1. To evaluate the various hypotheses that the basal ganglia preferentially control one mode of movement to the exclusion of others, we recorded the discharge of single neurons in the globus pallidus (GP) in rhesus monkeys during their performance of five trained wrist-movement tasks. The tasks were designed to dissociate several modes and parameters of movement to see whether pallidal neurons would discharge in relation to one and not the others. All tasks were performed by flexing and extending the wrist with opposing or assisting torque loads (0.2 Nm). The five tasks included 1) VisStep, a visually cued step tracking task; 2) VisRamp, a visually guided hold-ramp-hold tracking task; 3) VisSine, a visually guided rapid sinusoidal tracking task; 4) SelfRamp, a self-paced hold-ramp-hold task with delayed alternation, trained velocity, and no visual feedback of wrist position; and 5) SelfSine, a self-paced rapid sinusoidal movement without visual feedback of wrist position. Wrist position and velocity were monitored during all recordings; and wrist, arm, shoulder, and back electromyographs (EMGs) were monitored periodically. Unit discharge was recorded extracellularly from both segments of the GP. The results were similar in the present analysis and are considered together. As a control, units were also recorded in the dentate nucleus of the lateral cerebellum, and the EMGs of many muscles were recorded in limbs, neck and trunk. 2. For 100 GP neurons [41 in the internal segment (GPi) and 59 in the external segment (GPe)], the activity of which changed with task performance, the discharge patterns varied greatly across tasks. The discharge of 96/97 neurons (99%) changed during VisStep, 66/91 neurons (73%) changed during VisRamp, 41/81 neurons (51%) changed during VisSine, 7/34 neurons (21%) changed during SelfRamp, and 25/80 neurons (31%) changed during SelfSine. Of 74 neurons that were fully tested in four or more tasks, 16 (21%) were related only to one task; only 17 cells (23%) were related to all tasks; and, for the remaining 41 (55%) neurons, the relation of the discharge of a given neuron to one task did not predict its relation to other tasks. These task-dependent differences in the discharge of pallidal neurons were not correlated with differences in wrist position, velocity, load, or muscle activity (see also the following paper-Mink and Thach, 1991a). 3. From these data, we conclude that no one task engaged all pallidal neurons to the exclusion of other tasks. Nevertheless, pallidal neurons were better related to some tasks than to others-in terms of both magnitude of change in discharge frequency and proportion of neurons. Thus all neurons were better related to visually guided than to self-paced movements, and all but one neuron to step movements better than to ramps. Sixty-nine percent of pallidal neurons were related to visually guided movements and not at all to self-paced movements. Of the neurons related to visually guided movements, 26% were related to step tracking ("open-loop") movements and not to ramp tracking ("closed-loop") movements. Pallidal neurons were conspicuous for the relative lack of modulation in two tasks, self-paced ramps and self-paced rapid alternation. 4. By contrast, for 44 neurons in the dentate nucleus, the activity of which changed with task performance, the discharge patterns were relatively more uniform across tasks. The discharge of 39/44 neurons (87%) changed significantly in VisStep, 37/40 neurons (93%) changed in VisRamp, 26/30 neurons (87%) changed in VisSine, and 30/35 neurons (86%) changed in SelfSine. Cerebellar dentate neurons differed from pallidal neurons chiefly in their robust modulation during the self-paced rapid alternating movements, to which the pallidal neurons were seldom or weakly related. Six of 30 dentate neurons (20%) had activity better related to SelfSine than to VisStep. 5. Muscle activity EMG varied predictably across tasks to the extent that it was greater for faster movements (VisStep and SelfSine); least for slower movements (VisRamp and SelfRamp); and correlated with position, velocity, and load. No pallidal or dentate neuronal activity resembled EMG activity in these respects. 6. The nonexclusive relation of pallidal discharge to the five movement tasks suggested that it was used for purposes other than initiating and controlling any one of them.
