Eye, Brain, and Vision

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 re-
verse way. As with geniculate cells, we could distinguish two classes, red-
green 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 there-
fore 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 gold-
fish. 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 ob-
served 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 yellow-
blue) were ordinary broad-band, center-surround cells. Again, these broad-
band 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 compari-
son 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 yellow-
blueness 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 three-
dimensional 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 charac-
teristics, 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 mam-
mals. 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