Origins of the scalp-recorded somatosensory far field potentials in man and cat.

Short latency somatosensory evoked potentials preceding the primary cortical potential were recorded from the scalp in man and cats. Four positive potentials (P1, P2, P3 and P4 in man, and CI, CII, CIII and CIV in cats) were observed with non-cephalic reference recording and 3 or 2 positive components (P1a, P2a and P3a in man, and CIa and CIIa in cats) were recorded with earlobe reference. (1) The latencies of these potentials and the effects of lesions on them in man and (2) the effects of (i) supracollicular and medullo-cervical transections and (ii) intrathecal anaesthesia of the cervical cord on these components in cats suggest that: (1) P1 and CI originate in the brachial plexus beneath the clavicle; (2) P2, P3 and CII, CIII are generated in the cervical cord; (3) P4 and CIV reflect activity mainly in the brain stem or the cerebellum or both; (4) P1a and CIa arise in the cervical cord; (5) P3a and CIIa reflect activity mainly in the brain stem or the cerebellum or both. As the impulse initiated in the median nerve travels through various regions where the resistance might change suddenly, these far field potentials might be recorded as if generated at fixed sites such as just beneath the clavicle and foramen magnum.