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In a typical type I receptive field, the center receives excitatory input from red cones; the surround, inhibitory input from green cones.






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 entirely within areas of uniform color will be idle. The result is economy in handling the information. We thus find ourselves with two advantages to having 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 information 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 Gunnar 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 depolarization 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 probably concerned with the sensation of color.
In 1958, Russell De Valois (rhymes with hoi polloi) and his colleagues recorded 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 machinery 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 opponent-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' results 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 yellowblueness 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 overlapping 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 described in Chapter 3, whose receptive fields similarly must consist of two opponent circular overlapping areas having different-shaped sensitivityvcrsus-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 represent 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, consists 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 "blueyellow", 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
   
 


These graphs plot the sensitivity of a cell (measured, for example, by the response to a constant very small spot of light) against retinal position along a line AA' passing through the receptive-field center. For an r+ center-g- surround cell, a small red spot gives a narrow curve and a small green spot, a much broader one. The lower graph plots the responses to light such as white or yellow that stimulates both of the opponent systems, so that the two systems subtract. Thus the red cones dominate in the center, which gives on responses, whereas the green cones dominate in the surround, which yields off responses.
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Gunnar Svactichin and Edward MacNichol recorded the responses to color of horizontal cells in the teleost fish. Deflections pointing downward from the gray line indicate hypcrpolarization; those pointing upward indicate depolarization.
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