Investigating the role of the Primary Motor Cortex in Motor Learning using a combination of Robotics and Non-Invasive Brain Stimulation with Implications for Rehabilitation

Background
Humans possess an extraordinary capacity to make reaching movements in a constantly changing physical environment. The studies in this thesis used a combination of non-invasive brain stimulation techniques (transcranial magnetic stimulation and transcranial direct current stimulation) and robotics to explore the role
of the primary motor cortex (Ml) in the preparation and execution of both "overlearnt" stereotypical and novel (motor adaptation) reaching tasks.
Methods
In each study, healthy adults made planar reaching movements on a robotic manipulandum from a central target to 1-8 peripheral targets. The robot was programmed to either allow unimpeded movement (chapters 3-4) or to apply a force field perturbation (chapters 5-7). Corticospinal drive to specific upper limb muscles was assessed by measuring transcranial magnetic stimulation (TMS) induced motor evoked potentials (MEPs) in selected upper limb muscles at a series of time points before and during the reaching movements (Chapters 3-6). In chapter 7, transcranial direct current stimulation (tDCS) was applied over the Ml during motor adaptation to determine if there was a causal relationship between Ml function and motor
adaptation.
Main findings
In chapter 3, both corticospinal excitability and voluntary EMG had a similar pattern of directional "tuning" in selected upper limb muscles. Furthermore, temporal analysis of MEPs in chapter 4 showed that increases in corticospinal and intracortical excitability occur just before the onset of voluntary muscle activity, which suggests that changes in Ml output may drive muscle activity in a feedforward manner during upper limb reaching. Chapters 5 and 6 revealed that after a single session of motor
adaptation there were direction specific changes in corticospinal excitability and a release of intracortical inhibition that were associated with changes in voluntary EMG in the same muscles. These findings suggest that the Ml may also play an important role in driving feedforward muscle control during newly learnt dynamics. In chapter
7, anodal tDCS applied during motor adaptation was shown not to influence reaching error during the adaptation phase, but did induce increased error during deadaptation.
Preliminary evidence suggested that this may be due to enhanced internal model formation rather than a disruption to the deadaptation process per se. This latter finding suggests that the Ml may also play a contributory role in internal model formation in response to a changing physical environment.

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