PAIN PROCESSING DURING 3 LEVELS OF NOXIOUS-STIMULATION PRODUCES DIFFERENTIAL PATTERNS OF CENTRAL ACTIVITY

Previous functional imaging studies have demonstrated a number of disc rete brain structures that increase activity with noxious stimulation. Of the commonly identified central structures, only the anterior cing ulate cortex shows a consistent response during the experience of pain . The insula and thalamus demonstrate reasonable consistency while all other regions, including the lentiform nucleus, somatosensory cortex and prefrontal cortex, are active in no more than half the current stu dies. The reason for such discrepancy is likely to be due in part to m ethodological variability and in part to individual variability. One a spect of the methodology which is likely to contribute is the stimulus intensity. Studies vary considerably regarding the intensity of the n oxious and non-noxious stimuli delivered. This is likely to produce va rying activation of central structures coding for the intensity, affec tive and cognitive components of pain. Using twelve healthy volunteers and positron emission tomography (PET), the regional cerebral blood f low (rCBF) responses to four intensities of stimulation were recorded. The stimulation was delivered by a CO2 laser and was described subjec tively as either warm (not painful), pain threshold (just painful), mi ldly painful or moderately painful. The following group subtractions w ere made to examine the changing cerebral responses as the stimulus in tensity increased: (1) just painful - warm; (2) mild pain - warm; and (3) moderate pain - warm. In addition, rCBF changes were correlated wi th the subjective stimulus ratings. The results for comparison '1' ind icated activity in the contralateral prefrontal (area 10/46/44), bilat eral inferior parietal (area 40) and ipsilateral premotor cortices (ar ea 6), possibly reflecting initial orientation and plans for movement. The latter comparisons and correlation analysis indicated a wide rang e of active regions including bilateral prefrontal, inferior parietal and premotor cortices and thalamic responses, contralateral hippocampu s, insula and primary somatosensory cortex and ipsilateral perigenual cingulate cortex (area 24) and medial frontal cortex (area 32). Decrea sed rCBF was observed in the amygdala region. These responses were int erpreted with respect to their contribution to the multidimensional as pects of pain including fear avoidance, affect, sensation and motivati on or motor initiation. It is suggested that future studies examine th e precise roles of each particular region during the central processin g of pain. (C) 1997 International Association for the Study of Pain. P ublished by Elsevier Science B.V.