Schedule of Events | Symposia

Hippocampal Representational Shifts Underlie the Learning of Exceptions to Category Knowledge

Poster Session C - Sunday, March 30, 2025, 5:00 – 7:00 pm EDT, Back Bay Ballroom/Republic Ballroom

Yongzhen Xie¹ (yongzhen.xie@mail.utoronto.ca), Michael L. Mack¹; ¹Department of Psychology, University of Toronto

We can acquire categories by extracting regularities across experiences, but when we encounter exceptions to such regularities, how do we reconcile them with our prior category knowledge? Existing work indicates that the hippocampus supports the acquisition of category regularities and exceptions by updating object representations through pattern integration and differentiation. However, there is currently no direct characterization of the hippocampal representational shifts in exception learning. We aimed to fill this gap by having participants learn competing object categories in which two exceptions were introduced after repeated exposure to regular category items. One exception was a unique oddball, and the other was a structural exception resembling items from the competing category. We collected fMRI data before and after participants learned these exceptions and found that exception representations underwent distinct changes in different hippocampal subfields. Specifically, through learning, representations of both exceptions became more distant from their regular category members in the dentate gyrus. Also, the structural exception – but not the oddball – became overlapped with its regular members in the representational space of CA1. Furthermore, within the CA1, greater increases in representational similarities between the structural exception and other members of its category were associated with better categorization and generalization performance post-learning. These findings, while consistent with theorized roles of dentate gyrus and CA1 in pattern differentiation and integration, demonstrate that learning-driven representational shifts vary across exception types. In conclusion, our study helps delineate the flexible hippocampal operations underlying the acquisition of items exceptional to existing category knowledge.

Topic Area: LONG-TERM MEMORY: Semantic