Cortical adaption influences excitability in the dominant and non-dominant hands following complex novel motor training

Abstract

The primary motor cortex (M1) can dynamically control the magnitude of motor refinement through motor plasticity. Plasticity can be investigated by looking at changes to motor excitability, which has been established to be stronger in the non-dominant hemisphere of right handed participants. However, it is unclear if these differences in excitability following a training task would still be present if participants performed a task that was equally as difficult for both the dominant and non-dominant hands. Thus, the first goal of this thesis was to develop and validate a novel motor training task designed to be equally challenging for both hands. Participants were required to trace a novel sinusoidal pattern, varying in both amplitude and frequency that was mirrored in both the right and left hand. The time course of learning was plotted over two separate training sessions. The second study then utilized this task to answer the question of which hemisphere has a greater plastic potential, determine by quantifying changes motor excitability, and the time-course over which these changes occur. Motor cortical excitability before and after learning was investigated using recruitment curves which evoke MEPs at 7 different stimulation intensities in order to better capture a more robust measure of hemispheric excitability for both the left and right hand. The training task was indeed novel as it equally challenged both the dominant and non-dominant hands of a healthy right handed population and lead to remarkably similar learning curves over six blocks of learning and similar retention. In the second study, only the dominant hemisphere had significant decreases in excitability following the motor learning task. A secondary study indicated that the time-course over which these changes occurs suggests that when learning a novel training task which the participant is naïve to, motor training and increases to performance have a rapid onset as changes to excitability were only seen on the first day of training. This may indicate that even though the non-dominant hemisphere has greater initial excitability, the dominant hemisphere has a greater ability to modulate excitability levels showing a greater potential for plastic adaption.