The Phantom Hand [Preview]

The feeling of being touched on a fake hand illuminates how the brain makes assumptions about the world

IN ONE VERY STRIKING ILLUSION, you can become convinced that you can feel a rubber hand being touched just as if it were your own. To find out for yourself, ask a friend to sit across from you at a small table. Use blocks or coffee cups to prop up a vertical partition on the table, as shown in the illustration on the opposite page. A flat piece of cardboard will do. Rest your right hand behind the partition so you cannot see it. Then, in view beside the partition, place a plastic right hand—the kind you can buy from a novelty shop or a party store around Halloween. Ask your assistant to repeatedly tap and stroke your concealed right hand in a random sequence. Tap, tap, tap, stroke, tap, stroke, stroke. At the same time, while you watch, he must also tap and stroke the visible dummy in perfect synchrony.

If he continues the procedure for about 20 or 30 seconds, something quite spooky will happen: you will have an uncanny feeling that you are actually being stroked on the fake hand. The sensations will seem to emerge directly from the plastic rather than from your actual hidden flesh.

Why does this happen? Matthew Botvinick and Jonathan Cohen, then at the University of Pittsburgh and Carnegie Mellon University, who reported the so-called rubber-hand illusion in 1998, have suggested that the physical similarity between your real hand and the model is sufficient to fool the brain into attributing the touch sensations to the phony fingers. They believe this illusion is strong enough to overcome the minor discrepancy of the position of your real hand signaled by your body’s joint and muscle receptors versus the site of the plastic hand registered by your eyes.

But that is not the whole story. At about the same time that Botvinick and Cohen observed the rubber-hand effect, we and our colleagues William Hirstein and Kathleen Carrie Armel of the University of California, San Diego, discovered a further twist: the object your helper touches does not even need to resemble your palm and digits. He can produce the same effect if he just pets the table. Try the same experiment, but this time have your acquaintance rub and tap the surface in front of you while making matching movements on your real, concealed hand. (If using the table alone does not work, practice on a dummy hand first before graduating to furniture.) You may have to be patient, but you will eventually start feeling touch sensations emerge from the wood surface before you. The illusion is even better if you have a rubber sheet covering the tabletop to mimic the tactile qualities of skin.

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