Anodal Transcranial Direct Current Stimulation Applied over the Supplementary Motor Area Delays 4 Spontaneous Anti-phase to In-phase Transitions 5 6

25 Coordinated bimanual oscillatory movements often involve one of two intrinsically stable 26 phasing relationships characterized as in-phase (symmetrical) or anti-phase (asymmetrical). The in27 phase mode is typically more stable than anti-phase and if movement frequency is increasing during 28 anti-phase movements, a spontaneous transition to the in-phase pattern occurs. There is converging 29 neurophysiological evidence that the SMA plays a critical role in the successful performance of these 30 patterns, especially during anti-phase movements. We investigated whether modulating the excitability 31 of the SMA via offline tDCS would delay the onset of anti-to-in-phase transitions. Participants completed 32 two sessions (separated by ~48 hours), each consisting of a preand post-tDCS block in which they 33 performed metronome-paced trials of rhythmic inand anti-phase bimanual supination-pronation 34 movements as target oscillation frequency was systematically increased. Anodal or cathodal tDCS was 35 applied over the SMA between the preand post-tDCS blocks in each session. Following anodal tDCS, 36 participants performed the anti-phase pattern with increased accuracy and stability and were able to 37 maintain the coordination pattern at a higher oscillation frequency. Anti-phase performance was 38 unchanged following cathodal tDCS and neither tDCS polarity affected the in-phase mode. Our findings 39 suggest increased SMA excitability induced by anodal tDCS can improve anti-phase performance and 40 adds to the accumulating evidence of the pivotal role of the SMA in interlimb coordination. 41