Thesis defense Stephanie Theves (Donders series 437)

23 June 2020

Mapping conceptual knowledge acquisition in the hippocampal system

Promotor: prof. dr. C. Doeller Norges teknisk-naturvitenskapelige universitet, Trondheim, Norway, prof. dr. G. Fernández

A remarkable hallmark of human cognition is the flexible use of an extremely rich, yet limited, and continuously developing repository of knowledge, to deal with unfamiliar situations and infer never experienced relations to gain insight. How is conceptual knowledge acquired and represented in the brain to allow its flexible use? In this thesis, I address the question whether coding principles of the hippocampal system, originally known to serve the representation of space for successful wayfinding, support concept learning by organizing new information into map-like representations. To this end, I combine different concept learning paradigms, each targeting a critical aspect of knowledge structures (semantic distance; hierarchical structure), with representational and model-based fMRI analyses techniques. First, I probe whether the hippocampus creates a map-like representation of a new concept, which is defined within a two-dimensional space spanned along continuous feature dimensions, by testing for a representation of the two-dimensional distances between exemplars in concept space. Second, I embed the concept space within a higher dimensional feature space, in order to distinguish whether the hippocampus maps new information according to all feature dimensions, or specifically according to conceptually-relevant dimensions. Third, I turn towards the hierarchical structure of concepts, entailing different levels of abstraction that allow applying knowledge at the appropriate level of detail required in a given situation. Here, I probe whether a hierarchical category structure is indeed learned in a hierarchical fashion and how the hippocampus and prefrontal cortex support this process. In addition to this empirical work, I review animal and human research on hippocampal pattern completion, a key mechanism of hippocampal memory function. The work presented in this thesis provides evidence for a map-like representation of conceptual knowledge in the hippocampus during acquisition. As such, the hippocampal system might not only help us to navigate the physical environment by providing mental maps of space, but serve cognitive functions that build on relational codes for behaviorally relevant dimensions (e.g. concepts) more generally.