Forward model. A, Idealized color tuning curve, modeled as a half-wave rectified and squared sinusoid. B, The response of a voxel was fitted with a weighted sum of six idealized color tuning curves, evenly spaced around the color circle, in CIE L*a*b* space. C, Simulated response amplitude matrix, for each voxel and each color. D, Matrix of principal component scores, computed by projecting the vector of response amplitudes (across voxels) onto each of the principal component vectors, ordered by the amount of variance they explain in the original response amplitudes. E, Plotting the first two principal component scores as coordinate pairs reconstructs the original color space. F, Cone-opponency model. LMS cone responses were calculated for the stimulus colors. Four cone-opponency channels (M–L, L–M, −S, and +S) were computed from the cone responses, half-wave rectified. G, The first two principal components of the simulated cone-opponency responses revealed results similar to those observed in V1.