one axis, to which the rods evidently contribute. That would not be easy to do with r, g, and b axes.
At present we can only guess how double-opponent cells are wired up. For several reasons we suspect that they receive their inputs from type 2 cells: their field centers are about the same size as type 2 field centers and much larger than centers of type 1cells; they are intermixed with type 2 cells in the blobs;
and finally, as already pointed out, type 1cells, with their opponent-color antagonistic surrounds, seem especially inappropriate as building-blocks for double-opponent receptive fields. For a red-on center double-opponent cell, the simplest arrangement, as illustrated on the facing page, would be to have excitation from one or a few red-on green-off type 2 cells whose field centers were contained within the double-opponent cell's center, and excitatory inputs from red-off green-on type 2 cells whose centers were scattered throughout the double-opponent cell's periphery. Or the surround could be formed by inhibitory inputs from red-on green-off type 2 cells. (Originally we favored a scheme in which the inputs were made up of type i cells. Logically such an arrangement is possible, but it seems much more awkward.)
This leaves unsettled the part that type i cells play in color vision—if they play any part at all. These are the most common cells in the lateral geniculate body, and they supply the lion's share of the input to the visual cortex. Their obvious color coding makes it easy to lose sight of the fact that they are beautifully organized to respond to light-dark contours, which they do with great precision. Indeed, in the fovea, where their centers are fed by one cone only, they have no choice but to be color-coded. (The mystery is why the surround should be supplied by a single, different, cone type; it would seem more reasonable for the surrounds to be broad-band.) Given this massively color-coded input it is astonishing that interblob cells in the cortex show so little interest in color. The few exceptions respond to red slits but not to white ones, and are thus clearly color coded. For the most part it would seem that the information on wavelength carried by type i cells is pooled, and the information about color lost. In one sense, however, it is not discarded completely. In Freiburg, in 1979, Jiirgen Kriiger and Peter Gouras showed that cortical cells often respond to lines formed by appropriately oriented redgreen (or orange-green) borders at all relative intensities of red and green. A truly color blind cell, like a color blind person, should be insensitive to the border at the ratio of intensities to which the cones respond equally. These cells presumably use the type i color information to allow contours of equal luminance to be visible by virtue of wavelength differences alone—of obvious value in defeating attempts at camouflage by predators or prey.
The recognition of colors as such would thus seem to be an ability distinct from the ability to detect color borders, and to require a separate pathway consisting of type 2 cells and color-opponent blob cells. Our tendency to think of color and form as separate aspects of perception thus has its counterpart in the physical segregation of blobs and nonblob regions in the primary visual cortex. Beyond the seriate cortex the segregation is perpetuated, in visual area 2 and even beyond that. We do not know where, or if, they combine.
The subject of color vision illustrates so well the possibilities of understanding otherwise quite mysterious phenomena—the results of color mixing or the constancy of colors despite changes in the light source—by using a combination ofpsychophysical and neurophysiological methods. For all their complexity, the problems presented by color are probably simpler than those presented by form. Despite all the orientation-specific and endstopped cells, we are still a long way from understanding our ability to recognize shapes, to distinguish shapes from their background, or to interpret three dimensions from the flat pictures presented to each of our eyes. To compare the modalities of color and form at all may itself be misleading: remember that differences in color at borders without any differences in luminous intensity, can lead to perception of shapes. Thus color, like black and white, is just one means by which shapes manifest themselves.
A double-opponent cell could be built up from many geniculate type 2 cells. If the cell is r+'g- center, r-g+ surround, then its inputs could be a large number of r g type 2 cells with fields scattered throughout the cell's receptive field center, and r g type 2 cells with fields scattered throughout the cell's receptive field surround, all making excitatory contacts with the double-opponent cell. Alternatively, the surround could be formed from r+g- type 2 cells, making inhibitory contacts.