Rubber hand' illusion creates ghost sensations

IF YOU and several other people have your arms resting on a table, how do you know which one is yours? It seems that when more than one sensory signal tells the same story, your brain believes it, even if the hand it thinks is yours is a rubber dummy.

This explanation for the so-called "rubber hand illusion" was suggested last year, but sceptics argued that the illusion depends solely on sight. Now the research team that published the original study has shown that people are deceived even when blindfolded.

Last year, Henrik Ehrsson at the Institute of Neurology in London and his colleagues asked volunteers to place one hand under a table while a rubber one was placed in view. Then researchers used two paintbrushes to simultaneously stroke the rubber hand, which subjects could see, and their hidden hand, which they could feel. Within about 10 seconds, everyone was under the strange illusion that the rubber arm was actually their own.

As the illusion set in, activity intensified in the brain's premotor cortex, a region known to process multisensory inputs. Ehrsson and his colleagues suggested that we consider a body part to be our own when multiple sensory signals - from sight and touch, for instance - are put together (Science, vol 305, p 875).
“When more than one sense tells you the same story your brain believes it. Even if the hand it thinks is yours is a dummy”

But critics suggested that knowing your own body part does not involve several senses and that the experiment instead showed how vision can override other senses. So Ehrsson's group has now set up a new experiment with blindfolded volunteers. Researchers placed a volunteer's real index finger against a rubber knuckle while they simultaneously touched the real knuckle on the other hand. Once again, within seconds, volunteers were convinced they were touching their own hand. In fact, when asked to point to their other hand, they pointed at the fake. "The brain compares two sensory signals to figure out this is me or not me," says Ehrsson.

From issue 2505 of New Scientist magazine, 25 June 2005, page 10