Paper on constructive memory & generalization published

New paper on mnemonic construction and generalization out in Nature Communications.

In this study, we asked how we construct when events took place and how temporal sequences of events are represented in the brain. We ran a brain imaging experiment where participants learned sequences of events, which were images from the computer game The Sims. We previously showed that temporal distances between events are mirrored in the similarity of brain activity patterns. So when remembering a sequence of events, events that happened close together in the sequence lead to more similar activity patterns in regions of the brain called the hippocampus and entorhinal cortex.

In the new study, we wanted to know what these temporal distances really are: Is it real elapsing time between, the order of the events in the sequence or some psychologically constructed form of time that underlies this effect? To test this, we asked participants to learn the times of events in a sequence relative to a hidden “virtual” clock. Only every once in a while we would briefly show participants the current “virtual time”. To solve the task they had to use these time cues and combine them with their estimates of “real” elapsing time. This way they could mentally construct the times of the events in a given sequence.

Both the behavioral and the brain imaging data show that it is this mentally constructed time that is represented in their memory, beyond the effects of order and real time. A second main finding is that, in the hippocampus, we observe not only memories of specific sequences, but also an integrated representation of the temporal structure of all four sequences participants learned about. Specifically, we saw that brain activity patterns systematically reflected the temporal relations of events, even when they were from different sequences. So knowledge about the temporal structure was generalized across sequences. Lastly, we show that knowledge about the temporal structure of the sequences introduces a bias in participants’ behavioral responses in a memory test where we asked them to indicate when they thought the events took place.

You can find the Open Access paper on the Nature Communications page or download the PDF directly. If you really want to dive into the details, you can find extensive documentation of the analysis code and results on my project page on GitHub.



JLS Bellmund, L Deuker, ND Montjn, CF Doeller (2022). Mnemonic construction and representation of temporal structure in the hippocampal formation. Nature Communications. doi: https://doi.org/10.1038/s41467-022-30984-3
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Abstract
The hippocampal-entorhinal region supports memory for episodic details, such as temporal relations of sequential events, and mnemonic constructions combining experiences for inferential reasoning. However, it is unclear whether hippocampal event memories reflect temporal relations derived from mnemonic constructions, event order, or elapsing time, and whether these sequence representations generalize temporal relations across similar sequences. Here, participants mnemonically constructed times of events from multiple sequences using infrequent cues and their experience of passing time. After learning, event representations in the anterior hippocampus reflected temporal relations based on constructed times. Temporal relations were generalized across sequences, revealing distinct representational formats for events from the same or different sequences. Structural knowledge about time patterns, abstracted from different sequences, biased the construction of specific event times. These findings demonstrate that mnemonic construction and the generalization of relational knowledge combine in the hippocampus, consistent with the simulation of scenarios from episodic details and structural knowledge.

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