Topography and intracranial sources of somatosensory evoked potentials in the monkey. I. Early components

Averaged somatosensory evoked potentials (SEP) were recorded in the monkey from arrays of surface electrodes overlying the brain, cervical cord and peripheral nerve; from epidural electrodes over the cerebral convexity; and from movable intracerebral electrodes. The initial cortically generated responses peak at mean latencies of 10 and 12 msec following stimulation of the median nerve at the wrist. Preceding these potentials 5 small positive wavelets were identified in scalp and epidural recordings. The sources of the latter three of these waves have been identified, based in part on the observation of amplitude maxima in depth recordings within cerebrum and brain stem. P7.2 is primarily generated within the thalamocortical radiations, whereas P5.3 and P6.2 reflect bursts of highly synchronized action potentials travelling along the medial lemniscus. Recordings of multiple unit activity within these tracts confirmed the source identifications made on the basis of potential distribution. Continuing activity within the more caudal portions of the somatosensory pathways produces potentials that sum with those generated more rostrally. This circumstance precludes the identification of the intracranial source of a surface recorded potential by demonstrating a concurrent wave form at a single location within the brain. It is necessary to examine the intracranial potential distribution and trace the potential from the surface to its maximum in order to identify its source with confidence. P3.1 and P3.8 were identified only as farfield potentials in intracranial recordings from the pons and more rostral regions. They were ascribed to activity of primary somatosensory neurons ascending in the dorsal columns on the basis of their timing, surface distribution and amplitude vs. interstimulus interval functions. The early SEP components recorded in the monkey closely resembled in configuration and topography those recorded from human subjects, although the latter were longer in latency, reflecting differences in length of the somatosensory pathways in the two species.