Projecting a sensation to an alien limb involves the somatosensory cortex

Michael Schaefer

Abstract
Previous work has shown that the body image can be distorted as a consequence of injury (e.g., phantom sensations). A number of studies suggest similar illusions in healthy subjects. The present study investigated an illusion in which tactile sensations are referred to an alien limb. Subjects watched a video, which showed a hand where the first digit (D1) was stroked by a stick. At the same time subjects were stimulated on D1 either synchronously or asynchronously to the hand in the video. Subjects reported an illusion of tactile sensation in which the real hand stimulation was referred to the hand in the video. During the experiment the topography of the functional organization of the primary somatosensory cortex (SI) related to tactile stimulation of D1 was assessed using neuromagnetic source imaging. The cortical representation of D1 moved to a more inferior location during synchronous in comparison to asynchronous stimulation and rest. This modulation of the map in SI was significantly positively correlated with the feeling of the alien limb. Our data suggest that somatosensory cortical maps contribute to the experienced illusion in which the subjects seemed to feel the touching of the hand on an alien limb.

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