Responses of cells in foveal visual cortex of the monkey to pure color contrast

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Abstract
A visual stimulator, which provided moving slits or edges of either pure luminance or pure color contrast, was used to study the responses of 79 cells located mostly in layers 2 and 3 at the striate-prestriate border of foveal visual cortex of anesthetized rhesus monkeys (Macaca mulatta). Cells (62) (78%) responded to slits or edges that were brighter or dimmer than their backgrounds and became unresponsive within a range of minimum luminance contrast (a null point); within this range, color contrast could restore responsiveness in 36 (58%) of these cells, either partially or completely. Cells (15) (19%) responded to only 1 direction of luminance contrast, slits or edges either brighter or dimmer than their background, but not to both; the lack of a null point for luminance contrast made an unequivocal demonstration of a response to pure color contrast impossible; nevertheless, some of these cells seemed to respond over a larger range with the addition of color contrast. Two cells (3%) responded almost exclusively to color and not to luminance contrast. Information about wavelength differences across the contours of objects reaches a large fraction, at least half, of the cells in this region of foveal visual cortex; in most of these cells, this color signal cannot be detected with stimuli combining both luminance and color contrast. Evidence is presented to support the hypothesis that it originates mainly from cone-opponent interactions rather than from chromatic aberrations in the retina.