The "human visceral homunculus" to pain evoked in the oesophagus, stomach, duodenum and sigmoid colon.

The oesophagus, stomach, duodenum and sigmoid colon were electrically stimulated in 12 healthy volunteers with a thin nasal endoscope. The painful cortical evoked potentials (EPs) were recorded from 64 surface electrodes. The early EPs with latencies < 200 ms were studied and the corresponding dipole sources were calculated. The electrical current intensities needed to evoke pain were highest in the stomach and duodenum, compared to the other segments (F = 7.8; P < 0.001; post hoc analysis P < 0.05). The EP latencies after stimulation of the stomach and sigmoid colon were shorter compared with those to stimulation of the oesophagus and duodenum (all P values < 0.001). The EP amplitudes were higher to oesophagus stimulation (all P values < 0.001 except for the early positivity). The potential fields obtained after stimulation of the most distal segments (duodenum and sigmoid colon) were in general distributed more posteriorly compared to those recorded in the more proximal regions. The EP topographies to stimulation of all gut tracts were explained by a bilateral source in the second somatosensory (SII) area, by a dipole in the anterior cingulate cortex (ACC), and by a bilateral generator in the insular cortex. However, the position of the sources significantly changed depending on the stimulated gut tract. Moreover, while the SII and ACC sources were initially activated to oesophagus and stomach stimulation, the ACC and insular activities were the earliest ones after stimulation of the lower gut segments. The findings reflect differences in pathways and brain processing of visceral nociceptive inputs coming from either upper or lower gut and may improve our understanding of the brain-gut axis in health and disease.