Robust memory of a previous event has been shown to prevent the formation of new, flexible memories.

Researchers from the MRC Brain Network Dynamics Unit at the Nuffield Department of Clinical Neurosciences in Oxford have discovered a new neural mechanism in the hippocampus that prevents the formation of new ‘flexible’ memories due to a past ‘robust’ memory.

The article, “Organization of hippocampal coactivity structure from robust to flexible memory,” is published in Science.

Memories influence our daily actions and guide our behaviors. However, when new memories are formed, they are not created from a blank slate, but based on our previous experiences. So what if consecutive memories place opposing demands on the brain? For example, could a strong memory of a previous significant event prevent an individual from forming a new, related memory that would require flexible updating?

Researchers in the Dupret lab used “food conditioning” to train mice over several days to develop robust contextual memory by offering them a choice of high-fat foods or familiar food pellets in different environments. They then measured the mice’s ability to continuously track objects placed in their environment: remembering old objects and exploring new ones.

The results showed that mice with good memories for high-fat foods failed to distinguish novel objects placed in their environment, demonstrating how robust contextual memory can prevent new memories from forming. Robust memory recruits a constant number of nerve cells, a recruitment process that later attenuates the formation of new flexible memories.

The team also highlighted how the diversity of the hippocampal neuron population is crucial for the formation of everyday memories, including the most robust ones. It is the structure of the pattern of activity between these neurons that impacts how new information is stored in memory.

Researcher Dr Giuseppe Gava commented: “We found that memories that are too robust, supported by hippocampal neurons synchronizing their activity like choristers reciting a melody, are not susceptible to being updated by new information. Like singers too immersed in the melody they are carrying, they cannot pick up new tunes.”

The combination of results suggests a trade-off between robust and flexible memory computations, explaining why individuals may struggle to adapt their behavior in certain situations due to strong contextual memories impacting the present.

Professor David Dupret concludes: “The mechanism described in this study highlights the importance of exploring how nerve cells cooperate to support brain functions. Here we reveal the cellular underpinnings of interference between memories with different demands. This is relevant for future research that will explore how to mitigate the effects of pathological memories.”