The origins of short-latency somatosensory evoked potentials in humans.

Short-latency somatosensory evoked potentials (SEPs) to median nerve stimulation were studied in three patients with well-localized lesions of the lemniscal pathways. In two patients who had focal thalamic vascular lesions, the early scalp-recorded P15 far-field potential was unaltered while later SEP components were absent. In the third patient, operated on for astrocytoma of the dorsal medulla and right dorsal funiculus, the P15 potential was absent but the earlier negative potentials recorded from the neck persisted whichever side was stimulated. The data suggest a subthalamic origin for the scalp-recorded P15 far-field potential.