VISUAL MECHANISMS THAT SIGNAL THE DIRECTION OF COLOR CHANGES

Evidence is presented for a visual capacity specialized to sense the c hromatic direction of change in colors over time. Discrimination thres holds were measured between pairs of suprathreshold color changes pres ented in consecutive intervals. In one interval, the color of a spatia lly uniform disk was changed at a constant speed along the circumferen ce of a circle in an equiluminant color plane. In the other, an instan taneous change, which can be described as a vector in the equiluminant plane, was added to the circular color modulation. Averaging across c onditions showed that the threshold for discriminating between a pair of purely temporal color changes was approximately proportional to the cosine of the color angle between them. The model that is presented t o account for these results is based on parallel directional-color mec hanisms that are tuned to different directions in color-space and are responsive to change in one color direction but not its opposite.