Eye, Brain, and Vision

pear: the blue melts away, and all you see is the red background. Stabilizing
the borders on the retina apparently renders them ineffective, and without
them, we have no color.
These psychophysical demonstrations that differences in the spectral content
of light across the visual field are necessary to perceive color suggest that in our
retinas or brains we should find cells sensitive to such borders. The argument
is similar to the one we made in Chapter 4, about the perception of black or
white objects (such as kidney beans). If at some stage in our visual path color is
signaled entirely at color-contrast borders, cells whose receptive fields are en-
tirely within areas of uniform color will be idle. The result is economy in
handling the information. We thus find ourselves with two advantages to hav-
ing color signaled at borders: first, color is unchanged despite changes in the
light source, so that our vision tells us about properties of the objects we view,
uncontaminated by information about the light source; second, the informa-
tion handling is economical. Now we can ask why the system evolved the way
it did. Are we to argue that the need for color constancy led to the system's
evolving and that an unexpected bonus was the economy—or the reverse, that
economy was paramount and the color constancy a bonus? Some would argue
that the economy argument is more compelling: evolution can hardly have
anticipated tungsten or fluorescent lights, and until the advent of supersuds,
our shirts were not all that white anyway.

                                THE PHYSIOLOGY
                                   OF COLOR VISION:
                                      EARLY RESULTS
The first cell-level physiological information came 250 years after
Newton from the studies of the Swedish-Finnish-Venezuelan physiologist Gun-
nar Svaetichin, who in 1956 recorded intracellularly in teleost fish from what
he thought were cones but turned out later to be horizontal cells. These cells
responded with slow potentials only (no action potentials) when light was
directed on the retina. He found three types of cells, as illustrated on this page:
the first, which he called L cells, were hyperpolarized by light stimulation
regardless of the light's wavelength composition; the second, called r-g cells,
were hyperpolarized by short wavelengths, with a maximum response to
green light, and depolarized by long wavelengths, with a maximum response
to red; the third, which with Hering in mind he called y-b cells, responded like
r-g cells but with maximal hyperpolarization to blue and maximal depolariza-
tion to yellow. For r-g and y-b cells, white light gave only weak and transient
responses, as would be expected from white's broad spectral energy content.
Moreover, for both types of cell, which we can call opponent-color cells, some
intermediate wavelength of light, the crossover point, failed to evoke a response.
Because these cells react to colored light but not to white light, they are proba-
bly concerned with the sensation of color.
In 1958, Russell De Valois (rhymes with hoi polloi) and his colleagues re-
corded responses strikingly similar to Svaetichin's from cells in the lateral
geniculate body of macaque monkeys. De Valois had previously shown by
behavioral testing that color vision in macaque monkeys is almost identical to
color vision in humans; for example, the amounts of two colored lights that
have to be combined to match a third light are almost identical in the two
species. It is therefore likely that macaques and humans have similar machin-
ery in the early stages of their visual pathways, and we are probably justified in
comparing human color psychophysics with macaque physiology. De Valois
found many geniculate cells that were activated by diffuse monochromatic
light at wavelengths ranging from one end of the spectrum to a crossover
point, where there was no response, and were suppressed by light over a
second range of wavelengths from the crossover point to the other end. Again
the analogy to Hering's color processes was compelling: De Valois tound op-
ponent-color cells of two types, red-green and yellow-blue; for each type,
combining two lights whose wavelengths were on opposite sides of some
crossover point led to mutual cancellation of responses, just as, perceptually,
adding blue to yellow or adding green to red produced white. De Valois' re-
sults were especially reminiscent of Hering's formulations in that his two
classes of color cells had response maxima and crossover points in just the
appropriate places along the spectrum for one group to be judging the yellow-
blueness of the light and the other, red-greenness.
The next step was to look at the receptive fields of these cells by using small
spots of colored light, as Torsten Wiesel and I did in 1966, instead of diffuse
light. For most of De Valois' opponent-color cells, the receptive fields had a
surprising organization, one that still puzzles us. The cells, like Kufflcr's in the
cat, had fields divided into antagonistic centers and surrounds; the center could
be "on" or "off". In a typical example, the field center is fed exclusively by red
cones and the inhibitory surround exclusively by green cones. Consequently,
with red light both a small spot and a large spot give brisk responses, because
the center is selectively sensitive to long-wavelength light and the surround
virtually insensitive; with short-wavelength light, small spots give little or no
response and large spots produce strong inhibition with off responses. With
white light, containing short and long wavelengths, small spots evoke on
responses and large spots produce no response.
Although our first impression was that such a cell must be getting input
from red cones in the center region and green cones in the surround, it now
seems probable that the total receptive field is a combination of two overlap-
ping processes, as illustrated in the figure on this page. Both the red cones and
the green cones feed in from a fairly wide circular area, in numbers that are
maximal in the center and fall off with distance from the center. In the center,
the red cones strongly predominate, and with distance their effects fall off
much more rapidly than those of the green cones. A long-wavelength small
spot shining in the center will consequently be a very powerful stimulus to the
red system; even if it also stimulates green cones, the number, relative to the
total number of green cones feeding in, will be too small to give the red system
any competition. The same argument applies to the center-surround cells de-
scribed in Chapter 3, whose receptive fields similarly must consist of two
opponent circular overlapping areas having different-shaped sensitivity-
vcrsus-position curves. Thus the surround is probably not annular, or donut
shaped, as was originally supposed, but filled. With these opponent-color cells
in monkeys, it is supposed—without evidence so far—that the surrounds rep-
resent the contributions of horizontal cells.
The responses to diffuse light—in this case, on to red, off to blue or green,
and no response to white—make it clear that such a cell must be registering
information about color. But the responses to appropriate white borders and
the lack of response to diffuse light make it clear that the cell is also concerned
with black-and-white shapes. We call these center-surround color-opponent
cells "type 1".
The lateral geniculate body of the monkey, we recall from Chapter 4, con-
sists of six layers, the upper four heavily populated with small cells and the
lower two sparsely populated with large cells. We find cells of the type just
described in the upper, or parvocellular, layers. Type 1 cells differ one from the
next in the types of cone that feed the center and surround systems and in the
nature of the center, whether it is excitatory or inhibitory. We can designate
the example in the diagram on the facing page as "r+g-". Of the possible
subtypes of cells that receive input from these two cone types, we find all four:
r+g-, r-g+, g+r-, g-r+. A second group of cells receives input from the blue
cone, supplying the center, and from a combination of red and green cones (or
perhaps just the green cone), supplying the surround. We call these "blue-
yellow", with "yellow" a shorthand way of saying "red-plus-green".
We find two other types of cells in the four dorsal layers. Type 2 cells make
up about 10 percent of the population and have receptive fields consisting of a
center only. Throughout this center, we find red-green opponency in some