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.
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.
"Approximately 70-80 percent of the people experience the illusion very strongly," 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 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 wasn't perfect as I couldn't quite recognize Petkova's grip as my own, even though that's what the goggles meant to make me believe.
Perhaps the session was too short. The actual study, in which 87 volunteers participated, consisted of repeated sessions that gradually provided more accurate data. The results were published in PLoS One, the online journal of the Public Library of Science.
The principle finding was that under certain conditions a person can perceive another body as his or her own, even if it is of an opposite gender or an artificial body.
"These findings are of fundamental importance because they identify the perceptual processes that make us feel that we own our entire body," the study said.
Ehrsson said the study built on a previous experiment known as the "rubber hand illusion" in which participants were manipulated to experience a rubber hand as their own.
"This goes beyond other recent studies, where you've taken ownership of rubber hands and rubber legs," said Spence, who was not involved with the study.
His only concern was whether there might be any lasting effect on participants.
"The questions is what happens if you did it much longer? If you were in there for days and weeks. Would it be like something out of Total Recall?" Spence said, referring to the 1990 Arnold Schwarzenegger science fiction movie about a virtual vacation that turns into a nightmare.
Ehrsson suggested the findings could be applied in research on body image disorders by exploring how people become satisfied or dissatisfied with their bodies. Another possible application could be developing more advanced versions of computer games such as Second Life, he said.
"It could lead to the next generation of virtual reality applications in games, where people have the full-blown experience of being the avatar," Ehrsson said.
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