Delay of the execution of rapid finger movement by magnetic stimulation of the ipsilateral hand-associated motor cortex.

We investigated the influence of focal transcranial magnetic stimulation (TMS) of the hand-associated motor cortex on the execution of ipsilateral finger-lifting movements in six humans. In a simple reaction time paradigm, suprathreshold TMS (1.6- to 2.1-fold of the response threshold determined at rest) was performed at intervals of 40, 70, 80, 90, and 100 ms after the auditory "go" signal. Movement onset was measured with an accelerometer. TMS delayed the execution of ipsilateral finger movement when the cortex stimulus preceded the onset of the intended movement by about 25-65 ms. Taking the corticomuscular conduction times to the activated muscles into account, TMS suppressed the output from the motor cortex in a period 6-45 ms after the contralateral motor cortex was stimulated. Such timing would be compatible with an interhemispheric inhibition similar to the previously described ipsilateral inhibition of ongoing tonic motor activity. The delay of the movement was 40 ms. The function of the neuronal structures mediating interhemispheric inhibition might be to suppress the coactivation of the other hand during unilateral finger movements within bimanual motor tasks.