Neuronal activity in the primate motor thalamus during visually triggered and internally generated limb movements

Single-unit recordings were made from the basal-ganglia- and cerebellar-rec eiving areas of the thalamus in two monkeys trained to make arm movements t hat were either visually triggered (VT) or internally generated (IG). A tot al of 203 neurons displaying movement-related changes in activity were exam ined in detail. Most of these cells (69%) showed an increase in firing rate in relation to the onset of movement and could be categorized according to whether they fired in the VT task exclusively, in the IG task exclusively, or in both tasks. The proportion of cells in each category was found to va ry between each of the cerebellar-receiving [oral portion of the ventral po sterolateral nucleus (VPLo) and area X] and basal-ganglia-receiving [oral p ortion of the ventral lateral nucleus (VLo) and parvocellular portion of th e ventral anterior nucleus (VApc)] nuclei that were examined. In particular , in area X the largest group of cells (52%) showed an increase in activity during the VT task only, whereas in VApc the largest group of cells (53%) fired in the IG task only. In contrast to this, relatively high degree of t ask specificity, in both VPLo and VLo the largest group of cells (similar t o 55%) burst in relation to both tasks. Of the cells that were active in bo th tasks, a higher proportion were preferentially active in the VT task in VPLo and area X, and the IG task in VLo and VApc. In addition, cells in all four nuclei became active earlier relative to movement onset in the IG tas k compared with the VT task. These results demonstrate that functional dist inctions do exist in the cerebellar- and basal-ganglia-receiving portions o f the primate motor thalamus in relation to the types of cues used to initi ate and control movement. These distinctions are most clear in area X and V Apc, and are much less apparent in VPLo and VLo.