Systems memory consolidation during sleep
After learning, memory traces undergo a period of transformation and consolidation. During “synaptic consolidation”, synaptic potentiation induced during learning is mainly made permanent. However, during “systems consolidation”, which has been proposed to exist in the declarative memory domain, new memory traces are not simply strengthened, but they are thought to be transferred and integrated into existing semantic networks. Recent studies show that sleep can play a role in systems consolidation. Several properties of sleep electrophysiology and neurotransmitter activity have been linked to memory consolidation. During sleep, neuromodulators like acetylcholine and noradrenaline shift the brain into a state which favours internal stimulus generation over external stimulus reception. Thus, a dialogue between different brain regions, e.g. the hippocampus and the neocortex, becomes possible, allowing interference free transfer of information between regions. Several recent studies show that the representation of memory in the brain changes over longer time intervals, and that these changes are backed by sleep. In particular, directly sleep-related electrophysiological brain activity, like sleep spindles and slow oscillations, has been associated with memory consolidation. Together, a pattern of brain activity emerges which consists of a coordinated interplay between brainstem, subcortical and cortical areas, providing an optimal environment for off-line memory processing.