LETTER TO PAUL(Sanitized)

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• Collection: NGA Records (Formerly NIMA)
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• Document Creation Date: December 28, 2016
• Document Release Date: October 27, 2004
• Sequence Number: 2
• Publication Date: September 24, 1965
• Content Type: LETTER

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September 24, 1965
The contract just executed for literature search on vision and perception
stipulates that we shall report at once any work that appears to be of special
significance. I am taking advantage of this clause of the contract to report
briefly some recent important findings about the mechanism of wavelength
discrimination, which I learned of while doing pre-proposal background study,
and to submit for your consideration some tentative views on methods of col-
lecting, reporting, and evaluating data under the program.
In the early nineteenth century it was theorized that the retina of the ver-
tebrate eye probably contains three different light-sensitive substances,
eact} with a peak sensitivity in a different region of the spectrum, and that
the information from the excitation of each substance is separately trans-
mitted to the brain and combined there to reproduce the colors of the outside
world. This theory was supported by rather strong inferential evidence,
notably color-matching experiments which showed that people with normal
vision can match any given spectral color by combining three primary colors
in different parts of the spectrum. However, other psychological studies
seemed to support rival theories, and the matter remained doubtful until
just last year.
The tri-receptor hypothesis of wavelength discrimination has been confirmed
by physiological experiments on goldfish, carried out at Johns Hopkins
University in 1963 and reported in 1964. The article by MacNichol which de-
scribes this work is brief enough for office reproduction, so I am enclosing
a copy which I think you should keep as a primary reference in your library.
(In looking over this article, and indeed throughout the program, you will
S notice that physiological research on vision harps strangely upon goldfish.
The reason for this is that only monkeys, some of the bony fishes, and man
possess a fovea, the tiny central area of the retina in which cone receptors
are densely packed and which mediates high-acuity vision.)
The author's rather sobersided style may obscure the fact that his work is
as basically significant in vision research as, say, the wave theory of light.
In the course of four or five years, this work will find its way into text-
books and other general references, and its scientific importance will be
suitably emphasized. For us, however, the question immediately arises:
what is the bearing of such information, however scientifically significant,
on the subjective experience of the photo interpreter or other worker doing
a visual task? I am afraid there is no categorical answer to this question.
In the work statement of the proposal I listed among the subjects to be
investigated "mechanisms of wavelength discrimination and subjective pheno-
mena of color perception." For the evaluation of data on these subjects, it
is important to realize that wavelength discrimination in the retina does not
result directly in color perception in the mind, and that the relations be-
tween the two events are indirect, tenuous, and still largely unexplained.
Some facts about physiological wavelength discrimination lend themselves to
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direct interpretation in terms of subjective experience. Other facts are
part of a complex nervous organization for transmitting information to the
brain, and only confusion can ensue from attempts to force psychological
interpretations on them. For example, from the efficiency curve of wave-
length discrimination (which peaks in the green) it does not follow that
the photo interpreter "sees" green_bettgr than he "sees" red his perception
being conditioned by the interactions of adaptation, light intensity and
quantum phenomena, spatial and emotional properties of color, and constancy
effects. Subjective color. perception is fraught with such complications,
which make it a highly ambiguous, though interesting, subject for study.
We could easily spend on color perception the entire time authorized for
the program -- in fact it would be tempting to do so -- but in terms of the
uses of color in working situations, we might not be much wiser than before.
I suggest, therefore, that the information on wavelength discrimination and
color perception which we accumulate in the present program should be re-
garded, not as material for direct application, but rather as background
knowledge, which needs to be better integrated before it can be successfully
exploited. Unless I receive different instructions from you, I intend to
collect and report the basic facts about color perception, bringing in
physiological information where it seems important in relation to subjective
experience. When you review these basic facts you can tell me which of them
seem worthy of more intensive study. For example, I do not intend to make
a close study of defects in color perception, unless expressly instructed
to do so, on the assumption that color defects are not particularly relevant
to your working problems. I do intend to study and report the experiments
of Land in two-color projection, and any commentaries and criticisms of
these experiments which I find in open literature, on the assumption that
you are actively interested in the possible uses of color projection and
the creation of artificial color worlds. However, I do not suggest that
you should put this material to practical use -- even in an experimental
sense -- without a firm grasp of the physical and psychological principles
that are embodied in Land's results.
