Pain perception: Is there a role for primary somatosensory cortex?

Anatomical, physiological, and lesion data implicate multiple cortical regi ons in the complex experience of pain. These regions include primary and se condary somatosensory cortices, anterior cingulate cortex, insular cortex, and regions of the frontal cortex. Nevertheless, the role of different cort ical areas in pain processing is controversial, particularly that of primar y somatosensory cortex (S1). Human brain-imaging studies do not consistentl y reveal pain-related activation of S1, and older studies of cortical lesio ns and cortical stimulation in humans did not uncover a clear role of S1 in the pain experience. Whereas studies from a number of laboratories show th at S1 is activated during the presentation of noxious stimuli as well as in association with some pathological pain states, others do not report such activation. Several factors may contribute to the different results among s tudies. First, we have evidence demonstrating that S1 activation is highly modulated by cognitive factors that alter pain perception, including attent ion and previous experience. Second, the precise somatotopic organization o f S1 may lead to small focal activations, which are degraded by sulcal anat omical variability when averaging data across subjects. Third, the probable mixed excitatory and inhibitory effects of nociceptive input to S1 could b e disparately represented in different experimental paradigms. Finally, sta tistical considerations are important in interpreting negative findings in S1. We conclude that, when these factors are taken into account, the bulk o f the evidence now strongly supports a prominent and highly modulated role for S1 cortex in the sensory aspects of pain, including localization and di scrimination of pain intensity.