Foster, D. J., & Wilson, M. A. (2006). Reverse replay of behavioural sequences in hippocampal place cells during the awake state. Nature, 440(7084), 680-683.
The hippocampus has long been known to be involved in spatial navigational learning in rodents(1,2), and in memory for events in rodents(3,4), primates(5) and humans(6). A unifying property of both navigation and event memory is a requirement for dealing with temporally sequenced information. Reactivation of temporally sequenced memories for previous behavioural experiences has been reported in sleep in rats(7,8). Here we report that sequential replay occurs in the rat hippocampus during awake periods immediately after spatial experience. This replay has a unique form, in which recent episodes of spatial experience are replayed in a temporally reversed order. This replay is suggestive of a role in the evaluation of event sequences in the manner of reinforcement learning models. We propose that such replay might constitute a general mechanism of learning and memory.
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Louie, K., & Wilson, M. A. (2001). Temporally structured replay of awake hippocampal ensemble activity during rapid eye movement sleep. Neuron, 29(1), 145-156.
Human dreaming occurs during rapid eye movement (REM) sleep. To investigate the structure of neural activity during REM sleep, we simultaneously recorded the activity of multiple neurons in the rat hippocampus during both sleep and awake behavior. We show that temporally sequenced ensemble firing rate patterns reflecting tens of seconds to minutes of behavioral experience are reproduced during REM episodes at an equivalent timescale. Furthermore, within such REM episodes behavior-dependent modulation of the subcortically driven theta rhythm is also reproduced. These results demonstrate that long temporal sequences of patterned multineuronal activity suggestive of episodic memory traces are reactivated during REM sleep. Such reactivation may be important for memory processing and provides a basis for the electrophysiological examination of the content of dream states.