Fig. 3.

Areas of the brain where opioid-induced changes in neuronal response to noxious stimuli show a significant positive correlation with opioid-induced behavioral analgesia. Opioid-induced changes in neuronal response and behavioral analgesia are defined as [v opioid_{(preinfusion−infusion)}] − [v saline_{(preinfusion−infusion)}], where v is the neuronal response or the pain intensity of noxious stimuli. A positive value for the change in neuronal response indicates an opioid-induced reduction. A positive correlation means the higher the behavioral analgesia the higher the opioid-induced suppression of the neuronal response. Clusters of voxels in rostral ventromedial medulla (RVM), periaqueductal gray (PAG), bilateral thalamus (in the Upper row from Left to Right), left amygdala and hippocampus and the right amygdala (Lower row, Left to Right) are shown. Montreal Neurological Institute coordinates are denoted in millimeters below each slice.