The study, conducted by researchers from Hiroshima University, examined the brain functions in the resting state of 43 patients with depression, 41 patients with chronic pain (somatoform pain disorder) and 41 healthy controls, using regional homogeneity (ReHo) and functional connectivity analysis. The results showed that ReHo values for the dorsolateral prefrontal cortex (DLPFC) significantly decreased for chronic pain patients, and functional connectivity between the DLPFC and thalamus decreased only for these patients. These findings suggest that the DLPFC and thalamus are associated with the dysfunction of pain-controlling networks specific to chronic pain, and that further comparisons be conducted to determine the similarities and differences in pathological pathways which might contribute to the development of clinical treatment.
Certain experimental studies on pain perception using thermal, or electrical stimuli have shown that chronic pain patients exhibit higher pain sensitivity than healthy controls. However, other studies of depressive patients have indicated that they were less likely to perceive pain stimuli compared to controls. Normand *et al*. examined potential differences in experimentally induced pain perception and diffuse noxious inhibitory control efficacy (e.g “pain inhibits pain” phenomenon) between depressive and chronic pain patients by using a tonic thermal test and a cold pressor test. They reported that chronic pain could be distinguished from depression by pain ratings during the cold pressor test, suggesting that the two conditions might have different underlying mechanisms.