An interhemispheric asymmetry in motor cortex disinhibition during bimanual movement.

The release of short interval intracortical inhibition (SICI) induced by passive bimanual movement was assessed in dominant and non-dominant primary motor cortices (M1). Dual-pulse focal transcranial magnetic stimulation (TMS) was delivered over M1 while the limbs were at rest, and during the mid-flexion phase of contralateral rhythmical wrist flexion-extension. Test and conditioned responses were recorded from flexor carpi radialis (FCR) when the wrist was passively moving alone, during bimanual mirror symmetric passive synchronous movement, and during bimanual passive asynchronous movement. Tonic inhibition was released to a greater extent in the non-dominant M1 than in the dominant M1 during synchronous mirror symmetric movement. This interhemispheric asymmetry was not evident during asynchronous movement. The findings support the idea that the dominant M1 has the capacity to disinhibit homologous representations in the contralateral M1 during synchronous movement, but the non-dominant M1 does not reciprocate to the same extent.