Decreased corticospinal excitability after subthreshold 1 Hz rTMS over lateral premotor cortex

Objective: To study whether trains of subthreshold 1 Hz repetitive transcra nial magnetic stimulation (rTMS) over premotor, prefrontal, or parietal cor tex can produce changes in excitability of motor cortex that outlast the ap plication of the train. Background: Prolonged 1 Hz rTMS over the motor cort ex can suppress the amplitude of motor-evoked potentials (MEP) for several minutes after the end of the train. Because,rms can produce effects not onl y at the site of stimulation but also at distant sites to which it projects , the authors asked whether prolonged stimulation of sites distant but conn ected to motor cortex can also lead to lasting changes in MEP. Methods: Eig ht subjects received 1500 magnetic stimuli given at I Hz over the left late ral frontal cortex, the left lateral premotor cortex, the hand area of the left motor cortex, and the left anterior parietal cortex on four separate d ays. Stimulus intensity was set at 90%, active motor threshold. Corticospin al excitability was probed by measuring the amplitude of MEP evoked in the right first dorsal interosseous muscle by single suprathreshold stimuli ove r the left motor hand area before, during, and after the conditioning train s. Results: rTMS over the left premotor cortex suppressed the amplitude of MEP in the right first dorsal interosseous muscle. The effect was maximized (approximately 50% suppression) after 900 pulses and outlasted the full tr ain of 1500 stimuli for at least 15 minutes. Conditioning rTMS over the oth er sites did not modify the size of MEP. A control experiment showed that l eft premotor cortex conditioning had no effect on MEP evoked in the left fi rst dorsal interosseous muscle. Conclusions: Subthreshold 1 Hz rTMS of the left premotor cortex induces a short-lasting inhibition of corticospinal ex citability in the hand area of the ipsilateral motor cortex. This may provi de a model for studying the functional interaction between premotor and mot or cortex in healthy subjects and patients with movement disorders.