Intentional switches between coordination patterns are faster following anodal-tDCS applied over the supplementary motor area

The supplementary motor area (SMA) plays a critical role in the regulation of in-phase (IP) and anti-phase (AP) coordination [1,2], as it is thought to simultaneously code the actions of each limb, as well as their temporal sequencing [3]. Previously [4], we showed that applying offline anodal-tDCS for 10 minutes improved participants' ability tomaintain AP coordination at higher movement frequencies, which consequently delayed the spontaneous AP-to-IP switch; however, anodal-tDCS did not affect the more stable IP coordination. The SMA has been identified as a key neural correlate of spontaneous switching [2,5], yet its role during intentional switching is less clear, with some recent evidence suggesting that the SMA is more active during intentional IP-to-AP switches compared to the reverse direction [6]. Here, we used transcranial direct current stimulation (tDCS) to investigate the role of the SMA in mediating the interaction between pattern stability and intentional switching. In a randomized, double-blind crossover design, ten right-handed participants (Mage 1⁄4 24.7 years, SD 1⁄4 7.25; 6 males) completed two separate bimanual coordination testing sessions where either anodal-tDCS or sham-tDCS was applied between preand posttDCS testing blocks. The experiment was approved by the Research Ethics Board at the University of Ottawa and written informed consent was obtained from all participants before the start of the experiment. Trials began with participants performing synchronous coordination patterns with the forearms requiring either IP (simultaneous supination and pronation) or AP (alternating supination and pronation) cyclical movements at different movement frequencies (1.75, 2.0, or 2.25 Hz) paced by an auditory stimulus (1000 Hz, 25ms). Trials lasted 18 s and once on each trial, an auditory switch cue (650 Hz, 150 ms) was presented randomly between 7 and 12 s, which prompted participants to intentionally switch between patterns as quickly as possible and maintain the new pattern for the remainder of the trial (i.e., IP-to-AP or vice versa). Testing sessions were separated by at least 48 hours and both sessions consisted of preand post-tDCS blocks with 18 trials in each. These 18 trials included nine trials in each switch direction with three trials performed at each of the three different pacing speeds. tDCS was delivered through two scalp electrodes using a Dupel iontophoresis constant current delivery device (Empi) and stimulation order was counterbalanced. The active electrode (7.8 cm2) was saturated with sterile saline and positioned 1.8 cm anterior to Cz