The reports which I plan to submit will not be chronological descriptions
of work performed, but rather topical essays, each dealing with a particular
aspect of vision or perception. I should like first to submit a brief re-
port on visual acuity and the nature of visual thresholds; I believe that
fundamental misunderstanding of these concepts is at the root of many theor-
etical and practical problems of photo interpretation. This first report
will give you a chance to review my general method of approach and supply
any redirection that you may feel is necessary. The report.on color percep-
tion will be much more complex and difficult, and will have to await accumu-
lation and study of material as well as refinement of objectives.
I look forward to discussing the above ideas with you when you come to
visit us, and to showing you some of my preliminary material.
ST
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RETINAL MECHANISMS OF COLOR VISION'
E. F. MACNICHOL, Jr. .3
' ?
-Thomas C. Jenkins Department of Biophysics, The Johns Hopkins University, Baltimore, Maryland
Abstract-Over a century and a half ago YOUNG (1802) theorized that color discrimination is
mediated by three photoreceptor substances, each maximally sensitive to a different region of
the spectrum, and that the activity of each of these substances is somehow communicated in a
unique fashion to the brain. In the last few years the techniques of electrophysiology and
retinal spectrophotometry have made possible experiments which justify Young's conclusions.
In particular, the technique of single-cell microspectrophotometry has revealed that in the
goldfish, an animal knows( from behavioral experiments to have a well-developed ability to
discriminate colors, there are tliree groups of retinal cones, each containing a single pigment
maximally sensitive in a particular region of the spectrum. Although electrophysiology has not
yet yielded much definite information in. regard to photopigments, since it has not been possible
to record from single cones, it has provided much information on the function of the neural layers
of the retina. The most distal responses recorded with microelectrodes, the S-potentials, are
slow, graded potentials in response to, illumination. One kind, the C-response, has opposite
polarity indifferent regions of the spectrum. The retinal ganglion cells which initiate impulses
in the optic nerve fibers are acted upon by paired, excitatory and inhibitory influences from
groups of receptors maximally sensitive in different regions of the spectrum. Thus, whereas the
receptors have been demonstrated to behave in a manner consistent with the trichromatic
theory, the neural responses are encoded in a fashion consistent with Hering's opponent color
hypothesis.
INTRODUCTION
IN MODERN Physics, when new theoretical predictions are made, direct and satisfying results
that give unequivocal and clear-cut answers are frequently obtained in a year or two; often
through Herculean efforts requiring mile-long vacuum tunnels, thousands of tons of magnet
iron: and shielding, and expenditures of gigawatt hours. of electrical energy and many
millions of dollars. Biological science is not usually as fortunate in obtaining such prompt
results: the theoretical question which` forms the basis of this discussion was posed by
Thomas Young in 1802. The direct and unequivocal answers have just started coming in,
over a century and a half later. As in Physics, the. results have been made possible by
advanced physical instrumentation which has been available for only a very short time.
This equipment, though sophisticated, is fortunately not of the monumental size and
complexity required by the physicists.
Young stated that it was unlikely that there was a special kind of receptor (vibrating
particle) for each small wavelength region in the spectrum and that the then known facts.of
color-mixing suggested that there was only a small number of receptor types involved,
probably only three. He further hypothesized that these receptors each communicate their
activity to the brain in some unique manner. Psychophysical experiments during the
following century and a half have amply demonstrated the trichromacy of vision; for
example, it has been convincingly shown, that the. entire span of spectral colors .can be,
exactly matched by mixtures of lights of any three primary wavelength Jands provided
t This paper is ' based on material presented orally to the' 17th International Congress of Psychology,
Washington, D.C.,'21 August 1963.
