TASK DEPENDENCE OF RESPONSES IN 1ST DORSAL INTEROSSEOUS MUSCLE TO MAGNETIC BRAIN-STIMULATION IN MAN

1. The response of the first dorsal interosseous (1DI) muscle to non-i nvasive magnetic and scalp electrical stimulation of the brain have be en investigated during performance of different manual tasks. 2. The s ix tasks tested required activation of the 1DI muscle, either in isola tion (during abduction of the index finger) or as part of a more compl ex pattern of muscle synergies (e.g. during power grip). The level of 1DI EMG activity across tasks was kept constant by providing subjects with visual feedback of their muscle activity. 3. In every subject (n = 14) magnetic stimulation produced larger responses during performanc e of complex tasks than during the simple index abduction task. The po oled results from all subjects showed that four of the five complex ta sks were associated with significantly larger 1DI responses (paired t test, P < 0.05). 4. These results were confirmed at the single motor u nit level for nine motor units recorded from six subjects. Subjects we re requested to produce a steady discharge of the same motor unit duri ng performance of different tasks. The probability of motor unit disch arge in response to magnetic stimulation was significantly greater dur ing complex tasks (rotation or pincer grips) than during abduction. 5. Scalp electrical stimulation was performed in three subjects with the cathode at the vertex and the anode over the contralateral motor cort ex. The pattern of response amplitudes in the different tasks tended t o parallel that obtained for magnetic stimulation, but the task-relate d differences were smaller. 6. These results suggest that during perfo rmance of the different tasks, the corticospinal volleys evoked by mag netic stimulation may vary in amplitude. The task-related cortical mec hanisms that may contribute to this variability are discussed.