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 somatosensation there is not much known about the cortical projection and functional organization of visceral sensation in humans. In this study, we determined the cortical areas activated by non-painful electrical stimulation of visceral afferents in the distal oesophagus, and somatosensory afferents in the median nerve and the lip in seven healthy volunteers using whole-head magnetoencephalography. Stimulation of somatosensory afferents elicited short-latency responses (approximately 20-60 ms) in the primary somatosensory cortex (SI) contralateral (median nerve) or bilateral (lip) to the stimulated side, and long-latency responses (approximately 60-160 ms) bilaterally in the second somatosensory cortex (SII). In contrast, stimulation of visceral oesophageal afferents did not evoke discernible responses in SI but well reproducible bilateral SII responses (approximately 70-190 ms) in close vicinity to long-latency SII responses following median nerve and lip stimuli. Psychophysically, temporal discrimination of successive stimuli became worse 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 stimuli. Correspondingly, amplitudes of the first cortical response to oesophageal stimulation emerging in the SII cortex declined with increasing stimulus repetition rates whereas the earliest cortical response elicited by median nerve stimuli (20 ms SI response) remained unaffected by the stimulus frequency. Our results indicate that visceral afferents from the oesophagus primarily 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.