Imaging cognitive modulation of pain processing

TLDR: This study induced painful perception in patients with phantom-limb pain using hypnotic suggestion that the missing limb was in a painful position and was able to show that when subjects solved the maze task and the authors induced a painful stimulation they perceived less pain as compared with when there was no competition for attentional space.

Abstract: The intensity and unpleasantness of a painful experience is often described as correlating well with the degree of noxious stimulation. However, the perception of pain is not a linear phenomenon, reflecting the signal from the peripheral neuron. Rather, the noxious input may be modulated at every level of the neural axis. One of the most potent sources of modulation is the brain—although these mechanisms have only sparsely been studied. The supraspinal modulatory influences involve both lower order automatic response schemata and higher order dynamic cognitive mechanisms. This organizational pattern has developed as an evolutionary driven adaptation, in which both fast hardwired responses and slower dynamic responses increased the chance for survival. In line with this hypothesis, it has been suggested that the brain is initially processing noxious input in the brainstem supporting the demand for a fast response (Petrovic et al., 2000a; Price, 2000). Apart from autonomic changes and a wide range of defense reactions, the brainstem may induce powerful analgesia in direct response to noxious stimuli (Fanselow, 1994). At a higher level, cognitive processes may dramatically modulate the perception of pain (Melzack and Casey, 1968; Weisenberg et al., 1996). Recently, functional imaging tools, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have described some of the possible underlying mechanisms that are involved in cognitive modulation of pain perception. Several functional imaging studies have indicated that pain processing may be modulated by cognitive mechanisms (Bantick et al., 2001; Longe et al., 2001; Petrovic et al., 2001a,b; Rainville et al., 1997, 1999; Willoch et al., 2000). Rainville and colleagues used hypnotic suggestion to modulate the perception of unpleasantness during noxious stimulation. When the subjects were suggested to perceive the noxious stimulation as highly unpleasant there was a concomitant increase in the activity in the anterior cingulate cortex (ACC) significantly more than when the subjects were suggested to perceive the same stimulation as less unpleasant (Rainville et al., 1997). However, the activity in the somatosensory areas was unaltered. Since lesion studies and animal studies have indicated that the ACC is involved in processing pain unpleasantness (Vogt et al., 1993) this finding indicates that cognitive mechanisms may specifically modulate sub-systems of the pain network. We used a different but classical approach in order to show that pain networks may be modulated by cognitive demands (Petrovic et al., 2000b). Most people have probably experienced that pain perception can decrease and even disappear when actively engaging the mind in a distracting task. We tested this mechanism by involving the subjects in a highly attention demanding task (computerized perceptual maze test) during noxious stimulation. We were able to show that when subjects solved the maze task and we induced a painful stimulation they perceived less pain as compared with when there was no competition for attentional space. At a neural level, activity was significantly attenuated in somatosensory regions and the PAG in this condition. Recently, it has been shown that cognitive distraction also may attenuate the pain-evoked activity in the ACC, the insula and the thalamus (Bantick et al., 2001; Longe et al., 2001). One intriguing study has demonstrated the opposite cognitive modulation in which the pain network was activated without any noxious stimulation being given (Willoch et al., 2000). In this study, painful perception was induced in patients with phantom-limb pain using hypnotic suggestion that the missing limb was in a painful position. All the regions discussed above are involved in pain processing, and modulation in their activity coincides with the changes in a pain perception. Apart from attention dependent changes in the network processing the perception, another distinct set of structures may act as sources for Pain 95 (2002) 1–5