Body Swap Illusion Tricks Mind

Dec. 2, 2008 -- Shaking hands with yourself is an amusing out-of-body experience. The illusion of having your stomach slashed with a kitchen knife, not so much.

Both sensations, however, felt real to most participants in a Swedish science project exploring how people can be tricked into the false perception of owning another body.

In a study presented Tuesday, neuroscientists at Stockholm's renowned Karolinska Institute show how they got volunteers wearing virtual reality goggles to experience the illusion of swapping bodies with a mannequin and a real person.

"We were interested in a classical question that philosophers and psychologists have discussed for centuries: why we feel that the self is in our bodies," project leader Henrik Ehrsson said. "To study this scientifically we've used tricks, perceptual illusions."

It sounded intriguing enough for me to try it, though entering the laboratory on Monday, I was having second thoughts. The first props I saw were two kitchen knives, three naked dummies and a prosthetic hand sticking out from behind a curtain.

"You have the right to say stop at anytime if you feel uncomfortable," said Ehrsson's colleague, Valeria Petkova, as she rubbed my left hand with electrolytic gel and attached electrodes to the middle and index fingers.

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She assured me I was not in any danger. Still, a nervous tingle rushed through my body as she placed the headset over my eyes.

In the first experiment, the goggles were hooked up to CCTV cameras fitted to the head of a male mannequin, staring down at its feet. Through the headset I saw a grainy image of the dummy's plastic torso. I tilted my head down to create the sensation I was looking down at my own body.

At that point, it didn't feel very real. But when Petkova simultaneously brushed markers against my belly and that of the mannequin, the effect started setting in. As my brain processed the visual and tactile signals, I had a growing impression that the mannequin's body was my own.

That was good fun, until the gleaming blade of a bread knife entered my field of vision. Petkova slid it across the dummy's stomach, sending shivers down my spine and a pulse of anxiety through the electrodes. My heightened stress level was illustrated by a spike in a computer diagram shown to me after the experiment.