Summary: Researchers have identified a neural mechanism that allows memories to link across different times and experiences. In experiments with mice, the team found that memories are replayed and reactivated not only after a single significant event but also in ways that connect that event with earlier, related experiences.
The study shows that memories are continuously updated and reorganized, a process that supports adaptive learning and may also explain how harmful patterns, such as those seen in PTSD, persist. During rest periods—so-called “offline” moments—neuronal ensembles can co-reactivate, strengthening associations between distinct memories formed on different days.
Unexpectedly, the researchers discovered that this type of memory linkage happens more often during wakeful rest than during sleep, challenging previous assumptions about when memory integration mainly occurs. These results clarify how past events are used by the brain to shape present behavior.
Key Facts:
- Dynamic memory integration: Negative experiences can become linked to earlier, unrelated memories via ensemble co-reactivation.
- Wakefulness plays a major role: Memory linkage was observed more frequently during wakeful rest than during sleep.
- Relevance to PTSD and learning: The findings illuminate mechanisms that support adaptive learning as well as the persistence of maladaptive memory associations in conditions like PTSD.
Source: Mount Sinai Hospital
Mount Sinai researchers have, for the first time, identified a neural mechanism that links memories across days and across different personal experiences.
Published in Nature, the study demonstrates that neural ensembles in the brain are not static recorders of individual events. Instead, they are continually updated with salient new information so that memories remain relevant and useful for future decisions.
These results could help explain both adaptive memory functions—such as making causal inferences from past events—and maladaptive outcomes, like the way traumatic memories can intrude into unrelated aspects of a person’s life.
“Long-standing models assume memories are locked in at the time of learning and remain stable within neural ensembles,” says Denise Cai, PhD, Associate Professor of Neuroscience at the Icahn School of Medicine at Mount Sinai and senior author of the paper. “Our mouse studies show those models are incomplete because they don’t account for how memories are flexibly updated with new, relevant information.”
The ability to balance stability and flexibility in neural ensembles is essential for making everyday predictions and decisions in a changing environment, Dr. Cai adds.
To investigate how memories are updated, the researchers recorded neural activity in the hippocampus of adult mice while the animals learned new experiences, rested during offline periods, and later recalled past events. They observed that after an experience, the brain replays and stabilizes that memory during offline periods.
Crucially, following a strong negative event, the brain did not only replay the recent aversive memory. It also reactivated a neutral memory formed two days earlier, effectively searching for related past experiences to link together and integrate across time.
In the experiments, mice that experienced a highly adverse event—such as a foot shock in a particular environment—showed reactivation not only of the shock-related memory but also of a separate, neutral memory from a safe environment where no shock had occurred days earlier.
“When mice rested after an aversive event, they simultaneously reactivated the neural ensemble for that event alongside the ensemble for the earlier neutral memory, integrating the two memories,” Dr. Cai explains. “We call this ensemble co-reactivation, and we show it drives long-term linking of memories in the brain.”
Contrary to many earlier reports emphasizing sleep’s role in memory consolidation, this study found that ensemble co-reactivation linking separate memories was more prominent during wakeful rest than during sleep. This raises new questions about the distinct contributions of wakefulness and sleep to different memory processes.
The team also found that negative experiences tended to link retrospectively to earlier neutral memories rather than prospectively to future ones, and that more intense negative events more reliably produced retrospective linking.
“By revealing a complex neural mechanism that enables memories to be integrated across days, we’ve made a significant advance toward understanding real-world memory,” says Dr. Cai. “Our memories are not static archives; they are constantly remodeled by new experiences so we can adapt and function in a dynamic world.”
About this learning and memory research news
Author: Elizabeth Dowling
Source: Mount Sinai Hospital
Contact: Elizabeth Dowling – Mount Sinai Hospital
Image: The image is credited to Neuroscience News
Original Research: Open access. “Offline ensemble co-reactivation links memories across days” by Denise Cai et al., published in Nature.
Abstract
Offline ensemble co-reactivation links memories across days
Memories are encoded in neural ensembles during learning and are stabilized by post-learning reactivation. Integrating recent experiences into existing memories ensures that memories contain the most current information, but the neural mechanisms that allow this integration across days have been unclear.
In mice, a strong aversive experience drives offline reactivation not only of the recent aversive memory but also of a neutral memory formed two days earlier, thereby linking fear associated with the recent event to the prior neutral context. Fear links retrospectively to neutral memories across days, rather than prospectively.
Consistent with earlier work, the recent aversive memory ensemble is reactivated during the offline period after learning. However, a strong aversive experience also increases co-reactivation of aversive and neutral memory ensembles during offline periods, and this ensemble co-reactivation occurs more during wakeful rest than during sleep.
Finally, the expression of fear in the neutral context is tied to reactivation of the shared ensemble between the aversive and neutral memories. Together, these findings demonstrate that offline ensemble co-reactivation is a neural mechanism for integrating memories across days.