Application of bilateral tDCS over left and right M1 produces asymmetric training and retention effects when learning a rhythmic bimanual task

Many motor skills require precise coordination between the arms to accomplish. The use of transcranial direct current stimulation (tDCS) has helped to reveal hemispheric contributions to bimanual skills. In this study, three bilateral montages were used to explore hemispheric contributions to a rhythmic bimanual skill: anode left M1/cathode right M1 (LARC), anode right M1/cathode left M1 (RALC), and sham. Stimulation lasted 20-minutes during training. Retention was examined 6-hr after training. Participants (n = 46) learned a bimanual 90° relative-phase pattern with a half-cycle movement amplitude goal of 12 cm per arm at self-selected movement frequencies. Greater coordination variability in the 90° pattern emerged early under RALC compared to LARC, with no difference in performance accuracy. Larger movement amplitudes emerged in training with LARC compared to sham but not compared to RALC. tDCS montage had no impact on coordination variability and accuracy of the 90° pattern after the 6-hr delay. Montage was associated with a delayed movement amplitude effect emerging in retention, with larger amplitudes in LARC compared to RALC and sham. The asymmetries observed across training and retention emerged from of an interaction between tDCS and the left-hemisphere's role in the control of bimanual movements in right-handed individuals.