Functional imaging and pain: behavior, perception, and modulation

Time-dependent increases of local metabolic or blood flow rates have been described in spinal cord and brain nociceptive networks during acute and chronic pain states in experimental animals, in parallel with changes of different behavioral endpoints of pain and hyperalgesia. In healthy human volunteers, pain intensity-related hemodynamic changes have been identified in a widespread, bilateral brain system including parietal, insular, cingulate and frontal cortical areas, as well as thalamus, amygdala and midbrain, during different kinds of acute noxious stimulation. Specific patterns of nociceptive activity may characterize hyperalgesic states and some chronic pain conditions. Both animal and human imaging studies suggest that forebrain nociceptive systems are under inhibitory control by endogenous opioids; in humans, acute administration of -opioid receptor agonists blunts noxious heat-evoked activation in several brain areas. Anticipation of pain may in itself induce changes in brain nociceptive networks. Moreover, pain-related increases in cortical activity can be modulated by different cognitive processes, such as hypnotic suggestions, focussing or diverting attention, and placebo. These findings highlight the complexity of the pain system, and begin to disclose the spatio-temporal dynamics of brain networks underlying pain perception and modulation in health and disease.