Neural activity during a simple reaching task in macaques is counter to gating and rebound in basal ganglia-thalamic communication

Task-related activity in the ventral thalamus, a major target of basal
ganglia output, is often assumed to be permitted or triggered by changes
in basal ganglia activity through gating- or rebound-like mechanisms. To
test those hypotheses, we sampled single-unit activity from connected
basal ganglia output and thalamic nuclei (globus pallidus-internus, GPi,
and ventrolateral-anterior nucleus, VLa) in monkeys performing a reaching
task. Rate increases 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 both were equally positive and negative. Finally,
spontaneous GPi bursts and pauses were both followed by small, slow
reductions in VLa rate. These results appear incompatible with standard
gating and rebound models. Still, gating or rebound may be possible in
other physiological situations: Simulations show how GPi-VLa communication
can scale with GPi synchrony and GPi-to-VLa convergence, illuminating how
synchrony of basal ganglia output during motor learning or in pathological
conditions may render this pathway effective. Thus, in the healthy state,
basal ganglia-thalamic communication during learned movement is more
subtle than expected, with changes in firing rates possibly being
dominated by a common external source.