Increased Intra-Thalamic and Thalamo-Cortical Functional Connections during fMRI of human REM Sleep

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Rapid Eye Movement (REM) sleep, characterized by vivid dreaming and muscle atonia, raises fascinating questions about how such immersive experiences occur without external sensory input. An fMRI study has shown that REM-time-locked activations were found in multiple non-visual primary sensory cortices, along with the visual cortex, anterior cingulate cortex, and thalamus (Hong et al., 2009). These findings suggest that the thalamus is simultaneously functionally connected to multiple sensory cortices during REM sleep, a pattern not observed during wakefulness. For instance, Seitzman et al. (2019) identified five distinct thalamic subnetworks based on their unique functional connectivity to cortical networks. Given that pontine cholinergic neurons discharge in bursts just before each ponto-geniculo-occipital (PGO) wave-a process associated with eye movements-it is plausible that these neurons release acetylcholine to the thalamus (Datta & Siwek, 2002; Steriade & McCarley, 2013), which in turn activates the limbic system and sensory cortices. Taken together, we hypothesize that these five thalamic subnetworks function collectively as a sensory relay station during REM sleep, simultaneously connecting to sensory-related cortical networks (See Figure 1).