Medial temporal and prefrontal cortices encode structural task representations at different levels of abstraction

Memory generalisations may be underpinned by either encoding- or retrieval-based mechanisms. We used a transitive inference task to investigate whether these generalisation mechanisms are influenced by progressive vs randomly interleaved training, and overnight consolidation. On consecutive days, participants learnt pairwise discriminations from two transitive hierarchies before being tested during fMRI. Inference performance was consistently better following progressive training, and for pairs further apart in the transitive hierarchy. BOLD pattern similarity correlated with hierarchical distances in the medial temporal lobe (MTL) and medial prefrontal cortex (MPFC). These results are consistent with the use of representations that directly encode structural relationships between different task features. Furthermore, BOLD patterns in MPFC were similar across the two independent hierarchies. We conclude that humans preferentially employ encoding-based mechanisms to store map-like relational codes that can be used for memory generalisation. While both MTL and MPFC support these representations, the MPFC encodes more abstract relational information.