Narratives bridge the divide between distant events in episodic memory

Abstract

Many studies suggest that information about past experience, or episodic memory, is divided into discrete units called “events.” Paradoxically, we can often remember experiences that span multiple events. Events that occur in close succession might simply be bridged because of their proximity to one another, but many events occur farther apart in time. Intuitively, some kind of organizing principle should enable these temporally-distant events to become bridged in memory. We tested the hypothesis that episodic memory exhibits a narrative-level organization, enabling temporally-distant events to be better remembered if they form a coherent narrative. Furthermore, we tested whether a post-encoding consolidation process is necessary to integrate temporally-distant events. Participants learned and subsequently recalled events from fictional stories, in which pairs of temporally-distant events involving side-characters (“sideplots”) either formed one coherent narrative or two unrelated narratives. In three experiments, participants were cued to recall the stories either immediately, after a 24-hour delay, or after a 12-hour delay which elapsed during daytime (“wake”) versus nighttime (“sleep”). Participants recalled more information about coherent than unrelated narrative events, in most delay conditions, and the delay and sleep manipulations indicated that post-encoding consolidation was not necessary to integrate temporally-distant narrative events. Post-hoc modeling across experiments suggested that sentence-level semantic similarity could not solely account for the coherence benefit. This reliable memory benefit for coherent narrative events supports theoretical accounts which propose that higher-order semantic structures scaffold episodic memory.

Building bridges: the hippocampus constructs narrative memories across distant events

“…If so, then boundary-evoked voxel patterns should exhibit higher encoding-retrieval similarity for Coherent Narratives than for Unrelated Narratives. Furthermore, in line with our previous behavioral work (Cohn-Sheehy et al, 2020), we hypothesized that this kind of reinstatement would predict the number of details recalled about Coherent Narrative events.…”

“…Our next analyses focused on recall of CN and UN events. In a previous behavioral study, we found that, across three experiments, participants were able to recall more details about CN than UN events (Cohn-Sheehy et al, 2020). In the present study, the number of details recalled from CN events (mean=10.71 details per cue, SD=9.4, max=41.5) was numerically higher than recall of details from UN events (mean=8.65 details per cue, SD=9.1, max=40.0), though this effect did not meet the threshold for statistical significance (t(24) =1.98, p=.059, Cohen's d=0.22).…”

“…Namely, people can construct narratives (van Dijk and Kintsch, 1983) that bridge distant events in memory. A recent series of experiments demonstrated that when two distant events can form one larger narrative, people recall these events in greater detail, compared with two distant events that are taken from unrelated narratives (Cohn-Sheehy et al, 2020). This reliable finding contrasts with the notion that memory is simply segmented into individual, discrete events.…”

“…The stimulus design is depicted in Figure 1A-B, and has been described elsewhere (Cohn-Sheehy et al, 2020). We constructed four fictional stories in which we manipulated whether temporally-distant events could form a coherent narrative (Supplementary Data 1).…”

“…In the current study, we used functional magnetic resonance imaging (fMRI) to test whether the hippocampus supports the ability to integrate two distant events into one unified narrative. Participants were scanned while they listened to fictional stories, in which particular characters appeared in two temporally-separated events (previously implemented in behavioral experiments; Cohn-Sheehy et al, 2020). Critically, these two distant events could either form parts of one overarching narrative (Coherent Narrative), or they belonged to two different narratives (Unrelated Narratives; Figure 1A-B).…”