Spatial structure of cone inputs to cortical color cells

Summary and Conclusions

The spatial structure of primate cortical color-cell receptive fields is controversial. In this study, spots of light that selectively modulate one class of cones (L, M or S, or loosely red, green or blue) were flashed in and around the receptive fields of V-1 color cells to map the spatial structure of the cone inputs. The maps generated using these cone-isolating stimuli, together with an eye-position corrected reverse correlation technique, produced four findings. First, the receptive fields were Double-Opponent, an organization of spatial and chromatic opponency that, when complemented by other cues in the natural world (e.g. spectral highlights and 3-D shape, Kraft and Brainard, 1999), likely contributes to color constancy and color contrast. Optimally stimulating both center and surround subregions simultaneously with adjacent red and green spots excited the cells more than stimulating a single subregion. Second, red-green cells responded in a luminance-invariant way. For example, red-on-center cells were excited equally by a stimulus that increased L-cone activity (appearing bright red) and by a stimulus that decreased M-cone activity (appearing dark red). This implies that the opponency between L and M is balanced and argues that these cells are encoding a single chromatic axis. Third, most color cells responded to stimuli of all orientations and had circularly symmetric receptive fields. Some cells, however, showed a coarse orientation preference. This was reflected in the receptive fields as oriented Double-Opponent subregions. Fourth, red-green cells often responded to S-cone stimuli. Responses to M- and S-cone stimuli were usually aligned, suggesting these cells might be best described as red-cyan. In sum, red-green (or red-cyan) cells, along with blue-yellow and black-white cells, establish three chromatic axes that are sufficient to describe all of color space.

We need not examine the explanations of color constancy by Helmholtz and those who have followed him during the last century because ... the paradox [of color constancy] does not really exist: The color of an object is not determined by the composition of the light coming from the object.

— Edwin Land, 1977