Color Constant Representations in Early Visual Cortex

Abstract

The light entering our eyes is the product of the illumination and the surface reflectance of an object. Although this light changes considerably when the illumination changes, we are usually able to perceive objects as stable in color. To investigate how the brain achieves color constancy, we measured BOLD fMRI while 19 participants either observed patches of light that appear colored (yellow, blue) under a spectrally neutral illuminant, or spectrally neutral gray patches that appear colored under simulated blue and yellow illumination conditions. Under bluish illumination, the neutral gray patches appeared yellow; under yellowish illumination, the same gray patches appeared blue. We successfully trained a classifier to discriminate between the blue- and yellow-colored patches in V1-V4. Crucially, we then tested whether this same classifier could also distinguish between the apparent blue and yellow induced by the illuminants. The neural representations for apparent blue and yellow resembled colorimetric blue and yellow in V1, V3 and V4. A control experiment showed that apparent lightness cannot explain these effects. These findings suggest that not only colorimetric, but also apparent color is represented to some degree in retinotopic visual cortex, as early as in V1. Furthermore, a small frontal region, the Rolandic operculum, showed activation for apparent color, possibly playing a role in color constancy.