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.

Usually this process is adaptive in that it allows you to perceive the object as it really is: constant in size regardless of distance and retinal image size. But in the case of an afterimage, the processing backfires. The afterimage does not change size on the retina with changes in viewing distance, but your brain still interprets it as doing so. Thus, when the afterimage is superposed on a far wall, your brain expects the retinal image to have shrunk from the size it would have been on a near wall. Your brain therefore expands the apparent size to compensate. It is important to realize that all this occurs on a kind of autopilot. There is no conscious reasoning or decision making such as: “If the object is far, it must have a small image; therefore, object size must be....” That type of cogitation would be much too time-consuming to be effective. Why this entire process results in the image actually looking large rather than simply knowing it is large is a $64,000 philosophical question called the riddle of qualia. (We will stay away from this question in this column, even though we personally believe size constancy might one day help solve the riddle more readily than asking, “Why is red red?”)

Emmert’s law also works in complete darkness. This is because when you look at an imaginary object at different distances, the angle between the two eyes’ lines of sight (vergence angle) changes, and the brain measures this change in eye position. So the afterimage shrinks and expands in darkness, depending on how far away you gaze.

Next try the following experiment. Generate an afterimage with another flash. Then, in darkness, stand perfectly upright and move your head forward and back from the (invisible) wall in front of you. When you stick your neck out, you will find that the afterimage shrinks because the brain “assumes” it is a real object expanding and therefore applies a (false) correction. Perhaps signals from the neck muscles are sent to the visual centers to zoom the perceived size. Alternatively, when the motor-command centers in the brain send commands to neck muscles, they may send a kind of cc (as in e-mail) to the visual centers.

Ghostly Apparition
These facts about Emmert’s law are pretty straightforward, but the best is yet to come.

Affix a tiny, luminous spot on the center of your right palm and, in complete darkness, hold your hand out at arm’s length and look at the spot. Have a friend look over your shoulder, then take a flash aimed at your outstretched hand.

Now look straight head. You will see a vivid ghostly afterimage of your hand. Keep gazing forward so that the hand image is hovering in front of you—nothing surprising so far. But now move your real hand toward your nose, and you will get the impression that the hand image is shrinking. This miniaturization will happen even if there is an image in only one eye, so the source of distance information cannot be the vergence angle.

Gregory’s ingenious idea was that the proprioceptive information from muscle and joint sense in the arm must be going all the way to the brain’s size-perception centers; the messages do not have to originate in the eye muscles. The effect feels spooky, because you would expect your real hand image to grow as it approaches your nose, but (try it in a fully lit room) it actually shrinks because of proprioception, driven by Emmert’s law. The arm muscles are telling your brain that the glowing hand is approaching you, yet it appears to expand. So you are startled. Moreover, if you move the hand too close to yourself, the expansion of the ghost ceases. This result may occur because you do not usually bring or see your hand that close, so your size-constancy mechanisms are not “wired” for it. It might be equally interesting to affix a long dummy arm to artificially lengthen your arm to see what happens.

Crumbling Images
Here is another experiment with the same setup. Move your hand away from its afterimage so that the afterimage remains out in front, but the hand is not. If you are like most of us, you will see the afterimage suddenly starting to fragment, the so-called crumble effect reported in 1973 by P. Davies, then at the University of Aberdeen in Scotland. This breaking apart happens because the brain is confronted with a discrepancy between the visual location of the afterimage and the proprioceptive location of the arm. Abhorring discrepancies, the brain simply starts “shutting down” one image. It is easier to halt an evanescent, inherently unstable afterimage than to shut down muscle and joint sense from the arm. So the image starts to fade and fragment. (Our colleague Stuart Anstis of the University of California, San Diego, has pointed out to us that the effect also occurs for other body parts.)

Another surprising effect takes place if you hold your right hand out in front of you in complete darkness so that congruence is reestablished, and the afterimage of the hand once again robustly reappears. Now move your left hand in between your nose and outstretched right hand (and its afterimage). You would not normally expect anything to happen because, unlike a real glowing right hand, which would be occluded by the interposed left hand, the afterimage should not be occluded—it is still stuck on the retina and should now be seen “superimposed” on the (albeit invisible) left hand. Astonishingly, in at least some trials, the afterimage becomes “occluded,” just as a real hand would—as if the mere expectation is enough to make it fade.

Do these effects occur only with hands, or can they happen for the entire body? By using a suitable placement of the flash camera in front of you while you look down on your own body, it is possible to create an afterimage of your entire body. It helps to wear white clothes, so the afterimage is brighter. (We did this experiment in collaboration with Seckel.) If you now tilt your eyes and head up to look straight ahead, a ghostly apparition of your body will start floating upward away from your real body, creating a momentary feeling of instability. More surprisingly, when we tried the experiment on a patient with chronic intermittent bodily pain, the discrepancy seemed to alter the pain—sometimes increasing it momentarily but mostly reducing it. It remains to be seen if the effect is merely wish-fulfilling suggestibility or a real sensory phenomenon.

Using a powerful flashgun, the reader might wish to try other ingenious variations on the theme. What if you were to superpose the afterimage of the hand on your hand and wiggle your fingers? Have fun!