MEP-independent silent periods in hand muscles elicited by transcranial magnetic stimulation of the ventral premotor cortex: a non-invasive tool to explore premotor negative motor areas

OBJECTIVE: We investigate the possibility to disrupt motor activity via premotor and parietal cortex stimulation, by inducing cortical silent periods (SPs) in the voluntarily activated upper limb. METHODS: We analyzed data from 17 subjects with normal brain function, using navigated TMS (nTMS) on individual MR anatomies. We applied single-pulse biphasic stimulation at 120% of resting motor threshold (rMT) in blocks of 30 stimulations on each spot of a 10-16 point grid covering the inferior parietal and frontal lobes in the dominant hemisphere while participants performed voluntary submaximal contraction. Electromyography (EMG) was recorded bilaterally from intrinsic hand muscles. RESULTS: We observed SPs not preceded by a MEP in the contralateral hand in 16/17 participants. The maximum overlap, of individual areas where such MEP-independent SPs could be evoked, corresponded to the ventral precentral gyrus (MNI coordinates: [x = -57, y = 7, z = 33]). In a subset of stimulus sites, MEP-independent SPs were bilateral, with contralateral predominance. Canonical early-onset MEPs were observed in all patients, with maximum overlap over the primary motor cortex. We also observed rare contralateral late-onset (>23 ms) MEP-like responses from peri-Rolandic TMS. CONCLUSIONS: MEP-independent SPs are systematically elicitable in healthy participants They likely reflect interference with premotor representations of ongoing movements. They offer a novel possibility to investigate higher-order motor functions in the experimental and clinical neurosciences.