Hippocampal Neurons Encode Different Episodes in an Overlapping Sequence of Odors Task

Recent theoretical models of hippocampal function suggest that the hippocampus plays a critical role in the memory for the overlapping sequences of events that comprise episodic memory. Consistent with this idea, the firing of hippocampal place cells have been shown to represent not only location, but also the context or episode in which the location occurs. Thus, hippocampal neurons fire differently in the same location depending on the particular journey or sequence of places in which the subject is traveling. Further, recent work in rats has shown that hippocampal lesions impair memory for sequences of odors and the ability to disambiguate overlapping sequences of odors. We therefore recorded the activity of hippocampal complex-spike cells during a disambiguation of odor sequences task in which the two sequences shared three common odors. Consistent with data from spatial memory tasks, we found that 26 of 44 complex-spike cells fired differentially in the periods before, or during the presentation of the ambiguous odors depending on the sequence in which the odors were presented. This finding further supports the idea that the hippocampus is critical for episodic memory, and extends the physiological evidence to suggest that the hippocampal neurons play a broader role representing sequences of both spatial and nonspatial information.