Short-latency somatosensory evoked potentials.

Short-latency components of the somatosensory evoked potential (SEP) were studied in 20 subjects who had median nerve stimulation using knee, forehead, and ear reference recordings. Six potentials were identified (P10, P12, P14, N19, P20, P23). Potential P10 seems to originate in the brachial plexus, P12 most likely is generated in dorsal column nuclei and medial lemniscus, P14 is probably thalamic in origin but is frequently bilobed and may have a second generator source, and N19 may originate in sensory radiation or cortex. The origin of P20 is unclear, and P23 appears to be generated in the contralateral somatosensory cortex. Clinically, we recommend using right, left, and bilateral median nerve stimulation at 4 Hz. Intensity of stimulus should produce at least a small thumb twitch, it was only with knee reference recording that all early potentials were seen.