Neural control of predictive balance: Cortical and subcortical contributions to anticipatory postural adjustments in humans

The research outlined in this thesis aims to enhance understanding of the neural control of the anticipatory postural adjustment (APA) and validate the use of several techniques in elucidating neural measures. This thesis is divided into six chapters. The first and last chapter provide an opening introduction and overarching conclusion respectively. The second chapter is a literature review and all other chapters are comprise standalone experimental works.The literature review of chapter two describes the current understanding of the purpose of the APA. Particular attention is devoted to articulating the neural control of APAs and the varying techniques used to elucidate the functionality of the neural structures driving it. The circumstantial variation in the nature of postural adjustments is also discussed.Chapter three was the first experimental chapter, where the profile and composition of a motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS) before and during the APA were determined. Participants were prompted to reach to an unstable target and received TMS at randomised intervals in the time period between the prompt and movement onset. The time course of the MEP reflected cortical control throughout the APA in both the tibialis anterior (TA) and the soleus. Despite this, MEPs elicited in the soleus during a voluntary contraction were much smaller than those elicited prior to a reach, suggesting that subcortical centre controlling posture may not be inactive.Chapter four sought to probe the APA produced at height, reflecting a greater postural challenge. Participants received TMS and were separately recorded with electroencephalography (EEG) while reaching to a low, non-virtual target, and a low and high virtual target in virtual reality. TMS measures in the TA reflected cortical control, while the soleus was influenced by height and showed increased amplitude in MEPs during the time period prior to the APA. Coherence measures calculated with EEG verified greater cortical control during the APA rather than prior to the APA, which may have more subcortical excitability. Interestingly, we found the APA was conserved during non-stimulation reaching TMS trials compared to those with EEG, highlighting the need for further investigation.Chapter five aimed to clarify the mechanism of interaction between a startling acoustic stimulus (SAS) exciting the subcortical brainstem, and TMS and record their subsequent modulation of the APA. Participants completed a series of reaches where they randomly received an SAS, TMS, TMS + SAS or nothing. The stimuli in the TMS trials was timed such that the beginning of the TMS induced silent period overlayed the expected APA onset time. We expected the APA to resume inside the silent period if subcortical sources were influential in its release. This did not occur, and the APA was consistently pushed back to begin at the end of the silent period, indicating the cortical control of the APA, or at the least, the interaction of SAS induced motor patterns with cortical circuits.In conclusion, this thesis demonstrated that APAs are largely cortically driven activations, but subcortical pathways are not completely inactive. TMS evoked MEPs were successful at indicating varying excitability in cortical and subcortical sources, and virtual reality replicated real life height threats to APAs. We note the necessity to clarify the impact of TMS on the conservation of the APA itself and the continued need to elucidate the precise subcortical contribution to the APA.