Contrasting neuronal activity in supplementary and precentral motor cortex of monkeys. I. Responses to instructions determining motor responses to forthcoming signals of different modalities

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Abstract
The present report contrasts neuronal activity in two motor cortical fields after instructions that determine which of two sensory signals will trigger a movement and which will not. The goal of the study was to determine possible differential roles of the two cortical fields in the process of preparing to move in response to one external cue and to ignore another. Single-cell recordings were made from the supplementary motor area (SMA) and the precentral motor area (PCM) of monkeys trained to perform key-press movements in two different modes. In the auditory mode, an instruction signal warned the animal to prepare to start the movement promptly in response to a forthcoming 1,000-Hz tone burst (trigger signal), but to remain motionless if the signal was vibrotactile (nontrigger signal). In the tactile mode, the trigger and nontrigger signals were reversed: a different instruction signal warned the animal to prepare to perform the key-press movement in response to the vibrotactile cue, but to withhold it in response to the 1,000-Hz tone. The instruction signals were auditory tones of 300 Hz for the auditory mode and 100 Hz for the tactile mode. Out of 259 task-related SMA neurons, 128 (49%) responded to instructions. Three types of instruction responses were observed: 1) 95 neurons showed continuous instruction-induced activity changes lasting until the occurrence of the movement-triggering signal, regardless of whether an intervening nontrigger signal occurred. 2) 24 neurons showed increased activity until the occurrence of the nontriggering signal, after which the activity subsided. When there was no nontrigger signal, the activity increased during a period when the nontrigger signal might have been given. 3) Nine neurons responded with a transient, short-latency discharge after the instruction. The responses of SMA neurons to two instructions were often different. Forty-four SMA neurons exhibited a selective response to only one of the two instructions. In 43 neurons the response was differential, with the magnitude of activity increase or decrease being at least three times greater after one instruction than the other. In the remaining 41 neurons the response was nondifferential. Out of 112 task-related PCM neurons, 25 (22%) responded to the instructions. In the majority of them (21 neurons), the instruction response was nondifferential.(ABSTRACT TRUNCATED AT 400 WORDS)