified from Boschek, 1971.

Page 149: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

148 Processes in Biological Vision

267Smakeman, J. & Stavenga, D. (1986) Spectral sensitivity of blow,fly photoreceptors: Dependence onwaveguide effects and pigment concentration Vision Res vol 26(7), pp. 1019-1025, eq. (1)

“Synapse 10" appears to be an electrostenolytic power source drawing glutamic acid from the adjacent epithelialglial cell. “Synapse 9" is inadequately defined using Boschek’s words but appears to also be a site ofelectrostenolytic activity. “Synapses 1" and “synapses 5" appear to be the primary synapses between the SVF axonsand the LMC neurons. “Synapses 2" and “synapses 3" appear to be connecting the axons of the two labeled SVFwith the dendrites of the centrifugal fibers, α & β. The centrifugal fibers appear to be the analogy of “bipolarneurons” in the retinas of chordates. They sum signals from varous retinula within one ommatidium to create abroadband signal (channel R) associated with the putative but archaic “rods” of scotopic vision. “Synapses 4"appear to be connecting other LMC to the centrifugal fibers, α & β. “Synapses 8" appear to be an inadequatelydefined connection to the fragments labeled “U.” R7 and R8 appear to be LVF passing through the lamina withoutsynapsing.

Boschek indicates there are six pairs of retinula from a single ommatidium interconnecting with the centrifugalfibers. There are clearly multiple pairs of retinula synapsing with individual LMC cells. These numbers are morethan sufficient to support a full color capability in the fly with additional circuitry available to support e-vector angledetermination in at least one spectral region. More electrophysiological data is needed to identify the spectralperformance of the individual retinula participating in each actual pairing to determine if O–, P– & Q–channelsignals are available at the axons of the LMC of each cartridge. The pairing may actually vary with the location ofthe ommatidium within the overall compound eye. [xxx add more from Boschek or others on the list of files acquired on 27 April ]

3.6.3.4 Stage 3– Signal projection in Insecta

Signal projection from one stage to the next is less important in Insecta than in Chordata. The distances betweenvarious signal processing engines is much smaller and the brain itself is frequently dispersed (rather than be definedas a unitary brain as in chordata). Thus, the need for signal projection over long distances, greater than 2 mm, is rarein inects. The use of monopulse signals (usually described as Action potentials), as oppose to analog generatorpotentials, are not found in insects.

On the other hand the speed of signal projection within analog neural paths has been found to exceed any reasonableestimate of the speed of molecular or ionic diffusion along axons or other conduits. The fact that signaling occurs ata speed exceeding reasonable molecular or ionic diffusion rates is strong support for the Electrolytic Theory of theNeuron where signaling by electronic charge rather than by ionic charge transport is the proposed mechanism(Section xxx).

3.6.4 Confirmation of the Rhodonines as chromophores of Insecta

During the 1980's a considerable number of papers from virtually a single laboratory made many assertionsconcerning the chromophores, visual pigments, filtering pigments and their combination using what were believed tobe early intracellular recording techniques. There was scant proof that their spectra originated from only a singlesensory receptor cell. The author’s also appear to have relied upon their earlier training as no block diagrams,schematics or descriptions of the transduction mechanism were provided. In one notable case267, the nominalabsorption spectrum was modeled using the expression 1– 10 0.43xexpression where 10 0.43 = e. Thus, the term 100.43xexpression should be replaced by the term e expression as used throughout quantum mechanics. Treating the numeric0.43 as part of the “total extinction” of the pigment puts their values in conflict with those of most physicsinvestigations. Fortunately, they treated the value 0.43 as a constant of unspecified source and defined another“extinction coefficient per unit length, κ.” The conclusions drawn from these early studies will not be discussedhere.

[xxx edit to note their emmissive spectrum matched their sensitivity profile in the UV. Indirect evidence that theyisolated and recrystallized the liquid crystalline chromophore, Rhodinine(11). ]Arikawa et al. (1999a) claim they successfully isolated and defined the ultraviolet chromophore of vision in

Page 150: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 149

268Arikawa , K. Mizuno, S. Scholten, D. Kinoshita, M. Seki, T. Kitamoto, J. & Stavenga, D. (1999) Anultraviolet absorbing pigment causes a narrow-band violet receptor and a single-peaked green receptor in theeye of the butterfly Papilio Vision Res vol 39, pp 1-8269Wyszecki, G. & Stiles, W. (19198) Color Science: Concepts and Methods, Quantitiative Data and Formulae.NY: John Wiley & Sons pp 15-24 & 708

Papilio268 However, their identification was not totally complete from a chemical or absorption spectra perspective. They performed the re-crystallization of the liquid crystalline material by extracting it from the retina using theformaldehyde method (section 2.4) and precipitating it on a microscope slide substrate and then observed itsemission spectra through a light microscope-attached photodiode array (USP-410, Unisoku), equipped with an imageintensifier (V1366U, Hamamatsu photonics). The glass of the microscope slide and the optics of themicroscope/image intensifier probably limited the measurements (Section 3.6.4.1)

The “modified formaldehyde method” used by Arikawa et al. (1999a) is traceable via their citations back toSuzuki et al. as discussed in Section 5.5.15.4 where a question is raised with regard to using formaldehyde. Formaldehyde not only shares a terminal group with the retinal form of the chromophores of vision but is alsoknown as a strong reducing agent due to its electron-deficient carbon. This electron-deficient agent acts as anelectrophilic agent when exposed to a resonant molecule such as any of the retinines. It may destroy ormodify the resonant form of the retinine (with an i) required for a chromophore exhibiting a non-isotropicsensitivity to light accompanied by a shift in its spectral sensitivity.

