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

We investigated the influence of focal transcranial magnetic stimulation (T MS) of the hand-associated motor cortex on the execution of ipsilateral fin ger-lifting movements in six humans. In a simple reaction time paradigm, su prathreshold 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 au ditory "go" signal. Movement onset was measured with an accelerometer. TMS delayed the execution of ipsilateral finger movement when the cortex stimul us preceded the onset of the intended movement by about 25-65 ms. Taking th e corticomuscular conduction times to the activated muscles into account, T MS suppressed the output from the motor cortex in a period 6-45 ms after th e contralateral motor cortex was stimulated. Such timing would be compatibl e with an interhemispheric inhibition similar to the previously described i psilateral inhibition of ongoing tonic motor activity. The delay of the mov ement was 40 ms. The function of the neuronal structures mediating interhem ispheric inhibition might be to suppress the coactivation of the other hand during unilateral finger movements within bimanual motor tasks.