Perceptual interaction between real and synesthetic colors.

People with color-graphemic synesthesia experience vivid, reliable color upon viewing achromatic alphanumeric characters. Recent evidence indicates that synesthetic color experiences are as perceptually real as actual colors are for non-synesthetic observers. To investigate possible interactions between real and synesthetic colors, we tested two adult color-graphemic synesthetes on a pair of perceptual grouping tasks. In Experiment 1, we employed a well-known phenomenon of motion perception, bistable apparent motion, to explore whether synesthetic colors interact with real colors in grouping over time. Two-frame apparent motion sequences were presented with both path lengths and colors systematically manipulated. Results showed that synesthetic colors of motion tokens interacted with matching real colors of the corresponding motion tokens, which could subsequently bias perceived direction of motion. In Experiment 2, we exploited binocular rivalry, a condition under which two dissimilar monocular images compete with each other and result in perceptual switches, to explore whether synesthetic colors interact with real colors in grouping over space. Pairs of rival images with two different characters were presented dichoptically with colors of characters manipulated. Results showed that synesthetic and real colors of characters tended to group together, which, in turn, promoted the perceived global dominance during binocular rivalry. Therefore, the present results identify substantial interaction between synesthetic colors and real colors in perceptual grouping.