The left dorsal premotor cortex (PMd) is thought to play a dominant role in the selection of movements made by either hand. We used transcranial magnetic stimulation to study the functional connectivity of the left PMd and right primary motor cortex (M1) during an acoustic choice reaction time (RT) task involving contraction of the thumb and forefinger. The facilitatory and inhibitory pathways that can be demonstrated between left PMd and right M1 at rest were suppressed during most of the reaction period. However, they were activated briefly at the start of the reaction period, depending on whether the cue indicated that the forthcoming movement had to be made with the left or the right hand. The facilitatory pathway was active at 75 ms in those trials in which the subjects were required to move the left hand, whereas the inhibitory pathway was active at 100 ms in trials in which the subjects had to move the right hand. These changes in excitability did not occur in hand muscles not used in the task. There were no significant changes in the excitability of intracortical circuits [short intracortical inhibition (SICI) and intracortical facilitation (ICF)] in the right M1. Interhemispheric interactions between the right PMd and left M1 were mainly inhibitory at rest and showed the same temporal profile of interhemispheric inhibition as for left PMd-right M1, although no evidence was found for facilitatory interactions. The results illustrate the importance of PMd not only in facilitating cued movements but also in suppressing movements that have been prepared but are not used.

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