Isolated cortical computations during delta waves support memory consolidation

Special moments at cortical quiet states Delta waves are moments of widespread cortical silence that alternate with active states during slow-wave sleep. However, upon closer examination, single neuronal action potentials can be detected during delta waves. Todorova and Zugaro sought to determine whether this neuronal noise could instead be an important signal (see the Perspective by Ikegaya and Matsumoto). They found that persisting action potential firing during delta waves is an overlooked but widespread phenomenon, which could potentially involve all neurons and all delta waves. A critical role of the delta wave may be to insulate specific cortical computations taking place in response to hippocampal replay and involved in memory consolidation. Science, this issue p. 377; see also p. 306 During cortical down-state events, hippocampal output activates specific groups of cortical neurons. Delta waves have been described as periods of generalized silence across the cortex, and their alternation with periods of endogenous activity results in the slow oscillation of slow-wave sleep. Despite evidence that delta waves are instrumental for memory consolidation, their specific role in reshaping cortical functional circuits remains puzzling. In a rat model, we found that delta waves are not periods of complete silence and that the residual activity is not mere neuronal noise. Instead, cortical cells involved in learning a spatial memory task subsequently formed cell assemblies during delta waves in response to transient reactivation of hippocampal ensembles during ripples. This process occurred selectively during endogenous or induced memory consolidation. Thus, delta waves represent isolated cortical computations tightly related to ongoing information processing underlying memory consolidation.