RESEARCH HIGHLIGHTS SLEEP AND MEMORY Memory consolidation…while you are sleeping DOI: 10.1038/nrn2084 assumes that crosstalk between brain terns occuring in the visual cortex regions transforms recent memory and the hippocampus reflected the in the hippocampus into long-term patterns that had been expressed dur- memory in the cortex. Ji and Wilson ing awake behaviour. But differences have investigated whether sleep reac- between memory reactivation in the tivation is involved in the processing cortex and hippocampus emerged. and formation of episodic memory Although reactivation was detected by recording and comparing firing in the cortex during all sleep sessions, patterns in the hippocampus and the hippocampal reactivation was the visual cortex of rats during sleep most robust during sleep that imme- prior to a task, during task perform- diately followed awake behaviour, ance (maze-running) and during suggesting that cortical traces tended sleep after the task. The animals used to reflect older memories, whereas in these studies were trained daily hippocampal traces tended to reflect over a period of up to 3 weeks in recent memories. The cortex also this sleep–run–sleep pattern before seemed to lead the hippocampus in recordings took place. the initiation of these memory replay Initially the researchers showed events, but the hippocampus seemed During the first phase of sleep our that there was a direct correlation to lead the cortex in the detailed brain is occupied with our last awake between the firing patterns that replay of the memory sequences experiences. Does this ‘revisiting’ of occurred in the visual cortex and the themselves, suggesting different roles events consolidate these experiences hippocampus during SWS and that for these structures in the memory as memories? In Nature Neuroscience, this correlation was independent dialogue. Therefore, the authors con- Ji and Wilson have now shown that of when the sleep session occurred. cluded that cortical and hippocampal the firing patterns during awake Cortical firing patterns always pre- replays are coordinated to match the experiences are replayed in the ceded hippocampal firing patterns, same awake experience during SWS. hippocampus and cortex in a coordi- indicating that the cortex drives the These findings support a bidi- nated manner during slow wave sleep hippocampus. rectional interaction model between (SWS), suggesting a mechanism for During the maze-running task, the hippocampus and the cortex for memory consolidation. the researchers observed that cells memory consolidation. It would SWS describes the low frequency of the visual cortex have spatially also be interesting to see whether of brain waves recorded by electroen- localized firing patterns, very semantic memory, the recollection of cephalography (EEG); it is character- much like the ‘place cells’ in the facts and concepts, follows the same istic for deep sleep, which normally hippocampus, which are active if principle of memory consolidation as occurs within the first 3 hours of a the rat is in a specific location. This episodicORIGINAL memory. RESEARCH PAPER Ji, D. & night’s sleep. Multiple studies have finding allowed them to compare the Wilson, M. A. CoordinatedClaudia memory replay Wiedemann in the shown that recent experiences are firing sequence of cells during task visual cortex and hippocampus during sleep. Nature Neurosci. 10, 100–107 (2007) replayed (sleep reactivation) during performance and during sleep replay this first phase of sleep. The current in both brain areas. The researchers theory for memory consolidation showed that during SWS, firing pat- NATURE REVIEWS | NEUROSCIENCE VOLUME 8 | FEBRUARY 2007 © 2007 Nature Publishing Group .