Nonlinear analysis of macaque V1 color tuning reveals cardinal directions for cortical color processing

Understanding color vision requires knowing how signals from the three classes of cone photoreceptor are combined in the cortex. We recorded from single V1 neurons in awake monkeys while an automated, closed-loop system identified stimuli that differed in cone contrast but evoked the same response. We found that isoresponse surfaces for about half the neurons were planar, consistent with linear processing. The remaining isoresponse surfaces were nonplanar. Some were cup-shaped, indicating sensitivity to a narrow region of color space. Others were ellipsoidal, indicating sensitivity to all color directions. The major and minor axes of these nonlinear surfaces were often aligned to a set of three color directions that were previously identified in perceptual experiments. These results demonstrate that many V1 neurons combine cone signals nonlinearly and provide a new framework within which to decipher color processing in V1.