You're walking down an empty street alone, when suddenly, you have the eerie feeling that someone's following you. Is your mind playing tricks on you? Maybe so. According to a new study, when a specific region of the brain called the left temporoparietal junction (TPJ) is stimulated, it can create the illusion of a "shadow person." Given that such experiences are often heightened in psychotic disorders such as schizophrenia and paranoia--and even in those who believe they've been abducted by aliens--the results could lead to a better understanding of these neurological conditions.
The finding emerged by accident. Neurologist Olaf Blanke of the Brain Mind Institute in Lausanne, Switzerland, and his colleagues, were attempting to identify the source of epileptic seizures in a 23-year-old woman. They applied a mild current through surgically implanted electrodes to various regions of her brain. Not much happened until the researchers stimulated the woman's left TPJ, located roughly above the left ear. Suddenly, she reported feeling the presence of a mystery person behind her, a motionless and speechless shadow that imitated her body posture and actions. "He" lay beneath her when she lay down, sat behind her when she sat down, and attempted to take a test card from her when she tried to participate in a language exercise.
Such delusions are similar to those seen in patients with schizophrenia, says Blanke. Schizophrenics often mistake their own bodies to be someone else's, for example, and attribute their own actions to others. They also have frequent illusions of being followed, or controlled by a stranger, as do those who claim to have been manipulated by aliens.
Blanke says the shadow person phenomenon may shed light on how the brain perceives "self." In order to recognize its own body, he says, the brain uses sensory information, such as visual and proprioceptive cues (which indicate the position of body parts relative to each other and everything else). The TPJ is known to put some of these cues together. When this function is disrupted, the brain perceives two bodies instead of one and mistakes the second for that of a stranger, the researchers propose tomorrow in Nature.
It's a valid idea, says neurologist Pawan Sinha of the Massachusetts Institute of Technology in Boston. But this might be just one of many mechanisms that generate such hallucinations, he says.