Slow-wave sleep and the consolidation of long-term memory

Slow-wave sleep (SWS) has been shown to play an important role in the reinforcement of declarative memory. A dialogue between the neocortex and hippocampus is important during this consolidation and appears to be largely regulated by <1 Hz electroencephalographic (EEG) slow oscillations. Events experienced during wakefulness are encoded in the neocortex but, simultaneously they are encoded even more strongly in the hippocampus. Slow oscillations that characterize SWS originate in the neocortex and their amplitude increases with increased amounts of information encoded during prior waking. Neuronal activity is temporarily grouped by these slow oscillations into up-states of enhanced neuronal activity and down-states of neuronal silence. Grouping is not only induced in the neocortex but also in other relevant structures, such as the thalamus and the hippocampus, generating spindle activity and sharp-wave ripples, respectively. Sharp-wave ripples are known to accompany a memory replay of encoded information in the hippocampus during SWS which stimulates the transfer of this memory-related information to the neocortex. The slow oscillations synchronize this transfer with the thalamocortical spindles arriving at the neocortex at the same time as the hippocampal memory information. This synchronization is thought to be critical to the long-term storage of respective memories within neocortical networks.