Dynamic modulation of the primary somatosensory cortex during seeing and feeling a touched hand.

Previous work has demonstrated cross-modal links between vision and somatosensation at an early stage of sensory processing. Furthermore, recent behavioral studies have shown that viewing the stimulated body part can enhance tactile discrimination ability at the stimulated site. This study aims to investigate the role of the primary somatosensory cortex (SI) during visuotactile integration processes. Subjects looked at a hand in a video being touched on the first digit (D1) in synchrony with felt touches on their real hidden hand as compared with watching a video with asynchronous touches. During synchronous stimulation, subjects reported to feel the tactile sensation on the video hand, thus indicating that in this condition the subjects regarded the video hand as their own touched hand. This feeling disappeared in the asynchronous condition. Using neuromagnetic source imaging, we assessed the topography of the functional organization of SI related to tactile stimulation of D1. 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 that the seen touch in the video represented the touch on the real hand. Thus, only if the seen touch is attributed to the own body, SI seems to be modulated.