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?~ all colors can be matched by mixtures of three primary colors in various proportions. Abnor-
light in various proportions. In spite of these complications the basic facts are indisputable:
when subjects are presented with an image that consists only of mixtures of red and white
LAND (1959) show that the colors of most-objects in a photograph can be correctly named
object was a complicated photographic scene. In fact, the startling demonstrations by
theory and its derivatives. In addition, it has been found that a wide variety of color
sensations could be elicited with less than three.primary colors, particularly when the test
.cddish green or a yellowish blue such considerations led to:the Hering opponent color
only that they are chosen from appropriate regions of the spectrum. There are of course
some complications: to match some regions of the spectrum the energy in one of the primary
wavelength bands must have a negative value; that is, it must be added to the light being
matched rather than to the m atchine light. There is also the problem of the uniqueness of
1 ~ the sensation of yellow, and that while there are bluish greens.there is no such thing asp
this information subsequently is decoded in the brain. .
are discriminated and encoded in the retina for transmission along the optic nerve; and how
answer in a direct and unequivocal manner are how the three primary wavelength regions
the primary colors. The questions that psychophysical experiments have not been able to
contain a single substance on which only a selected region of the spectrum is allowed to fall,
sensitive substance with maximum sensitivity in a different region of the spectrum, or would
different ways by the three primary colors. This could be brought about in a number of,
ways; possibly there are three different kinds of receptors. Each would contain a photo-
simple and direct way.. Young hypothesized that the retina, must be affected in three
Thanks to modern physical apparatus, these questions are beginning to be answered in a
due to the interposition of some sort of an optical filter. Alternatively, there might be a
in the colored oil droplets of the cone, ellipsoids of birds, or they could be physical filters
the different colored lights it received. The filters could be pigments similar to those found
kind of receptor would in some unknown way produce nervous effects that were different for
that is somehow affected differently by lights from three wavelength regions. This single
single kind of receptor that contains three different kinds of pigment, or a single pigment
are in fact three or even more kinds of cone, or only one.
Obviously, some direct, objective experiment is needed to determine whether or not there
was so constructed. However, the three-receptor hypothesis is at least equally plausible.
circuit. The theory is entirely consistent with the facts of color-mixing and of color defect. It
spectrum at about 20 c/s. The color information is recovered by a neuronal phase detection
proposed a single receptor theory in which a physical filter is scanned through the visible
observed by EN?cH (1963) in receptor outer segments. For example BIERNSON (1963) has
.. such as multilayer interference filters or filters based in the "waveguide" propagation modes
Cone pigments are difficult to extract and have not been separated chemically from one
pigments would presumably differ only -slightly in the structure of the protein part of the
another. This fact favors hypotheses involving a single receptor pigment. However, the
usual biochemical methods such as precipitation, chromatography or electrophoresis has
molecule and, furthermore, the pigments are not directly soluble. Thus, separation by the
rod figment rhodonsin. M* a mixture of the two (WALD et 1955). 'thus. new methods
j technique has not yielded a clear indication of different kinds of cone pigments in higher
vertebrates although it was successful in distinguishing the cone pigment iodonsin from the
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receptor, or whether there is a . single pigment with several kinds of filters in front of it.
not tell us whether the pigments occur in different receptors or are mixed in a single type of
reflection densitometry on areas of the retina containing large numbers of receptors does
One of these was maximally photosensitive in the yellow (chlorolabe) and the other in the red
(erythrolabe). He measured the kinetics of bleaching and regeneration, finding that they
agreed well with those of iodopsin in solution. These pigments also agreed well (provided
certain assumptions were made) with the receptor sensitivity functions determined by
STILES (1959) who used psychophysical method involving light-adaptation at various
wavelengths (RUSHTON, personal communication, 1963; MARRIOTT, 1962). Unfortunately
RUSHTON (1958) reported finding two photolabile pigments in the rod-free area of the fovea.
Retinal Mechanisms of Color Vision 121
were required for distinguishing them in situ. The first successful method was that of
CAMPBET-L and RUSHTON (1955), who constructed a device for measuring the intensity of
light reflected from the living human fundus after having passed twice through the retina.
entering from behind through the sclera. Unless the filter completely surrounded the outer
color matches could be made between light entering normally through. the pupil and light
BRINDLEY and RUSHTON (1959 ruled out the filter hypothesis by showing that accurate
segments of the receptors such a match would be impossible.
microspectrophotometric measurements on the outer segment of frog rods. However, their
instruments apparently do not have sufficient sensitivity to detect photopigments in the
much smaller outer segments of cones. WOLKEN (1962, 1963) was able to obtain a composite
measurement of the spectral response of frog cones. He used a separate cone to measure
absorption at each wavelength since the light he needed for his measurements was so strong
that it bleached the pigments very rapidly. This technique, because it used a population of
receptors to obtain a single curve, cannot, of course, distinguish between different kinds of
cones in the same retina.
