Your body is mine

WASHINGTON — It sounds like a lost episode of The Twilight Zone. A man enters a laboratory, dons a special headset and shakes hands with a woman sitting across from him. In a matter of seconds, he feels like he’s inside the woman’s skin, reaching out and grasping his own hand.

Strange as it sounds, neuroscientists have induced this phenomenon in a series of volunteers. People can experience the illusion that either a mannequin or another person’s body is their own body, says Valeria Petkova of the Karolinska Institute in Stockholm. She and Karolinska colleague Henrik Ehrsson call this reaction the “body-swap illusion.”

“Our subjects experienced this illusion as being exciting and strange, and often said that they wanted to come back and try it again,” says Petkova, who reported the findings November 17 at the annual meeting of the Society for Neuroscience.

Illusory body-swapping could provide a new tool for studying the nature of self-identity and psychiatric disorders that involve distortions of body image, she suggests. This phenomenon might also be tapped to enhance user control over virtual reality applications and to prompt a person’s sense of really being part of a virtual world.

Volunteers experienced the body-swap illusion by receiving simultaneous visual and motor input from another’s body. In one experiment, each participant stood across from a male mannequin, and in another experiment volunteers faced a female experimenter. A headset covering participants’ eyes displayed a three-dimensional view of the other’s visual perspective, transmitted from a small video camera positioned on the mannequin’s or the woman’s head.

In the mannequin situation, an experimenter simultaneously touched the participant’s belly and the mannequin’s belly with separate probes. So the volunteer felt a poking in the abdomen but saw the poking happen as if he or she were the mannequin. In the real-person situation, participant and experimenter shook hands. Thus, while volunteers felt the sensation of hand shaking, it appeared to them that they were shaking their own hand. After 10 to 12 seconds of abdominal touch or hand-shaking, male and female participants spontaneously had the experience of looking out from the body of the male mannequin or the female experimenter. They literally felt that they were in the mannequin’s body getting poked or had embodied the female experimenter and were shaking their own hands.

“In the body-swap illusion, we can see that multisensory information powerfully affects the brain,” says neuroscientist Patrick Haggard of University College London, who was not part of the research team.

Petkova and Ehrsson first confirmed that 16 male and 16 female volunteers experienced an illusory body-swap with a mannequin. After undergoing the procedure, participants indicated on a questionnaire that they had experienced the mannequin’s body as their own. They didn’t feel that they had become plastic like a mannequin, Petkova notes. Volunteers reported having had an expectation that, if they moved, the mannequin’s body would move accordingly.

In a subsequent experiment, the researchers found that 10 volunteers experiencing a body-swap with a mannequin displayed elevated electrical responses in the skin on their fingertips — a physiological indication of heightened emotion — when a knife was passed just over the mannequin’s arm. No such response occurred when a knife was passed just over volunteers’ arms during the illusion.

In a third experiment, 12 volunteers experiencing a body-swap with a female experimenter exhibited comparable physiological signs of emotional arousal when a knife was passed just over the experimenter’s arm, but not just over their own arms.

Gender had no affect on the illusion. Men had no difficulty experiencing a body-swap with a female experimenter, Petkova notes. Women readily experienced the illusion of being in a male mannequin’s body.

“This illusion is so strong that one can face one’s physical body and shake hands with oneself while still experiencing owning another person’s body,” Petkova says.

When a researcher stroked a brush along a volunteer’s own arm, the body-swap illusion vanished. In this way, each participant’s personal sense of touch became disengaged from the other individual’s visual perspective, Petkova proposes.

The new findings build on Ehrsson’s earlier research documenting a “rubber-hand illusion.” To induce that effect, a rubber hand is plausibly positioned on a table to extend from a volunteer’s outstretched arm, while the person’s actual hand is hidden. As an experimenter strokes the rubber hand with a brush, the volunteer eventually experiences the fake hand as his or her own and feels the sensation of being stroked.