The completeness of context learning impacts hippocampal neural ensemble activity during memory formation but not retrieval

Abstract

Studies investigating the neural basis of memory typically focus on well-formed memories in which subjects have ample time to learn about, and therefore remember, the target information. In reality, memories may not be formed under ideal circumstances, leading to the formation of weak or incomplete memories for past events. Here, we used a contextual fear conditioning paradigm to examine how memories of varying completeness are formed and retrieved within the hippocampus. Within this paradigm, we manipulated how well the memory was initially formed by altering the amount of time given to explore the context prior to shock (placement-shock interval, PSI). We then examined the effect of PSI on memory completeness by using immediate-early gene (IEG) expression as an indicator of hippocampal memory related activity. In Chapter 2 we found that hippocampal IEG expression continued to increase well past the point at which animals develop robust conditioned fear, suggesting that the level of conditioning is not an accurate indication of the extent of context learning and that conditioning may proceed to incomplete contextual representations. In Chapter 3 we extended upon this work by investigating the effect of PSI on the recruitment of hippocampal neuronal ensembles during learning. We found that extended context exposure resulted in the recruitment of more cells, suggesting that animals may be encoding more information as the session continues. In Chapter 4 we examined how PSI alters the rate and magnitude of hippocampal pattern completion-based memory retrieval. Surprisingly, the number of hippocampal cells activated after retrieval was not influenced by the extent of previous experience in the context. Finally, in chapter 5 we relate these findings to models of hippocampal function and discuss the potential consequences of conditioning to incomplete contextual representations.