An instrument sensitive enough to measure spectral absorption curves of small cones
without irreparable distortion due to bleaching has been constructed by MARKS and
MACNICHOL (1962, 1963) and MARKS (1963). This work will be described more fully in a
forthcoming publication.
Figure 1 shows a cone of the stage of our microspectrophotometer. The arrows indicate
two spots of light, the measuring beam and a reference beam.
In the absence of any photopigment the energy received by the light-measuring system in
our instrument from the two illuminated spots is not equal throughout the spectrum. This
is due to the effects of stable pigments and wavelength dependent scattering by particulate
matter in the receptors; chromatic aberration and diffraction effects in the optical system.
-own more recent work. BROWN (1961) and LIEBMAN (1962) have subsequently published
Clearly, the simplest way conceptually to distinguish between the single receptor and
multi-receptor hypotheses is to measure the absorption or action spectra of photopigments
in situ in single receptors. Absorption measurements are difficult since cone outer segments
have a volume of only a fewp3?and their absorption turns out to be less than, at most, 10 per
cent of the incident light. Some preliminary work on. carp cones was reported by HANAOKA
and FUJIMOTO (1957) who indicated that there are at least five different kinds of cones having
absorption maxima at abouf `495, 530, 570, 630 and 680 mp. For some reason these authors
have not published any more work in this field and what they did publish conflicts with our
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 vertical scale divisions indicate percentage absorption. The spectrum was scanned from long to short wavelengths in a and
b, from short to long wave- lengths in c. The differences between the first and last curves in the records are maximal at 455,
545 and 610 mu. Correction for progressive bleaching moves the maxima to about 450, 530 and 625 mu.
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780 700 650 600 550 500 450 400 350
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true absorption spectrum of the bleachable pigment. In our measurements sufficient light is
used to bleach about half the photopigment during a single scan through the spectrum.
Subtracting absorption atcorresponding wavelengths between a pair of absorption spectra
made on a single receptor gives an uncorrected bleaching difference spectrum for the photo-
pigment. The difference spectrum must then be corrected for the loss of pigment molecules
during a single measurement.
Figure 2 shows the raw data obtained for three different cones from goldfish retinae. It
is evident that they indicate maximum'absorption by different receptors in three distinct
regions of the spectrum. The maxima of these curves are displaced from their true wave-
length because there is progressively less pigment. available as the measurement continues..
Each ab sorption value can be corrected by dividing it by an estimate of the fraction of
pigment remaining at that time. Marks has worked out a correction procedure which
consists of first obtaining a difference spectrum and estimating the amount of pigment
remaining as each wavelength was scanned. The curves are also smoothed by averaging a
number of adjacent data points and plotting the average ~s the center point. A computer
FIG. 3. Histograms from 113 experiments on single cones such as those shown in Fig. 2,
showing the number ofoccurrences ofcones having maxima in their corrected difference spectra
at a given wavelength plotted as a function of wavelength. The entries at 565 and 580 mu are the
peak wavelengths of composite spectra of overlapping twin cones. Those at 470,475 and 485 MU
are not explained. Otherwise the receptors can be seen to fall into three distinct groupings.
(From MARKS, 1963.)
program for accomplishing this smoothing of curves (and estimating the number of photons
absorbed by the visual pigment and its photoproducts) was worked out by Marks in
collaboration with Dr. W. E. Love. The spectrophotometric data is automatically digitized
and punched on paper tape as it is being recorded. The computer then makes the necessary
calculations and plots the results by' means of an electric typewriter. Figure 3 is a histogram
of the corrected absorption maxima from 113 experiments, each on an individual cone. They
were not selected in any way. It is evident that the population falls into three groups. The
maxima of the three distributions are at 455,, 530 and 625 mp.
: The two measurements that gave spectral maxima at 565 and..580 mu are composite
spectra of twin cones and thus do not represent a single pigment.
