Different cortical organization of visceral and somatic sensation in humans

Sensory stimuli from the visceral domain exhibit perceptual characteristics different from stimuli applied to the body surface. Compared with somatose nsation there is not much known about the cortical projection and functiona l organization of visceral sensation in humans, in this study, we determine d the cortical areas activated by non-painful electrical stimulation of vis ceral efferents in the distal oesophagus, and somatosensory afferents in th e median nerve and the lip in seven healthy volunteers using whole-head mag netoencephalography. Stimulation of somatosensory afferents elicited short- latency responses (approximate to 20-60 ms) in the primary somatosensory co rtex (SI) contralateral (median nerve) or bilateral (lip) to the stimulated side, and long-latency responses (approximate to 60-160 ms) bilaterally in the second somatosensory cortex (SII). In contrast, stimulation of viscera l oesophageal afferents did not evoke discernible responses in SI but well reproducible bilateral SII responses (approximate to 70-190 ms) in close vi cinity to long-latency SII responses following median nerve and lip stimuli . Psychophysically, temporal discrimination of successive stimuli became wo rse with increasing stimulus repetition rates (0.25 Hz, 0.5 Hz, 1 Hz, 2 Hz) only for visceral oesophageal, but not for somatosensory median nerve stim uli. Correspondingly, amplitudes of the first cortical response to oesophag eal stimulation emerging in the SII cortex declined with increasing stimulu s repetition rates whereas the earliest cortical response elicited by media n nerve stimuli (20 ms SI response) remained unaffected by the stimulus fre quency. Our results indicate that visceral afferents from the oesophagus pr imarily project to the SII cortex and, unlike somatosensory afferents, lack a significant SI representation. We propose that this cortical projection pattern forms the neurophysiological basis of the low temporal and spatial resolution of conscious visceral sensation.