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!
This article was originally published with the title The Ghost Hand Illusion.