Study objectives: To determine whether thalamocortical signaling between the thalamus and the neocortex decreases from wakefulness to nonrapid eye movement (NREM) sleep.
Design: Electroencephalography and functional magnetic resonance imaging data were collected simultaneously at 02:30 after 44 h of sleep deprivation.
Setting: Clinical research hospital.
Participants: There were six volunteers (mean age 24.2 y, one male) who yielded sufficient amounts of usable, artifact-free data. All were healthy, right-handed native English speakers who consumed less than 710 mL of caffeinated beverages per day. Psychiatric, neurological, circadian, and sleep disorders were ruled out by reviewing each patient's clinical history. A standard clinical nocturnal polysomnogram was negative for sleep disorders.
Interventions: N/A.
Measurements and results: A functional connectivity analysis was performed using the centromedian nucleus as the seed region. We determined the statistical significance of the difference between correlations obtained during wakefulness and during slow wave sleep. Neocortical regions displaying decreased thalamic connectivity were all heteromodal regions (e.g., medial frontal gyrus and posterior cingulate/precuneus), whereas there was a complete absence of neocortical regions displaying increased thalamic connectivity. Although more clusters of significant decreases were observed in stage 2 sleep, these results were similar to the results for slow wave sleep.
Conclusions: Results of this study provide evidence of a functional deafferentation of the neocortex during nonrapid eye movement (NREM) sleep in humans. This deafferentation likely accounts for increased sensory awareness thresholds during NREM sleep. Decreased thalamocortical connectivity in regions such as the posterior cingulate/precuneus also are observed in coma and general anesthesia, suggesting that changes in thalamocortical connectivity may act as a universal "control switch" for changes in consciousness that are observed in coma, general anesthesia, and natural sleep.
Keywords: Electroencephalography; functional connectivity; functional magnetic resonance imaging; neocortex; nonspecific nuclei; sleep; thalamus.