Somatosensory evoked potentials following median nerve stimulation. I. The cervical components.

Median somatosensory evoked potentials were studied in 20 normal adult volunteers. Recording electrodes were positioned over posterior, anterior and lateral neck, as well as on the scalp. Three distinct cervical potentials were identified. Immediately after the afferent volley passes Erb's point, a travelling wave is recorded in the lateral cervical electrodes ipsilateral to the side of stimulation. This represents the afferent volley approaching the spinal cord in the proximal brachial plexus and cervical roots and has been designated the 'proximal plexus volley' (PPV). Following PPV, a second travelling wave is recorded which increases in latency from low to high cervical levels. It represents the afferent volley in the dorsal column, and has been designated the 'dorsal column volley' (DCV). Following DCV, a stationary potential, designated CERV N13/P13, is recorded with characteristics of a transverse midline dipole having maximal negativity posteriorly and maximal positivity anteriorly. This potential may be generated by interneurons in the dorsal grey of the cervical cord. Each of these cervical travelling waves is accompanied by a negative far-field potential recorded at the scalp. The PPV is accompanied by a negative scalp deflection with a nominal latency of 10 ms (N10), and the peak of DCV at SC1 is accompanied by a scalp negativity with a nominal latency of 12 ms (N12). In view of these observations, it is necessary to reexamine assumptions regarding the polarity of scalp-recorded potentials generated by remote neural events.