Differential activation in somatosensory cortex for different discrimination tasks

Maps of the body surface in somatosensory cortex have been shown to be high ly plastic, altering their configuration in response to changes in use of b ody parts. The current study investigated alterations in the functional org anization of the human somatosensory cortex resulting from massed practice. Over a period of 4 weeks, subjects were given synchronous tactile stimulat ion of thumb (D1) and little finger (D5) for 1 hr/d. They had to identify t he orientation of the stimuli. Neuroelectric source localization based on h igh-resolution EEG revealed that, when subjects received passive tactile st imulation of D1 or D5, the representations of the fingers in primary somato sensory cortex were closer together after training than before. There was a lso an apparently correlative tendency to anomalously mislocalize near-thre shold tactile stimuli equally to the distant finger costimulated during tra ining rather than preferentially to the finger nearest to the finger stimul ated in a post-training test. However, when the stimulus discrimination had to be made, neuroelectric source imaging revealed that the digital represe ntations of D1 and D5 were further apart after training than before. Thus, the same series of prolonged repetitive stimulations produced two different opposite effects on the spatial relationship of the cortical representatio ns of the digits, suggesting that differential activation in the same regio n of somatosensory cortex is specific to different tasks.