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To minimize these effects bleaching difference spectra are obtained. These are classically
determined by first obtaining an absorption spectrum, then bleaching the photosensitive
pigment with white light, obtaining a second absorption spectrum, and subtracting the
absorptions at corresponding wavelengths. Thus, provided colored photoproducts are not
produced (often they are and must be corrected for) the difference spectrum should give the
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red region. (From MARKS, 1963.)
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FIG. 4. Corrected transmission difference spectra of 28 experiments selected only for signal-to-
noise ratio and normalized to the same height. Curve f (arrows); the composite spectrum of a
pair of touching twin cones shows the greatest departure from the center of a group. The.
horizontal dotted line represents zero difference in bleaching. A number of the spectra that peak
in the blue become negative at the center of the spectrum and then go through submaxima in the
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Some idea of scatter in the data can be obtained. from Fig. 4, which is a plot of the
computed spectra from twenty-eight experiments which gave the greatest maximum absorp-
tion but which were: not selected in any other way. It is evident that they fall into three close
groups, except for ('f) which is the composite spectrum of a pair of twin cones. Figure 5
shows the average: of the curves in each of the three groups and the standard deviations of
the data points. The large spots are points derived from the DARTNALL (1953) nomogram
for three hypothetical pigments having the maxima of the three averaged curves, 455, 530
and 625 m,u. The Dartnall nomogram is based on rhodopsin. Porphyropsin, iodopsin and
other visual.pigments that have been extracted are consistent with it. The relative density
function of these pigments is invariant if plotted on a wave number rather than a wavelength
scale. It can be seen that the green absorbing pigment is a typical Dartnall pigment, whereas
the blue pigment has a somewhat broader and the red a narrower absorption.
An explanation for the narrower absorption of the red-sensitive pigment has been
suggested by Lewis and some of its implications confirmed by Brindley and by Stiles (for
discussion, see BRINDLEY, 1960).
Although many details need to be explained, a direct and unequivocal conclusion can be
drawn from our experiments: for"at least one species known behaviorally to be capable of
color discrimination (MCCLEARY and BERNSTEIN, 1959). Thomas Young's prediction that
there are three kinds of receptors has been verified. In addition, Marks has found that the
pigment density in cones is about 106 molecules/p3, which is nearly the same as in rods. It
is also present throughout the outer segments and the chromophores are aligned perpendi-
cular to the axis of the receptors as in rods (SCHMIDT, 1938; LIEBMAN, 1962). Thus previous
failures to extract large quantities of cone pigments can no longer be explained on the basis
suggested by RuSHTON (1962), that each cone contains a minute granule of pigment that is
used efficiently because of a focusing effect of the ellipsoid and outer segment. Marks's
measurements were made with the receptors on their sides, light entering transversely to the '
axis. Furthermore, it was possible to make measurements on several portions of the same
outer segment with identical results. Thus filtering and focusing, while they may play some
part, as for example in accounting for the Stiles-Crawford effect, are not essential for color
discrimination in the goldfish.
As yet, we have no significant data on human or other primate receptors. Primate foveal
cones are smaller and more difficult to measure, but we have high hopes of ultimate success.
i THE MICROELECTRODE TECHNIQUE
The other type of physical measurement which is yielding direct, though not always as
', unambiguous information, is the microelectrode technique for recording the electrical
activity of single neurons. This method has been in use much longer than single cell spectro-.
photometry and was first described independently by GRANIT and SVAETICHIN (1939) and
by WILSKA (1940).' These workers developed their microelectrodes specifically for recording
from retinal ganglion cells only a short time after HARTLINE (1938) had accomplished the
same result by the more difficult dissected fiber technique. Since its development the
microelectrode has been used successfully to trace the nerve message through many parts of
the visual pathway as well as in other parts of the nervous system. In this paper only its
contribution to our knowledge of the mechanism of color discrimination will be discussed.
Electrical responses have been demonstrated in some invertebrate photoreceptor cells..
(Especially convincing are the experiments of HAOINS et al. (1962) who clearly demonstrated
local potential generated only in the illuminated, region of squid photoreceptor cells.