Arikawa et al. identified the chromophore as a retinene, 3-hydroxyretinol, but immediately qualified their remarks bysuggesting it might be the form 3-hydroxyretinal (paragraphs 2.4 & 3.2). A resonant form of the two retinenes,could lead to confusion if one assumes the resonant retinene must be one or the other of these forms. The use offormaldehyde in extraction may have introduced an unexpected complication into their molecular identifications. Their method involved a presumptive identification as they did not demonstrate the character of the atom or groupattached to Carbon 5 of these (potentially conjugated diol, and therefore resonant) arenes. This work demonstratesthe actual chromophores of vision in the UV are conjugated diols with an oxygen atom at positions 11 and 15 (or 1and 5 depending on the convention employed) of the aliphatic chain. As a result, the molecule is more appropriatelynamed Rhodonine(11), a retinine where the underlined “i” indicates the diol form of this family (Section 5.5, andspecifically Section 5.5.8).

Section 3.2 of Arikawa et al. does not provide a strong conclusion. The final paragraph of that sectionsuggests the chromophores of the eye were in fact resonant Rhodonines and not retinenes (with an e). Theynote, “The fluorescing pigment appeared to be rather labile. Prolonged UV-illumination with themicroscope’s mercury lamp caused a rapid fading of the fluorescing stars (ommatidia sensitive to UV light),within half a minute. The fluorescence pattern fully regenerated after a dark adaptation time of several hours,however.” These temporal intervals are suggestive of a diol based chromophore.

The prefix 3-hydroxy- indicates this retinine family is based on vitamin A3 as described in Section 5.3.3. Isolationof this family from the adult form of Palilio is suggestive of two points; first it is likely the earlier caterpillar andpupa stages of their life cycle also used vitamin A3 in their visual chemistry and unless the family is catadromouslike some fish (Section 1.2), it would use that form throughout its life cycle. Second, the fact that both selectedbutterflies and selected moths employ vitamin A3 would suggest that the Order Lepidoptera also employs vitaminA3, like the Order Diptera (flies), as the principle form of the vitamin used (at least) in their visual modalities.

3.6.4.1 The measured spectra of the UV chromophore, Rhodonine(11) of Papilio

Figure 3.6.4-1 reproduces figure 1a from the Arikawa et al. (1999a) paper. It appears to display the same measuredvalues as used in [Figure 3.6.5-2] below. The graph shows a well formed set of measurements with a peak at ~365nm and left and right half amplitude points at ~322 and ~410 nm respectively. These values can be compared withthe values for the human eye in Table 5.5.10-1 and illustrated in [Figure 3.6.3-3]. These values suggest the shortwavelength of the left half amplitude point and the peak amplitude have been impacted by the use of soda-glassoptics in the microscope, the light source and the microscope slide used as a substrate for precipitating thechromophore. While the system can be calibrated to eliminate the absorption by the envelope of the light source andthe microscope/intensifier, it is not clear the calibration accounted for the microscope slide. In either case, thesignal to noise in their measurements is degraded using glass optical components. Arikawa et al. only described theoptical equipment used for the emission measurements and not the absorption measurements of the chromophore. Use of quartz optics, or other adequately transmitting optics, are preferred and possibly necessary in thesemeasurements269.

Page 151: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

150 Processes in Biological Vision

3.6.5 Light and dark adaptation amongInsecta

This area will not be explored in detail at this time. The literature related to dark adaptation among Insectaappears to follow the archaic assumptions showingdark adaptation following a two-step exponential curvethat is not compatible with the actual mechanisms ofthe visual modality (Goldsmith & Bernard, 1974, page233). Goldsmith & Bernard did not provide any datapoints in their figure describing the dark adaptationfunction but they did indicate the process was completein less than 30 minutes. The actual mathematicalframework of Section 17.6.1 follows a morecomplicated path that appears similar to the archaicassumption under specific stimulus conditions (figuresin Section 17.6.1). The actual mechanism isrepresented by a graph of an “exposine,” a product of alow frequency sine wave and an exponential. Depending on the intensity of the last stimulus level,and the position and diameter of the test area, thefunction becomes a single exponential and then progresses into the exposine form as illustrated.

Figure 17.6.1-4 of this work can be compared to figure 14 of Goldsmith & Bernard.

The remarks of Nilsson et al. in Section 3b about behavioral observations related to dark adaptation can bereinterpreted based on the above citations. When they speak of a dark adapted eye, they are referring to an eye thathas not been stimulated for a period on the order of 30 minutes. Under this condition the opacity of the rhabdom isvery high and “very little or no light comes back.” This is a theoretical statement. As soon as any stimulation isapplied, the rhabdom becomes light adapted within a few milliseconds and its opacity is greatly reduced. As theopacity is reduced, applied orthodromic light is reflected back through the rhabdom and eye shine is observed. Thisis the applicable empirical statement. Whereas, the mode patterns are the same in the light and dark adapted eyes, itis only in the light adapted eye that the opacity is low enough for investigators to observe light that has passedthrough the rhabdom in both directions.

3.6.6 Summary performance of the visual modality of Insecta

Insecta vision: form, function and neural circuitry can be described in considerable detail based on the abovematerial in Section 3.6.

The figure in Section 3.6 presents the relevant block diagram for organizing the technical information knownconcerning the visual modality of Insecta and defining the major stages and neural engines employed within thatmodality of Insecta.

