The Effects of Anodal Oscillatory Transcranial Direct Current Stimulation on Top-Down Cortico-Muscular Control: A Pilot Study

Oscillatory transcranial direct current stimulation (otDCS) is a novel non-invasive stimulation protocol that combines the characteristics of both transcranial direct current and alternating current stimulation. Therefore, it has the ability to simultaneously modulate the potential and oscillatory activities of neuronal membranes. In this pilot study, anodal otDCS in different frequencies (20 Hz, 25 Hz, and 30 Hz) in the beta band were applied to 4 healthy subjects. High-density EEG and EMG were collected during a simple isometric finger contraction task. The modulation effects of otDCS on the top-down motor control were explored using generalized partial directed coherence (GPDC), which is a directed measurement of cortico-muscular interactions in both top-down and bottom-up directions. The individual peak frequencies for top-down GPDC was $20.25\pm 2.06$. There was a trend toward an increase in top-down GPDC after 20 Hz anodal otDCS, but not after 25 Hz or 30 Hz otDCS. The findings in this pilot study imply the effect of 20 Hz otDCS on top-down motor control, which can be applied to patients with impairment in motor control, such as stroke survivors. Our results also suggest the potential effectiveness of otDCS in the individual peak frequency in the beta band, which might be investigated in future studies.