All daily activities require the precise interaction and coordination of several brain regions to facilitate purposeful movements of the upper limbs. The mechanisms responsible for cross facilitation between the primary motor cortices are poorly understood and are important in understanding the neurophysiology of everyday upper limb movements and customizing task- and deficit- specific rehabilitation protocols following brain injury. Researchers have demonstrated activity-dependent changes in the primary motor cortex (M1) ipsilateral to the moving limb; however, the characteristics mediating this interaction between the hemispheres are not well understood. The aim of this thesis is to examine sensorimotor manipulations that modulate excitability of the resting M1 and determine the neural substrates that may be mediating these interactions. This thesis is comprised of 4 studies and we investigated corticomotor excitability changes of a resting upper limb muscle during (1) rhythmical movement at increasing force requirements, (2) rhythmical movement at increasing force requirements with the addition of sensory input (3) interhemispheric interactions and somatotopic relationships, and (4) convergence of multiple effectors. This dissertation identifies various sensorimotor manipulations that increase excitability of M1 and further informs the neurophysiological mechanisms that may be responsible for these interactions. Understanding the extent to which these mechanisms mediate activity between the upper limbs has implications in bimanual coordination and ultimately experience-dependent plasticity. The findings in this thesis have important applications for improving motor recovery with rehabilitation interventions post brain injury.
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Factors influencing bilateral interactions in the human motor cortex: investigating transcallosal sensorimotor networks
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