A Brief Communication by Arzy et al in today’s Nature describes how the illusion of a shadowy person was induced in an epileptic patient.
The patient, a 22-year-old woman, had suffered complex partial seizures since the age of 7, and was undergoing a neurological evaluation before a left temporal lobectomy.
When focal electrical stimulation was applied to the left temporoparietal junction, she reported the presence of a sinister shadowy person in her extrapersonal space. The patient, who had no history of psychiatric illness, described the shadow as a young person of an undetermined sex, who neither spoke nor moved, but mimicked her movements. During a seizure in the presurgical evaluation, the patient also reported having an out-of-body experience.
The above figure shows a three-dimensional surface reconstruction of the patient’s left hemisphere. The region that induced the sensation of a doppelgänger is indicated by the pink arrow. Red dots indicate locations which evoked movements when stimulated; blue dots indicate locations which evoked sensations of being touched; speech was induced by stimulating the locations indicated by green dots; and the foci of the patient’s epileptic seizures are indicated by white stars.
Previous studies have implicated the temporoparietal region in distinguishing self from non-self and integrating sensory information. The authors conclude that the sensation reported by the patient was in fact an altered perception of her own body, caused by a disturbance of the integration of proprioceptive and tactile information. (Proprioception is the sense of how one’s own body is oriented in space.) They also note that overactivity in the temporoparietal region of schizophrenics is associated with somatoparaphrenia, a condition in which one’s actions or limbs are misattributed as being performed by, or belonging to, other people.