The Effects of Altered Partial Pressures of Gas on Cognitive and Neural Function

Oxygen is central to neural function, yet the precise mechanisms and effects by which varying <br/>oxygen levels, whether through Hypoxia, Hyperoxia, or Hyperbaric Oxygen Therapy (HBOT), <br/>shape cognition and brain activity remain incomplete. This thesis adopts a novel, multi-modal <br/>framework that integrates normobaric gas manipulations, cognitive testing, Transcranial <br/>Magnetic Stimulation (TMS), Electroencephalography (EEG), Functional Magnetic Resonance <br/>Imaging (fMRI), and HBOT to examine how gas and pressure variability influences cognitive, <br/>motor, and neural processes. <br/>Chapter 3 presents the findings of a preliminary experiment investigating normobaric oxygen <br/>manipulations on cognition. Standardised cognitive assessments revealed domain-general <br/>impairments (i.e. memory and executive function) under Hypoxia, whereas Hyperoxia <br/>produced smaller, and more inconsistent domain-specific changes. Notably, both conditions <br/>increased movement time but left reaction time unaffected, implicating the motor system <br/>rather than broad cognitive slowing. Chapter 4 extends this by probing the motor system with <br/>TMS to measure Corticospinal Excitability (CSE). Hypoxia increased early motor neuron <br/>recruitment at lower stimulation intensities yet lowered maximum excitability, while <br/>Hyperoxia raised the saturation threshold for excitability, highlighting distinct motor <br/>responsiveness under different levels of oxygen. Based on these motor findings, Chapter 5 <br/>explores neural oscillations and evoked responses with EEG. Hypoxia reduced Critical Flicker <br/>Fusion (CFF) thresholds and altered Visual Evoked Potentials (VEPs), while Hyperoxia <br/>generated smaller more transient changes in CFF and VEPs, with a specific reduced motor <br/>Beta power, suggesting more localised oscillatory disruptions. <br/>Chapter 6 then investigates HBOT using mobile EEG during a hyperbaric “dive,” to understand <br/>the neural impacts of HBOT. The results showed Delta power decreased cumulatively <br/>throughout the session, whereas Alpha, Beta, and Theta power increased during transitions <br/>to a relatively lower partial pressure of oxygen, pointing to “relative Hypoxia” as a potential <br/>driver of neuroplasticity. These results also showed heightened neural entropy during <br/>transitions to higher oxygen levels, emphasising the importance of dynamic pressure changes <br/>for neural adaptability. Chapter 7 examines CO₂-induced anxiety via a Carbon Dioxide <br/>Challenge Model (CCM) and fMRI, revealing transient anxiogenic responses that increased <br/>functional connectivity within networks involving the insula, amygdala, and frontal regions. A <br/>correlation between subjective anxiety and connectivity between the brainstem and frontal <br/>cortex was observed, highlighting the role of top-down emotional regulation and how <br/>physiology interacts with anxiety. <br/>Collectively, these findings demonstrate that oxygen variability significantly impacts cognition, <br/>motor systems, and neural plasticity, with relative Hypoxia emerging as a particularly potent <br/>stimulus for adaptive changes. By illustrating how normobaric manipulations, HBOT, and CO₂<br/>induced anxiety each alter neural excitability and connectivity, this thesis offers an integrated <br/>perspective on oxygen’s role in shaping brain function. It further establishes a framework for <br/>potential novel therapeutic interventions, ranging from enhanced neurorehabilitation <br/>protocols to strategies for managing anxiety and cognitive decline, that leverage controlled <br/>oxygen variability for clinical and performance benefits.<br/>

Author dc:creator

• Graham, Daniel

Contributors dc:contributor

• Stephen Hall, Gary Smerdon, Alastair Smith, Jonathan Marsden

Language dc:language

eng

Repository record dc:identifier

https://pearl.plymouth.ac.uk/psy-theses/157

OAI identifier oai:identifier

oai:pearl.plymouth.ac.uk:psy-theses-1157