Swedish researchers create body-swap illusion

WASHINGTON Tue Dec 2, 2008 4:16pm EST

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WASHINGTON (Reuters) - Researchers using closed-circuit televisions to create an illusion have made volunteers virtually swap bodies, even making women believe they were in a man's body and vice-versa.

The experiment, reported in the Public Library of Science journal PLoS ONE on Tuesday, shows it is possible to manipulate the human mind to create the perception of having another body, the Swedish researchers said.

It helps explain how humans understand the limits of their own bodies, Valeria Petkova and Henrik Ehrsson of the Karolinska Institute in Stockholm reported.

They set up a series of experiments aimed at fooling their volunteers, each an extension of a common illusion in which people can be fooled into thinking a rubber hand is their own.

For the illusion, the volunteer's real hand is concealed and stroked at the same time the visible rubber hand is. The brain will often trick the volunteer into truly feeling as if the rubber hand is his or her own hand.

Petkova and Ehrsson went further, using a closed-circuit camera to fool their volunteers into believing a rubber mannequin was in fact their own body -- and eventually, that another human being was.

"This effect is so robust that, while experiencing being in another person's body, a participant can face his or her biological body and shake hands with it without breaking the illusion," they wrote.

They started with a life-sized mannequin.

"Two closed-circuit television cameras were positioned on a male mannequin such that each recorded events from the position corresponding to one of the mannequin's eyes," they wrote.

"A set of head-mounted displays connected to the cameras was worn by the participants, and connected in such a way that the images from the left and right video cameras were presented on the left and right eye displays, respectively, providing a true stereoscopic image," they added.

"Participants were asked to tilt their heads downwards as if looking down at their bodies. Thus, the participants saw the mannequin's body where they expected to see their own."

As in the rubber hand illusion, the experimenters touched the volunteers' bodies at the same time as they touched the mannequin.

"...it was evident that the participants had felt the mannequin's body to be their own body," they wrote.

Finally, they tried a full body swap between two volunteers and it worked.

The illusion only goes so far. The researchers said they could not fool their volunteers into thinking they were a box, for example.

(Reporting by Maggie Fox; Editing by Julie Steenhuysen)

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