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f the
sorp-
close
sure 5
ms of
gram
i, 530
n and
-nsity
ngth
Iereas
been
s (for
,an bi
.ble of
n that
,at the
ds. It
endi-
Wious
basis
.hat is
,arks's
to the
same
r some
color
The results with vertebrate photoreceptors are not nearly so convincing. It used to be
assumed that a major portion of the electroretinogram was generated in the receptor cells.
Recent work with microelectrodes (ToMITA, 1963) has indicated that most, if not all, the
ERG is generated in other structures. Even Brown, who for a long time has been perhaps the
strongest proponent of the idea that the a-wave or PIII process is a receptor potential, has
been forced by his own experimental evidence to conclude that the source of this potential
is no more distal than the receptor synaptic endings (BROWN and WATANABE, 1962). All
that can be said at the present time about the mechanism of excitation and conduction in
vertebrate photoreceptors is that very little is known about it and that it presents a real
challenge to investigators. A few leads have been suggested by WALD et al. (1963), by
MCCONNELL and SCARPELLI (1963) and by JAHN (1963).
'ays as
ctrical
,)ectro-
9) and
ording
led the
;nt the
arts of
)nly its
-used.
r cells.
strated
response showing negative (downward) deflection only. Lower trace: Chromatic (C) response
with both negative and positive components. (From SVAETICHIN and MACNICHOL, 1958.)
The most peripheral signs of color-related electrical activity that have thus far been
described in the vertebrates are the S-potentials which were found by SVAETICHIN (1956) in
fish retinae. Initially these were thought to be receptor potentials, a conclusion questioned by
TOMITA (1957). Further work (MACNICHOL and SVAETICHIN, 1958) showed clearly that
the potentials arise more proximally. The S-potentials are of two types as shown in Fig. 6.
The most distal, known as the luminosity or L-response, is a negative hyperpolarization of
the structure responsible, and has a broad spectral maximum. Histological electrode
marking techniques show that it arises from giant horizontal glial cells which are abundantly
present in the fish retina. Similar potentials have been obtained in the cat (GROSSER, 1957)
and the frog (TOMITA et a!., 1961), which presumably have homologous, although much
smaller, cells in their retinae. The color-related S-potential is called the C- or chromatic
response., his potential is in the hyperpolarizing (negative) direction in part of the spectrum
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E. F. MAcNICHOL, Jr.
and depolarizing (positive) in other parts of the spectrum. Stimulation at a particular
wavelength or with white light gives no sustained response, but only brief transients at the
beginning and end of the stimulus period. The C-responses appear to be signs of a definite
color discrimination mechanism which is reminiscent of the Hering opponent color hypo-
thesis. In at least one fish, the mullet (Mugil) there are two kinds of C-response, which are
shown in Fig. 7: a red-green opponent pair and a blue-yellow pair. There are also more
FIG. 7. S-potentials from the Mullet (Mugil) showing luminosity response and two kinds of
chromatic responses. (From MACNICHOL and SVAETICHIN, 1958.)
complicated triphasic responses found in some species..of: fish (SvAETICHIN et al., 1961 ;
TOMITA, 1963; MOTOKAWA et al., 1961). The C-responses were found to arise from more
proximal structures than those giving rise to the L-responses and to occur in much more
highly localized regions (MACNICHOL and SVAETICHIN, 1958). SVAETICHIN et al. (1961)
have shown by histological electrode marking methods that they arise in Muller fibers which
are glial elements surrounding the bipolar cells.
It is clear that the S-potentials are quite different in nature from the kind of electrical
activity usually ascribed to nerve cells. They are steady potentials which are sustained during
strong illumination and which adapt but little to a prolonged stimulus. SVAETICHIN et al.
(1961) have stated the belief that they are signs of metabolic changes in glial cells induced by
the activity of adjacent neurons. The altered metabolism of the glial cells would, in turn,
alter the activity of the neurons. These observations give promise of opening up a whole new
field in Neurophysiology: the study of neuron-glial interaction. Traditionally, the glia have
been considered to play only a passive role as supporting structures in the nervous system.
