Integration and differentiation of hippocampal memory traces.

Prevailing theories of hippocampal function argue that memories are rapidly encoded by non-overlapping memory traces. Concurrently, the hippocampus has been argued to integrate across related experiences, enabling generalization. The cognitive neuroscience of memory has been transformed by the recent proliferation of studies using pattern similarity analyses to investigate the neural substrates of memory in humans, marking an exciting and significant advance in our understanding of population-level neural representations. We provide an overview of hippocampal pattern similarity studies published to date. By considering the effects of stimulus type, time-scale, and hippocampal subregions, we account for both increases and decreases in representational similarity. We argue that hippocampal representations for related memories are not fixed. Instead, the evoked representations are flexibly modulated, depending on whether the current goal is to extract generalities or to reinstate specific experiences. In the first comprehensive review of hippocampal pattern similarity analyses, we provide insight into the mechanisms of memory representation and implications for the interpretation of pattern similarity more generally.