Electrophysiologic analysis of the sensorimotor cortex in primates

Abstract

Somatosensory evoked potentials (SEPs) index nervous system integrity; movement-related potentials (MRPs) index a process of self-initiated efferent pathway activation. Generators of surface waveform components have not been clearly elucidated in primates. This project was designed to study spatiotemporal aspects of processing in sensorimotor cortex following electrical stimulation and associated with simple movement initiation. Experiments involved epidural mapping and intracranial recordings using a multi-channel electrode and three complementary techniques: intracranial profiles of SEPs/MRPs, multiple unit activity (MUA) and one-dimensional current source density (CSD).;The data confirms participation of pre- and postcentral cortex in generation of early SEP components. The onset latencies of areas 1, 2, 3, and 4 appear to reflect unique thalamocortical input to each. Initial activation of SI is localized to lamina 4 and represents depolarization of thalamocortical terminals and activation of local stellate cells. Subsequent sinks in laminae 5 and 3 principally reflect pyramidal cell activation. Two patterns are detected following initial cortical activation: (1) localized MUA inhibition associated with a current source and (2) a second burst of MUA (laminae 3-5), preceded by laminae 4 and 2/1 sinks and consistent with pyramidal cell activation via thalamorecipient lamina and depolarization of pyramidal apical dendrites.;An antecedent portion of MRPs, N2a, begins at approximately {dollar}-{dollar}100 msec; its generator appears restricted to a small portion of area 4 immediately rostral to the lip of the anterior bank of the central sulcus. N2a may reflect processing within a small sub-set of cells. N2B is also antecedent; MUA occurs within 50 msec of EMG onset and is recorded in the anterior bank of the central sulcus. Early and late N2b may reflect corticospinal discharge and short-latency reafferent input, respectively. Movement-related activity in SI was not anteeedent. MUA recorded in banks of the intraparietal sulcus was coincident with EMG onset and may represent central input.;Area 6, anteromedial to area 4, did not exhibit pre-movement MUA but a lesion there produced dramatic changes in epidural N2. This region may have a supportive role in organizing movement-related activity. The sensitivity of MRP for detecting sub-clinical CNS motor dysfunction is noted.