ITHACA, N.Y. -- 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 (March 6, 1997). It all began with Isaac Newton, he said.
"Almost 300 years ago Newton used a prism to decompose light into different colors. Today we do the reverse experiment with our computer's color monitor," Nijhawan explained. "Every pixel that we see as white on the screen is produced by the simultaneous composition of red, green and blue. And yellow is produced by the composition of red and green." Nijhawan's experiment showed that the brain itself can decompose yellow into its constituent red and green on the basis of motion of the stimulus.
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. Still, vision scientists couldn't agree.
Nijhawan's color experiment exploited a human capability that he previously demonstrated in black and white: 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. But special circumstances can trick our brain: If we watch the tail light of a car moving past us on a dark and stormy night -- and lightning flashes to momentarily illuminate the body of the car -- we will see the tail light in the middle of the car. That is because our visual apparatus continues to compensate for the movement of the tail light. However, the abrupt exposure of the car's body causes its visual image to be seen -- after a 50-millisecond delay -- with the tail light in the "wrong" place. (If lightning provided continuous illumination, the car would soon catch up with its tail light.)
The Cornell psychologist brought the lightning-lit car phenomenon into the laboratory for his visual decomposition experiment. He asked volunteers to watch a moving green bar with a thin line, produced by the red flash of a strobe light, in two slightly different situations. When the viewers saw the green bar only for an instant (through a slit in a partition between the observer and the bar) the red strobe superimposed on the green bar appeared yellow.
But when the partition was removed and viewers could perceive the moving green bar until the red strobe flashed, no one saw yellow where the flash actually occurred -- inside the green bar. Instead, they saw a red line lagging behind the green bar. Their visual systems could not compensate for the neural delays in registering the sudden flash, so the sensation could not be re-mapped to the appropriate place in the visual cortex, where red and green would have been composed into yellow.
Nijhawan succeeded in decomposing a color sensation that was made of two primary colors, using movement. This composition and decomposition can occur only in the brain, he said, because human retinas -- and retinas of other primates -- are not capable of sensing motion on their own. However, some animals can.
"A frog's eye can detect motion and send signals directly to its muscles without passing through the brain, Nijhawan said. "That's why frogs can react so quickly and catch flies.
"But we're not frogs," he added. "We need our brains to perceive motion and color, too."