Rats replay episodic memories in context

Vivid episodic memories in people have been characterized as the replay of multiple unique events in sequential order.^1,2,3,4 An important feature of this type of remembering is that we remember the identity of specific items and the contexts in which they occurred.^5,6,7,8,9 Animal studies demonstrate that rats remember multiple items and the contexts in which they occurred using episodic memory,¹⁰ and they replay the sequence of episodic memories.^11,12,13,14 However, whether rats remember the specific contexts in which event sequences occurred is not known. Here, we show that rats remember the flow of events and the contexts in which those events occurred. We trained rats to identify the third-to-last odor from lists presented in two distinct arenas using lists of trial-unique odors of unpredictable lengths. We first established that rats remember (1) ordinal information about two lists and (2) the encoding context of the lists. Next, we showed that rats simultaneously remember the order of events and the contexts in which they occurred. Finally, by interleaving contexts at unpredictable points in the lists, we demonstrated that rats replay episodic memories in a context-specific manner, with memory performance remaining robust even when the interleaving of lists was interrupted by a 30-min delay. These findings are consistent with the hypothesis that rats can replay streams of episodic memories within specific contexts. This capability suggests that rats may serve as a model for complex cognitive processes, which may ultimately provide insights into the biological mechanisms of memory, disorders of memory, and therapeutic interventions.