SAN2020

The perceived color of a stimulus depends not only on its spectral properties but also on those of its surround. For instance, a patch that looks gray on an achromatic background appears reddish on a green background, and greenish on a red background. Previous studies showed that the effect of the surround is repulsive: It enhances the perceptual difference between stimulus and surround. We performed psychophysical experiments to quantify the repulsion. To report the results, a notion of distance in color space was required. We therefore proposed an individually tailored metric in color space that captured the perceptual abilities of each observer. To define the metric, we determined the minimal chromatic difference between a stimulus and its surround required by each subject to detect the stimulus. Next, observers performed discrimination experiments between two spatially localized stimuli presented in a surround of a different chromaticity. The surrounding color affected the discrimination thresholds. Quite remarkably, when these thresholds were expressed in the color coordinates defined before, the change in thresholds followed a simple law that only depended on the distance between the surround and the two compared stimuli. Perceptual coordinates, hence, reveal the symmetry of the repulsion effect. This finding was confirmed with a third experiment, in which subjects were asked to match the color of two stimuli presented in two different surrounds.