Rubber hand shows brain can be fooled on skin colour

Taking pointers from rubber hands, psychologists have shown that racial differences really are only skin deep.

When someone sees a rubber hand being stroked while their actual hand is hidden from view and stroked simultaneously, they can begin to "embody" it ? to feel that the rubber hand is their own ? and lose feeling in their real hand.

But can you embody a rubber hand of a different skin colour from your own? Manos Tsakiris and colleagues at Royal Holloway, University of London, induced the illusion in 22 white participants, using both white and black rubber hands. Later, the subjects claimed, on average, to have identified more strongly with the white hand.

Objective measures suggested otherwise, though. For instance, one measure of the strength of the illusion is a shift in the perceived location of the real hand ? known as proprioceptive drift. The volunteers showed the same amount of drift regardless of the colour of the rubber hand.

Surface features

The team also tested changes in skin conductance ? a measure of stress ? when the subjects saw a needle being stuck into the embodied rubber hand. In theory, the more the subject identifies with the rubber hand, the greater this stress response. Yet they were just as stressed watching the needle puncture the black hand as the white one.

"The processes that are involved in the illusion aren't particularly sensitive to the skin colour of the hand," says team member Harry Farmer.

"The way that the brain defines who we are doesn't care that much about the surface features, it cares about actual sensory experiences," says Patrick Haggard of University College London, who was not involved in the work.

Journal reference: Consciousness and Cognition, DOI: 10.1016/j.concog.2012.04.011

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