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We compared neuronal activity in the premotor (PM) and supplementary motor cortex (SM) of two rhesus monkeys as they performed two tasks. In an externally-instructed task, a visuospatial instruction stimulus indicated which of two touch pads should be the target of a forelimb movement. In an internally-instructed task, the visuospatial stimulus was either irrelevant or not presented, but in either case the target alternated between the two touch pads in blocks of 20 trials each. In both tasks, the monkey withheld movement for a self-timed delay period. Neuronal activity modulation during the delay period (set-related activity) and immediately before movement (movement-related activity) was comparable in PM and SM, both in terms of the proportion of cells with both of those activity patterns and their depth of modulation. Thus, our findings do not provide strong support for a clear-cut functional division between PM and SM regarding the control of externally- and internally-instructed limb movements. Within PM, 57 out of 96 cells with set-related activity showed similar modulation during the two tasks, supporting the proposition that such activity contributes to the preparation for a limb movement. In 32 of the 39 PM set-related neurons that showed a significant activity difference between the two tasks, activity was greater in the externally-instructed task. This finding supports the hypothesis that set-related activity in PM contributes more to sensorially-instructed than to other movements.

Most of our knowledge about motor cortex is based on studies of primates or cats, but rodents may become increasingly useful subjects for studies in which a number of neurobiological methods are to be used concurrently. The present review is meant to provide a background for such neurobiological investigations, and will focus on three issues: the definition of rodent motor cortex; its input—output organization; and the role of motor cortex output in the control of movement.

Pyramidal tract neurons (PTNs) were identified in precentral motor cortex (MI) and in postcentral cortex (PoC) of a monkey trained to pronate and supinate its forearm. PTN responses to passive, ramp-form displacements of the forearm were examined in relation to the size of the neuron (as reflected by its antidromic latency). Larger PTNs tended to exhibit transient responses to passive limb displacement, whereas smaller PTNs more frequently showed sustained responses. These findings suggest that smaller PTNs, that make up the majority of the total PTN population, receive continuous feedback during posture as well as during the dynamic phase of movement.

The somatotopic organization of the supplementary motor area (SMA) is commonly held to consist of a rostrocaudal sequence of orofacial, forelimb, and hindlimb representations. Recently, however, this somatotopy has been questioned. Studies of regional cerebral blood flow in humans and the movements evoked by intracortical electrical stimulation in cynomolgus monkeys have been unable to reveal evidence of distinct orofacial, forelimb, and hindlimb representations rostrocaudally situated along the medial cortex of the hemisphere. Partly on the basis of those results, it has been suggested that the SMA functions as a nontopographically organized “higher-order” motor center. The present study reexamines SMA organization by observing stimulation-evoked movements. The medial frontal cortex of 2 rhesus monkeys was mapped using a modified intracortical microstimulation technique. We observed a forelimb representation mainly on the medial surface of the hemisphere in both animals. Rostral or rostrolateral to the forelimb representation, depending on the individual, we evoked orofacial movements (including eye movements). Hindlimb movements were evoked from tissue overlapping, but largely caudal to, the forelimb representation. Thus, we conclude that there is a clear rostrocaudal progression of orofacial, forelimb, and hindlimb movement representations in the SMA.

The first motor (MI) cortex of the rat was identified as the region from which movements could be evoked by the lowest intensity of electrical stimulation. The location of this region was correlated with cytoarchitecture in the frontal and parietal cortex. Two frontal areas can be discerned in Nissl-stained sections: (1) the medial agranular field, marked by a pale-staining layer III and a compact layer II, and (2) the lateral agranular field, which has more homogeneous superficial layers and a broad layer V containing large, densely staining cells. Both of these regions project to the spinal cord and can therefore be included in the somatic sensorimotor cortex. MI in the rat coincides with the lateral agranular field but also overlaps with part of the adjacent granular cortex of the first somatic sensory (SI) representation. We conclude that the rat MI cortex can be identified by microstimulation techniques and by cytoarchitecture in the rat.

It has been proposed that the premotor cortex plays a role in the selection of motor programs based on environmental context. To test this hypothesis, we recorded the activity of single neurons as monkeys learned visuomotor associations. The hypothesis predicts that task- related premotor cortical activity before learning should differ from that afterward. We found that a substantial population of premotor cortex neurons, over half of those adequately tested, showed the predicted learning-dependent changes in activity. The present findings support a role for premotor cortex in motor preparation, generally, and suggest a specific role in the selection of movements on the basis of arbitrary associations.

A technique that combines local injection of GABAergic drugs, neuronal recording, and electrical stimulation in awake, behaving monkeys has been used to study the mid-brain eye-movement system. Cells in the substantia nigra pars reticulata (SNr) that project to the superior colliculus tonically suppress saccadic eye movements and do so via a GABAergic mechanism. Removal of this suppression leads to facilitation and elicitation of saccadic eye movements. Furthermore, the integrity of the nigrotectal pathway is especially important for eye movements guided by memory. Since the SNr is thought to be part of the basal ganglia output system, this work may point to two general principles of basal ganglia function: a mechanism involving ‘disinhibition’ and a relative specialization for memory-guided movements.