A “body-swap” effect that convinces people they inhabit a different body from their own has been induced by scientists for the first time.

The experiment, in which volunteers were tricked into perceiving the bodies of other people or mannequins as their own, offers powerful new insights into how the brain constructs the sense of self. It also promises practical implications for treating body image disorders such as anorexia, for designing robotic technology and remote surgery tools, and even for developing better virtual reality games.

The illusion was created with a combination of special goggles and tactile stimulation, which fooled participants into sensing that they had moved into another body. The effect was so powerful that when a tailor’s dummy perceived as a volunteer’s body was threatened with a knife, he or she would exhibit physiological signs of stress, such as increased sweating. Even when the other body was a real person of a different sex or race, subjects said they felt a strong sense of ownership over it. “This shows how easy it is to change the brain’s perception of the physical self,” said Henrik Ehrsson, of the Karolinska Institute in Stockholm, who led the research.

“By manipulating sensory impressions, it’s possible to fool the self. This effect was so strong that people could experience being in another person’s body when facing their own body and shaking hands with it. Our results are of fundamental importance because they identify the perceptual processes that produce the feeling of ownership of one’s body.”

The findings could have therapeutic applications for patients with anorexia, bulimia or mental illnesses in which the sense of self and body image is abnormal, or for confronting ingrained sexual or racial prejudice. “You can see the possibilities, putting a male in a female body, young in old, white in black and vice versa,” Dr Ehrsson said.

Other uses could include improving robotics, so that operators could control artificial arms or surgical instruments more precisely, or in recreation. “It could lead to the next generation of virtual reality applications in games, where people have the full-blown experience of being the avatar,” he said. It could also have military applications, such as robotic soldiers.

In the study, published in the journal Public Library of Science One, the scientists used special goggles to change the perspective from which volunteers see the world. A stereo camera was mounted on a mannequin where the eyes would be and its images were projected on screens inside the goggles worn by the volunteers.

When the volunteers looked down at their bodies, the camera pointed down too and they saw the mannequin’s torso. When each body was stroked simultaneously in the stomach area, they felt that the mannequin’s body was their own.

The scientists then induced a “full-blown body swap”, in which people experienced another body as their own. The camera was put on an experimenter’s head instead of a mannequin’s, and experimenter and volunteer shook hands. Again, the subjects perceived the experimenter’s hand, not their own, as part of their body.

The effect suggests that the brain builds its sense of which body belongs to it less from signals from the muscles, joints and skin, and more from to what it sees. Split-second body awareness is important to co-ordination and to physical survival, and so the brain uses a short-cut that is almost always correct. The body we see as ours through our eyes is usually ours, and so changing the visual input can easily trick our perception.

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