Temporal coding in the hippocampus
Salz, Daniel M.
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There is a large body of evidence that the hippocampus is involved in temporal aspects of memory. It remains unclear what neural processes within the hippocampus contribute to this ability. The following experiments aim to quantify and qualify these neural processes while rats perform temporal memory tasks. First we examined the firing of neurons in the hippocampus while rats compared a current series of odors to a learned sequence of odors. We found evidence of neural correlates which might represent whether a stimulus odor was in the correct ordinal sequence or not. Next we examined the delay intervals in between learned sequences of events with the goal of identifying the origin of “time cells” in the hippocampus. We used a delayed alternating T-maze task that our lab has used before to record time cells in area CA1 of the hippocampus. We found time cells in CA3, one of the major inputs to CA1 and demonstrated that they behave in many ways like place cells previously observed in these two regions. Time cells had previously been reported to occur only when an animal is engaged in a task with memory load. We demonstrated that memory load isn't necessary to observe time cells. Our observations of the similarities between place and time cells led us to conjecture that the hippocampus might process space and time similarly. In a final study I examined time vii cell firing properties with an aim at constraining models of time cells. We defined time cells in several ways including a new methodology that is promising as a future unbiased selection criteria. All of our findings help further elucidate several different ways that neural coding in the hippocampus contributes to temporal processing.
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