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See Inside December 2008/January 2009

When Your Mind Disowns a Limb

When the mind is fooled into disowning a limb, body functions go awry

Look down. There isn’t a doubt in your mind that the body you are looking at is yours. But what if you could be fooled into thinking that one of your hands belonged to someone else? Scientists at the University of Oxford recently incited this false perception through an illusion—and they found that when people felt dissociated from a limb, their brain devoted less processing power toward that limb and even interfered with its temperature regulation. These findings, building on a smattering of other studies in disembodiment, suggest that the conscious mind’s control over basic body function is much stronger than scientists once thought.

The science of out-of-body exper­iences seems to have begun with an impromptu party trick. In 1998 Matthew Botvinick, a psychiatrist at the University of Pittsburgh, attended a gathering where the hostess happened to have a rubber hand as a Halloween prop. Botvinick, who was interested in how we distinguish our body from other objects, immediately thought up an experiment. He asked a fellow guest to hide one of his hands behind an opaque screen, then placed the rubber hand next to the screen where the person could see it. When Botvinick simultaneously tickled the man’s hidden hand and the rubber hand with brushes, the person felt as if the rubber hand was his own. Thus, the “rubber hand illusion,” as it is now called, was born. [For more, see “The Phantom Hand,” by V. S. Ramachan­dran and Diane Rogers-Ramachandran; Scien­tific American Mind, December 2004.]

But how does it work? Seeing the touch delivered on the rubber hand “captures the sense of touch experienced by the subject’s real hand,” explains Manos Tsakiris, a psychologist at Royal Holloway, University of London. “The rubber hand illusion shows that the integration of different senses is powerful enough to fool the brain into treating a fake hand as a real one,” he says.

Last year scientists used a similar tactic to fake an entire out-of-body ex­perience: they gave subjects goggles that played live feeds from two video cameras located eye distance apart two meters behind them. The experimenter stood beside the subject and used two rods to touch the person’s actual chest and the “illusory chest,” the space that would correspond to the chest of some­one whose eyes were located at the two video cameras. After two minutes, subjects began to feel as if they were sitting two meters behind their bodies.

The stronger a person experiences these types of illusions, the stronger the activity in his or her brain’s premotor and parietal cortices, which integrate sensory and movement information. The brain’s fear circuits are also affected: although subjects know they are experiencing an illusion, they become protective of their new body part. When experimenters threaten the fake hand, brain areas corresponding to threat responses and withdrawal urges become more active.

In the latest study, University of Oxford scientists used the illusion to figure out what happens to the hand that becomes “disowned.” Immediately after the brain begins thinking of the rubber hand as its own, the temperature of the disowned hand drops (while the rest of the body’s temperature remains the same). When an experimenter touches the disowned hand, the subject’s brain responds more slowly than it does when his or her other real hand is touched. These results suggest that when the brain forgets about a limb, the body responds accordingly.

Disease-associated problems in temperature regulation have always been attributed to central nervous system defects, never to thought alone—but these new results offer the tantalizing suggestion that conscious control may be possible, too. These types of illusion-based experiments could help scientists understand what is required for ampu­tees to accept prosthetic limbs as “real” body parts, and they could lead to treat­ments for people who disown their limbs, such as stroke patients who stop recog­nizing their paralyzed body parts.

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