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.
Assimilating the Hand
This illusion is extraordinarily compelling the first time you encounter it. But how can scientists be certain that you have now perceptually assimilated the table into your body image (rather than merely assigning ownership to it the same way you own a house)? In 2003 Armel and one of us (Ramachandran) learned that once the illusion has developed, if you “threaten” the table or dummy by aiming a blow at it, the person winces and even starts sweating, as she would if she were facing a real threat to her own body. We demonstrated this reaction objectively by measuring a sudden decrease in electrical skin resistance caused by perspiration—the same galvanic skin response used in lie detector tests. It is as if the table becomes incorporated into a person’s own body image so that it is hooked up to emotional centers in the brain; the subject perceives a threat to the table as a threat to herself.
These illusions demonstrate two important principles underlying perception. First, perception is based largely on extracting statistical correlations from sensory inputs. As you feel your unseen hand being tapped and stroked and see the table or dummy hand being touched the same way, your brain in effect asks itself, “What is the likelihood that these two sets of random sequences [on the hidden hand and on the visible table or dummy] could be identical simply by chance? Nil. Therefore, the other person must be touching me.”
Second, the mental mechanisms that extract these correlations are based on automatic processes that are relatively impervious to higher-level intellect. With information gathered by sensory systems, the brain makes its judgments automatically; they do not involve conscious cogitation. Even a lifetime of experience that a table is not part of your body is abandoned in light of the perceptual decision that it is. Your “knowing” that it cannot be so does not negate the illusion (just as some people cling to superstitions even while recognizing their absurdity).
The experiment was inspired by earlier work we had done with patients who had phantom limbs. After a person loses an arm from injury or disease, he may continue to sense its presence vividly. Often the phantom seems to be frozen in a painfully awkward position. We asked a patient to put his phantom left arm on the left side of a mirror propped vertically on a table in front of him. He then put his intact right arm on the right side, so its reflection was seen in the mirror superimposed on the phantom, creating the visual illusion of having restored the missing arm. If the patient now moved his right arm, he saw his phantom move. Remarkably, this “animated” the phantom so it was felt to move as well—sometimes relieving the cramp. Even more surprising: in some cases, if the physician touched the real hand, the patient not only saw his phantom being touched but experienced the touch as well. Again the brain regards this combination of sensory impressions as unlikely to be a coincidence; therefore, it quite literally feels the touch emerging from the phantom hand.
Consider what these illusions imply. All of us go through life making certain assumptions about our existence. “My name has always been Joe,” someone might think. “I was born in San Diego,” and so on. All such beliefs can be called into question at one time or another for various reasons. But one premise that seems to be beyond question is that you are anchored in your body. Yet given a few seconds of the right kind of stimulation, even this axiomatic foundation of your being is temporarily forsaken, as the table next to you seems to become part of you. As Shakespeare aptly put it, we are truly “such stuff as dreams are made on.”