October 2024
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Introduction: Memory interference poses a challenge to accurate recall when memories share similarities. Previous research (e. g. Favila et al., 2016) has highlighted hippocampal repulsion as a mechanism to reduce interference. More specifically, Wanjia et al. (2021) have shown that hippocampal repulsion occurs precisely when memory interference is resolved. However, it is unclear how training (frequent studying of associations) affects hippocampal repulsion. When experiences are similar (overlapping), this can lead to interference-related forgetting. However, recent research has demonstrated that interference can be minimized via targeted differentiation of activity patterns in the hippocampus (e.g. Hulbert & Norman, 2015; Favila et al., 2016; Wanjia et al., 2021)—a phenomenon we term ‘hippocampal repulsion.’ Hippocampal repulsion is thought to critically depend on the degree of similarity between overlapping memories (Wammes et al., 2022) and the amount of experience with those memories (Favila et al., 2016), but in ways that are complex and not fully understood (Ritvo et al., 2019). Here, we sought to characterize potential interactions between stimulus similarity and experience in determining hippocampal repulsion. Method: Participants completed an experiment with three phases. During the study phase, participants learned unique associations between scene and object images. Importantly, the scene images were drawn from two categories (beaches vs. gazebos), where the image similarity is higher within, relative to between category scenes. Because we included 24 scenes per category, this allows us to investigate repulsion as a function of pairwise scene similarity. Participants received high training on associations from one category and low training on the other, counterbalanced across participants. FMRI was used to acquire the BOLD signal during the second phase, in which participants viewed the scene images. In the third phase, participants were presented with each scene object and had to select the associated object from a list. Results: Using pattern similarity analyses, we computed a relative similarity score of brain activity as the difference between within-scene-category similarity and between scene-category similarity. This score allows us to compare different brain regions. Congruent with previous observations on hippocampal repulsion, this score was more negative in the CA23DG relative to the early visual cortex and the parahippocampal place area, indicating hippocampal repulsion of similar memories. Training showed differential effects between these areas.
