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The next task was obvious: we had to record from the striate cortex again, monitoring the experiments histologically with the cytochrome-oxidase stain to see if we could find anything different about the cells in the blobs. Margaret Livingstone and I set out to do this in 1981. The result was quite unexpected.
In traversing a distance of one-quarter millimeter, the diameter of a blob, it is possible to record from roughly five or six cells. Each time we crossed a blob, the cells we saw completely lacked orientation selectivity, in marked contrast to the high orientation selectivity shown by the cells outside the blobs.
One might explain this absence of orientation selectivity in either of two ways. First, these cells might receive their input unselectively from oriented cells in the nonblob neighborhood and consequently still respond specifically to lines (slits and so forth)—but by pooling all the various orientations, still end up with no preference. Second, they could resemble geniculate cells or cells in layer 4C and thus be simpler than the nonblob orientation-selective cells. The question was quickly settled: the cells were mostly center-surround.
A few more experiments were enough to convince us that many of them were color coded.
Over half the blob cells had opponent-color, center-surround receptive fields, but they behaved in a decidedly more complicated way than type i cells in the lateral geniculate body. They gave virtually no responses to white spots of any size or shape. But to small colored spots shone in the center of the receptive field they responded vigorously over one range of wavelengths and were suppressed over another range: some were activated by long wavelengths (reds) and suppressed by short (greens and blues): others behaved in the reverse way. As with geniculate cells, we could distinguish two classes, redgreen and blue-yellow, according to the position of the maximum responses.
(Here, as before, red, green, and blue stand for the respective cone types, and yellow implies an input from the red and green in parallel.) So far, then, these cells closely resembled opponent-color, center-only geniculate cells (type 2).
Their field centers, like centers of type 2 cells, were large—several times the size of type 1cell centers. They were unresponsive to small shone anywhere in their receptive fields. Most surprising was the finding that these color-coded blob cells, unlike type 2 cells, were mostly unresponsive to large colored spots, regardless of wavelength content. They behaved as though each center system was surrounded by a ring ofopponency. To take the commonest type, the r+g- center seemed to be surrounded by a ring that was r-g+.
Margaret Livingstone and I have called these cells double-opponent because of the red-green or yellow-blue opponency in the center and the antagonism of the surround to any center response, whether "on" or "off". They are therefore unresponsive not only to white light in any geometric form but also to large spots, regardless of wavelength content. As already mentioned, Nigel Daw coined the term double-opponent for cells he saw in the retina of the goldfish. Daw suggested that cells like these might be involved in color-spatial interactions in man, and a few years later, with Alan Pearlman, he searched carefully in the macaque monkey lateral geniculate for such cells, without success.
From the late 1960s on, double-opponent cells had occasionally been observed in the monkey cortex, but they were not clearly associated with any anatomical structure. We still do not understand some things about these cells.
For example, in the r+g- just described, a red spot surrounded by green often gives a poor response, not the vigorous one we might expect.
Mixed with the two classes of double-opponent cells (red-green and yellowblue) were ordinary broad-band, center-surround cells. Again, these broadband cells differed from cells in the upper geniculate layers and from cells in 4C Bata in having several times larger center sizes. Blobs also contain cells that are indistinguishable from geniculate type 2 cells, resembling double-opponent cells but lacking the receptive-field surround.
Margaret Livingstone and I have proposed that the blobs represent a branch of the visual pathway that is devoted to "color", using the word broadly to include blacks, whites, and grays. This system seems to separate off from the rest of the visual path either in the lateral geniculate body or in layer 4 of the striate cortex. (The geniculate probably projects directly but weakly to the blobs. It seems likely that layer 4C Bata also projects to them, and it may well form their main input. Whether 4C alpa: projects to them is not clear.) Most blob cells seem to require border contrast in order to give responses at all: either luminous-intensity borders, in the case of the broad-band, center-surround cells or color-contrast borders, in the case of the double-opponent cells would respond. As I argued earlier, this amounts to saying that these cells play a part in color constancy.
If blob cells are involved in color constancy, they cannot be carrying out the computation exactly as Land first envisioned it, by making a separate comparison between a region and its surround for each of the cone wavebands. Instead ^ they would seem to be doing a Hering-like comparison: of red-greenness in one region with red-greenness in the surround, and the same for yellowblueness and for intensity. But the two ways of handling color—r, g, and b on the one hand and b-w, r-g, and y-b on the other—are really equivalent. Color requires our specifying three variables; to any color there corresponds a triplet of numbers, and we can think of any color as occupying a point in threedimensional space. We can plot points in such a space in more than one way.
The coordinate system can be Cartesian, with the three axes orthogonal and oriented in any direction or we can use polar or cylindrical coordinates. The Hering theory (and apparently the retina and brain) simply employ a different set of axes to plot the same space. This is doubtless an oversimplification because the blob cells making up the three classes are not like peas in pods but vary among themselves in the relative strengths of surrounds and centers, in their perfections in the balance between opponent colors, and in other characteristics, some still not understood. At the moment, we can only say that the physiology has a striking affinity with the psychophysics.
You may ask why the brain should go to the trouble to plot color with these seemingly weird axes rather than with the more straightforward r, g, and b axes, the way the receptor layer of the retina does. Presumably, color vision

 

 

 

 

 

 

 

 

 

 

was added in evolution to the colorless vision characteristic of lower mammals. For such animals, color space was one-dimensional, with all cone types (if the animal had more than one) pooled. When color vision evolved, two more axes were added to the one already present. It would make more sense to do that than to throw out the pooled system already present for black-white and then have to erect three new ones. When we adapt to the dark and are using only our rods, our vision becomes colorless and is again plotted along

   
 
Top: Land's original formulation of the color-constancy problem seems to call for three kinds of cells, which compare the activation of a given set of cones (red, green, or blue) in one region of retina with the average activation of the same set in the surround. The result is three numbers, which specify the color at the region. Thus yellow, brown, dark gray, and olive green each has a corresponding triplet of numbers. We can therefore plot colors in a color space specified by three axes, for red, green, and blue. Bottom: A mathematically equivalent system also gives three numbers, and is probably closer to the way the brain specifies color. At any point on the retina, we can speak of red-greenness, the reading an instrument would give if it were to record the relative stimulation of red and green cones (zero for yellow or white).
This value is determined for a particular region, and an average value is determined for the surround; then the ratio is taken.
The process is repeated for yellow-blueness and black-whiteness. These three figures together are enough to specify any color.





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The dark areas are blobs seen tace on, about 50 of which form a polka-dot pattern. This section, through layer 3 of area 17, is parallel to the cortical surface and about 0.5 millimeter beneath it. (The yellow circles are blood vessels cut transversely.)


 
 
 
 
 

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