Cerebral evoked potentials after rectal stimulation.

We obtained reproducible cortical evoked potentials (EPs) in response to electrical stimulation of the rectum with 1 Hz frequency. We found 2 distinctly different EPs in response to rectal stimulation. In 5 females, the EP had an early onset latency (mean 26 msec) with multiple positive and negative peaks. In 10 females, the EP had a later onset latency (mean 52 msec) and a trifid configuration, having a very prominent negative peak. The early onset EPs after rectal stimulation appeared very similar to the wave form of the cortical EPs recorded after pudendal nerve stimulation. Finding similar interpeak latencies in the early onset EP after rectal stimulation and the EP after pudendal nerve stimulation suggests that either the same pathway was used or that rectal stimulation also stimulated the pudendal nerve. It appears that we stimulated visceral afferents when we recorded late onset EPs, because the large EP amplitude declined rapidly with faster stimulation rates and also with greater number of averaging, and the sensation threshold was very unstable, all different to somatosensory EPs.