Contributions of cortical and subcortical circuits to reaction times and excitability levels in complex tasks

The way humans interact with their environment can range from simple reaching and grasping movements to remarkably complex movement patterns. Complexity increases reaction time (RT) and alters the excitability of the motor system. However, this area of research lacks a detailed description of the locus of the processes involved in the successful preparation and execution of complex movements. As the motor system can be influenced by cortical, spinal, and peripheral components, the overall purpose across the four studies of this thesis was to assess separately how they contribute to complex movement preparation and execution. The purpose of study 1 was to describe how movement complexity affected whole-body anticipatory postural adjustments (APAs). In this study, it was shown that APA onset times increased prior to an imperative stimulus (IS) as movements became more complex, demonstrating the motor system integrated movement complexity into a global motor plan. To isolate corticospinal contributions to this motor plan, study 2 used transcranial magnetic stimulation (TMS) to demonstrate that motor evoked potentials (MEPs) increased as a consequence of movement complexity. Study 3 used a combination of TMS and transmastoid stimulation to assess the cortical and spinal contributions to the increase of the MEP. This study demonstrated that increases in MEP due to movement complexity were mediated at the spinal level. Furthermore, it was shown that motoneuron excitability increases at least 50% earlier than previously described in the literature. As the spinal cord also receives descending input independent of cortical influence (e.g. extrapyramidal pathways), the purpose of study 4 was to examine if the vestibulomotor system had a role in the preparation of complex movements. This study demonstrated that vestibular-evoked responses are greater during the preparation of complex movements in both the upper and lower limbs. Prior to this study, the vestibulomotor pathway was shown to be involved in the online control of arm movements; however, this demonstrates the vestibulomotor pathway is also involved in the preparation of movement. Overall, this thesis demonstrates a complex interaction of multiple elements of the motor system in the execution of movement.Health and Social Development, Faculty of (Okanagan)Health and Exercise Sciences, School of (Okanagan)Graduat