Neuroimaging of brain oscillations during human sleep

Abstract

Sleep is not simply a state with a decreased level of consciousness. During sleep, the human brain exhibits a variety of neural activities across sleep stages. Functional neuroimaging studies have investigated these activities and identified their underlying neural networks in different stages of sleep. These studies have characterized neural activations related to major sleep oscillations such as sleep spindles and slow waves during non-rapid-eye-movement (NREM) sleep, as well as ponto-geniculo-occipital (PGO) waves during rapid-eye-movement (REM) sleep. Neuroimaging thus identified structures possibly involved in the generation of phasic brain oscillations and provided insights into their importance in regulating sleep. Most notably, neuroimaging studies showed that neuronal oscillations of sleep regulate the interplay between the sleeping brain and external stimulations. Precisely, spindles preserve sleep by contributing to a gating process during which transmission of external auditory stimuli to the cortex is inhibited. In contrast, slow waves induced by external acoustic cues-K-complexes-are associated with enhanced processing of external information at the level of primary auditory cortex. Finally, recent neuroimaging studies have highlighted the role of spindles in sleep-dependent memory consolidation.