The dorsal premotor cortex exerts a powerful and specific inhibitory effect on the ipsilateral corticofacial system: a dual-coil transcranial magnetic stimulation study

A rich pattern of connectivity is present in non-human primates between the dorsal premotor cortex (PMCd) and the motor cortex (M1). By analogy, similar connections are hypothesized in humans between the PMCd and the ipsilateral hand-related M1. However, the technical difficulty of applying transcranial magnetic stimulation (TMS) with a dual-coil paradigm to two cortical regions in such close spatial proximity renders their in vivo demonstration difficult. The present work aims at assessing in humans the existence of short-latency influences of the left PMCd on the ipsilateral corticofacial system by means of TMS. A dual-coil TMS paradigm was used with 16 participants. Test TMS pulses were applied to the left orofacial M1, and conditioning TMS pulses were applied to three distinct points of the ipsilateral PMCd along the caudal part of the superior frontal sulcus. The inter-stimulus interval (ISI) between condTMS and testTMS varied in 2-ms steps between 2 and 8 ms. Motor evoked potentials (MEPs) in the active orbicularis oris muscle were recorded. CondTMS exerted a robust effect on the corticofacial system only when applied to one specific portion of the PMCd and only at one specific ISI (6 ms). The effect consisted in a systematic suppression of facial MEPs compared to those obtained by testTMS alone. No other effect was found. We provide evidence for a specific short-latency inhibitory effect of the PMCd on the ipsilateral M1, likely witnessing direct corticocortical connectivity in humans. We also describe a novel paradigm to test ipsilateral PMCd-M1 in humans.