RESEARCH HIGHLIGHTS SLEEP and cortical neurons fire during Peyrache et al. investigated replay an experience is replayed by recording in both the hippocampus during slow-wave sleep. This replay and the neocortex during a learn- Play it again is assumed to contribute to memory ing experience in a Y maze and in consolidation. Now, Karlsson and subsequent sleep. In each trial, rats Repetition is the mother of learning, Frank demonstrate the existence of had to select the arm of the maze that states an old Latin proverb. Recent replay of previous, spatially remote contained a reward, but the position studies in rodents have shown that experiences during waking (“awake of the reward arm changed during the the sequence in which hippocampal remote replay”) and Peyrache et al. task, so that the animals had to learn a show that during sleep, the neural fir-fir- new rule to keep obtaining the reward. ing pattern that appears during new First, the authors recorded pat- learning is replayed in the medial terns of neural activation in the prefrontal cortex (mPFC) in concert mPFC during rule learning. They with hippocampal activity. then observed that during subsequent In the first paper, Karlsson and slow-wave sleep, preferential reactiva- Frank monitored the firing patterns tion of learning-related activation of hippocampal place cells in two patterns in the mPFC coincided with different environments. Between trials hippocampal sharp waves or ripples animals were placed in a rest box. (which have been associated with When in the rest box after trials in hippocampal replay), suggesting that environment 1, the rats frequently hippocampal–neocortical interac- showed replay of place cell activation tions were taking place. sequences from their experience in These studies show that replay of environment 1. Surprisingly, these neuronal activation patterns occurs sequences were more prevalent when during sleep after learning experi- the animals were awake than when ences and also during quiet waking, they were in a sleep-like, quiescent providing more evidence that neu- state. This awake replay of environ- ronal reactivation underlies learning ment 1 was also seen when animals and memory. paused between bouts of movement Claudia Wiedemann in environment 2. Thus, the hippocampus can replay past, ORIGINAL RESEARCH PAPER Karlsson, M. P. & Frank, L. M. Awake replay of remote experiences remote experiences during waking. in the hippocampus. Nature Neurosci. 14 Jun 2009 These findings suggest that awake (doi:10.1038/nn.2344) | Peyrache, A. et al. Replay replay might have a particularly of rule-learning related neural patterns in the prefrontal cortex during sleep. Nature Neurosci. important role in memory 31 May 2009 (doi:10.1038/nn.2337) BRANDX consolidation. NATURE REVIEWS | NEUROSCIENCE VOLUME 10 | AUGUST 2009 © 2009 Macmillan Publishers Limited. All rights reserved.