Spinal and early scalp-recorded components of the somatosensory evoked potential following stimulation of the posterior tibial nerve.

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the posterior tibial nerve (PTN) in 12 normal adults. Recording using both cephalic and non-cephalic references were obtained from multiple electrodes placed over the spine and scalp. Following PTN stimulation, the fastest recorded potentials of the afferent sensory volley proceeds up the spinal cord at constant velocity. After arrival of the volley at cervical cord levels, 3 widely distributed waves, P28, P31 and N34, are recorded from scalp electrodes. These 'far-field' potentials are followed by a localized positivity (P38) which has a peak voltage either at the vertex or just laterally toward the side of stimulation. A contralateral negativity (N38) was present in most individuals. We propose that P28 arises from medial lemniscus; that P31 is generated by ventrobasal thalamus; and that N34 is probably the result of further activity in thalamus and/or thalamocortical radiations. The P38/N38 complex represents the primary cortical response to PTN stimulation. Its most consistent characteristic is a positivity at the vertex or immediately adjacent scalp areas ipsilateral to the stimulated leg. The topography of the P38/N38 potential varies slightly from individual to individual in a manner consistent with a functional dipole situated in the leg and foot area on the mesial aspect of the postcentral gyrus, whose exact location and orientation changes in accordance with known variations in the location of the leg area.