tDCS over left M1 or DLPFC does not improve learning of a bimanual coordination task

Previously, transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) has resulted in improved performance in simple motor tasks. For a complex bimanual movement, studies using functional magnetic resonance imaging and transcranial magnetic stimulation indicated the involvement of the left dorsolateral prefrontal cortex (DLPFC) as well as left M1. Here we investigated the relative effect of up-regulating the cortical function in left DLPFC and left M1 with tDCS. Participants practised a complex bimanual task over four days while receiving either of five stimulation protocols: anodal tDCS applied over M1, anodal tDCS over DLPFC, sham tDCS over M1, sham tDCS over DLPFC, or no stimulation. Performance was measured at the start and end of each training day to make a distinction between acquisition and consolidation. Although task performance improved over days, no significant difference between stimulation protocols was observed, suggesting that anodal tDCS had little effect on learning the bimanual task regardless of the stimulation sites and learning phase (acquisition or consolidation). Interestingly, cognitive performance as well as corticomotor excitability did not change following stimulation. Accordingly, we found no evidence for behavioural or neurophysiological changes following tDCS over left M1 or left DLPFC in learning a complex bimanual task.