Highly Correlated Activity across Higher-Order Thalamic Nuclei in Awake and Anesthetized States

The thalamus is an egg-shaped structure deep inside the brain and is of central importance for our cognitive functions. The thalamus can be divided into first- and higher-order (HO) nuclei. Unlike first-order thalamic regions which are known to primarily relay sensory signals to the neocortex, the functions of HO thalamic nuclei remain far less clear. Although previous studies indicate HO thalamic nuclei influence cortical processing and consciousness, most studies examined single thalamic nuclei in isolation. To investigate the relation between distinct HO thalamic nuclei, we recorded local field potentials (LFP) and multi-unit activity (MUA) across multiple thalamic nuclei simultaneously in awake and anesthetized mice with sensory stimulation. The effect of voluntary locomotion was also studied in the awake state. Surprisingly, we found that both LFP and MUA were strongly correlated across HO nuclei in both states, which cannot be explained by volume conduction alone but suggests the existence of shared synaptic inputs. Furthermore, in the awake state locomotion and sensory stimuli activated many HO nuclei, including those that have not classically been regarded as sensory or motor. These findings challenge the classical view of the thalamus being the aggregate of isolated, independent nuclei and suggest that the thalamus serves as an ideal bottleneck whose coordination resulted in effective orchestration of the entire brain.