Slow wave activity as the default mode of the cerebral cortex.

The function of sleep remained one of largest enigmas of neuroscience for most of the 20th century. However in recent years different evidence has accumulated in support of a critical role of sleep on functions such as replay and memory consolidation. In particular slow wave sleep, and its underlying corticothalamocortical activity, slow oscillations, could be critical not only for memory but also for the maintenance of the brain's structural and func- tional connectivity. In this article we ask: why slow oscillations? To answer this question we put forward the idea that slow oscillations are the default activity of the cortical network based on both experimental and theoretical evidence. We go on to discuss why slow oscillations emerge from the cortical circuits and what are the dynamic advantages of this activity pattern, such as the resilience to perturbances and the facilitation of transitions between a disconnected (e.g. deep sleep) brain and a connected, awake brain.