Do S cones contribute to color-motion feature binding?

Wu et al. [Nature 429, 262 (2004)] describe a visual illusion in which color and motion are incorrectly bound: green dots moving downward and red dots moving upward are seen as green dots going up and red dots going down. The present study determined whether S cones contribute to color-motion feature-binding errors, in order to assess the neural representation of color at the level of binding. The specific experimental question is whether binding errors depend on S-cone responses from the objects perceived to have an illusory direction of motion. Alternatively, only L and M cones may determine the neural representation of color that regulates color-motion feature binding. In two experiments, the chromatic difference was manipulated between central objects, which induce color-motion binding errors, and peripheral objects, where color-motion binding errors occur. The chromaticity difference was varied along only the L/M-cone axis or only the S-cone axis. As in Wu et al. [Nature 429, 262 (2004)], color-motion binding was frequently observed in the periphery when there were no central versus peripheral chromatic differences. Further, the results showed that the frequency of color-motion binding errors in the periphery depended on the difference in S-cone excitation between center and periphery, thereby demonstrating that the neural representation of color at the level of feature binding depends on signals from not only L and M cones but also S cones.