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Body-swap illusion tricks the mind in study

— PHOTO: AP

SURREAL: Valeria Petkova of the Karolinska Institute in Stockholm prepares student Andrew Ketterer to test the ‘body-swap’ illusion.

STOCKHOLM: Shaking hands with yourself could be 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 on Tuesday, neuroscientists at Stockholm’s 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 Mr. Ehrsson’s colleague, Valeria Petkova, as she rubbed my left hand with electrolytic gel and attached electrodes to the middle and index fingers.

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 Ms. 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. Ms. 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.

“Approximately 70-80 per cent of the people experience the illusion very strongly,” Ms. Petkova said. Apparently, I was one of them.

The second experiment was more benign. This time my headset was connected to cameras mounted on a round hat that Ms. Petkova was wearing. We faced each other, extended our right arms and shook hands.

Now that was weird: I was supposed to have the sensation of shaking hands with myself. The illusion was not perfect as I couldn’t quite recognise Ms. Petkova’s grip as my own, even though that is what the goggles meant to make me believe. The session was too short. The actual study, in which 87 volunteers participated, consisted of repeated sessions that provided more accurate data. — AP

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