cells, blue-yellow in others. The centers of these type 2 cells tend to be large, several times larger than the centers of type I cells. The other 15 percent or so of cells in the four upper geniculate layers, and all the cells in the two lower (magnocellular) layers, are center-surround but show no such color preferences; it is as if their field centers and surrounds received the same relative contributions from the three cone types. We refer to these cells as broad-band, and in the upper layers we call them type 3 cells.
All these findings are remarkably compatible with Hering's model: we have two classes of color-opponent cells, one red-green, the other yellow-blue, and a third showing no color opponency at all but a broad-band spatial opponency instead. What seemed not to fit any theory was the spatial organization of the opponent-color, or type i cells. You might think, at first glance, that this organization would have something to do with color contrast, with the tendency for one color, say blue, to look more vivid if surrounded by another, say green, or for a gray piece of paper to look yellowish if surrounded by blue.
But a moment's thought will convince you that type i cells can hardly be useful for that kind of color contrast: the r+ center-g- surround cell just described, far from being strongly excited by a red spot surrounded by green, gives little or no response, because one effect cancels the other—the reverse of what would seem to be required for color contrast.
What we can say of type i cells is that they are likely to play an important part in high-precision form perception, given their tiny field centers and their responsiveness to black-and-white contours. As we saw in Chapter 6, we have several ways to measure visual acuity, the ability of our visual system to discriminate small objects; these include the smallest separation between two dots that can just be discriminated and the smallest detectable gap in a circle (called the Landolt C). Acuity measured in either of these ways turns out, for the fovea, to be about 0.5 minute of arc, or about i millimeter at a distance of 8 meters. This corresponds well with the distance between two cones in the fovea. Type i geniculate cells that get their input from near the fovea have receptive-field centers as small as about 2 minutes of arc in diameter. It seems likely that in the fovea one cone only contributes to a field center. So we find a reasonable fit between acuity and smallest field-center sizes of lateral geniculate cells.
The ventral pair of geniculate layers differs from the dorsal four in being made up entirely of cells whose field centers are broad-band. The cells do show a curious form of color opponency that no one understands and that I will say no more about. Most people assume that these cells are color-blind.
Their field centers are several times larger than centers of parvocellular cells, and they differ in several other interesting ways. We presently suspect that these cells feed into parts of the brain that subserve depth and movement perception. To elaborate further would take us far from color and require another book.
Most of the cells I have been describing for the lateral geniculate have also been observed in the retina. They are more conveniently segregated in the geniculate and are easier to study there. We do not know what the geniculate contributes to the analysis of visual information in the monkey, besides its obvious function of handing on to the cortex the information it receives from the eyes.
THE NEURAL BASIS OF COLOR CONSTANCY
Since type i cells in the lateral geniculate body seem not to be geared to make color-spatial comparisons, we probably have to look beyond the retina and geniculate. To test the idea that such computations might go on in the cortex. Land's group and Margaret Livingstone and I examined a man who had had his corpus callosum severed surgically to treat epilepsy. Spatialcolor interactions did not take place across the visual-field midline, that is, the color of a spot just to the left of the point at which the subject was looking was not affected by drastic changes in the colors in the right visual field, whereas normal subjects observed marked differences with such changes. This suggests that the retina by itself cannot mediate the color-spatial interactions. Although no one had seriously claimed that it could, the question continued to be debated, and it was satisfying to have some experimental evidence. The experimental results are consistent with our failure to find retinal ganglion cells that could plausibly be involved in color-spatial interactions.
The goldfish, which makes spatial comparisons very much like ours, has virtually no cerebral cortex. Perhaps the fish, unlike us, does make such computations with its retina. Nigel Daws' discovery in 1968 of double opponent cells in the fish retina seems to bear this out. In the monkey, as I describe in the next section, we find such cells in the cortex but not in the lateral geniculate or the retina.
This cross section through the striate cortex shows the layers stained for the enzyme cytochrome oxidase. The darker zones in the upper third of the section are the blobs.