Published online 20 September 2006 | Nature | doi:10.1038/news060918-4

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Simple stimulation may underpin complex mental illusions.

Brain stimulation can trigger the spooky feeling of a shadowy person lurking behind you. Punchstock

Simple stimulation of the brain can cause the mind to play complex and creepy tricks on itself, neurologists have discovered. They found that, by inserting electrodes into a specific part of the brain, they could induce a patient to sense that an illusory 'shadow person' was lurking behind her and mimicking her movements.

Doctors treating the patient, a 22-year-old woman with epilepsy, found that when they stimulated a brain region called the left temporoparietal junction, the patient sensed the presence of a sinister figure behind her who copied her actions. They suspect that the effect is due to the mind projecting its own movements onto a phantom figure conjured up by the brain, an effect that is seen in some patients with serious psychiatric conditions.

"It was quite astonishing — she definitely realized the 'person' was taking the same posture as she did, but she didn't make the connection," says Olaf Blanke of the École Polytechnique Fédérale de Lausanne in Switzerland, who led the research. "To her it remained a different person, an alien — exactly what you find in schizophrenics."

The patient had no history of psychiatric problems. So the results suggest that this type of illusion, despite being an apparently complex psychiatric symptom, can be caused by a very simple switch in the brain. The mechanism might help to explain schizophrenic feelings such as paranoia, alien control, and the notion that parts of one's body belong to somebody else.

The phenomenon may also be linked to the 'out-of-body' experiences reported by many people. Previously, Blanke's group has shown that similar brain stimulation can induce a sensation that one is rising up out of one's body (see 'Electrodes trigger out-of-body experience').

Extrasensory perception

Doctors were investigating the patient's brain in preparation for an operation to remove scar tissue that was causing persistent epileptic fits. Up to a third of adult epileptics suffer in this way and cannot be helped by drugs.

The team inserted electrodes directly into the patient's brain to accurately pinpoint the regions that control language and right-hand movement, to ensure that these were not damaged during the subsequent operation. But in doing so, they accidentally interfered with a brain region that coordinates different sensory information to give a sensation of the body's location in space.

"There's a lot of information coming in from your body to your brain," Blanke explains. If you are talking on the telephone, for instance, you will hear your own voice, feel the handset in your hand, and have feedback from your arm muscles to tell you what position you are in. Your brain integrates the information and forms a picture of where your body is and what it is doing.

But in this patient such integration seems to have been scrambled, Blanke says. For the few seconds that the electrical stimulation was occurring, she described a sensation of a shadowy man hovering behind her. And, as the researchers report in this week's Nature1, when she was asked to lean forward and hug her knees, she said it felt as if the man was (unpleasantly) reaching around to grasp her.

The feeling persisted even though the researchers pointed out that it was her mind playing tricks and projecting its own movements. "She was aware of this, but said it remained quite scary. She still had to turn around to check [there was no one there]," Blanke recalls.

Not me

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The work may provide some insight into schizophrenia. Blanke notes that this condition frequently includes problems with perceiving one's own body. In experiments in which subjects view an image of themselves in which one of their arms is rotated by an abnormal amount, for example, schizophrenics will readily declare that the appendage is not theirs. Normal volunteers will disown it only when the rotation reaches about 90° out of normal.

Others caution that this one experiment will have a limited impact on our understanding of such symptoms, however. "Schizophrenia is a syndrome and not a single phenomenon," says Sabine Bahn, a psychiatrist at the University of Cambridge, UK.

Blanke plans to try to replicate the result in other volunteers.

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École Polytechnique Fédérale de Lausanne