Color cell groups in foveal striate cortex of the behaving macaque

Color-tuning curves were obtained for 218 cells in the foveal striate cortex of behaving macaques. Each cell was tested with its optimal spatial stimulus. Test colors (14 interference filters, 4 Wratten filters, and white) were matched for human photopic luminosity and presented at luminance levels sufficient to induce vigorous responding from most cells. One hundred eighty-four cells were selected for further analysis on the basis of a color-tuning index. Of these, 130 with tuning curves that correlated well (0.9 or better) with other tuning curves were studied in detail. Individual cells were found with peak responses to every color tested. Sixty-three tuning curves fell into the six largest cross-correlation groups, containing 15, 14, 12, 9, 7, and 6 cells, with mean tuning-curve peaks at 450, 656, 656, 506, 577, and 506 nm, respectively. Cross-correlation groups having the same peak location (656 nm, 506 nm) were distinguishable on the basis of tuning-curve width. Response patterns, cone input estimates, and comparison with human psychophysics suggest that two of these cell groups function as an opponent pair processing the colors red and green. Two other cell groups process the colors blue and yellow but show less well-developed opponency. Microdrive depth readings, correlated with histological lesion sites, show these "red," "green," "blue," and "yellow" cells to be most common in layer 4 of the striate cortex.