Increased intra-thalamic and thalamo-cortical functional connections during human REM sleep: Insights from a two-night EEG-fMRI study

Rapid Eye Movement (REM) sleep, characterized by vivid dreaming and muscle atonia, raises the fascinating question about how such immersive experiences occur without external sensory input. To investigate this, we analyzed data from a whole-night 8-hour EEG-fMRI concurrent recording study with 12 non-sleep-deprived participants. Our findings confirmed that during REM sleep, all thalamic subnetworks simultaneously connect with multiple sensory networks, including visual, motor, auditory, and action-planning networks, which was not observed during wake. Additionally, intra-thalamic connection was significantly higher during REM sleep compared to wake in all 11 participants with high-quality fMRI data during REM, and compared to Non-REM stages in nine out of 11 participants. We validated these results on seven participants who achieved REM sleep on the first (adaptation) night. Building on these findings, we further hypothesized that the observed increases in thalamus-related FCs were specifically linked to phasic REM, a state associated with vivid dreaming, as opposed to tonic REM. Using the Hidden Markov Model to classify phasic and tonic REM states, we found that phasic REM exhibited significantly greater intra-thalamic and thalamo-cortical FCs than tonic REM. These results suggest that during REM sleep, specifically during phasic REM, thalamic subnetworks function collectively to distribute internally generated sensory information to sensory-related cortical networks, which might be the neural mechanisms underlying vivid dreaming.