Morphing the body: Illusory feeling of an elongated arm affects somatosensory homunculus

Recent studies suggest that in contrast to traditional views of the body map the topographic representation in primary somatosensory cortex (SI) reflects the perceived rather than the physical aspects of peripheral stimulation. Here, we created a simple illusion of feeling an elongated arm by using the dominance of the visual domain over the tactile sense: employing an artificial hand and arm, which were connected to the body, subjects were given the visual impression that they had an extended arm. Since it is known from animal studies that tactile illusions alter early sensory processing in SI, we expected a modulation of the topography in SI corresponding to this illusion. Behavioral results showed that during the illusion the participants felt that their arm was elongated. Neuromagnetic source imaging of the functional organization in SI revealed that the cortical distance between first (D1) and fifth digit (D5) decreased when subjects felt the arm elongated. Since this modulation was significantly positively correlated with the illusionary feeling of an extended arm, the results suggest an involvement of SI during perceived changes in the size of body parts. We discuss the results as possible top-down modulations of SI by higher order somatosensory areas.