Activity of ventrolateral thalamic neurons during arm movement.

1. Monkeys were trained to maintain the handle of a manipulation within a small zone despite perturbations to the handle and to perform slow and rapid arm movements triggered by a visual stimulus. The activity of neurons in the region of the ventrolateral thalamus which projects directly to the arm area of the motor cortex (VL arm area) was then examined during performance of the task and while the animal's limb was passively manipulated. 2. Thalamic units related to arm movement were confined to the VL arm area. Microstimulation (less than 25 muA) in the VL arm area could, in some instances, evoke localized contractions of shoulder, arm, and hand musculature. Thalamic units whose discharge appeared to be related to jaw, tongue, and neck muscle activity occurred only medial to arm movement-related units, and units related to spontaneous leg movements occurred only laterally. 3. Most VL arm area neurons, although well related to active arm movements, were not influenced by gentle passive manipulation of the animal's limb. Few VL arm area neurons responded at short latency to perturbations applied to the handle of the manipulandum which the animal was holding. Thus, VL arm area neurons are unlike many motor cortex arm area neurons which are responsive to passive manipulation and are influenced at short latency by similar perturbations to the animal's limb. 4. Most arm movement-related VL neurons discharge during both fast and slow arm movements. Although a few neurons varied their discharge only during fast movements, none varied only in relation to slow movements. VL arm area neurons are, therefore, unlike the neurons in one source of its input, the globus pallidus, where some neurons are preferentially related to slow movements. 5. Some VL arm area neurons vary their discharge before any change in muscle activity during a fast and slow movement. Such neurons were recorded in VL regions which project to motor cortex areas representing proximal and distal musculature. This suggests that VL neurons might play a role in initiating activity in muscles concerned with controlling body posture, as well as those involved in discrete limb movements.20