FIGURE1|
Schematic of the proposed memory processes. Upper panel:
Memory processes can be divided into theree steps (Encoding, Consolidation, and Retrieval). Encoding is the process of information up take during learning (in this case of card pair locations of a concentration game) and is most effective during wakefulness, while retrieval is the process of recalling the memorized content at a later time point. Consolidation takes place after encoding and is necessary to transform the initially labile traces into enduring representations. During sleep after learning, sleep-dependent memory consolidation strengthens the acquired memory traces by reactivating (“replaying”) them. Lower panel: During wake encoding, information is taken up by the sense organs and flows to the hippocampus via the association cortices. In the hippocampus a trace is formed fast but only for temporary storage (thick lines in the hippocampus represent the hippocampal trace, dashed lines in the cortex represent the corresponding cortical trace). During slow wave sleep the traces in the hippocampus are reactivated leading to their integration into the long-term store of the cortex (active system consolidation), where they are strengthened through synaptic consolidation (the consolidated trace is represented by thick lines in the cortex). Additionally, processes of synaptic downscaling during sleep (shown as the hippocampal trace vanishing) lead to the preparation of the brain to encode new information during the next wake phase (again depicted by thick and dashed lines in hippocampus and cortex, respectively). While the synaptic downscaling hypothesis has been most rigorously applied to cortical learning tasks, recent research has provided evidence for downscaling in the hippocampus during REM sleep. Processes of local synaptic strengthening may work in concert with global downscaling processes to provide reliable and efficient memory traces. Bottom panel is adapted from.