Study finds colors composed by brain, not eyes

17 Jun 2002

Trying to cope with red flashing lights on green moving objects, the human visual system is tricked into revealing where yellow -- and all other colors -- apparently are composed: in the visual cortex of the brain. The red, green and blue cone receptors in the retina merely pass along signals for the brain to make sense of, Cornell University psychologist Romi Nijhawan concludes from an experiment that may confirm, once and for all, the "central synthesis" theory of human color vision.

A visiting scholar in the laboratory of Beena Khurana, assistant professor of psychology, Nijhawan reported his study, "Visual decomposition of colour through motion extrapolation," in the journal Nature.

How yellow is perceived became the crux of differing explanations of human color vision, Nijhawan said. One theory has the composition of yellow occurring in the eyes, with nerves from the red and green cones feeding their inputs to special cells. Another, the central synthesis theory, has separate signals from the red and green cones traveling to the visual cortex and composing yellow there. A simple experiment -- shine green light in the left eye and red in the right, and we perceive yellow from binocular fusion -- would seem to favor the central synthesis theory, since the optic nerves from the left and right eyes don't converge until the brain.Because visual signals take about 50 milliseconds to travel from the eye to the brain, moving objects would not appear in exactly the same place in the visual cortex "map" as they were in the retinal "map" without a computation to correct this discrepancy. This correction, which Nijhawan calls "motion extrapolation," kicks in and continues perfectly as long as the object undergoes smooth, regular motion.