Take this test. In the top image to the right, you will find a small white dot just below the center of the figure. Stare at that dot for 10 to 20 seconds and then shift your gaze to the dot in the empty space to the right. Now what do you see? If you see anything at all, you most likely see one of the figures shown in the bottom image: gray disks, turquoise Pacmen or a turquoise square. Reporting in today's issue of Science, Shinsuke Shimojo of the California Institute of Technology and NTT Communication Science Laboratory and colleagues suggest that not all of these afterimages come from the same place. Some, they say, are generated within the eye, but others stem from perceptual adaptations in the brain.

Researchers long assumed that the bleaching of photochemical pigments or neural adaptation within the retina caused afterimages in response to visual stimuli. The first two figures in the bottom image are examples of such local, or eye-based, visions. But in a series of careful experiments with five subjects, Shimojo and team demonstrated that some afterimagessuch as the turquoise squareare filled in more globally by perception. This square, though perceived by way of suggestion in the original image, does not actually exist among the white disks and red pie wedges. But adaptation in the brain's cortical neural circuits produces the square as an afterimage.