High-definition transcranial direct-current stimulation of left primary motor cortices modulates beta and gamma oscillations serving motor control.

Recent studies have linked non-invasive transcranial direct-current stimulation (tDCS) with altered neural processing near the site of stimulation and across a distributed network of brain regions, with some evidence for a possible therapeutic role. However, negative results also exist and the potential impacts on motor-related neural oscillations have rarely been studied. Herein, we applied high-definition tDCS to the left primary motor cortex of 62 healthy adults in three sessions (anodal, cathodal and sham). Participants then performed a motor task with two conditions (i.e. cognitive interference and no interference) during magnetoencephalography (MEG). The MEG data were imaged in the time-frequency domain and whole-brain, voxel-wise maps were probed for task condition and stimulation effects. Our results indicated the classic pattern of slower behavioural responses and stronger neural oscillations in frontal and parietal cortices during interference relative to no-interference trials. Importantly, we found task condition-by-stimulation interactions involving motor-related gamma oscillations, with weaker interference effects after cathodal stimulation relative to anodal and sham in the right prefrontal, left temporoparietal junction and left cerebellar cortices. Conversely, stronger gamma interference responses were found in the right motor and superior parietal cortices following anodal relative to cathodal and sham. Lastly, main effects of stimulation indicated stronger beta oscillations following anodal stimulation in the left supplementary motor area. Taken together, these data provide key mechanistic insight into the polarity-specific effects of tDCS on the neural oscillatory dynamics serving motor control. Such findings reflect the modulatory effects of tDCS on population-level neural oscillatory responses distant from the stimulation site. KEY POINTS: Neurophysiological studies have indicated that beta and gamma oscillations are critical to motor control and that their dynamics are modulated by higher-order features of the task. Recent investigations have shown that transcranial direct-current stimulation (tDCS) affects neural activity both locally and in brain regions distant from the stimulation site, but the mechanisms remain poorly understood. Sixty-two adults underwent anodal, cathodal and sham high-definition tDCS of the left motor cortices and completed a motor task with two levels of cognitive interference during magnetoencephalography (MEG). Task condition by stimulation-type interactions on movement-related gamma oscillations were observed across a distributed network of higher-order brain regions, including parietal cortices, right prefrontal and left temporoparietal junction. In sum, our results indicate polarity-specific effects on beta and gamma oscillations across a distributed network of brain regions that contribute to motor control in the context of interference and hold implications for understanding the therapeutic capacity of tDCS.