The default mode network during sleep: spindle oscillations and long-range cortical interactions in memory consolidation

The distributed nature of cortical function is becoming increasingly supported by experimental evidence. The dynamics of the interaction between cortical areas has been studied in multiple species with a variety of techniques covering spatial and temporal scales, going from the details of neural ensemble activity in a brain area to the characterization of functional networks spanning the entire brain. The interaction between the hippocampus and the neocortex has attracted much attention as supporting memory consolidation and information exchange between memory systems, a basic tenet of contemporary memory theory. Recently, the emphasis has shifted from the study of the exchanges between the hippocampus and one select cortical area to characterization of cortico–hippocampal networks spanning large cortical swaths. We review evidence on the prominent involvement of the default mode network with the hippocampus and its role in memory consolidation. We then look at the statistics of cortical activations in sleep and wakefulness, discussing to what extent they support the hypothesis that the brain operates in a near-critical state, which would have profound consequences on the propagation of information across distant areas. Last, we link that body of literature to data on brain oscillations, hypothesizing that near-critical cortical activations shape sleep spindles, organizing activity conducive to memory consolidation.