The direction of associations: prior knowledge promotes hippocampal separation, but cortical assimilation

Summary What does it mean to say, “a new association is learned”? And how is this learning different when adding new information to already-existing knowledge? Here, participants associated pairs of faces while undergoing fMRI, under two different conditions: a famous, highly-familiar face with a novel face or two novel faces. We examined multivoxel activity patterns corresponding to individual faces before and after learning. In the hippocampus, paired novel faces became more similar to one another through learning. In striking contrast, members of famous-novel pairs became distinct. In the cortex, prior knowledge led to integration, but in a specific direction: the representation of the novel face became similar to that of the famous face before learning, but less so vice versa, suggesting assimilation of new into old memories. We propose that hippocampal separation might resolve interference between existing and newly learned information, allowing cortical assimilation. Associations are formed through divergent but specific neural codes, that are adaptively shaped by the internal state of the system – its prior knowledge.