Somatosensory evoked potential in man: far field potentials.

Three short latency positive potentials evoked by median nerve stimulation were recorded from the scalp, nose and ear of 11 normal adult subjects in leads where the hand or knee contralateral to the side of stimulation was used as the reference site. The short latencies and positive polarity of these components suggest that they are volume conducted far field potentials. Evidence is presented which suggests that the first potential arises in peripheral nerve fibers. Brain stem and dicencephalic structures are suggested as possible sources for the second and third potentials. The configuration of the response to median nerve stimulation recorded over the cervical spine in ear and hand reference recordings was different. Evidence is presented which suggests that this occurs because the electrode placed on the ear records the far field potentials described in the above paragraph. A far field potential was also recorded in hand--knee leads contralateral to the stimulated median nerve.