High-frequency transcranial magnetic stimulation of the supplementary motor area reduces bimanual coupling during anti-phase but not in-phase movements.

Previous electrophysiological and neuroimaging studies have provided evidence that the supplementary motor area (SMA) has an important role in the control of bimanual coordination. The present experiment investigated the effects of high-frequency repetitive transcranial magnetic stimulation (rTMS) over the SMA region on kinematic variables during cyclical bimanual coordination, with a particular focus on the quality of coordination. Subjects performed metronome-paced trials of in-phase and anti-phase bimanual index-finger movements at near-maximal cycling frequency. During movement execution, rTMS (20 Hz, 0.5 s, 120% hand motor threshold) was applied over one of three positions in the sagittal midline 2.0, 4.0 and 6.0 cm anterior to the primary motor leg area. Sham rTMS was included as a control condition. After rTMS, the mean relative phase error between hands increased, but only in the anti-phase trials. The maximum increase in phase error occurred immediately after rather than during the rTMS train. The effect was largest after stimulation 4 or 6 cm anterior to the leg area of the primary motor cortex. We did not observe any changes in the variability of relative phase or in cycle duration or movement amplitude. Findings are discussed in light of recent functional models on the role of the SMA in bimanual movement control.