Color and Spatial Frequency Provide Functional Signatures of Retinotopic Visual Areas

The visual cortex contains multiple retinotopic representations of the visual field. A long-standing question of visual neuroscience concerns the role of these different regions in visual computations and perception. Differences in psychophysical properties have also been noted across the visual field, varying with eccentricity and with the upper and lower hemifields. Inspired by studies that used linear-systems-defined stimuli to characterize single neuron responses, we collected fMRI data from two macaque monkeys as they passively viewed gratings that varied in color, saturation, and spatial frequency. We apply a novel variant of multivariate linear discriminant analysis to discover combinations of these features that best separated visual cortex, along three dimensions of representation: eccentricity; classic retinotopic progression (V1, V2, V3, V3a, V4, MT+); and upper vs lower visual field. As expected from prior work, eccentricity across all visual areas was characterized by responses to color and spatial frequency: foveal regions, compared to peripheral regions, showed greater responses to colors not restricted to the S-cone-axis and high spatial frequency. Retinotopic visual areas were characterized by their responses to saturation: relative to anterior visual areas (V3-V4, V3a, MT+), posterior areas (V1-V2) were more responsive to increases in saturation. And upper-vs-lower representations were distinguished by responses to colors modulating along the Daylight locus: ventral areas showed relatively higher responses to colors associated with the Daylight locus (orange/blue) as compared to responses to achromatic luminance. Together, these results provide a data driven approach that recovers functional signatures of retinotopic organization using color and spatial frequency, providing clues toward the different roles that the components of retinotopic cortex play in visual perception.