Precentral cortex unit activity during the M-wave and contingent negative variation in behaving squirrel monkeys

Abstract

The firing of 96 units in the precentral cortex of four trained squirrel monkeys was examined in relation to the M-wave and contingent negative variation (CNV) evoked by two tone cues 1 s apart, to which the monkey had to respond by a bar-press within 1 or 2 s in order to receive a food reward. Many (92%) units showed increased or decreased firing during at least one of the two event-related potentials [(ERPs), the M-wave and CNV]. The finding of both increases and decreases in firing during the CNV is consistent with the hypothesis that the CNV represents a combination of EPSPs on superficial neuronal elements and IPSPs on deeperlying elements. Like the M-wave and CNV, many of the ERP-related changes in unit firing were apparently influenced by the animal's level of motivation; i.e., they differed in magnitude in successful vs. unsuccessful trials; in early vs. late successful trials; and/or in successful trials with more vs. less preferred food rewards. The timing of some of the firing increases during the CNV suggests that they correlate with two postulated components of the CNV, one related to the first cue, the other to preparation for the motor response. The increases in firing after cue 2, observed in 91% of the units and often influenced by motivation, occurred before and during the bar-press and suggest that the ERP-related neurons are also involved in the well-established role of the precentral cortex in motor output. We suggest that these neurons are the anatomic substrate by which the influence of motivation is transmitted to other neurons of the motor system, including those more directly related to the effectors.