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.
The team also tested whether the hidden hand became less sensitive. The researchers touched volunteers' index fingers on both real hands in extremely rapid succession and then asked them to guess which one was touched first. Participants tended to ignore information from the hidden hand, often guessing that both fingers had been touched at the same time when really the hidden hand had been tapped first. Overall, the findings suggest that the body begins to forget--or even disown--an appendage once a convincing substitute is present, the authors say.
"I think this study is really cool," says Henrik Ehrsson, a neuroscientist at the Karolinska Institute in Stockholm, Sweden (and, yes, he says the pun is intended). "The body stops caring about its own hand, in a way." Manos Tsakiris, a psychologist who studies sense of self at Royal Holloway, University of London, says the finding is a novel one, showing "a very strong link" between our conscious sense of self and the physical regulation of our body. He adds that the research could potentially open avenues for helping people who have body-perception disorders, such as stroke victims with damage to the self-sensing part of the brain, regain awareness of their forgotten limbs.