Weak modulation of thalamic discharge by basal ganglia output in association with a reaching task

Task-related activity in the ventral thalamus, a major target of basal ganglia (BG) output, is often assumed to be permitted or triggered by changes in BG activity through gating- or rebound-like mechanisms. To test those hypothesized relationships, we sampled single-unit activity from connected BG-output and thalamic nuclei (globus pallidus-internus, GPi, and ventrolateral-anterior nucleus, VLa) in non-human primates performing a reaching task. Increases in firing were the most common peri-movement change in both nuclei. Moreover, peri-movement changes generally began earlier in VLa than in GPi. Simultaneously-recorded GPi-VLa pairs rarely showed short-timescale spike-to-spike correlations or slow across-trials covariations and the latter were equally positive and negative. Finally, spontaneous bursts and pauses in GPi activity were both followed by small yet significant reductions in mean VLa rate. These results appear incompatible with gating and rebound models. Thus, BG-thalamic communication is more subtle than expected and may be dominated by a common external source.