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See Inside March / April 2011

The Ghost Hand Illusion

Spooky fun with afterimages

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.

Why does the cross fade? Partly because the fatigued receptors recover slowly as the bleached pigment regenerates. In contrast, with real images our eyes are in constant motion—images sail and jerk across the retina as we scan rooms, roads, texts or faces to identify novel or important bits. This continual movement prevents adaptation or fatigue because new patterns are constantly on any retinal area. With intense focus, you can eliminate all voluntary movements, and you should notice certain objects slowly fade away, as in b (termed the Troxler effect or Troxler fading). This fading is intermittent because your eyes never completely stop moving. Microscopic involuntary trembling characterizes even the steadiest fixation. This “physiological nystagmus” allows the brain’s edge-detecting neurons to avoid being fatigued, even during fixation, by providing moment-to-moment refreshing. But an afterimage, unlike a real image, remains stuck to the retina so the neurons are not refreshed and fatigue quickly kicks in.

All of what we have discussed so far is the conventional story. But there is much more to afterimages than meets the eye, as shown by the late Richard L. Gregory of the University of Bristol in England, who was the world’s preeminent perceptual psychologist. His 1966 book Eye and Brain launched many a student (including both of us) on a career in visual psychology and neurophysiology. The word “genius” is rarely used these days, but if anyone deserves the title, it would be Gregory.

Gregory studied positive afterimages because they fade more slowly and are more intense with more clearly defined borders, making them easier to study. In collaboration with Elizabeth L. Seckel of our laboratory, we have confirmed the results of many little-known experiments Gregory did on afterimages in the late 1960s. The reader might wish to try them out today.

A Shot in the Dark
Have a friend aim a flash camera at you in a dimly lit room while you gaze at a tiny, luminous dot affixed to the center of the flash. When he “takes your picture,” you will get a positive afterimage. The persistent firing of photoreceptors makes you see a bright white disk long after the actual flash has gone.

Because the afterimage is glued to the retina, if you move your eyes around the room, the afterimage moves along with them. Now, while you have an afterimage, look at surfaces at different distances. The afterimage will appear on each surface as you fixate on it, and, amazingly, its apparent size will expand or shrink depending on how far or close the surface of regard is. What fun! Hold a piece of paper at arm’s length, move it toward your nose and watch the afterimage on it change in apparent size from a Ping-Pong ball to a pea. Cast your view back to a distant wall, and instantly the afterimage appears beach-ball-sized.

Why does this effect occur? Consider real objects. For example, if a friend standing five feet from you starts walking away, her retinal image size shrinks as she leaves. At 10 feet, it is half as tall (simple geometry). But of course, you do not see her shrinking—only as moving farther away. Perceived size varies directly with perceived distance (known as Emmert’s law). And in judging distance, the brain weighs information from motion, stereo, perspective, vergence angle, and so forth and applies the necessary “corrections”—a process called size constancy.

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