Out of body

Brain scientists have succeeded in fooling people into thinking they are inside the body of another person or a plastic dummy.

The out-of-body experience - which is surprisingly easy to induce - will help researchers to understand how the human brain constructs a sense of physical self.

The research may also lead to practical applications such as more intuitive remote control of robots, treatments for phantom limb pain in amputee patients and possible treatments for anorexia.

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The research follows a related study from the same group last year in which the scientists convinced volunteers that they were having an out-of-body experience. It was the first time it had been done in the lab and showed that the intensely spiritual experiences that patients sometimes have while on the operating table, for instance, can have a scientific explanation.

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"We are interested in how normal perception works, how we recognise our own body.

And we do that by studying these perceptual illusions," said Dr Henrik Ehrsson at the Karolinska Institute in Sweden.

"Critically it depends on the visual perspective and the so-called multisensory integration or the combination of visual signals and tactile signals."

In the new study Ehrsson and his colleague, Valeria Petkova, attached two cameras to the head of a dummy. These were hooked up to two small screens placed in front of their subjects' eyes. This gave the illusion that the person was looking through the mannequin's eyes.

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For example, when they looked down they saw the dummy's body and not their own.

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To create the illusion of occupying the dummy's body, the team stroked the abdomen of the subject and the dummy at the same time while the subject watched the stroking via the cameras on the dummy's head. As a result, subjects reported a strong feeling that the dummy's body was their own.

The technique is similar to the "rubber hand illusion", in which a subject can be convinced that a rubber hand is his or her own, but this is the first time the illusion has been extended to a whole body.

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The illusion was so convincing that when the researchers threatened the dummy with a knife they recorded an increase in the subject's skin conductance response - the indicator of stress that polygraph lie detector tests rely on. "This shows how easy it is to change the brain's perception of the physical self," said Ehrsson, who led the project.

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"By manipulating sensory impressions, it's possible to fool the self not only out of its body but into other bodies too."

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Things got even weirder when the researchers dispensed with the dummy and put the cameras on the head of another person. After carrying out the same double stroking routine the subjects were convinced that they were occupying another person's body.

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The illusion persisted even when the other person came over and shook the subject's hand, producing the sensation of the subject feeling as if they were shaking hands with themselves.

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The researchers plan to use the out-of-body illusion to try to treat amputee patients that experience phantom limb pain in the arm or leg they have lost. "We have begun to realise that there could be a link between pain perception and the feeling of ownership of the body," said Ehrsson.

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Another potential angle for research is body image in patients with anorexia. These people become obsessed with reducing their own weight even when they become dangerously thin.

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"Possibly this approach could be used for new diagnostic tools and maybe therapeutic tools to train people better to recognise their actual body size," he said.

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Another application is in remotely operated robots, for example in nuclear power plants or surgery.

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"The hope is to elicit a full-blown illusion that you are the robot," said Ehrsson.

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The results are reported today in the journal PLoS One.