Many studies suggest that the hippocampus can build memories that integrate information across overlapping experiences 20– 26, although this might not depend on narrative coherence. Little is known about how the brain assembles narratives from individual events, but there is reason to think that the hippocampus plays a critical role. In the present study, we assessed whether narrative coherence shapes the way the brain encodes and retrieves events in memory. Our findings suggested that episodic memory might be organized above and beyond words, sentences, or events, on the basis of narrative coherence: the degree to which individual units of information can be interrelated within a single narrative 9, 11. Although some models have suggested that semantic associations can bind words or sentences in memory 19, we found that the memory advantage for events that formed a larger narrative was over and above any advantage at the word or sentence level 10. Furthermore, narratives are more than the semantic information conveyed in words and sentences alone 11– 13, 16. We recently demonstrated that, when temporally separated events could be assimilated into a larger narrative, they were recalled in greater detail than events involving an overlapping character in different narratives 10. Organizing events into a narrative can be beneficial for memory 12, 15– 18. Although Melvin appears in temporally separated events, they coalesce to form a narrative: a larger unit of information which encompasses multiple events, and in which one’s understanding of each event is dependent on information from other events 9– 14. The next day, Melvin might call and tell you that his kitchen is covered in ash. For example, in one event, you might encounter your neighbor Melvin, who tells you that he is locked out of his house while a pizza is baking in the oven. However, there is reason to think that real-life memory might be organized at a level above and beyond events. It is well-established that although experience unfolds over a continuous timeline, memory for one’s experience, or episodic memory 1, can be segmented into individual units of time called “events” 2– 8. These findings demonstrate a key function of the hippocampus: the integration of events into a narrative structure for memory. One day later, the hippocampus preferentially supported detailed recall of coherent narrative events, through reinstatement of hippocampal activity patterns from encoding. During encoding of fictional stories, patterns of hippocampal activity, including activity at boundaries between events, were more similar between distant events that formed one coherent narrative, compared with overlapping events taken from unrelated narratives. We used functional brain imaging to test whether the opportunity to form a larger narrative (narrative coherence) drives hippocampal memory integration. However, the conditions which lead to hippocampus-dependent memory integration are unclear. Prior research suggests that the hippocampus, which supports memory for past events, can support the integration of overlapping associations or separate events in memory.
Life’s events are scattered throughout time, yet we often recall different events in the context of an integrated narrative.
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