However, the electron microscopists have shown that there is very little extracellular space
in the retina or in any part of the central nervous system. Ions and metabolites must pass
through glial cells to get in and out of neurons. These cells when stimulated by neuronal
activity are therefore in a position to exert regulatory and control functions on the activities
of the nerve cells. The exquisitely sensitive microchemical studies by HYOEN and PIGON
(1960) have indicated that there are changes in DNA and other constituents of the glia which
correlate with nervous activity. At present we can only say that we know very little about
their significance, and that this is a very challenging area for further experimentation.
If the S-potentials have any significance in color vision they should bear some relation
to the pattern of discharge in the fibers of the optic nerve which appears to he the only way
in which the information collected by the retina is communicated to the brain. The first
indication that there was such a relation was provided by DE VALOIS et al. (1958), who
showed that many of the neurons in the lateral geniculate body of the monkey responded
only at the beginning of a prolonged stimulus ("on"-response) at one extremity of the
spectrum and only after the end of the stimulus ("off"-response) at the other extremity of
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the spectrum. At intermediate wavelengths there was an "on-off" response with the propor-
tion of "on"- to "off"-impulses changing with wavelength. GRANIT (1948) had previously
described this change in "on-off ratio" in the retinal ganglion cells of the cat, but presumably
due to his preoccupation with the Dominator-Modulator theory did not assign it an impor-
tant role as a mechanism of color discrimination. From the known facts of electrical
excitation of neurons one would expect that at wavelengths where the C-responses are
depolarizing there would be an "on"-response in the retinal ganglion cells. Where they are
articular
is at the
definite
)r hypo-
hich are
so more
hyperpolarizing one would expect inhibition during illumination followed by an "off"-
response. Unfortunately, no one has yet recorded C-responses from the monkey retina.
However, they are obtainable from the goldfish and other Cyprinids. Accordingly, WAGNER
et al. (1960, 1963), MACNICHOL et at. (1961) and WOLBARSHT et al. (1961) undertook a
study of the effects of wavelength on the responses of retinal ganglion cells in the goldfish.
As shown in. Fig. 8, we found responses similar to those recorded by de Valois. In fact, it
v!., 1961 ;
,om more
,ch more
n!. (1961)
rs which
electrical
ed during
tiIN et al.
duced by
,in turn,
hole new
glia have
is sy.
lar space
must pass
neuronal
activities
nd PIGON
glia which
ttle about
ation.
e relation
only way
The first
958), who
responded
pity of the
tremity of
600
FIG. 8. Responses of a single ganglion cell of the goldfish at various wavelengths (shown at
the right of each trace in m?). Spikes occurring before the onset of a stimulus are off-responses
from a previous stimulus. A repetitive 0.5 sec flash was applied at 2-sec intervals to keep the
retina light adapted. The records shown are taken from a series of several hundred nearly
identical responses. (From WAGNER el a!., 1960.)
was possible to obtain a shift in some units from a pure "on"- to a pure "off"-response
with a wavelength change of less than 10 m,u, as shown in Fig. 9. This appears to indicate
the existence of a very precise wavelength discriminating mechanism.
Figure 10 is a threshold vs. log intensity plot of a typical "green-on"-"red-oft" ganglion
cell. These curves bear little resemblance to the absorption spectra found by Marks. There
is no reason why they should, since they represent the interaction of at least two populations
of receptor cells at the ganglion cell level. DE VALOIS and JONES (1961) have indicated how
the simple responses of the receptors can be greatly distorted by such interaction.
Units have been found that gave "on"-responses in the long wavelength region of the \1,
spectrum and "off"-responses in the short wavelength region of the spectrum. Others gave
the opposite pattern of response. The "off"-response was associated with inhibition of
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 Approved For Release 2004/11/30 : CIA-RDP78BO477OA000900040002-8
activity during illumination regardless of whether it was spontaneous, the result of back-
f ground illumination, or a continuation of the "on" burst elicited by the onset of a stimulus.
