TEMPORARY INTERFERENCE IN HUMAN LATERAL PREMOTOR CORTEX SUGGESTS DOMINANCE FOR THE SELECTION OF MOVEMENTS - A STUDY USING TRANSCRANIAL MAGNETIC STIMULATION

It is known that damage to the left hemisphere can lead to movement de ficits, and that patients with apraxia have difficulty in selecting mo vements. Neurophysiological recording studies and lesion studies have shown that the premotor cortex is important for the selection of movem ents in monkeys, In this study we used transcranial magnetic stimulati on (TMS) to disrupt the processing in human premotor cortex. We applie d TMS to normal healthy volunteers over the premotor and primary motor areas while they carried out choice reaction time and simple reaction -time tasks. We measured response times of either hand as subjects wer e stimulated over the left and right hemisphere separately. We found t hat we were able to delay responses by stimulating at short cue-stimul us intervals (100-140 ms) over premotor cortex and at longer cue-stimu lus intervals (300-340 ms) over primary motor cortex while subjects pe rformed the choice reaction-time task with the contralateral hand. We were also able to delay responses with the ipsilateral hand while stim ulating over the left premotor cortex, but not while stimulating over the right premotor cortex or either sensorimotor cortex, Premotor cort ex stimulation alone disrupts an early stage of movement selection; mo tor cortex stimulation disrupts the movements at a later stage of exec ution, There was no distinguishing short cue-stimulus interval effect when premotor cortex was stimulated in the simple reaction time paradi gm, where the movement selection demands of the task are kept to a min imum, We conclude that the premotor cortex is important for selecting movements after a visual cue and that the left hemisphere is dominant for the rapid selection of action.