Body-swap illusion tricks mind in new study
Dec 2, 7:55 AM (ET)
By KARL RITTER
STOCKHOLM, Sweden (AP) - 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.
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.
Charles Spence, a professor of experimental psychology at the
University of Oxford, said the Karolinska study was a "step up" from
other research on the subject.
"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.