NEURAL ACTIVITY IN THE MONKEY ANTERIOR VENTROLATERAL THALAMUS DURING TRAINED, BALLISTIC MOVEMENTS

1. To examine the role of the anterior ventrolateral thalamic nucleus (VLa) in motor control, extracellular single-cell recordings were made from the VLa nucleus in three conscious monkeys performing visually triggered, rapid movements requiring flexion and extension of the wrist joint. The movement paradigms consisted of three components: an initial hold period, a ballistic movement, and a final hold period. 2. Cerebellar nuclear stimulation was used to physiologically identify thalamic neurons receiving input from the cerebellum. Neurons subsequently confirmed histologically as lying within the VLa nucleus were located anterior to those ''driven'' by cerebellar stimulation. 3. The activities of 261 VLa neurons displaying movement-related behavior were examined. In the absence of movement, the activity of these neurons ranged from 5 to 80 spikes/s, the majority of cells (222/261; 85%) firing between 10 and 30 spikes/s. Neural activity was not influenced by afferent input from skin, joint, or muscle but altered markedly when the animal engaged in active movement. 4. The relationship between neural activity and movement at a specific joint was determined in 189 of the 261 neurons. The majority of these neurons (161/189; 85%) modulated their activity in response to movement confined to a single joint; the remaining neurons displayed multijoint motor responses, their activity being best related to a motor act, such as grasping, reaching, or feeding. There was no clear segregation in the representation of body parts in the VLa nucleus to suggest the presence of clearly defined somatotopy. 5. The activities of wrist-related neurons (45 of the 261) were examined to determine the motor characteristics of the VLa nucleus. Three distinctive activity patterns were exhibited by these neurons during the performance of the movement paradigms: 1) a phasic burst of <300 ms in duration (34/45; 76%); 2) a sustained change in neural activity lasting >300 ms (typically up to 1,000 ms; 4/45; 9%); and 3) both a phasic and a sustained response, the sustained response occurring either prior or subsequent to the phasic response (7/45; 15%). 6. The movement-related activity of the wrist-related neurons occurred late, 52% firing after the onset of the forearm electromyogram (EMG). In 47% of the neurons, discharge rate was related to movement direction and, in a smaller proportion (13%), joint position. No relationship was found between neural activity patterns and amplitude of movement or velocity of movement. 7. The timing and pattern of neural activity in VLa indicates that it is unlikely that cells in this nucleus are involved in the selection or activation of muscles for the initiation of a motor action or the coding of the physical parameters of movement.