Somatotopic representation of pain in the primary somatosensory cortex (S1) in humans

In contrast to tactile inputs, the organization and processing of nociceptive inputs in the primary somatosensory cortex (S1) remain largely unexplored. Few studies have examined the arrangement of nociceptive inputs in S1. The aim of this study was to investigate the representation of nociceptive inputs in the human cortex, including the somatosensory and posterior parietal cortices, from widely separated cutaneous sites. We examined the somatotopic organization of the nociceptive system in S1, opercular and posterior parietal cortices by measuring the magnetoencephalographic responses (somatosensory-evoked magnetic fields) of four healthy controls in response to intraepidermal electrical stimulation applied to the face, neck, back, elbow, wrist, hand, finger, knee, and foot, which selectively activated the Aδ fibers. Magnetoencephalography demonstrated clear somatotopy in the S1 responses to noxious stimuli, with the foot representation in the extreme posteromedial position of S1 and the facial area in the extreme anterolateral position. There was little evidence of any clear somatotopic organization in the secondary somatosensory and posterior parietal cortices. These findings suggest that the nociceptive system uses the large body surface map in S1. This is the first MEG study to demonstrate the cortical representation of nociceptive inputs in the human S1. We showed that widely separated cutaneous sites clearly supported Penfield's homunculus.