Here’s a trick to make a rubber hand come to life. Hide your right hand under a cloth and stick the rubber hand where your right hand should be. Now have someone stroke your right hand and the fake hand at the same time. Before you know it, you’ll begin to “feel” sensation in the rubber hand. But what happens to your real right hand? New research suggests that your body begins to disown it. Psychologists have used the rubber-hand illusion for years to study how people perceive body boundaries. How, for example, does your brain know where you stop and a bicycle begins? Brain scans reveal that the premotor cortex, the part of the brain that integrates vision and touch, helps the body adopt the rubber hand, but no one had looked at what was going on with the hidden, real hand.
Lorimer Moseley, a neuroscientist who studies pain at Oxford University in the U.K., and colleagues repeated the rubber-hand experiment on 11 volunteers, but they added a twist: They took the temperature of the hidden hand. During the 7-minute illusion, the researchers found that the average temperature of the hidden hand dropped 0.27°C in all participants; the temperature of other body parts, including the person’s other real hand, remained the same. The researchers also tried stroking the rubber hand and the experimental hand asynchronously, a trick that diminishes the illusion. In this case, the hidden hand cooled down but slightly less than when the hands were stroked at the same time. The more strongly volunteers rated the vividness of the illusion, the colder their hidden hands became, the team reports online this week in the Proceedings of the National Academy of Sciences.