Dynamic organization of the somatosensory cortex induced by motor activity

Intensive and long-lasting experience of altered sensory input induces perm anent changes in the functional organization of the somatosensory cortex. I n addition, an increasing body of evidence suggests the existence of dynami c, short-term and task-dependent adaptation of representational maps within somatosensory cortex. It is hypothesized that somatosensory maps can, not only, be acquired within a short period of time, but might also be set up d uring periods of training related to specific tasks and subsequently activa ted dynamically upon performance of that particular task. In order to test this hypothesis we studied the functional organization of somatosensory cor tex for a heavily overlearned and frequently performed task for which no ne w acquisition of a sensory map had to be assumed. To this end, the function al organization of somatosensory cortex for handwriting was compared with t he organization during rest in healthy humans. Functional organization of t he somatosensory cortex was assessed using non-invasive, neuromagnetic sour ce imaging based on tactile stimulation of the thumb (D1) and little finger (D5) during writing and rest. In different blocks, subjects wrote with the ir right, dominant and their left hand, respectively. During writing, D1 an d D5 of the writing hand were stimulated. To test the reliability of our re sults all measurements were repeated after 1 week. It was found that amplit udes of somatosensory evoked magnetic fields with latencies of 45 ms were r educed during writing compared with rest. This finding is in accordance wit h the sensorimotor gating effect. Using source localization we could show t hat cortical representations of D1 and D5 are more distant during writing w ith either hand compared with rest. Our data suggest that somatosensory cor tical maps undergo rapid modulation depending on task-specific involvement of sensory processing in daily-life overlearned movements. As it is unlikel y that a new sensory map is always acquired when a frequently used task suc h as writing is performed, we suggest that somatosensory cortex switches be tween different, concurrently preexisting maps depending on actual requirem ents. Task-dependent activation of pre-existing maps might be a powerful me chanism to optimize stimulus processing.