[xxx modify to show polarization channels ]Figure 3.6.6-1 displays a re-labeled schematic of stages 0, 1 and 2 proposed to represent the schematic diagram forInsecta adopted from a previous version developed for Chordata. The acceptance patterns shown between stage 0and stage 1 represent the acceptance pattern of the rhabdom considered as a waveguide for light. The laminarepresents the lamina/medulla of the above figures. The axons emanating from the photoreceptors are labeled RXwhere X can be UV, S, M or L to represent the spectral sensitivity resulting from the rhodonine chromophoreemployed in the rhabdomere of each sensory receptor. Similarly, the axons emanating from the lamina are labeledLX where X can be O, P, Q or R depending on whether the circuit in the lamina is a differencing circuit or asumming circuit.

Figure 3.6.4-1 Absorption spectrum of the UV channelreceptor of Papilio ADD. The experimental data (circles)are compared with visual pigment spectra (bold curves)predicted by a template (Stavenga et al., 1993). Bothspectra are limited by the transmission of the optics used toprepare them. See text. From Arikawa, et al. 1999 withadded annotation.

Page 152: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 151

270Fulton, J. (2004) Processes in Biological Vision. http://neuronresearch.net/vision/pdf/5Photochem.pdf#page=47

Details of the chemistry and quantum-molecular performance of the chromophores of animal vision are developed inSection 5.5 of the text, “Processes in Biological Vision,” available on line270. The chromophores, given the name ofRhodonines, are the active part of the conceptual material rhodopsin. The Rhodonines are chemically derived fromretinol (vitamin A) but are not chemically attached to the protein named Opsin. When in use, the Rhodonines aredeposited on the microvilli of the rhabdomere portion of the sensory neural receptors in the form of a liquid crystal. The detailed structure of the microvilli has not been located in the literature of Insecta. They may consist of a neuralportion and an Opsin portion. As shown in the figure, each sensory neuron receptor contains two electronicamplifiers, defined as Activa, in series plus associated circuit elements (not shown). The first amplifier is called theadaptation amplifier. It dominates the adaptation mechanism controlling the performance of the entire modalitybased on its level of stimulation. Since these adaptation amplifiers are independent of the amplifier in other sensoryneurons within a ommatidium, adaptation is a spectrally selective process. In the compound eye of Insecta,adaptation is not only spectrally selective but independent for each ommatidium. Thus, adaptation level variesindependently for the field of view associated with each nominal 1.5 degree field of view of the compound eye.

Figure 3.6.6-2 describes the close match between the theoretical spectra of the photoreceptors of Insecta (based onthe similar spectra of Chordata developed in this work (Section 5.5.10.1.2) and the measured spectra from Bandaiet. al. (1992). The theoretical first order peak sensitivities are 342, 437, 532 & 625 nm. for the UV–, S–, M– & L–channels respectively The precise peak in the theoretical L–channel response is still a question. First order theorysays it is at 625 nm. but this may move closer to an effective 610 nm. due to details of the molecular arrangement of

Figure 3.6.6-1 Signal Processing within stages 0, 1 & 2 of Insecta. The figure is modified from a similar schematic forChordata. For Insecta, the acceptance patterns for all sensory neuron receptors are the same and controlled by theaperture of the rhabdom, and the optical properties of the cornea and crystalline lens. The labeling of the axonsemanating from the stage 1 sensory neuron receptors and the stage 2 lamina are those developed in the text. See text.

Page 153: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

152 Processes in Biological Vision

271Fulton, J. (2004) Processes in Biological Vision, http://neuronresearch.net/vision/pdf/5Photochem.pdf#page=74

Rhodonine(5) or the 2 photon —>1 exciton mechanism introduced in Section 3.6.3. The measured responses showwhat appear to be some extraneous inputs, inadequate sampling interval to achieve sufficient precision and somepotential problems with calibration. The figure has involved multiple scaling activities by editors.. It isrecommended that any future investigators refer to the original Bandai et al. data and calculate the theoretical spectrausing the equations in Table 5.5.1 of this work271. Note the important parameters in the fundamental Helmholtz-Boltzmann Equation are the half amplitude values for the short and long wavelength skirts of each chromophore. The values n that Table were derived to match the spectra of the human eye. Future measurement activities shouldemploy narrower sample spacing in the wavelength interval of maximum interest and narrower band width filters inthe monochromators used. Such precision would provide measured half-amplitude values for the species underevaluation and for purposes of recomputing the Helmholtz-Boltzmann Equation for that species.

The microvilli of the sensory receptors form very effective grid-type polarizers that act as analyzers of thepolarization of the incident light. There may be considerable variation in the arrangement of these grid-typepolarizers. In the case of Papilio as a minimum, the polarizers associated with sensory neurons R1-R4 rotate the e-vector of the light reaching R5-R8 by 45 degrees, thereby explaining the 45 degree rotation of the latter neuronsrelative to the former. The same polarization mechanism is shared with other members of Insecta and Mollusca.

To confirm the above specifications, investigators will need to update their protocols and test equipment. The

Figure 3.6.6-2 Comparison between theoretical and measured spectral responses of Insecta based primarily on the datafor the butterfly, Papilio The comparisons stress the necessity of making measurements at intervals of 5 nm or less toachieve adequate precision. The lower left frame suggests a potential calibration problem in the protocol for measuringsuch spectra. The measured spectra far from the theoretical waveforms that are not equal to zero (shown as dashed &in color) suggest some signal pickup from adjacent sensory, or other, neurons. See text. Data from Bandai et al., 1992.

