Primary motor cortex excitability is modulated with bimanual training

Bimanual visuomotor movement has been shown to enhance cortical motor activity in both hemispheres, especially when movements require simultaneous activation of homologous muscle groups (in-phase movement). It is currently unclear if these adaptations are specific to motor preparatory areas or if they also involve changes in primary motor cortex (M1). The present study investigated the representation of wrist muscles within motor cortex before and following bimanual movement training that was in-phase, anti-phase with or without motor preparation. Motor evoked potentials (MEPs) for the extensor carpi radialis muscle (ECR) cortical territory were acquired and analyzed before and following bimanual movement. The cortical representation was quantified and compared in terms of spatial extent and MEP amplitude, in two different experiments involving distinct movement training types. In Experiment 1, participants performed bimanual wrist flexion/extension movements to targets which involved in-phase movements, either following a 2s preparation period (In-phase preparation), or without the preparation period (In-phase no preparation). In Experiment 2, training involved antagonist muscle groups activated simultaneously (Anti-phase) with the addition of the 2s preparation period. In-phase bimanual movement enhanced the spatial representation of ECR in M1, and did not show a difference in MEP amplitude of the cortical area. It may be that simultaneous activation of homologous M1 representations in both hemispheres, in combination with activity from premotor areas, leads to a greater increase in plasticity in terms of increased M1 spatial extent of trained muscles.