The upshot of up states in the neocortex: From slow oscillations to memory formation

A sleeping brain is by no means dormant: most cortical neurons, primarily detached from the influence of stimuli in the environment, are nevertheless active, just as they are during behavior. Although neural activity is preserved during sleep, the structure of the activity changes significantly, yielding bouts of widespread high-amplitude synchronized fluctuations in activity. These fluctuations, known as “slow oscillations” (Steriade et al., 1993), alternate between intervals of strong activity (“Up states”) and intervals of almost complete silence (”Down states“), which spread coherently throughout the cortex at a typical frequency of ∼1 Hz during the sleep stage known as slow-wave sleep (SWS), as well as under certain types of anesthesia. The neocortical slow oscillation is thought to be generated intrinsically, depending on recurrent excitatory synaptic transmission (Sanchez-Vives and McCormick, 2000; Petersen et al., 2003).

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