Scalp distribution of the earliest cortical somatosensory evoked potentialto tibial nerve stimulation: proposal of a new recording montage

Objective: To investigate the most reliable method to record the earliest c ortical somatosensory evoked potential (SEP) after tibial nerve stimulation . The 'gating' phenomenon was used to dissociate the overlapping cortical S EP components. Methods: In 11 subjects we recorded the scalp SEPs at rest, during the volu ntary (active gating) and passive (passive gating) foot movement and during the isometric calf muscle contraction (isometric gating). Results: At the vertex the P40 amplitude was reduced in all the gating cond itions. Instead, both the P40 response recorded in the parietal region ipsi lateral to the stimulation (indicated as P40par) and the fronto-temporal N3 7 potential were reduced in amplitude only during the passive foot movement . Conclusions: The same behaviour of the N37 and P40par potentials suggests t hat they can represent the opposite counterparts of the same dipolar genera tor. Instead, the real P40 amplitude, which is affected in all the gating c onditions, is recorded at the vertex and might be generated by a different source. We conclude that the montage obtained by referring a temporal elect rode contralateral to the stimulation to an ipsilateral parietal lead can r eliably record the earliest cortical component (N37/P40par) after tibial ne rve stimulation. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved .