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The default mode network (DMN) is a distributed set of brain regions engaged during wakeful rest. Spontaneous thoughts — internally directed mental experiences that arise with relative freedom and few constraints on their content or transitions — are ubiquitous during wakeful rest. Spontaneous memory reactivation — neural events in which recently learned information is reinstated — also occurs at rest and has a fundamental role in long-term memory consolidation. Here, we synthesize emerging behavioral, theoretical, and neurophysiological evidence suggesting that DMN activity associated with spontaneous thought provides a neural context that promotes the propagation of reactivated information to aid consolidation. We highlight )ndings linking spontaneous thought to improved memory performance, discuss how speci)c DMN subsystems coordinate with the hippocampus during reactivation, and highlight recent studies that connect spontaneous thought content to memory reactivations. We conclude by outlining future directions to clarify the mechanistic role of spontaneous thought in memory consolidation mediated by the complex interplay between the DMN, hippocampus, and broader cortical networks. 

The brain is a complex, multiscale dynamical system composed of many interacting regions. Knowledge of the spatiotemporal organization of these interactions is critical for establishing a solid understanding of the brain’s functional architecture and the relationship between neural dynamics and cognition in health and disease. The possibility of studying these dynamics through careful analysis of neuroimaging data has catalyzed substantial interest in methods that estimate time-resolved fluctuations in functional connectivity (often referred to as “dynamic” or time-varying functional connectivity; TVFC). At the same time, debates have emerged regarding the application of TVFC analyses to resting fMRI data, and about the statistical validity, physiological origins, and cognitive and behavioral relevance of resting TVFC. These and other unresolved issues complicate interpretation of resting TVFC findings and limit the insights that can be gained from this promising new research area. This article brings together scientists with a variety of perspectives on resting TVFC to review the current literature in light of these issues. We introduce core concepts, define key terms, summarize controversies and open questions, and present a forward-looking perspective on how resting TVFC analyses can be rigorously and productively applied to investigate a wide range of questions in cognitive and systems neuroscience.