Memory signals are temporally dissociated within and across human hippocampus and perirhinal cortex

In the endeavor to understand how our brains enable our multifaceted memories, much controversy surrounds the contributions of the hippocampus and perirhinal cortex (PrC). Here we recorded functional magnetic resonance imaging (fMRI) in healthy controls and intracranial Electroencephalography (EEG) in patients during the same recognition memory paradigm. Although conventional fMRI analysis showed indistinguishable roles of the hippocampus and PrC in familiarity-based item recognition and recollection-based source retrieval, event-related fMRI and EEG time-courses revealed a clear temporal dissociation of memory signals within and across these regions. Whereas an early source retrieval effect was followed by a late, post-decision item novelty effect in hippocampus, an early item novelty effect was followed by a sustained source retrieval effect in PrC. Although factors like memory strength were not experimentally controlled, the temporal pattern across regions suggests that a rapid item recognition signal in PrC triggers a source retrieval process in the hippocampus, which in turn recruits PrC representations/mechanisms – evidenced here by increased hippocampal-PrC coupling during source recognition.