DIRECT RECORDING OF SOMATOSENSORY EVOKED-POTENTIALS IN THE VICINITY OF THE DORSAL COLUMN NUCLEI IN MAN - THEIR GENERATOR MECHANISMS AND CONTRIBUTION TO THE SCALP FAR-FIELD POTENTIALS

Somatosensory evoked potentials (SEPs) in the vicinity of the dorsal column nuclei in response to electrical stimulation of the median nerve (MN) and posterior tibial ne e (PTN) were studied by analyzing the wave forms, topographical distribution, effects of higher rates of stimulation and correlation with components of the scalp-recorded SEPs. Recordings were done on 4 patients with spasmodic torticollis during neurosurgical operations for microvascular decompression of the eleventh nerve. The dorsal column SEPs to MN stimulation (MN-SEPs) were characterized by a major negative wave (N1; 13 msec in mean latency), preceded by a small positivity (P1) and followed by a large positive wave (P2). Similar wave forms (P1'-N1'-P2') were obtained with stimulation of PTN (PTN-SEPs), with a mean latency of N1' being 28 msec. Maximal potentials of MN-SEPs and PTN-SEPs were located in the vicinity of the ipsilateral cuneate and gracile nuclei, respectively, at a level slightly caudal to the nuclei. The latencies of PI and NI increased progressively at more rostral cervical cord segments and medulla, but that of P2 did not. A higher rate of stimulation (16 Hz) caused no effects on P1 and N1, while it markedly attenuated the P2 component. These findings suggest that P1 and N1 of MN-SEPs, as well as Pl' and N1' of PTN-SEPs, are generated by the dorsal column fibers, and P2 and P2' are possibly of postsynaptic origin in the respective dorsal column nuclei. The peak latency of NI recorded on the cuneate nucleus was identical with the scalp-recorded far-field potential of P13-14 in all patients, while no scalp components were found which corresponded to P2. These findings support the previous assumption that the scalp-recorded P13-14 is generated by the presynaptic activities of the dorsal column fibers at their terminals in the cuneate nucleus.