Understanding non rapid eye movement sleep through neuroimaging

Non rapid eye movement (nonREM) sleep has long been considered as a state of brain quiescence, partly because brain oxygen metabolism, cerebral blood flow and glucose metabolism were shown to be decreased during nonREM sleep compared with wakefulness and REM sleep. However, emission techniques used for these measurements are characterized by relatively low temporal and spatial resolutions, whereby brain activity is averaged over minutes (the time required to obtain a single acquisition). Event-related functional magnetic resonance imaging (fMRI) acquires images with relatively high spatial and temporal resolution. Simultaneous electroencephalography (EEG) and fMRI can detect transient increases in activity associated with slow waves and slow and fast spindles during nonREM sleep. EEG/fMRI data show regionally specific increases in activity in relation to slow waves (brainstem, cerebellum, ventral prefrontal cortex, and posterior cingulate cortex/precuneus, parahippocampus areas) and slow (thalamus, anterior cingulate, insula) and fast spindles (additional responses in cingulate motor areas and sensorimotor contrices); some of which are involved in memory processing. These data demonstrate that nonREM sleep is not a state of brain quiescence, but a highly active state during which brain activity is consistently synchronized to slow waves and spindles in specific brain regions. These neural activity patterns have possible implications for memory processing.