An Investigation of the Neural Correlates of Cognitive Modulation of Pain Perception in the Human Brainstem and Cervical Spinal Cord Using Functional Magnetic Resonance Imaging
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Pain is a multifaceted and malleable sensory experience that is processed at all levels of the central nervous system. It can be influenced by cognitive factors such as attention, expectation, suggestion, and attitudes; the role of attention in cognitive modulation of pain is the focus for the work that is presented in this thesis. Behavioural studies showed that pain perception was reduced under cognitive loads that place a continuous demand on executive functions such as working memory. Neuroimaging, pharmacological, and electrophysiological studies provide evidence that the underpinnings of cognitive modulation of pain involve a network of descending modulation of pain among cortical and brainstem structures. However, the role and relationship of subcortical regions in the brainstem and spinal cord (SC) during cognitive modulation of pain is not well understood. Therefore, the aim of this thesis was to confirm the behavioural effect of cognitive modulation of pain using a working memory task and to investigate the neural correlates of this modulatory mechanism in the brainstem and cervical SC using a combination of functional Magnetic Resonance Imaging (fMRI) and Structural Equation Modelling (SEM). Twelve female participants, ages 24-52, was given noxious thermal stimulation on the C6 dermatome while simultaneously performing three levels of the N-Back task. Our findings demonstrated a significant attenuation in pain perception across the group as a result of the task, along with high inter-subject variability in the degree of modulation. Functional MRI results exhibited neural responses in brainstem regions known to be involved in descending modulation of pain and the ipsilateral dorsal horn of the SC. SEM analysis revealed connectivity between the brainstem and SC at the group level and individual level that varied depending on the cognitive load and the degree of pain modulation, respectively. Altogether, the findings of this thesis demonstrate the behavioural effect of cognitive modulation on pain perception and give insight into the subcortical neural response to this process.