The notion that a memory is a dynamic, living system rather than a static snapshot of the past is among the most important lessons to be learned from this semester’s talk on memory presented by Dr. Stephanie Grella. We talked about how memories are physically stored as engrams, which are sparse ensembles of neurons that were active during the initial experience and are dispersed throughout the brain. Importantly, new research keeps refuting the traditional theory of the “stable memory engram,” indicating that these representations are much more malleable and susceptible to linking, forgetting, and updating than we previously believed. I was reminded of this theme of memory flexibility after reading a study from Steve Ramirez’s lab that was published in the Proceedings of the National Academy of Sciences in 2022. Finkelstein et al.’s paper, “ social reactivation of fear engrams enhances memory recall,” delves into the intriguing relationship between personal memory and social behavior. The key finding is that male mice that had previously learned a non-social fear memory much better just by later encountering a stressed conspecific in their home cage. Stated differently, an individual's pre-existing, non-social negative memory was reactivated and amplified by an unrelated social experience.
To determine the neural mechanism underlying this phenomenon, the researchers employed activity-dependent tagging and optogenetics. They discovered that during the following stressful social event, the cellular ensembles (the engrams) linked to the initial fear memory in the hippocampus were naturally reactivated. This indicated that the memory trace was strengthened because the person's brain activated its preexisting "fear memory file" when they witnessed stress in a cagemate. The reactivation was strong enough to cause fearful behaviors in fear-conditioned animals but not in naïve ones when this same hippocampus ensemble was artificially stimulated. The trigger's simplicity is hat makes it so appealing. In addition to direct physical conflict, surrounding olfactory-auditory cues from a stressed familiar species also increased fear recall. This illustrates that memory is not just a log of discrete personal experiences; rather it is continuously influenced and interconnected with out social surroundings.
This study has significant ramifications for us a neuroscience consumers who attempt to understand the outside world. What does it mean for people navigating difficult, high-stress social situations if a stressed mouse can unintentionally trigger its companion's preexisting fear memories? We frequently consider our traumas, anxieties, and fears to be entirely internal. However, this study implies that our recollections are vulnerable to a form of social transmission. An external stressor may be sufficient to inadvertently reactivate and reinforce your own dormant negative memory if you had a negative experience in the past (forming an engram) and are later surrounded by a group of people displaying high levels of stress and anxiety (the social cue). The outcome is an abrupt, contextual reinforcement of your pre-existing mental state rather than a new memory. This viewpoint reinterprets resilience and mental health. It demonstrates that treatment strategies for conditions like PTSD and phobias need to take into consideration both the social environment, which is always trying to reactivate those engrams, and the internal neural pathways. A protective evolutionary trait is the ability to quickly access and intensify a prior fear memory in response to a social cute or a signal of danger in the environment; supporting the claim in the study that memory systems are made for survival. Finkelstein et al.'s results present that our brains are continuously wired into social environments rather than being isolated hard drives, which demonstrates that in order to be effective life learners, we need to understand not only what our memories are but also with whom we are recalling them.
References:
A.B. Finkelstein, H. Leblanc, R.H. Cole, T. Gallerani, A. Vieira, Y. Zaki, & S. Ramirez, Social reactivation of fear engrams enhances memory recall, Proc. Natl. Acad. Sci. U.S.A. 119 (12) e2114230119, https://doi.org/10.1073/pnas.2114230119 (2022).
Josselyn, S. A., & Tonegawa, S. (2020). Memory engrams: Recalling the past and imagining the future. Science (New York, N.Y.), 367(6473), eaaw4325. https://doi.org/10.1126/science.aaw4325
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