Activities of neurons in 'motor' thalamus during control of limb movement in the primate

Experiments were designed to study the role of the motor area of the thalamus in the performance of a precise, controlled movement. (The motor area of thalamus is defined as that area projecting in a specific manner to motor cortex, and includes ventrolateral nucleus pars oralis (VLo), ventroposterolateral pars oralis (VPLo), and parts of candal ventroanterior parvocellularis (VApc), area X, and VL pars medialis (VLm).) Extracellular recordings of 244 motor thalamic units were made in awake primates, trained to perform a visually guided tracking task involving wrist flexion and extension. Responses following a visual cue to initiate movement and following a visual or somatic perturbation were analyzed. One hundred and seventy-five cells (72%) altered their firing following the visual cue, 27% of these responding earlier than 100 ms (the earliest time for a similar response in motor cortex). One hundred and fifty-five units (64%) responded following the visual perturbation. Responses to the somatic perturbation (torque step) were most numerous (197 cells, 81%). One third of the responses following the torque had a latency of less than 50 ms, but the earliest was 30 ms. Thus, the thalamus may play a role in relaying to motor cortex the earliest responses to a visual stimulus. However, this area apparently does not contribute to the earliest motor cortical responses to somatic input (<30 ms). One group of cells (35) was observed to be specifically related to one direction of wrist movement. This finding suggests a role for the motor thalamus in the control of single muscles or groups of synergists acting at a joint. The latency data are compatible with a role in movement initiation. It is suggested that these cells could be connected to the clusters in motor cortex related to these muscles. Responses to somatic or visual input, but independent of the wrist movement, were also observed. Such responses might produce a nonspecific activation of wide areas of motor cortex or, alternatively, might be related in a specific way to cortical areas controlling muscles other than those acting at the wrist. A possible relationship is drawn between this group of response patterns and cortical slow-wave activity, such as the readiness potential and contingent negative variation.