The discharges during movement of cells in the ventrolateral thalamus of the conscious monkey.

1. Monkeys were trained to perform a stereotyped movement task in return for food rewards. On completion of training a headpiece which allowed microelectrode access to the thalamus for single cell recordings during performances of the task was attached at a surgical operation. The location of each cell studied was determined by histological examination of the fixed brain and precise identification of electrode tracks. 2. Ninety-three of ninety-seven cells discharging in association with arm movements but not responding to natural activation of peripheral receptors in the forelimb were located predominantly in the rostral part of nucleus VPLo and the caudo-ventral part of VLo. These cells appeared to be associated with active movement in one direction of a specific joint. 3. 52% of these 'motor' cells discharged in association with movements of either forelimb. The other 48% discharged in association with movement of the contralateral arm and hand only. 4. Sixty cells responded to stimulation of deep receptors or to passive limb manipulation in the relaxed and cooperative animal. These cells were predominantly located in the ventro-caudal part of VPLo and all the responses were obtained from contralateral receptors. Their discharges during performances of the motor task were indistinguishable from those of 'motor' cells. 5. Ninety-two cells were driven by limb manipulation and by natural activation of superficial cutaneous receptors and these were found predominantly in VPLo and VPLc. All responses were from contralateral receptors. These cells discharged during performance of the motor task; for some of them, their afferent input zones were not being stimulated by contact with the manipulandum when their 'motor' discharges commenced. 6. Although responses in each of the above groups of cells were sought by imposing a sudden perturbation of the limb during the performance of the active movement task, no responses were seen.