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WASHINGTON–Swedish researchers have used closed-circuit cameras to craft an illusion so powerful that volunteers were tricked to think their bodies had been swapped, to the extent of a woman believing she was in a man's body and vice-versa.
The experiment, reported in the Public Library of Science journal PLoS ONE yesterday, shows it is possible to manipulate the human mind to create the perception of having another body, the researchers said, and it helps explain how humans understand the limits of their own bodies.
Valeria Petkova and Henrik Ehrsson of the Karolinska Institute in Stockholm set up a series of experiments aimed at tricking their volunteers, expanding on a common illusion in which people can be fooled into thinking a rubber hand is their own.
In that example, a person's real hand is concealed and stroked at the same time the visible rubber hand is. The brain will often trick the subject into truly believing the rubber hand is his or her own hand.
Petkova and Ehrsson went further, using a closed-circuit camera to fool volunteers into believing a rubber mannequin – and eventually, the body of another human being – was in fact their own body.
"This effect is so robust that ... a participant can face his or her biological body and shake hands with it without breaking the illusion," they wrote.
The first test was with a life-sized mannequin.
"Two closed-circuit television cameras were positioned on a male mannequin such that each camera recorded events from the position corresponding to one of the mannequin's eyes," they wrote.
Each subject was fitted with a head-mounted display linked to the cameras, "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 downward as if looking down at their bodies. Thus, they saw the mannequin's body where they expected to see their own."
Later, they tried a full body swap between volunteers and it worked.
The illusion has limits. Volunteers couldn't be fooled into thinking they were inside a box, for example.