Page 154: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 153imaging equipment used to investigate the insect eye should have a spectral response of 300-700 nm (minimum of325-650 nm). Electronic imaging devices of this spectral bandwidth are commercially available but probably inblack and white (broad spectrum) only. However, by using tailored illumination to collect light from only onespectral band at a time to create a set of “separation” images and then print composite images using false color toindicate the UV channels (and potentially the polarization of individual spectral channels), useful composite imagescan be obtained. Such separation images (or separation negatives) or commonly used in the printing industry. Torecord the image light from specimens, it is necessary that all of the optics involved support the required spectrum. Quartz optics are available to support the imaging devices and light sources involved. The monochromators andrecording photocell equipment used to record spectral data must also accommodate the above spectral requirement. Such equipment is readily available, however separate monochromators and photocell equipment are frequently usedto accommodate the complete spectrum. In these cased the calibration of the equipments should all be calibrated toan accuracy of 5 nm or better.

It is proposed that the appearance of the bulk pigments by reflected light is significantly different than theappearance of monolayers (liquid crystalline) of chromophore deposited on the microvilli of the sensory neuronreceptors. These two appearances are typically complementary in the language of the printing industry and thecommunity dealing in definitions related to light. In those communities, “magenta” is quite different than “reddish.” It can be argued that many of the references to a reddish pigment in histology actually refers to a “magenta” pigmentthat includes reddish and bluish components; the actual absorption of the related chromophore peaks in the greenisharea of the visible spectrum. Use of the label “pigment” should not be used to describe the morphology orelectrophysiology of the sensory neurons unless care is taken to specify whether the appearance relates to reflectedor transmitted light. Arikawa and Stavenga encountered this problem in 1997 (page 2502, lower right) where theynote, “In transmitted light, the ommatidium appear either yellow or (more or less saturated) red.” As noted inSection 3.6.3.1, their yellow is actually a yellowish-green based on the spectral peak at 536 nm of theirphotoluminescent indicator, Lucifer-yellow. Yellowish-green has a complementary color at 536c nm. This worksupports and this paragraph provides the foundation for the last sentence in their abstract, “The differentpigmentations are presumably intimately related to the various spectral types found previously inelectrophysiological studies.”

Investigators need to carefully evaluate the Excitation/De-excitation Equation of Section 3.6.3.2 to recognize thecritical necessity to define the intensity of the stimulant in their test protocol. Defining this stimulant intensity iscomplicated by the spectral sensitivity of the rhabdom as a whole and individual groups of spectrally sensitiverhabdomere within a single rhabdom. It is also complicated by the spectrally sensitive adaptation mechanismswithin each wensory neuron associated with a specific signaling channel.

Page 155: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

154 Processes in Biological Vision

TABLE OF CONTENTS 4/30/17

3 Description of the Retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3.1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23.1.1.1 Order and tetra-chromaticity in the photoreceptor arrays of the retinas . . . . . . . 23.1.1.2 Comparison of retinas of different phyla . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1.2 A framework for discussion of the chordate retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.1.2.1 The plan view perspective of the retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1.2.1.1 A panoramic view of the photoreceptors of the retina . . . . . . . . . . . . 53.1.2.1.2 Global recording of other layers of the retina . . . . . . . . . . . . . . . . . . . 53.1.2.1.3 Local recordings of the plan view of the retina . . . . . . . . . . . . . . . . . 6

3.1.2.2 The profile view perspective of the retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.1.2.3 The signaling architecture of the retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1.2.3.1 Expansion of spatial processing within the Top Level Schematic . . . 63.1.2.3.2 Subdivision of retinal layers or interdigitation . . . . . . . . . . . . . . . . . . 73.1.2.3.3 Initial tabulation of signal processing roles within the retina . . . . . . . 8

3.1.3 Additional concerns in experiment design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.3.1 Lack of a detailed model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.3.2 Lack of consistent control of the motion of the eyes . . . . . . . . . . . . . . . . . . . . . 103.1.3.3 Lack of precise control of stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.3.4 Importance of controlling the stimuli orientation . . . . . . . . . . . . . . . . . . . . . . . 11

3.1.4 Matters specific to the organizational structure of the chordate retina . . . . . . . . . . . . . . . . 113.1.4.1 Matters of scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.4.2 Fovea versus other terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.4.3 Amercine cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1.4.4 Matters of architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1.5 Matters of photoreceptor cell classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1.5.1 Background--Rods and cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1.5.1.1 The clinical attempt of Shultze to identify rods in 1860-70 . . . . . . . 133.1.5.1.2 Attempts at redefining the retina during the 1940-70s . . . . . . . . . . . 143.1.5.1.3 Attempts at redefinition during the 1980-90s . . . . . . . . . . . . . . . . . . 173.1.5.1.4 Attempts at locating rods during 2000-11 & culminating in 2016 . . 19

