Concurrent- not independent- M1 anodal and cerebellar cathodal tDCS enhances skill acquisition of a dexterous rhythmic-timing videogame

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that can alter the excitability of targeted brain regions and influence motor learning. For the first experiment, we studied the effects of several individual stimulation montages (2mA) on motor learning in a complex rhythm-timing video game task (n=79, M1 anodal tDCS [M1 a-tDCS], Cerebellar anodal tDCS [CB a-tDCS], Cerebellar cathodal tDCS [CB c-tDCS], and SHAM). Performance was assessed using a performance index (PI) incorporating keystroke timing accuracy, tap distribution ratio, and key error rate. All groups demonstrated significant improvements in PI ( P < 0.001), but no significant interaction effect of tDCS group by practice block was observed. However, a non-significant trend towards improved PI scores was observed in both the M1 a-tDCS and CB c-tDCS groups, which lead us to perform a second experiment using same video game task, but with concurrent M1 anodal and cerebellar cathodal tDCS (n=24, M1a+CBc tDCS). With concurrent stimulation, there was a significant main effect of stimulation group on PI gain scores across practice blocks ( P = 0.021) with M1a+CBc showing greater gains than SHAM. When controlling for baseline performance, the M1a+CBc group had significantly higher post-test PI scores compared to the sham group ( P = 0.034). The results of this study suggest concurrent M1a+CBc stimulation causes neuroplastic changes between M1 and the cerebellum that enhance motor learning in complex tasks beyond what single site stimulation can provide.