In 19 healthy volunteers, we used transcranial magnetic stimulation (TMS) to probe the excitability in pathways linking the left dorsal
premotor cortex and right primary motor cortex and those linking the left and right motor cortex during the response delay and the
reaction time period while subjects performed a delayed response [symbol 1 (S1) - symbol 2 (S2)] Go–NoGo reaction time task with
visual cues. Conditioning TMS pulses were applied to the left premotor or left motor cortex 8 ms before a test pulse was given to the
right motor cortex at 300 or 1800 ms after S1 or 150 ms after S2. S1 coded for right-hand or left-hand movement, and S2 for release
or stopping the prepared movement. Conditioning of the left premotor cortex led to interhemispheric inhibition at 300 ms post-S1,
interhemispheric facilitation at 150 ms post-S2, and shorter reaction times in the move-left condition. Conditioning of the left motor
cortex led to inhibition at 1800 ms post-S1 and 150 ms post-S2, and slower reaction times for move-right conditions, and inhibition at
300 and 1800 ms post-S1 for move-left conditions. Relative motor evoked potential amplitudes following premotor conditioning at
150 ms post-S2 were significantly smaller in ‘NoGo’ than in ‘Go’ trials for move-left instructions. We conclude that the excitability in
left premotor / motor right motor pathways is context-dependent and affects motor behaviour. Thus, the left premotor cortex is
engaged not only in action selection but also in withholding and releasing a preselected movement generated by the right motor
cortex.