Instabilities during antiphase bimanual movements: are ipsilateral pathways involved?

The spatial and temporal coupling between the hands is known to be very robust during movements which use homologous muscles (in-phase or symmetric movements). In contrast, movements using nonhomologous muscles (antiphase or asymmetric movements) are less stable and exhibit a tendency to undergo a phase transition to in-phase movements as movement frequency increases. The instability during antiphase movements has been modeled in terms of signal interference mediated by the ipsilateral corticospinal pathways. In this study we report that participants in whom distal ipsilateral motor-evoked potentials could be elicited with transcranial magnetic stimulation (TMS), exhibited higher variability during a bimanual circling task than participants whose ipsilateral pathways could not be transcranially activated. These results suggest that ipsilateral control of the limb affects the level of bimanual coupling, and may contribute to uncoupling phenomena observed during asymmetric coordination.