Memory is a very widely known and recognized cognitive function. Despite this common knowledge of its existence, how memory actually works behind the scenes has been widely debated for decades. Dr. Stephanie Grella presented a talk at Loyola University Chicago about engrams, a theory gaining more traction in recent years that suggests the physicality of a memory in the brain. Something that particularly struck me was the discussion of silent engrams. Silent engrams are memories that are no longer retrievable by natural means (Josselyn & Tonegawa). Scientists have experimented with this concept by messing around with optogenetics and disrupting the normal consolidation process using protein inhibitors (commonly known to cause amnesia). They found that administering anisomycin, a protein inhibitor, would block the formation of memories. In this context, scientists were fear-conditioning mice and after administering anisomycin, mice would show little fear response. However, reactivating the associated neurons using optogenetics was sufficient to bring back the memory.
This concept has been used in the study of Alzheimer's Disease (AD). AD is a neurodegenerative disease that mainly affects the elderly by slowly eating away at their brain. Studies have been surrounding this disease for years, desperate to find a cure or specific causes. Silent engrams have been used to study AD, with it being used as a possible explanation for the early stages of AD. Transgenic mice used for studying AD and expressed genes that were associated with the onset of the disease underwent an optogenetic treatment to reactivate an area of the brain associated with memory (Josselyn & Tonegawa). Miraculously, these mice essentially got their memories back. This has been compared to reports of humans in early-stage AD having enhanced memory retrieval if specific retrieval cues are used. This is a very promising step in figuring out how to combat AD, and with more research on engrams, perhaps a medicinal solution is not entirely out of reach.
Other research surrounding AD has also been undertaken. Another focus besides engrams is hippocampal neurogenesis and how to maximize it. Studies have shown that neurogenesis occurs into adulthood, however it is difficult to study specifically in humans. However, in rodents and primates, hippocampal neurogenesis tends to decrease with age, along with cognition (Lazarov et. al). Scientists have found that growing within an enriched environment and partaking in physical activity is able to retain these levels of neurogenesis from young mice to old mice. Despite this being somewhat difficult to fully apply to human subjects in a scientific setting, it shows promise in how we can potentially make certain lifestyle choices to avoid AD. In the future, more work surrounding this will hopefully allow us to bring a scientific backing for ways on how people can fight back against AD.
References:
Lazarov, O., Gupta, M., Kumar, P., Morrissey, Z., & Phan, T. (2024). Memory circuits in dementia: The engram, hippocampal neurogenesis and Alzheimer's disease. Progress in neurobiology, 236, 102601. https://doi.org/10.1016/j.pneurobio.2024.102601
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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