Neural Networks Involved in Spatial and Temporal Pattern Separation
Critical to episodic memory is pattern separation (PS), the storage of similar inputs as distinct and nonoverlapping. Spatial and temporal PS have been shown to be related to disparate subfields of the hippocampus (HC) in rodents. Extra-HC structures involved have not yet been elucidated. The current work provides an exploratory investigation into the neural correlates of spatial and temporal PS, employing functional magnetic resonance imaging and univariate and multivariate analysis techniques. In Experiment 1, behavioural spatial and temporal memory tasks were developed that assess varying PS demands. Objectives for the experiment were met, in that accuracy was lower and reaction time higher for conditions requiring more engagement of PS. In Experiment 2, whole-brain regions as well as the neural networks involved in spatial and temporal PS were examined, and functional connectivity of the HC was observed. Univariate data revealed unique areas of activation based on information type being encoded (i.e., spatial vs. temporal). The cuneus and HC were uniquely involved in the spatial task, while a wider area of regions including middle occipital and medial frontal areas were activated in the temporal task. Multivariate analyses were convergent with the spatial and temporal context memory literature. The HC, parahippocampal gyri, prefrontal cortices, and precuneus were part of a correlated network in the spatial task. Bilateral prefrontal cortices, including the orbitofrontal cortex were involved in the temporal task. Further, the multivariate analysis revealed qualitatively distinct networks based on memory processing stage (i.e., encoding vs. retrieval). Interestingly, the network included anterior HC in spatial encoding, and posterior HC in spatial and temporalretrieval, consistent with an influential theory positing a rostrocaudal gradient along the HC for encoding and retrieval. Functional connectivity analyses revealed connectivity of the posterior HC seed with temporal and superior parietal areas in the spatial task, and with frontal areas in the temporal task, suggesting the right posterior HC interacts with regions differently based on information type. Results confirm and extend findings from previous literature demonstrating HC involvement in PS, and also suggest HC and extra-HC involvement varies based on processing stage and information type.