Color sensitivity of human visual areas

Purpose. To understand the cortical representation of color, we measured color sensitivity of several visual areas in the occipital lobe of the human brain. Methods. Subjects viewed color contrast-reversing patterns (4 Hz flicker-rate) that alternated with a uniform field at 1/42 Hz. Four planes of functional magnetic resonance imaging (fMRI; TE = 40, TR = 300, FA = 35, voxel size = 1.2 x 1.2 x 5 mm) data were acquired in the occipital lobe every 3 seconds. We defined the response to each stimulus as the amplitude of the 1/42 Hz harmonic that best fit the fMRI signal. Responses were spatially averaged within each of four human visual areas (V1 and three nearby areas) that were segregated based on their retinotopic organization. Results. Color sensitivity measured using fMRI resembles sensitivities measured using psychophysical methods. The fMRI responses are well-accounted for by a model consisting of a sum-of-squares combination of three linear opponent-colors mechanisms. Responses are monotonic with stimulus contrast, well-fit by a simple saturating function, and approximately parallel for different color directions. Hence, a single iso-response surface summarizes color sensitivity. When plotted in cone-contrast coordinates, iso-response surfaces are ellipsoidal, and the shape of the ellipsoid indicates the presence of opponent-colors mechanisms. For example, sensitivity to stimuli in the L + M color direction is lower than sensitivity to stimuli in either the L or M directions. Conclusions. Measurements using fMRI can provide quantitative estimates of the color sensitivity of cortical areas. Our data show that human V1 and nearby cortical areas represent color using three opponent-colors mechanisms. Cortical areas may differ in how these mechanisms contribute to their sensitivity; such differences will be informative about the role of these areas in color perception.