The main aim of this thesis was to explore the neural and behavioural responses underpinning upper-limb motor control in a novel (force-field) robot-mediated reaching task using a non-invasive brain stimulation method known as transcranial magnetic stimulation (TMS). A new TMS-based network mapping technique was used to target different regions of the motor circuit (i.e. network nodes) using a ‘virtual disruption’ approach.
Seven cortical regions including the left and right primary motor cortex (M1), the supplementary motor area (SMA), the left and right posterior parietal cortex (PPC) and the left and right dorsal pre-motor cortex (PMC) were targeted with TMS at nine different time points during the preparation phase of upper-limb reaching towards a north-west target (i.e. reaching away from the body). Both neural mechanisms (corticospinal excitability with left M1 stimulation) and kinematic (behavioural) responses such as, movement onset, movement offset, maximum velocity, movement duration, summed error (reaching errors quantified by the calculating the difference between the subject’s reaching trajectory and the ideal reaching trajectory) and maximum force were explored offline. When exploring the impact of TMS on each cortical region individually, the results demonstrated a behavioural effect on reaching responses because 1) TMS caused a significant disruption in reaching trajectories during motor adaptation compared to normal reaching (no force-field) at most time points and 2) TMS caused a significant delay in movement onset, particularly during motor adaptation. As well as exploring the effect of TMS on each region separately, it was important to determine the network of regions that may play a more functional role in novel reaching. Therefore a comparative analysis was performed between all stimulated regions for each kinematic parameter. The comparative analysis revealed a region specific relative influence on summed error. More specifically, the left M1 and left PPC were the principle structures that were involved in novel reaching because TMS to these structures resulted in significantly greater reaching trajectory errors. Based on this finding, it can be concluded that the left M1 and left PPC play a pivotal role in the preparation phase of upper-limb novel reaching compared to other regions in the motor network, including the right M1, SMA, left and right dPMC and right PPC.
Overall, the findings from this project can not only help 1) refine our understanding of the mechanistic elements that operate during reaching and 2) gain an insight into the functional role of the different regions that are involved in novel reaching, but they also have a wide range of applications, ranging from brain machine interfaces (BMI) to neurocomputational models where data-based virtual lesions have been introduced into models of stroke patients.