3.1.5.2 “Rods” and “cones” are not functional descriptors . . . . . . . . . . . . . . . . . . . . . . 203.1.5.3 On the subject of “red rods” and “green rods” . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2 Morphology of the chordate retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.1 Anatomical Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2.1.1 The brain/blood barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2.1.1.1 Membranes separating the laminates . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2.1.2 Layers of the Retina and some statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.2.1.2.1 The neural laminate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.2.1.2.2 The photoreceptor laminate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.2.1.2.3 The RPE laminate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.2.1.2.4 Laminate dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2.1.3 Other anatomical features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2.1.3.1 The visual streak versus an elongated fovea . . . . . . . . . . . . . . . . . . 313.2.1.3.2 The optical disk (or blind spot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2.1.3.3 The macula or macula lutea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2.1.3.4 The signal paths on the neural laminate surface . . . . . . . . . . . . . . . . 333.2.1.3.5 The tapetum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.2.1.4 The optic nerve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.2.2 Gross histology of the retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.2.2.1 The sensing laminate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2.2.1.1 The RPE sub-laminate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2.2.1.2 Orientation of photoreceptors in the outer segment sub-laminate . . 383.2.2.1.3 Spatial parameters of the mosaics of the outer segment sub-laminate

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Page 156: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 1553.2.2.2 Geometrical patterns in retinal arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.2.3 Statistical parameters of the complete mosaic of the outer segment sub-laminate

RE-OUTLINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.2.2.3.1 Statistical parameters of the complete mosaic(s) . . . . . . . . . . . . . . . 453.2.2.3.3 Statistics of the foveola only . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.2.2.3.4 Major axes of the foveola mosaic . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.2.2.3.5 Limiting resolution of the foveola of the retinal mosaic . . . . . . . . . . 51

3.2.2.4 Statistical parameters of the chromatic mosaics of the outer segments . . . . . . . 533.2.2.4.1 Chromatic mosaics based on trichromatic . . . . . . . . . . . . . . . . . . . . 543.2.2.4.2 Chromatic mosaics based on tetrachromatic assumption . . . . . . . . . 553.2.2.4.3 Statistical parameters of the individual spectral channel mosaics . . 55

3.2.2.5 The neural laminate RE-OUTLINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573.2.2.5.1 Signal paths through the laminate . . . . . . . . . . . . . . . . . . . . . . . . . . 583.2.2.5.2 Sign conserving amplifiers found in the luminance channels . . . . . . 613.2.2.5.3 Amplifiers for both sign conserving and sign reversing paths . . . . . 613.2.2.5.4 Cell configurations within the laminate . . . . . . . . . . . . . . . . . . . . . . 633.2.2.5.5 Axon sizes within the Optic Fiber Layer . . . . . . . . . . . . . . . . . . . . . 633.2.2.5.4 Cell sizes within the laminate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.2.2.5.4 Hydraulic elements within the laminate . . . . . . . . . . . . . . . . . . . . . . 64

3.2.2.6 Subdivisions of the neural laminate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.2.2.6.1 Spatial parameters of the mosaics of the ganglion cells in cat . . . . . 65

3.2.3 Fine histology of the photoreceptor layer of the retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663.2.3.1 The archaic representations of Curcio et al. showing inner segment ellipsoids . 673.2.3.2 The more recent work of Roorda & Williams showing the retinal face . . . . . . 683.2.3.3 Putative arrangement of photoreceptors in the human eye . . . . . . . . . . . . . . . . 703.2.3.4 Candidate photoreceptor groupinng based on this work . . . . . . . . . . . . . . . . . . 713.2.3.5 Putative arrangement of glia cells acting as light pipes . . . . . . . . . . . . . . . . . . 733.2.3.6 The “text rewriting” work of the Roorda team with optimized AOSLO . . . . . . 74

3.2.4 Electronic architectural level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793.3 Metabolism of the chordate retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

3.3.1 Static considerations related to a cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793.3.1.1 The Neuron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793.3.1.2 The RPE cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

3.3.2 The vascular supply to the retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803.3.2.1 Blood flow to the INM and most neural laminates . . . . . . . . . . . . . . . . . . . . . . 813.3.2.2 Blood flow to the IPM, RPE and photoreceptor cells . . . . . . . . . . . . . . . . . . . . 823.3.2.3 Block Diagram of the Metabolic Components . . . . . . . . . . . . . . . . . . . . . . . . . 82

3.3.3 Dynamic considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833.3.3.1 Bulk characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

3.3.3.1.1 Studies in heat generation within the retina . . . . . . . . . . . . . . . . . . . 843.3.3.1.2 Studies in Oxygen consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

3.3.3.2 Detailed characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.3.3.3 Steady state characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.3.3.4 Transient characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3.3.3.4.1 Slow transients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863.4 Functions of the Chordate Retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3.4.1 Functional levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.4.1.1 The morphological level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.4.1.2 The physiological or signal function level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

3.4.1.2.1 The spectral signal level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893.4.1.2.2 The signal channel level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893.4.1.2.3 The signal projection level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

3.4.2 The center-surround phenomenon (temporary home) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 923.4.2.1 Types of experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 923.4.2.2 Span of stimuli versus span of neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.4.2.3 Interpretation of experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

3.5 Electrophysiology, morphology & function of the eyes of Mollusca . . . . . . . . . . . . . . . . . . . . . . . . . 953.5.1 The compound eye of Mollusca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

3.5.1.1 Multispectral mollusc retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 963.5.1.2 Details of the rhabdom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 973.5.1.2.xxx The retina of Pecten maximus . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

3.6 Electrophysiology, morphology & function of visual modality of Insecta . . . . . . . . . . . . . . . . . . . . 993.6.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

3.6.1.1 Diversity among eyes of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Page 157: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

156 Processes in Biological Vision

3.6.1.2 Simple versus compound eyes–apposition versus superposition etc. . . . . . . . 1043.6.1.2.1 Potentially more complex simple eyes . . . . . . . . . . . . . . . . . . . . . . 104

