Researchers have unveiled a neural mechanism that enables the integration of memories across both time and personal experiences.
A team from Mount Sinai has made a groundbreaking discovery regarding a neural mechanism that integrates memories across time and individual experiences. These findings, published in Nature, illustrate how memories stored in neural ensembles in the brain are consistently updated and reorganized with important information. This research represents a crucial advance in understanding how our memories remain current with the latest information. Such a discovery could provide valuable insights into both adaptive memory processes (like making causal connections) and maladaptive ones (such as PTSD).
“Traditionally, it was believed that memories were established during the initial learning phase and stayed stable within the neural ensembles over time, allowing us to recall experiences,” stated Denise Cai, PhD, Associate Professor of Neuroscience at the Icahn School of Medicine at Mount Sinai and the senior author of the study. “Our research with mouse models indicates that this perspective falls short since it overlooks how the brain effectively stores memories while also updating them with new, relevant information. The balance of stability and adaptability in these neural ensembles is vital for us to make daily predictions and decisions while engaging with our ever-evolving surroundings.”
The key question of how we update memories dynamically when faced with new information has been a long-standing challenge in neuroscience. In this study, the Mount Sinai team observed the behavior and neural activity in the hippocampus of adult mice as they learned new experiences, rested after learning (during periods known as “offline” periods), and recalled past memories in the subsequent days. Researchers discovered that following each event, the brain consolidates and stabilizes the memory by re-experiencing it. After a negative experience, the brain not only replayed that particular event but also brought up memories from days prior, seemingly attempting to connect related events and integrate memories over time.
The examination of mice that underwent a significant negative event (for example, a foot shock in a certain environment) showed that such experiences triggered the reactivation of not only the immediate negative memory but also a “neutral,” non-threatening memory established days earlier (a safe environment free from shocks). “Our findings revealed that while the mice were resting after an intense negative encounter, they simultaneously reactivated the neural ensemble of that experience along with the earlier neutral memory, thereby linking these two distinct memory types,” explained Dr. Cai. “We describe this occurrence as ensemble co-reactivation, which we now understand to facilitate the long-term connection of memories in the brain.”
In contrast to existing research suggesting that sleep enhances memory storage, the findings indicated that memory linking was more common while mice were awake than asleep. This observation sparked intriguing questions regarding the different functions of wakefulness and sleep in various memory processes. Additionally, the research indicated that adverse experiences were more likely to be linked with previous memories in a “retrospective” manner than “prospectively” over time. Furthermore, more severe negative events were associated with a higher likelihood of retrospective memory linking.
“By unraveling a complex neural mechanism that supports memory integration, we’ve made significant progress toward a deeper understanding of how we utilize memory in our daily lives, as we continue to adapt and reshape our memories with new experiences in a constantly changing world,” remarked Dr. Cai.
