The paper, published in the British journal Nature, describes the case of a 22-year-old woman with no history of psychiatric problems who was being evaluated for treatment of epilepsy. When a region of her brain called the left temporoparietal junction was electrically stimulated, the woman described encounters with a ‘shadow person’ who mimicked her bodily movements.

“Electrical stimulation repeatedly produced a feeling of the presence of another person in her extra-personal space,” said Olaf Blanke, co-author of the study conducted by a team of researchers from University Hospital in Geneva, Switzerland.

When the patient was lying down, stimulation of this brain region caused her to feel that someone was behind her. She described the person as young, of indeterminate sex, “a shadow who did not speak or move, and whose position beneath her back was identical to her own”, according to the researchers.

When the patient sat up, leaned forward and clasped her knees, she felt that the figure was also sitting, embracing her in its arms – a feeling she described as “unpleasant”.

During a language task, in which the seated patient held a card in her right hand, she described the person sitting next to her and trying to interfere with the task. “He wants to take the card … he doesn’t want me to read,” she said.

Because it was possible to induce the sensation repeatedly, and because the ‘shadow person’ closely mimicked the patient’s posture and movements, the researchers conclude that the patient was experiencing a perception of her own body.

“The strange sensation that somebody is nearby when no one is actually present has been described by psychiatric and neurological patients, as well as by healthy subjects,” said Blanke. Until now, however, it was not understood how the illusion was triggered in the brain.

The temporoparietal junction is known to be involved in creating the concept of ‘self’, and the distinction between ‘self’ and ‘other’. According to the researchers, stimulation of this region interfered with the patient’s ability to integrate information about her own body, leading to her experience of a ‘shadow person’.

Although the woman was aware of the similarity between her own movements and those of her doppelganger, she didn’t recognise the experience as an illusion of her own body.

Similar shadowy encounters have been described by people with schizophrenia, as well as by healthy subjects, leading the researchers to believe that: “Our findings may be a step towards understanding the mechanisms behind psychiatric manifestations such as paranoia, persecution and alien control