3.6.1.3 Fundamental versus complex ommatidia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043.6.1.4 Reconciling the definition of the “pigments” of Insecta vs other animals . . . . 1063.6.1.5 Color vision potential of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1073.6.1.6 A generic eye of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

3.6.1.6.1 The dimensions of the rhabdom of butterflies EMPTY . . . . . . . . . 1123.6.2 The morphology of the compound eye of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

3.6.2.1 The compound eye of the butterfly according to Arikawa & colleagues . . . . . 1123.6.2.1.1 The high frequency limitations of butterfly visual sensory neurons

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213.6.2.1.2 Is R9 an eccentric cell? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3.6.2.2 Special features of the polarization mechanism . . . . . . . . . . . . . . . . . . . . . . . . 1213.6.2.3 Detailed specific features of the optics of the ommatidia . . . . . . . . . . . . . . . . 122

3.6.2.3.1 The tapetum & secondary pigment found in many butterfly and mothspecies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

3.6.2.3.2 Index of refraction for ommatidia of blowfly and honey bee . . . . 1263.6.2.3.3 The rhabdom as a waveguide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1283.6.2.3.4 The circular waveguide modes of Heteronyrnpha merope . . . . . . 1293.6.2.3.5 The rectangular dielectric waveguide modes . . . . . . . . . . . . . . . . . 131

3.6.2.4 The superposition (scotopic) eye of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . 1323.6.2.5 Morphology of stage 4, information extraction, and stage 5 cognition . . . . . 134

3.6.3 The electrophysiology of the compound eye of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . 1353.6.3.1 The neural circuitry of stage 1 of the eye of Insecta . . . . . . . . . . . . . . . . . . . . 1373.6.3.2 The Photo excitation/De-excitation equation of the eye of Insecta . . . . . . . . . 138

3.6.3.2.1 The stage 1 P/D response to an impulse function . . . . . . . . . . . . . . 1393.6.3.2.2 The stage 1 P/D response to square pulse stimulation ADD . . . . . 1413.6.3.2.3 Inversion of the P/D response by stage 2 neurons . . . . . . . . . . . . . 1423.6.3.2.4 Generator potentials versus action potentials in Insecta . . . . . . . . . 143

3.6.3.3 Empirical signals associated with the ommatidium and stage 2 lamina . . . . . 1443.6.3.3.1 A cartridge of stage 2 of the domestic fly . . . . . . . . . . . . . . . . . . . 145

3.6.3.4 Stage 3– Signal projection in Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1483.6.4 Confirmation of the Rhodonines as chromophores of Insecta . . . . . . . . . . . . . . . . . . . . . 148

3.6.4.1 The measured spectra of the UV chromophore, Rhodonine(11) of Papilio . . . 1493.6.5 Light and dark adaptation among Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1503.6.6 Summary performance of the visual modality of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . 150

Page 158: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 157

Figure 3.2.1-1 CR Cross section through a human retina, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Figure 3.2.1-2 The fovea of monkey, Macaca, irus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 3.2.1-3 Embryogenesis of the retina showing cellular origin of the various layers . . . . . . . . . . . . . . . . . . . 26Figure 3.2.1-4 CR Electron micrograph of the photoreceptor and nuclear laminates of the bullfrog. . . . . . . . . . . 28Figure 3.2.1-5 CR A fundus photograph matched with a meridional light micrograph of the macular region . . . . 29Figure 3.2.1-6 Cross-sections of a macaque retina taken in 460 nm (blue) and 525 nm . . . . . . . . . . . . . . . . . . . . 32Figure 3.2.2-1 A caricature of the central one-third of the human fovea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Figure 3.2.2-2 Ultrahigh-resolution spectral OCT image of living human macula . . . . . . . . . . . . . . . . . . . . . . . . . 37Figure 3.2.2-3 Tangential section through inner segment layer of Macaca nemestrina near the fovea . . . . . . . . . 39Figure 3.2.2-4 The human photoreceptor mosaic in the fovea centralis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Figure 3.2.2-5 CR Micrographs of foveal “cone” inner segments at fixation point of monkey . . . . . . . . . . . . . . . 41Figure 3.2.2-6 The proposed fundamental array used in the spherical human retina . . . . . . . . . . . . . . . . . . . . . . . 42Figure 3.2.2-7 Fermat spiral patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Figure 3.2.2-8 Dubra’s analysis of the regularity of the peripheral photoreceptor mosaic. . . . . . . . . . . . . . . . . . . . 44Figure 3.2.2-9 The reinterpreted photoreceptor and neural densities of Osterberg . . . . . . . . . . . . . . . . . . . . . . . . . 47Figure 3.2.2-10 Ratio of rods, cones and total photoreceptor cells to ganglion cells in a primate retina . . . . . . . . 48Figure 3.2.2-11 CR Horizontal section through a region of a fixed human retina . . . . . . . . . . . . . . . . . . . . . . . . . . 49Figure 3.2.2-12 Change in photoreceptor spacing with eccentricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Figure 3.2.2-13 A measurement of contrast sensitivity versus spatial resolution in the human eye . . . . . . . . . . . . 52Figure 3.2.2-14 Caricature of the signal paths found in the retina of the Chordate . . . . . . . . . . . . . . . . . . . . . . . . . 60Figure 3.2.2-15 CR Capillary bed in the neural laminate behind the macular region . . . . . . . . . . . . . . . . . . . . . . . 64Figure 3.2.2-16 Caricatures of bipolar neurons within the retina of rat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Figure 3.2.3-1 CR [Color] Pseudo-color images of the human retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Figure 3.2.3-2 Potential mosaic of photoreceptors based on a trichromatic approach . . . . . . . . . . . . . . . . . . . . . . . 72Figure 3.2.3-3 Potential photoreceptor array based on a tetrachromatic human retina . . . . . . . . . . . . . . . . . . . . . . 73Figure 3.2.3-4 Eye motion trace computed from an image sequence showing vertical (y) component of eye motion

