People with chronic pain are often afflicted with psychiatric depressive disorders. Even the commonplace, everday experience of pain can be modified by ordinary emotion. However, the neural mechanisms underlying these phenomena are still unclear. Investigating the effects of sad mood on pain perception at the behavioral and the neural levels helps to understand the co-morbidity of depression and pain problems. This dissertation focuses on: (1) examining the effects of sad mood on perception of painful stimuli, and (2) identifying the functional brain networks that could provide the neural substrate for pain perception altered by sadness.In Chapter 2, a task-evoked functional magnetic resonance imaging (fMRI) method has been used to investigate which brain areas are activated by painful and emotional stimuli. The hypotheses are that the subjective perception of pain intensity would be greater during sadness compared to neutral mood, and significantly increased neural activation would be observed in the primary cortical areas for pain perception. To test these hypotheses, the fMRI study has recruited sixteen healthy female subjects. The experimental trials include the delivery of painful electrical shocks concurrent with or without sad mood induced by the presentation of pictures from the International Affective Picture System (IAPS). The study identifies widely-distributed cortical and subcortical areas involved in the processing of pain and the altered pain perception by sadness. The cortical areas for pain processing are mainly located in the primary/secondary somatosensory, insular and cingulate cortices. Specifically, the posterior insular and the adjacent secondary somatosensory cortices (pIn/SII) have significantly increased neural activation, when sadness intensifies subjective perception of pain.In Chapter 3, using the resting-state fMRI and path analysis, it further examines the functional networks of pain processing and altered pain perception by sadness. The pain-relevant brain areas, such as primary somatosensory cortex, anterior cingulate cortex and thalamus, are intrinsically correlated with pIn/SII during rest. The path coefficients in the pain processing functional brain network increase during painful stimuli. The path analysis has also revealed a functional network in which the pIn/SII connects with emotion-relevant areas, such as the subgenual cingulate cortex, anterior insula and amygdala. These findings provided evidence of specific neural pathways that may be relevant to understanding the comorbidity of depression and pain problems.
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