Investigating the Effect of Beta tACS on Sensorimotor Neural Oscillations in Healthy Adults

Kym Wansbrough

Voluntary movement is an important ability that allows us to complete day-to-day activities. Evidence suggests that sensorimotor beta oscillations (13-30Hz) are functionally important for movement, and that individuals with movement impairments show abnormal beta activity. Transcranial alternating current stimulation (tACS) is thought to modulate these oscillations to improve motor function, however, the mechanisms by which tACS modulates oscillations is not fully understood. To address this gap, this thesis investigated the effect of beta tACS on sensorimotor oscillations. First, computational modelling was used to systematically identify the best electrode montage for targeting the left primary motor cortex (M1) in the average adult brain (Chapter 2). A 4 × 1 montage using 20 mm electrodes, with return electrodes set at a radius of 50 mm from the target site, provided the best balance between electric field strength and focality. Next, this thesis empirically tested the effect of beta tACS on sensorimotor beta power (Chapter 3) and M1-M1 connectivity (Chapter 4), at various stimulation intensities. Results showed that beta tACS can modulate sensorimotor oscillations, however, the aftereffects depend on the stimulation intensity, brain state, and oscillatory feature. The fourth part of this thesis critically examined a novel approach to quantify tACS-induced changes in corticospinal neurons during stimulation, by analysing intramuscular coherence (Chapter 5). While the examined study did not observe tACSinduced changes in intramuscular coherence, suggested adjustments to the protocol may lead to significant effects. For example, systematic investigations into the parameters of the protocol (e.g., stimulation intensity of tACS or oscillatory feature of intramuscular coherence) may improve the efficacy of the approach. The final part of this thesis assessed the current state-of-the-field in a perspectives paper, which provided recommendations for how future studies may overcome challenges inherent in tACS application and maximise its utility in research and clinical settings (Chapter 6). Overall, this thesis provided valuable insights into the effects of beta tACS, which are crucial for developing effective and reliable tACS protocols to improve motor performance in individuals with movement-related issues.

Investigating the Effect of Beta tACS on Sensorimotor Neural Oscillations in Healthy Adults

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