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76Figure 3.3.1-1 Operation of the typical living cell with the input and output signals added . . . . . . . . . . . . . . . . . . 79Figure 3.3.1-3 Vascular circulation within the retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81Figure 3.3.1-4 The equivalent circuit of the vascular system of the eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Figure 3.3.2-1 Perceived brightness in response to a high contrast image transition . . . . . . . . . . . . . . . . . . . . . . . 85Figure 3.4.1-1 Overall Schematic Diagram of the Processes in Animal Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Figure 3.4.1-2 Block diagram of principal signal paths of the eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Figure 3.5.1-1 Diagram of the retina of Octopus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Figure 3.5.1-2 Early prototypical photoreceptor of Mollusca. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Figure 3.5.1-3 Structure of the dual retina of Pecten maximus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99Figure 3.6.1-1 Anatomy of the head of an insect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Figure 3.6.1-2 Annotated visual modality block diagram applicable to Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . 103Figure 3.6.1-3 Schematic diagram of the anatomical organization of the fly, Diptera . . . . . . . . . . . . . . . . . . . . . 105Figure 3.6.1-4 A generic simple eye and component of the compound eye of Insecta REDRAW . . . . . . . . . . . . 108Figure 3.6.2-1 Stimulus condition and typical responses for Bandai et al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114Figure 3.6.2-2 Polarization sensitivity curves predicted from the microvillar orientation of the Papilio xuthus . . 114Figure 3.6.2-3 Correlation between spectra of Chen et al. and theoretical rhodonine spectra of this work . . . . . . 116Figure 3.6.2-4 Electron micrographs of transverse sections of Papilio retina . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Figure 3.6.2-5 The ommatidia of Graphium Sarpedon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119Figure 3.6.2-6 Spectral sensitivities of photoreceptors of type 1 ommatidium of Graphium sarpedon retina . . . 120Figure 3.6.2-7 Electron micrograph of the grid polarizers within the rhabdom of Papilio . . . . . . . . . . . . . . . . . . 122Figure 3.6.2-8 The demonstrated stage 0 optical system of the simple and compound eye of Insecta ADD . . . . 124Figure 3.6.2-9 A tapetum based on reflection from a stack of dielectric plates in Precis lavinia . . . . . . . . . . . . . 125Figure 3.6.2-10 Longitudinal sections of dioptric portions of ommatidia with indices . . . . . . . . . . . . . . . . . . . . . 128Figure 3.6.2-12 Modal patterns for rectangular dielectric waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132Figure 3.6.2-13 Superposition eye of the nocturnal dung beetle, Onitis westermanni . . . . . . . . . . . . . . . . . . . . . 134Figure 3.6.2-14 Visual modality of monarch butterfly visual modality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135Figure 3.6.3-1 Proposed morphology and circuit diagram of the sensory neurons of Insecta ADD . . . . . . . . . . . 137Figure 3.6.3-2 Theoretical impulse responses based on the P/D Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139Figure 3.6.3-3 Theoretical P/D response to a square pulse stimulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Figure 3.6.3-4 Synaptic connections in the cartridge of a worker bee based on data . . . . . . . . . . . . . . . . . . . . . . 143Figure 3.6.3-5 Diagrammatic representation of two retinula neurons and one LMC . . . . . . . . . . . . . . . . . . . . . . 144Figure 3.6.3-6 A semischematic summary drawing of the structure and synapses . . . . . . . . . . . . . . . . . . . . . . . . 147Figure 3.6.4-1 Absorption spectrum of the UV channel receptor of Papilio ADD . . . . . . . . . . . . . . . . . . . . . . . . 149Figure 3.6.6-1 Signal Processing within stages 0, 1 & 2 of Insecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151Figure 3.6.6-2 Comparison between theoretical and measured spectral responses of Insecta . . . . . . . . . . . . . . . . 152

Page 159: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

158 Processes in Biological Vision

Page 160: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 159

(Active) SUBJECT INDEX (using advanced indexing option)

2 photon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116, 136, 1523D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84, 11895% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114, 143action potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61, 91, 104, 121Activa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 59-62, 80, 86, 88, 97, 110, 111, 121, 137, 141-145, 151adaptation . . . . . . 18, 20, 21, 33, 34, 50, 55, 57, 66, 77, 82, 83, 86, 101, 117, 121, 126, 134, 137, 138, 149-151, 153adaptation amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66, 82, 137, 151afterimage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88arborization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2, 3, 6, 92, 93attention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31, 37, 54, 93, 120autocorrelation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45axoplasm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110, 137, 141, 142Bayesian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74, 145Bayesian trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145bilayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79bioelectrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84bistratified . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59, 62bleach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113bleaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 70, 86, 117, 126, 134blood-brain barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25BOLD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 149Bragg dielectric mirror . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125broadband . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 20, 66, 105, 111, 120, 148b-wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39, 144, 149, 152, 153cerebellum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89CIE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71, 78, 113, 130CIE 1976 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78coma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 36, 37, 56, 69, 74, 145compound eye . . . . . . . . . . . . . . . . . . . . . 1, 95, 100, 104-106, 108, 110-112, 114, 116, 123-125, 132-136, 148, 151computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113, 131computational . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31, 62confirmation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 33, 148consonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70continuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23, 32, 36, 38, 68, 78, 92, 97, 105, 125, 129, 134cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 22, 25, 31, 35, 38, 107, 108, 122dark adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83, 149, 150data base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 6DG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79diol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149disparity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105drusen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Duplex Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 21dynamic range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 72, 85, 88eccentric cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110, 111, 118, 121, 145electrostenolytic process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84, 137entrainment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134ERG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 63, 99evoked potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4, 13, 84, 95, 99, 101, 112, 143exothermic animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141expanded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 65, 74exposine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150external feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59, 65FD-OCT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Page 161: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

