Colors distinguishable with trichromatic vision can be defined by a 3-dimensional color space, such as RGB (Red-Green-Blue) or HSL (Hue-Saturation-Lightness) space, but it remains unclear how the cortex represents colors along these dimensions, unifying their perception. Using intrinsic optical imaging and electrophysiology, and systematically choosing color stimuli from HSL-coordinates, we examined how perceptual colors are mapped in visual area V4 inbehaving macaques.

Shiming Tang's lab report that any color activates 1-4 separate cortical patches within “globs”, millimeter-sized color-preferring modules. Most patches belong to different  hue - or  lightness -clusters, in which sequential representations follow the color order in HSL space. Some patches overlap greatly with those of related colors, forming stacks, possibly representing invariable features, while few seem positioned irregularly. However, for any color,  saturation  increases the activity of all its patches. These results reveal how the color map in V4 is organized along the framework of theperceptual HSL-space, whereupon different multi-patch activity patterns represent different colors. We propose that such distributed and combinatorial representations may expand the encodable color.

Ming Li, Fang Liu, Mikko Juusola  and Shiming Tang (2014) Perceptual Color Map in Macaque Visual Area V4.  The Journal of Neuroscience . 34(1):202–217