Limitations
handedness, upper-limb motor learning
There are some limitations to this study. First, only single-session tDCS in the initial stage of upper-limb motor learning was evaluated. The tDCS-induced gain in motor learning of the non-dominant hand should be evaluated in long-term continuous training, rather than in a single efficacy test. Moreover, retention of acquired skills should be evaluated. This could further enhance clinical application of tDCS to individuals obliged to change their handedness. Second, the learning effect due to repetition was shown in two PPT tasks. This is an unavoidable problem in the early phase of motor learning when rapid performance gains were obtained. Nevertheless, we found the effectiveness of tDCS beyond learning effects in the task with high cognitive demand, although the potential tDCS effects may have been underestimated. Third, physiological evidence underlying the behavioral data is lacking. Functional neuroimaging can provide more robust evidence and better understanding of the impact of tDCS on motor learning. Finally, precise electrode placement based on each individual’s brain structure could not be performed. A recent study evaluating the precision of electrode placement in electroencephalography based on the international 10/20 system referring to structural magnetic resonance imaging (MRI) data revealed the variation in electrode location in Montreal Neurological Institute coordinates [