Cortical representation of pain: functional characterization of nociceptive areas near the lateral sulcus

Many lines of evidence implicate the somatosensory areas near the lateral sulcus (Sylvian fissure) in the cortical representation of pain. Anatomical tracing studies in the monkey show nociceptive projection pathways to the vicinity of the secondary somatosensory cortex in the parietal operculum, and to anterior parts of insular cortex deep inside the Sylvian fissure. Clinical observations demonstrate alterations in pain sensation following lesions in these two areas in human parasylvian cortex. Imaging studies in humans reveal increased blood flow in parasylvian cortex, both contralaterally and ipsilaterally, in response to painful stimuli. Painful stimuli (such as laser radiant heat) evoke potentials with a scalp maximum at anterior temporal positions (T3 and T4). Several dipole source analyses as well as subdural recordings have confirmed that the earliest evoked potential following painful laser stimulation of the skin derives from sources in the parietal operculum. Thus, imaging and electrophysiological studies in humans suggest that parasylvian cortex is activated by painful stimuli, and is one of the first cortical relay stations in the central processing of these stimuli. There is mounting evidence for closely located but separate representations of pain (deep parietal operculum and anterior insula) and touch (secondary somatosensory cortex and posterior insula) in parasylvian cortex. This anatomical separation may be one of the reasons why single unit recordings of nociceptive neurons are scarce within regions comprising low-threshold mechanoreceptive neurons. The functional significance (sensory-discriminative, affective-motivational, cognitive-evaluative) of the closely spaced parasylvian cortical areas in acute and chronic pain is only poorly understood. It is likely that some of these areas are involved in sensory-limbic projection pathways that may subserve the recognition of potentially tissue damaging stimuli as well as pain memory.