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

Abstract

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.

Keywords: Consolidation; Cortical excitability; Movement amplitude; Relative phase.

Fig. 1

A-D. (A) Orientation of the arms, handles, and movement direction are represented. Abduction-adduction motion of the handles on the horizontal plane (x-axis) is defined with respect to the body midline and required motion of the whole arm. (B) The positioning of a participant with respect to the camera, feedback display, and handles is Portrayed. (C) The Lissajous plot feedback display. The crossed lines represent in-phase (positive slope) and anti-phase (negative slope) coordination for the familiarization trials, and the circle represents a 90° relative phase for the training and retest trials. (D) The two simulated time series representing right, and left arm motion have a 90° phase offset with an amplitude of 12 cm, which would move the dot around the circle in (C)

Fig. 2

A-B. A) The three tDCS montages used in this experiment are depicted: (1) LARC– left M1 anode (A+) and right M1 cathode (C-), (2) RALC right M1 anode (A+) and left M1 cathode (C-), and (3) sham. The direction of current flow is from the red to blue square for the LARC and RALC montages. The sham condition montage is shown with two squares with minus signs to indicate no prolonged stimulation (only ramp up– ramp down current) during the training period. B) The flow chart represents the experimental timeline, from familiarization trials (in-phase and anti-phase) to stimulation and training of 90° to retest of 90° after a 6-hr consolidation window

Fig. 3

A-F. Sample trajectories from an individual in the LARC group (A, B, C), RALC group (D, E, F), and sham group (G, H, I). The trials in A (trial 4), D (trial 7), and G (trial 6) are from early in training, and the trials in B (trial 21), E (trial 21), and H (trial 14) are from later in training. The trials in C (trial 4), F (trial 2) and I (trial 4) are examples from the four retest trials, 6-hrs. after the end of practice. Left-arm motion is plotted on the y-axis and right-arm motion on the x-axis. Half-cycle displacement was 12 cm

Fig. 4

A-L. In all plots, LARC data is plotted as a circle, RALC as a triangle, sham as a square. A-D) Training data plotted by Block and Montage. E-H) Retention data plotted by Montage, group means and individual participant data. I-L) Delta values representing performance difference between the last 4 training trials and the 4 retention trials, group means, and individual participant data are plotted. A, E, and I) BW22 - relative phase accuracy measure. B, F, and J) - Coordination variability measure. C, G, and K) AMP - bimanual amplitude measure (radius). D, H, and L) FRQ– averaged frequency across both arms. The error bars represent standard deviation of the group mean. The asterisks and brackets represent significant differences between blocks in A, C and D. In B, within group differences found in the interaction tests are highlighted for the RALC (*) and LARC (+) montages