160 Processes in Biological Vision

feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28, 59, 65Fourier transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 51GABA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 137Gaussian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35, 120, 136genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57glia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 73glutamate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 65, 83, 84glycolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84half-amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120, 152Hodgkin Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Hodgkin solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139homogeneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64, 133hyperacuity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56inverting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 61, 62, 104, 110, 123, 133, 134, 143, 145lactate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61, 139lateral geniculate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 62Lepidoptera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102, 105, 106, 108, 129, 133, 145, 149light adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 126, 138Limulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110, 111, 118, 121, 141, 143liquid-crystalline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106, 135locomotion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143marker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53microvillar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114microvilli . . . . . . . . . . . . . . . . . . . . . . . . 90, 97, 107-112, 114, 117, 118, 121, 122, 125, 129, 135, 137-139, 151-153midbrain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91monopulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148morphogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 26, 42, 43MRI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87myelin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125narrow band . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113, 136navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109, 134neural coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92neurite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111neurites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 89, 100, 110, 111, 124, 134, 137, 144neuropil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111, 124, 143neurotransmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Node of Ranvier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143, 145noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 27, 28, 142, 149non-inverting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61, 62, 104, 110, 123, 133, 134, 143Nyquist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42, 51-53, 55OCT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 26, 36, 37, 74, 79P/D equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121, 139, 142, 145parametric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85parvocellular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 35, 59, 61, 63, 78, 89, 91, 92perceptual space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78phylogenic tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1plasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42poda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60, 62podites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111poditic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61, 62, 65, 92, 93, 134, 143-145point of regard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31POS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Pretectum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44, 51, 59, 61, 62, 89, 91protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 57, 77, 114, 119, 152, 153pulvinar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75Pulvinar pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75quantum-mechanical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106, 117, 138resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67, 87reticulated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Page 162: ELECTROCHEMISTRY OF THE NEURONneuronresearch.net/vision/pdf/3description.pdfThe Retina 3- 3 12Kaneko, A. (1979) Physiology of the retina.Ann. Rev. Neurosci., vol. 2, pp. 169-191 Kaneko

The Retina 3- 161retinine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1, 149retinitis pigmentosa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25rhabdom . . . . . . . . . . . 29, 96, 97, 99, 101, 104, 105, 107-110, 112, 117-125, 127-134, 137, 138, 145, 150, 151, 153rhabdome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104, 108, 112, 126, 129, 131rhabdomere . . . . . . . . . . . . . . . . . . . 96-98, 104, 105, 107-110, 114, 117, 118, 120, 122, 126, 137, 139, 150, 151, 153rod intrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13simple neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79stage 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107, 110, 119, 123, 124, 129, 150stage 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75, 88, 102, 107, 110, 111, 121, 124, 137, 139, 141, 143, 147, 150, 151stage 1A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88stage 1B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88stage 2 . . . . . . . . . . . . . . . . . 57, 72, 73, 75, 77, 88, 102, 103, 105, 107, 110, 111, 118, 121, 124, 142-145, 147, 151stage 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89, 100, 102, 104, 118, 143, 144stage 3A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89, 121, 143stage 3B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89stage 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102, 103, 111, 134, 145stage 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75, 103, 134stage 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134stereopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Stiles-Crawford . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78stratified . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107, 108, 134stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 55, 152synapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 17, 58, 59, 75, 77, 79, 110, 111, 143, 145, 146, 148threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 12, 14, 50, 60, 61, 77, 78, 136tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 36, 74topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-13, 29, 46, 56, 67, 74, 78, 79, 87topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 13, 45, 79, 88, 111, 137transduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1, 63, 80, 88, 106, 121, 138, 148translation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42, 63, 88trans- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54tremor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 10, 12, 13, 51-53, 56, 68, 69, 74-78, 97type 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 120type 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8type I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 116, 119, 120type II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 119, 136type III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119Verhoeff’s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 36, 37, 80, 82visual cortex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 66vitamin A1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116, 120vitamin A3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112, 120, 149Voroni diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44waveguide . . . . . . . . . . . . . . . . . . . . . . . . . 18, 53, 55, 78, 107, 108, 110, 118, 122-124, 128-131, 133, 145, 148, 150white matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Wikipedia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44, 102xxx . . . . . . . . . . . . . . . . . . 1, 2, 4, 24, 32, 36, 38, 46, 57, 67, 73, 78, 98, 104, 112, 120, 122, 124, 126, 133, 137, 148[xxx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76, 78, 97, 105, 106, 118, 122, 124, 129, 131, 133, 138, 145, 148, 150