An Examination of Contextual and Temporal Contributions to Rodent Episodic Memory Processes

Abstract

This thesis explored how the environmental context in which an event is encoded impacts later recall on the WWWhich task of rodent episodic memory. In study 1, as presented in chapter 3 in experiments 1, 2, and 3, rats completed the WWWhich task in a new maze design that was being validated for use in behavioural memory tasks. The results of this pilot study indicated that rats performed the WWWhich task when contextual cues were available but could not when contextual cues were made to be identical. Various methodological considerations led to the conclusion that this maze was an appropriate and valid tool that could be used to explore behavioural tasks of memory. Study 2, as presented in chapter 4 in experiments 4 and 5, used the validated corridor maze in an experimental design that compared behavioural results on the WWWhich task of episodic memory to a previous study by Spiers et al., (2015) who used a similarly configured maze to record place cells in freely-moving rats. Task demands were varied along two dimensions: access to extra maze cues and contextual richness. When access to extra maze cues in visually and tactually rich unique chambers was available (Ex 4), rats could indeed complete the WWWhich task. When the task demands were changed to be maximally difficult (Ex 5), rats could not identify the novel configuration on the continuous trials version of the WWWhich task. These results followed the pattern of place cell results reported by Spiers et al., (2015) where place cells remapped in the presence of distinct environmental context changes, but not when the local environmental context of the adjacent chambers were identical contextually. These findings show that episodic memory performance is not only dependent on the local contextual environment, but that the results follow the pattern of results found in hippocampal place cells. These data show the importance of context in how events are encoded in episodic memory processes, and how behaviour on a memory task is later impacted on a recall, test phase that requires discrimination. In study 3, experiments 6, 7, and 8, explored if temporally spacing trials would lead to increased memory performance on the identical chambers version of the WWWhich task. To this end, performance was compared along two factors: context (unique versus identical chambers) and cognitive load (trial presentation as continuous (massed) or spaced (alternating short (4 hr) and long (20 hr) delays) to determine if the episodic memory deficits previously observed under identical contextual conditions were overcome and maintained after experiencing temporally spaced trials between events. When rats were presented with contextually rich and uniquely decorated chambers in experiment 6, they easily identified the novel configuration on the WWWhich task. Experiencing spaced trials had no effect on performance when continuous trials were presented again. In Experiment 7, experiencing spaced trials had a significant effect on performance. In identical chambers, rats showed an increased preference for the novel configuration when trials were spaced over time, but most interestingly, this preference was maintained when rats later completed the same task but with trials again presented continuously. These data confirm that context is a critical factor in rats’ ability to separate events and identify the novel configuration on the WWWhich task, but also implicates temporal spacing as being a means of potentially overcoming deficits produced by contextual similarity between events. Chapter 6 (Ex 9), presents electrophysiological data whilst the rat performed the WWWh task to examine how spatial and episodic aspects of memory are represented in the hippocampus. Neural representations reflected behavioural data with field repetition present where rats could not task discriminate.
These data show the importance of context in how events are encoded in episodic memory processes, how encoding context impacts behaviour on a discrimination memory task, and how context drives neural representations of events.