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1. Monkeys were trained to 1) hold a handle in a central zone midway between "push" and "pull" while awaiting 2) an instruction telling them how to respond to a subsequent 3) perturbation, which triggered the instructed movement and was followed by 4) a reward if the movement was correct. 2. There were two sorts of instructions: push and pull. When the pull instruction had preceded the perturbation, the monkey responded to the perturbation by pulling, whereas after a push instruction, the monkey responded to the perturbation by pushing. 3. Recordings in pre- and postcentral sensorimotor cortex revealed instruction-induced changes of neuronal activity during the period intervening between the instruction and the perturbation-triggered movement. Effects of the instruction were differential depending on which of the two instructions was given, such differential responses to the instruction being detected in 61% of precentral pyramidal tract neurons (PTNs), 44% of precentral non-PTNs, and 11% of postcentral neurons. 4. Since motor cortex PTN axons end on alpha and gamma motoneurons and on interneurons of the spinal cord, changes of PTN activity with "intention" or "motor set" provide a mechanism for suprasegmental control and presetting of spinal cord reflex excitability specific to the nature of an impending movement.

Previous microelectrode recordings in the putamen of monkeys have revealed a class of tonically active neurons without apparent behavioral correlates. The present study shows that such neurons have responses to stimuli that trigger movement but that these responses disappear when motor responses to the stimuli are extinguished. The short latency of the responses (less than for other putamen neurons) suggests that they may play a role in linking conditioned stimuli and responses.The putamen is an input stage for a group of interconnected subcortical neuronal aggregates collectively referred to as the basal ganglia. It receives inputs from the cerebral cortex and thalamus, is reciprocally connected with the substantia nigra pars compacta, and projects to the globus pallidus, a basal ganglia output stage. Microelectrode recordings in monkeys have shown that relatively few putamen neurons have tonic spontaneous discharge in the absence of movement; the more numerous putamen neurons that are phasically related to voluntary movement do not exhibit tonic activity in the absence of movement and, conversely, the tonically active neurons do not appear to be related to movement (1).The present study confirms the finding that tonic putamen neurons are unrelated to body movements per se, but it reveals that under certain circumstances they may exhibit highly reliable responses to external events. It was found that an auditory stimulus can elicit short-latency responses in tonic putamen neurons when the stimulus is a cue for juice delivery and consumption, but that such a stimulus fails to elicit responses when juice delivery has repeatedly failed to follow the sound, with the result that the sound no longer triggers movements associated with consuming the juice. MATERIALS AND METHODSTwo monkeys (Macaca mulatta) made repeated self-paced extension-flexion movements of the elbow for a juice reward that was delivered after eight movement cycles. The delivery of the reward was preceded by the click of a solenoid valve, which came to be a trigger for movements to consume the juice. Extracellular microelectrode recordings in the putamen yielded a number of tonic neurons that were unrelated to body movements and that had action potentials of greater duration (mean ± SD, 1.2 ± 0.2 msec) than action potentials of the more numerous putamen cells (0.9 ± 0.1 msec) that were silent except during movement. On casual inspection, these tonic neurons did not appear to be related to any aspect of the behavioral situation, but in raster displays aligned on the occurrence of the solenoid click that signaled reward (Fig. 1A), it was apparent that there was a greatly increased probability of impulse occurrence '60 msec after the click. This observation led to an examination of tonic neuron responsiveness in three behavioral conditions: (i) self-paced movement, as already described, in which a series of elbow movements resulted in a solenoid click and a juice reward; (ii) free reward, in which click and juice occurred at regular intervals (...

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