Synopsis: A functional brain imaging study by Cohn-Sheehy and colleagues from the Center for Neuroscience at the University of California, Davis looked at people’s ability to learn and recall fictional stories with either a coherent narrative or overlapping yet unrelated narrative. Results showed that the hippocampus preferentially supports the integration of distant, separate events into a coherent narrative structure for memory.

Imagine yourself telling a friend about a book that you recently enjoyed. Do you find yourself jumping from one event to another? One key moment to another? Interestingly, when telling the story of our lives or memorizing others’, we follow the same episodic pattern. It turns out, there’s a neurological explanation behind that. A study by Ph.D. candidate Cohn-Sheehy and colleagues at the University of California, Davis suggests that the hippocampus is an architecture connecting distant and separated events into a coherent narrative, thus shaping the way we encode and retrieve events in memory.

Before diving into the details of the study, what, in this case specifically, are “events” and “narrative”? An event, formally defined by Zacks & Tversky (2001) in Psychological Bulletin, is a segment of time at a given location perceived as having a beginning and an end. The length of each event varies from seconds to hours, and the spatial and temporal gaps between each are identified as event boundaries. And finally, when the segmented yet dependent events connect, the larger unit that encompasses them is called a narrative. 

Recent studies suggest two key points that power the work of Cohn-Sheehy and colleagues. First, the hippocampus, a brain region in the temporal lobe, plays a crucial role in memory integration of overlapping experiences as well as coherent narratives with linked events. Second, the hippocampus is active when people perceive the transition between events or event boundaries; this indicates that narrative or the formation of narrative using past and current events modulates activity in the hippocampus. With those two moving pieces, the researchers saw an opportunity to do something that no studies have attempted before: test the effects of narrative coherence on hippocampus-dependent memory tasks such as learning and recalling across temporally distant events.

To conduct the experiment, the research team created four fictional stories with main and side plots, each manipulated to either have separated events that could form a narrative or not. They then monitored the participants’ brain activity using an fMRI scanner during the story presentation or learning stage on day one and the recall stage the next day. After that, researchers analyzed the data to determine whether there are differences in hippocampus activity between the coherent and the unrelated narrative for the two phases.

Indeed, there were differences. Right hippocampus patterns are significantly more similar for coherent narrative events than unrelated overlapping narrative events. This means that during the learning phase, related events are tied together. Furthermore, as participants recall the story a day later, the hippocampus preferably restores more information on coherent narrative events, especially that of the second event. The team shares that the second event is connected to the first, so its activity restoration correlates with participants’ ability to recall the overall narrative details. These results establish the hippocampus as a memory architecture, bridging and connecting distant events to a narrative that allows people to better remember and retell the story of their lives.

This study unlocks a brand-new ground for research on how the brain processes memory and on the specific roles of each memory site. Additionally, it’s remarkable that only the right hippocampus shows significant pattern similarity during the learning phase. Further research should plan to reveal the relative contribution of the left and right hippocampus to episodic memory, Cohn-Sheehy and colleague suggest. Future work should also determine whether the hippocampal activity at event boundaries for preceding and subsequent events reflects one common, underlying process or multiple processes. These efforts have and will aid greatly in the diagnostic and caring processes for patients with memory loss. Lastly, this added knowledge of the hippocampus brings us one more step closer to deciphering and understanding the capability of the human brain, the organ that makes us who we are today.

References:

Cohn-Sheehy BI, Delarazan AI, Reagh ZM, Crivelli-Decker JE, Kim K, Barnett AJ, Zacks JM, Ranganath C (2021) The hippocampus constructs narrative memories across distant events. Current Biology 31:4935-4945.e7.

Zacks JM, Speer NK, Swallow KM, Braver TS, Reynolds JR. Event perception: a mind-brain perspective. Psychol Bull. 2007 Mar;133(2):273-93. doi: 10.1037/0033-2909.133.2.273. PMID: 17338600; PMCID: PMC2852534.

Zacks JM, Tversky B (2001) Event structure in perception and conception. Psychol Bull 127:3.