Representations of ongoing experience within the rodent hippocampal subfield CA1
Sheehan, Daniel Joseph
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The hippocampus is critical for the encoding and retrieval of episodic memories. During ongoing experience, the hippocampus exhibits activity patterns related to the current spatiotemporal context. How hippocampal firing patterns relate to the representation of mental maps important for behavioral and cognitive processes is still an open question. Here a series of experiments aimed to test how the hippocampus represents the spatiotemporal context of ongoing experience. Extracellular recordings from the dorsal CA1 region of the hippocampus were collected from rats engaged in a blocked serial reversal object-association task. Behaviorally, rats did not utilize the temporal segregation between task blocks as a way to correctly match object valence and rather treated each block of trials as separate episodes. This lack of an alternating context was further uncovered in the neural coding of the rat’s hippocampal firing patterns. Furthermore, gradual drift in the hippocampal ensemble representation of experience was discovered, correlating with the temporal duration of the task and not the blocked organization of the behavioral paradigm. In the next two experiments, extracellular recordings from dorsal CA1 were collected from rats traversing a linear track environment, with different environmental manipulations. During variable starting location recording sessions, it was found that positional coding by the hippocampal population was relative to starting location and that place field allocation was biased towards the reference frame at the start of the journey, demonstrating that hippocampal place fields are not uniformly distributed and express compressed activity patterns referenced to the beginning point of trajectories. During blocked manipulation of lighting condition, individual units showed preference to specific lighting conditions and the hippocampal population rapidly remapped between lights ‘ON’ and lights ‘OFF’ blocks of trials, suggesting that hippocampal maps of space are not solely governed by internal dynamics and that alterations in sensory input can modify hippocampal motifs of ongoing experience. Overall, the findings of the three experiments further our understanding of how the hippocampus represents ongoing experience, highlighting the role of temporal drift as well as demonstrating how both external and internal stimuli and frames of reference coalesce into a comprehensive cognitive map of experience.