Coordination of uncoupled bimanual movements by strictly timed interhemispheric connectivity.

Independent use of both hands is characteristic of human action in daily life. By nature, however, in-phase bimanual movements, for example clapping, are easier to accomplish than anti-phase movements, for example playing the piano. It is commonly agreed that interhemispheric interactions play a central role in the coordination of bimanual movements. However, the spatial, temporal, and physiological properties of the interhemispheric signals that coordinate different modes of bimanual movements are still not completely understood. More precisely, do individual interhemispheric connectivity parameters have behavioral relevance for bimanual rapid anti-phase coordination? To address this question, we measured movement-related interhemispheric interactions, i.e., inhibition and facilitation, and correlated them with the performance during bimanual coordination. We found that movement-related facilitation from right premotor to left primary motor cortex (rPMd-lM1) predicted performance in anti-phase bimanual movements. It is of note that only fast facilitation during the preparatory period of a movement was associated with success in anti-phase movements. Modulation of right to left primary motor interaction (rM1-lM1) was not related to anti-phase but predicted bimanual in-phase and unimanual behavior. These data suggest that strictly timed modulation of interhemispheric rPMd-lM1 connectivity is essential for independent high-frequency use of both hands. The rM1-lM1 results indicate that adjustment of connectivity between homologous M1 may be important for the regulation of homologous muscle synergies.