Remembering sequences of events defines episodic memory, but retrieval can be driven by both ordinality and temporal contexts. Whether these modes of retrieval operate at the same time or not remains unclear. Theoretically, medial prefrontal cortex (mPFC) confers ordinality, while the hippocampus (HC) associates events in gradually changing temporal contexts. Here, we looked for evidence of each with BOLD fMRI in a sequence task that taxes both retrieval modes. To test ordinal modes, items were transferred between sequences but retained their position (e.g., AB 3 ). Ordinal modes activated mPFC, but not HC. To test temporal contexts, we examined items that skipped ahead across lag distances (e.g., AB D ). HC, but not mPFC, tracked temporal contexts. There was a mPFC and HC by retrieval mode interaction. These current results suggest that the mPFC and HC are concurrently engaged in different retrieval modes in support of remembering when an event occurred.
Significance Statement
Memory for sequences of events is a defining aspect of everyday episodic memory allowing our brain to separate unique experiences that otherwise have overlapping sensory and spatial content. Sequence memory is impaired in typical aging and in disorders such as Alzheimer’s disease. The results of the current study provide new evidence that two retrieval modes concurrently arise during sequence memory, and they have distinct neural correlates. The medial prefrontal cortex contributes to an ordinal retrieval mode, while at the same time, the hippocampus contributes a gradually-changing temporal context mode of retrieval. These data shed new light on why typical episodic memory requires both the medial prefrontal cortex and the hippocampus, and suggests a functional dissociation between the medial prefrontal cortex and hippocampus across these modes of retrieval.
