Neuronal Network Oscillations in the Control of Human Movement
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The overarching aim of this thesis was to use neuroimaging and neuromodulation techniques to further understand the relationship between cortical oscillatory activity and the control of human movement. Modulations in motor cortical beta and alpha activity have been consistently implicated in the preparation, execution, and termination of movement. Here, I describe the outcome of four studies designed to further elucidate these motor-related changes in oscillatory activity.
In Chapter 3, I report the findings of a study that used an established behavioural paradigm to vary the degree of uncertainty during the preparation of movement. I demonstrate that preparatory alpha and beta desynchronisation reflect a process of disengagement from the existing network to enable the creation of functional assemblies required for movement. Importantly, I also demonstrate a novel neural signature of transient alpha synchrony, that occurs after preparatory desynchronisation, that underlies the recruitment of functional assemblies required for directional control.
The study described in Chapter 4 was designed to further investigate the functional role of preparatory alpha and beta desynchronisation by entraining oscillatory activity in the primary motor cortex (M1) using frequency-specific transcranial alternating current stimulation. No significant effects of stimulation were found on participant response times. However, no clear conclusion could be drawn due to limitations of the stimulation parameters that were used.
In Chapter 5, I explored the inverse relationship between M1 beta power and cortical excitability using single-pulse transcranial magnetic stimulation to elicit motor-evoked potentials (MEPs). The amplitude of MEPs collected during a period of beta desynchronisation was significantly greater than during a resting baseline. Conversely, the amplitude of MEPs collected during the post-movement beta rebound that follows the termination of a movement was significantly reduced compared to baseline. This finding confirms the inverse relationship between M1 beta power and cortical excitability.
The study in Chapter 6 explored the effect of experimental context on M1 beta power. When the participant was cued to expect an upcoming motor task, resting beta power was significantly increased, then when the likelihood of an upcoming motor requirement decreased, there was a significant concurrent decrease in resting beta power. This reflects increased coherence and functional connectivity within M1 and other motor areas, to ‘recalibrate’ the motor system in preparation for a synchronous input signal to more readily recruit the required functional assembly.
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