Transcranial direct current stimulation of the supplementary motor area modulates behavioral and peripheral electrophysiological measures of motor preparation.

Converging evidence has shown that the Supplementary Motor Area (SMA) is involved in the preparation of movements. Such preparatory activity has been traditionally thought to lead to shorter reaction times by allowing the motor network to settle into states favourable to motor execution without directly engaging the peripheral musculature, limiting such preparatory states to central neural processes only. We revisited this question by recording motor unit related activity using high-density surface EMG grid electrodes, from the Anterior Deltoid muscle, a proximal muscle, during a delayed reach task. We observed that anodal stimulation of the supplementary motor area (SMA) enhances motor preparation by shortening reaction times, while simultaneously and preferentially recruiting smaller amplitude motor units. Additionally, we found that the SMA stimulation also produced a subsequent increase in movement velocities due to the increase activation of larger amplitude motor units. Taken together, we suggest that motor preparatory states involve an active interaction between central and peripheral processes with direct descending SMA outputs to the spinal cord providing a link between cortical preparation and muscle readiness, resulting in decreased reaction times. In addition, the increased gain of such central to peripheral interactions also increases the subsequent gain of during the execution phase via modulating larger amplitude motor unit activation, resulting in increased response vigour.