Effects of anodal transcranial direct current stimulation over lower limb primary motor cortex on motor learning in healthy individuals

Transcranial direct current stimulation (tDCS) is a neuromodulatory technique which alters motor functions in healthy humans and in neurological patients. Most studies so far investigated the effects of tDCS on mechanisms underlying improvements in upper limb performance. To investigate the effect of anodal tDCS over the lower limb motor cortex (M1) on lower limb motor learning in healthy volunteers, we conducted a randomized, single-blind and sham-controlled study. Thirty-three (25.81 ± 3.85, 14 female) volunteers were included, and received anodal or sham tDCS over the left M1 (M1-tDCS); 0.0625 mA/cm² anodal tDCS was applied for 15 min during performance of a visuo-motor task (VMT) with the right leg. Motor learning was monitored for performance speed and accuracy based on electromyographic recordings. We also investigated the influence of electrode size and baseline responsivity to transcranial magnetic stimulation (TMS) on the stimulation effects. Relative to baseline measures, only M1-tDCS applied with small electrodes and in volunteers with high baseline sensitivity to TMS significantly improved VMT performance. The computational analysis showed that the small anode was more specific to the targeted leg motor cortex volume when compared to the large anode. We conclude that anodal M1-tDCS modulates VMT performance in healthy subjects. As these effects critically depend on sensitivity to TMS and electrode size, future studies should investigate the effects of intensified tDCS and/or model-based different electrode positions in low-sensitivity TMS individuals.