When two motor cortices are better than one

When performing an action with only one hand, like playing a melody on the piano, our typical understanding is that the contralateral motor cortex controls all movements of the acting hand. Yet, it has been found that sometimes when moving just one hand, both the contralateral and ipsilateral motor cortex become active. In addition, certain clinical neurological disorders are characterized by an inability to move just one hand. The goal of the current research endeavor is to understand the mediating factors behind this bilateral engagement of the motor cortices during unimanual actions in both normal and clinical populations. In particular, the research project was designed to assess 4 questions. First, does the presence of activity in the ipsilateral precentral gyrus depend on the complexity of the action being executed? Second, does the activation on the ipsilateral precentral gyrus reflect the recruitment of the primary motor cortex or is it showing the engagement of an adjacent premotor region? Do congenital mirror movements arise as consequential activation of ipsilateral muscles due to ipsilateral collaterals of descending corticospinal fibers or do they result from an adaptive strategy to control hand movements due to abnormal organization of the corticospinal pathways? During actions that increase the bilateral activation of cortical motor areas, is there an involuntary coupling of the two hands: e.g., do complex actions result in subthreshold mirror movements? The results of this dissertation project show that even when a hand movement becomes mildly complicated, the brain's tendency is to recruit both motor cortices. This strategy reflects an adaptive mechanism by which the brain overcomes obstacles in the execution of difficult movements. These findings provide novel insights into how the brain performs even the most simple of coordinated actions and why involuntary bimanual movements are sometimes expressed following damage to upper motor neuron systems.