Is there a role for the parietal lobes in the perception of pain?

Converging lines of evidence confirm a role for the anterior parietal cortex in pain processing and extend the traditional view of SI to include discriminative aspects of somatic stimulation that is potentially tissue-damaging (e.g., painful). Recent studies more specifically implicate SI in the sensory aspect of pain perception by demonstrating that SI activation is modulated by cognitive manipulations that alter perceived pain intensity, but not by manipulations that alter unpleasantness, independent of pain intensity. Nevertheless, despite the probable role of SI in the encoding of the various sensory features of pain, considerable evidence suggests that nociceptive input to SI may also serve to modulate tactile perception. Thus, SI cortex may be involved in both the perception and modulation of both painful and nonpainful somatosensory sensations. Defining a role in pain processing for the parietal operculum is somewhat more problematic. The absence of a fine somatotopic organization of cutaneous (or visceral) receptors virtually eliminates a substantial role for this region in localizing noxious stimuli. Several studies suggest separate representations for pain and touch within the posterior parietal cortex and SII, respectively; however, inter-species differences in cortical anatomy and inconsistencies in the designation of SII proper preclude a clear reconciliation of the data. Likewise, suggestions that SII activation is predominantly related to processing the nociceptive quality of the stimulus (60,61) are inconsistent with many studies in both human and nonhuman subjects, which show a strong functional relationship between SII activity and innocuous (especially, vibrotactile) stimulation. Nevertheless, the numerous studies indicating pain-related activation within the parietal operculum (and/or SII) underscore the potential importance of this region in the perception of pain and the need for continued research. Finally, a possible role of posterior parietal cortex (BA 5/7, 39/40) in orientation and attention toward painful sensory stimuli is consistent with existing literature describing this region as a poly modal association area concerned with intrapersonal and extrapersonal space; however, results from studies that actually manipulate the subjects' level of attention relative to painful stimuli have not uniformly supported this hypothesis (75). Future studies assessing both attentional demand and direct manipulation or motor interactions involving noxious stimuli may help to resolve this issue. In spite of some discrepant results concerning specific details of the nociceptive process, the weight of human pain research now firmly establishes a role for the parietal lobes in the conscious appreciation of the sensation of pain.