Descending Pain Modulation in Humans: A Multimodal Investigation of Conditioned Pain Modulation
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Authors
Keast, Brieana
Date
2025-09-19
Type
thesis
Language
eng
Keyword
pain , fMRI , pupil dilation , conditioned pain modulation , descending pain modulation , spinal cord , brainstem
Alternative Title
Abstract
Descending pain modulation plays a critical role in shaping the perception of nociceptive input; however, its underlying autonomic and neural mechanisms are not fully understood. This thesis explores conditioned pain modulation (CPM) through a multimodal approach that combines behavioural assessments with autonomic and functional neuroimaging measures to deepen our understanding of endogenous pain control.
In the first study, eye-tracking was used to measure pupil area during the application of both a noxious test stimulus alone and simultaneously with a noxious conditioning stimulus. We found that the CPM condition was associated with variation in pupil size depending on whether the individual exhibited a facilitatory or inhibitory response. Importantly, differences in pupil diameter between inhibitory and facilitatory groups emerged specifically during the control conditions suggesting the potential for trait-level effects or influences.
The second study used functional magnetic resonance imaging (fMRI) of the brainstem and spinal cord to identify neural correlates of descending modulation. Decreases in BOLD signal in relation to the CPM condition were observed in the spinal dorsal horn and brainstem nuclei, consistent with inhibition within the descending pain modulatory pathway. Subject-specific connectivity analysis using Structural and Physiological Modeling (SAPM) further revealed modulation of ascending and descending pathways during CPM.
These findings provide evidence that descending pain modulation involves both autonomic suppression and targeted neural inhibition in the descending pain pathway. The integration of behavioural, eye-tracking, and fMRI data highlights the distributed and dynamic nature of endogenous analgesia.