Illusions: The Ghost Hand Illusion; March / April 2011; Scientific American Mind; by Vilayanur S. Ramachandran; Diane Rogers-Ramachandran; 3 Page(s)

Stare at the tiny, central black fixation spot on the white cross in a. After 30 seconds, transfer your gaze to a neutral gray background. You should see a dark—almost black—cross fading in and out. It is especially pronounced if you blink your eyes to revive the image to slow down the fading.

This effect is called a negative afterimage because the persistent ghost of the cross is the opposite of what you were looking at—it is dark instead of light. When you fixated on the white cross, you “fatigued” the retinal light receptors by bleaching out the cone pigments. So when you look at neutral gray, the region corresponding to where the white cross had been fires less vigorously than the surrounding area, and the net result is that it is seen as a dark cross.