Swiss researchers made the discovery while evaluating a young woman for surgery to treat epilepsy. They believe their finding could help explain feelings such as paranoia which afflict patients suffering from schizophrenia.

When they electrically stimulated the left temporoparietal junction in her brain, which is linked to self-other distinction and self-processing, she thought someone was standing behind her.

If they repeated the stimulus while she leaned forward and grabbed her knees she had an unpleasant sensation that the shadowy figure was embracing her.

"Our findings may be a step towards understanding the mechanisms behind psychiatric manifestations such as paranoia, persecution and alien control," said Olaf Blanke, of the Ecole Polytechnique Federale de Lausanne, in the journal Nature.

The feeling that someone is lurking nearby has been described by patients suffering from psychological and neurological problems. The researchers believe the woman was experiencing a perception of her own body.

"Although our patient was aware of the similarity between her own postural and positional features and those of the illusory person, she did not recognise that that person was an illusion of her own body, like many deluded schizophrenia patients," Blanke added.