Stare at some sights long enough, and you'll continue to see an afterimage even after you've looked away. Now vision scientists report that the brain can see afterimages of things that weren't even there in the first place, but were created by optical illusions. The cerebral cortex is responsible for the trick.
Afterimages come in two basic types: those formed on the eye's retina and those conjured up in the brain. For example, a bright light can bleach pigments in the retina and cause the retina's neurons to adapt; these effects linger after the light disappears, creating an afterimage. But most other aftereffects, such as continuing to hear a sound when it has ceased or seeing color-based patterns after looking away, are formed in the brain's cerebral cortex.
Now cognitive scientist Shinsuke Shimojo and his colleagues at the California Institute of Technology in Pasadena have discovered that the cerebral cortex can create afterimages even of illusory surfaces. They worked with a well-known effect called perceptual color spreading, or filling in (see figure). If you stare at the illustration long enough, you will start to see a box between the four circles with wedges cut out of them (researchers call them pacmen for their resemblance to the video game heroes).
Shimojo and his colleagues noticed that if you shift your gaze to the blank area at right, you continue to see the box. Illusions, they found, can create afterimages. Then they fiddled with the pacmen to find out whether the newfound perception relies more on the retina-based afterimages created by the pacmen or on a cortex-based process of filling in. The cortex won, they report in the 31 August issue of Science.
The demonstration that illusions can create afterimages is "cute," says vision scientist Mary Hayhoe of the University of Rochester in New York state. "No matter how much we study perception we still find it puzzling when we see something that isn't there," Hayhoe says.