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).