Spectral coding in cat retinal ganglion cell receptive fields.

We have examined the spectral-coding properties of ganglion cell receptive fields in the cat retina. Two classes of spectral coding were found. The first class consists of cells in which color opponency is spatially local. That is, the opponent cone types cover the same (center or surround) region of the receptive field. The second class consists of cells that show color opponency only to large stimuli (relative to center diameter). Center and surround regions of cells of this class have different spectral sensitivities. Individually the regions are nonopponent. When both regions are stimulated, a spectral opponency is revealed. For example, we recorded from one unit in which the ON-center was mediated by the 556-nm cone type and the OFF-surround was mediated by both 450- and 556-nm cone types. Large-field, threshold-level stimulation in the short-wavelength end of the spectrum produced OFF-responses, while in the long-wavelength end produced ON-responses. For a small stimulus, cells of the second class could mediate spatial vision, largely unaffected by the chromatic properties of the stimulus. Cells of the second class (center/surround opponent) were more commonly encountered than cells of the first class (locally opponent). Color-opponent units of X, Y, and W types were all found.