Thus we have tended to regard the "off"-response as a post-inhibitory rebound pheno-
menon which may serve to accentuate the termination of an inhibitory stimulus. Whatever
i its importance, it is clear that, in the goldfish at least, the information with regard to wave-
length is carried up the optic nerve in the form of discharges of the axons of a population of
ganglion cells which are acted upon by groups of receptors having sensitivities in different
parts of the spectrum. These groups of receptors, presumably acting through the bipolar
560
570
Fio. 9. Change from pure "on"-response to pure "off "-response with a wavelength shift of
only 10 mp. Same ganglion cell as that whose responses are shown in Fig. 8. (From WAGNER
et a!., 1960.)
cells, exert either excitatory or inhibitory effects on the ganglion cells. For example, a given
ganglion cell may be excited mainly by a group of red-sensitive receptors and inhibited
mainly by a group of green-sensitive receptors. Other cells may be oppositely affected.
Some ganglion cells show no wavelength discriminatory responses at all: they have about
equal on- and ofd thresholds throughout the spectrum. It would be expected that ? such
cells receive equal amounts of both excitatory and inhibitory connections from each type of
receptor. A few cells respond by a maintained discharge during steady illumination. These
presumably have only excitatory connections. Others discharge in the dark and responses
are inhibited throughout prolonged illumination. These presumably have only inhibitory
sypapses.
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back-
iulus.
heno-
atever
wave-
ion of
Xemnt
Bipolar
Thus we have at the level of the S-potential, and later on at the level of the optic nerve
fiber discharge, elements that behave in a way consistent with Hering's hypothetical red-
green, blue-yellow, and black-white processes. Yet these elements are found in retinae of
animals which have been shown to have three kinds of cones maximally sensitive in three
spectral regions in accordance with Young's original predictions. Thus, as the originators
of the various stage or zone theories have predicted, a retina may be consistent with the
Young theory of color vision at the receptor cell level and with the Hering theory at the
level of the optic nerve fibers (JUDD, 1951; SVAITICHIN and MACNICHOL, 1958;
BRINDLEY, 1960; MARRIOTT, 1962.)
OFF
l e e o 9 0
400 500, 600 ?00
WAVELENGTH IN MILLIPAICR0N5
Fio. 10. Thresholds of on- and off-responses of one impulse as a function of the logarithm of
the energy of an 0.5 sec stimulus at various wavelengths. Log intensity of zero represents 0.2,u
W/cm2. (From WOLBARSHT et a!., 1961.)
a given
-hibited
cted.
ie about
at such
type of
These
sponses
hibitory
At present there is some question as to whether the same mechanisms operate in the
primates, since we have not yet recorded either bleaching or action spectra from single cones
in any of these animals. However, the three cone pigments found by RUSHTON (1961), the
three receptor processes uncovered by the increment threshold method of STILES (1959) and
by the artificial monochromacy method of BRINDLEY (1960) furnish a strong presumption
that similarities will be found. Furthermore, DE VALOIs et al. (1962), by partial bleaching
experiments, have derived response functions from the lateral geniculate cells of the monkey
that probably represent rather closely the action spectra of the chlorolabe and erythrolabe
pigments in these animals. In addition, HUBEL and WIESEL (1960) have shown by recording
from m.icroelectrodes inserted into the monkey optic nerve that the pattern of responses
recorded by de Valois in the lateral geniculate body is already encoded in the same form in
the retina. Thus, the retinal mechanisms in the fish, monkey and man are not likely to differ
very greatly.
It may be that Granit's modulators, which are responses to very narrow regions of the
spectrum recorded by him in the retinae of some animals, represent a later step in the
processing of retinal data than the wavelength sensitive "on-off" responses. Similar
responses were recorded by DE VALOIS et al. (1958) from certain layers of the monkey's
lateral geniculate body. They have not been recorded from the retinal ganglion cells of
either monkeys or fishes.
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In. summary, it appears that the broad outlines of the retinal mechanism of color vision
have now, been determined by conceptually simple and direct, though sometimes difficult,
experiments. The results conform to some rather,ancient predictions which were supported,
though not conclusively proven, by a wealth of psychophysical data.
For the sake of brevity many important details have been left out of this paper. Many
more experiments need to be done before our knowledge is complete, but current progress
gives promise that we will soon have a thorough knowledge of the mechanism responsible
for that most interesting and useful phenomenon, color vision.
Acknowledgements-The work of the author and his colleagues described herein was supported by grants
from N.S.F. and N.I.H. and by the Naval Medical Research Institute.
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