Memories are generally thought of as out of our control. Whether they are positive or negative, they follow us around and affect our emotions and behavior in daily life. They influence our relationships and fear-based ones may even keep us from activities needed to accomplish our dreams and goals. However, traumatic and fearful memories are surprisingly not undefeatable, which recent research has been proving in various ways. From researchers alleviating negative memories in mice through complex processes to therapists using eye-movement desensitization and reprocessing (EMDR) on PTSD/trauma patients, the dark clouds following people around may actually be blown away.
A research team with the goal of figuring out how memories can be influenced in the brain in order to lessen the effects of fear was successful, locating specific regions and circuits that are central in fear. This research team and paper was led by Grella et al. 2022, and they groundbreakingly demonstrated, using mice, that memories can be updated, which can be helpful when it comes to bad experiences. The encoding of fear memories occurs in the dorsal dentate gyrus (dDG) of the hippocampus, so this team targeted this region during the reconsolidation process of memory retention. The mice were fear conditioned and the neurons in their dDG were tagged with the method of Tet-tag, and during recall these neurons were stimulated and reactivated. The researchers decided to intervene during recall by reactivating specific dDG cells that had shown activity during recall of a positive memory, which ultimately ended up reducing the intensity of the fear memory, thereby “updating” it. They found that the reduction in fear was significant, and it also lasted long enough to be enduring, meaning the negative feelings did not simply return after a short time.
One especially intriguing aspect of this study is that the intervention in which dDG neurons were reactivated was done optically, through optogenetics. This raises the question of any possible link it may have with a popular form of therapy used by trauma specialists on PTSD patients: eye-movement desensitization and reprocessing, or EMDR. EMDR involves alternating bilateral sensory stimulation (ABS), in which patients move their eyes from side to side while following a light or the therapist’s fingers while recounting painful memories and breaking down the emotions behind them. This therapy has successfully treated many patients over the last few decades since it was discovered and put into use, but neuroscientists are not sure what exact neural processes form the basis for its effectiveness. A research team led by Baek et al. 2019 belonging to the Institute of Basic Science in South Korea took on this initiative. They also used mice, first planting fear through conditioning, then used ABS via light stimulation. They found a significant and long-lasting decrease in fear. Interestingly, other fear reduction methods they attempted had nowhere near this same impact on reduction. While investigating further, the scientists found that when a pathway linking the superior colliculus (SC) to the mediodorsal thalamic nucleus (MD) was blocked, the method of ABS no longer worked and the fear response returned. This formed a more solid basis of understanding how EMDR works in patients via ABS.
Although Grella and Baek’s teams were investigating different methods of fear reduction, both of the studies are significant in furthering scientific knowledge about basic neural processes behind fear, trauma, and their link to memories. With even more research, EMDR may become more effective and trusted, and there could even be new therapies that emerge due to the research done on the dorsal dentate gyrus neurons. The exact future may be uncertain, but there is new hope for so many individuals who feel haunted by their past and are unable to move on with their lives, thanks to these researchers.
References
Grella, S.L., Fortin, A.H., Ruesch, E. et al. Reactivating hippocampal-mediated memories during reconsolidation to disrupt fear. Nat Commun 13, 4733 (2022). https://doi.org/10.1038/s41467-022-32246-8
Baek, J., Lee, S., Cho, T. et al. Neural circuits underlying a psychotherapeutic regimen for fear disorders. Nature 566, 339–343 (2019). https://doi.org/10.1038/s41586-019-0931-y
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