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

Higher-order (HO) thalamic nuclei are enigmatic. 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 nuclei in isolation. To investigate the effect of conscious state across nuclei, we simultaneously recorded local field potentials (LFP) and multi-unit activity (MUA) across multiple thalamic nuclei in awake and anesthetized mice during sensory stimulation. The effect of voluntary locomotion was also studied in the awake state. Surprisingly, we found that LFP traces were strongly correlated across HO nuclei in both states, which cannot be explained by volume conduction alone, suggesting the existence of shared synaptic inputs. Anesthesia significantly reduced MUA across HO nuclei, while in the awake state locomotion and sensory stimuli activated many nuclei, including those that have not classically been regarded as sensory or motor. Moreover, we found that sensory responses of HO nuclei depended on the sensory modality and conscious state of the animal. These findings challenge the classical view of the higher-order thalamus being the aggregate of isolated, independent nuclei.