Episodic memory enhancement versus impairment is determined by contextual similarity across events

For over a century, stability of spatial context across related episodes has been considered a source of memory interference, impairing memory retrieval. However, contemporary memory integration theory generates a diametrically opposite prediction. Here, we aimed to resolve this discrepancy by manipulating local context similarity across temporally disparate but related episodes and testing the direction and underlying mechanisms of memory change. A series of experiments show that contextual stability produces memory integration and marked reciprocal strengthening. Variable context, conversely, seemed to result in competition such that new memories become enhanced at the expense of original memories. Interestingly, these patterns were virtually inverted in an additional experiment where context was reinstated during recall. These observations 1) identify contextual similarity across original and new memories as an important determinant in the volatility of memory, 2) present a challenge to classic and modern theories on episodic memory change, and 3) indicate that the sensitivity of context-induced memory changes to retrieval conditions may reconcile paradoxical predictions of interference and integration theory

Individual Differences in Memory Functions and Their Relation to Hippocampal Connectivity

The hippocampus plays an important role in many aspects of learning and memory. It is most known for its role in episodic memory and spatial navigation, though it has also been shown to contribute to other processes like prioritizing memory for motivationally salient information and connecting related memories to form generalized knowledge. How can a single structure support different types of learning? As the hippocampus does not work in isolation to support memory, one proposal is that it may form connections with different brain regions to support different functions of memory. Recent work has shown how stable, trait-like connections can be leveraged to predict individual behavior. Thus, the present dissertation aims to explore 1) how different hippocampal connections relate to different memory processes, and 2) whether intrinsic hippocampal connections can be linked to individual memory performance. In three empirical chapters, I demonstrate how distinct hippocampal connections are associated with different functions of memory, including reward motivated learning, generalization and memory specificity. Moreover, I show how anterior and posterior hippocampus form distinct connections that may further support different aspects of memory. Finally, the dissertation demonstrates how stable, trait-like hippocampal connections can be linked to individual behavior. Together, these findings provide insight into the different functions of hippocampal connectivity and the utility of intrinsic connections in understanding individual memory abilities

Representations of ongoing experience within the rodent hippocampal subfield CA1

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