Memories: Hidden, Rewritten, or Forgotten?

In the beginning of September, Loyola neuroscience faculty member, Stephanie Grella, came to give a talk about engrams, their history, and her research on them. Before her talk she gave us literature to read for some background knowledge on engrams. It was about the history of engrams and how they became accepted as a part of the brain. Then the article moved on to silent engrams, which are memories that someone has but cannot naturally recall. However, they can be recalled artificially. Finally, the last major point of the article was when engrams compete for recall, one can get forgotten while the other is simultaneously strengthened. It stuck out that a lot is unknown about how a memory is lost. These ideas are brought to light in a newer article about how an engram is forgotten.

In “Memory engram stability and flexibility” it explains possible reasons for memory loss or alteration. In order for a memory to be lost there is two main problems. The memory is plain forgotten, or it has a reconsolidation error, meaning it is altered from the original memory. In forgetting, there could be a few things happening. One might be increased neurogenesis in the hippocampus. As new neurons are being made it erases the memories that are already there. Another common way to forget is sleep. It may seem counterintuitive but recent research has shown that sleep downregulates neural activity. However, it is said that sleep increases memory specificity, perhaps signaling that sleep helps you forget the unnecessary clutter in the brain. Then with reconsolidation there are also a few things that can occur. The memory first can be written over. This is demonstrated well in extinction learning; a type of learning that attempts to get rid of fears or phobias by separating a bad experience from the object of fear itself. It has also been found that administration of drugs can impact memory reconsolidation, demonstrating that memories are constantly being updated and aren’t stably stored within the brain.

When looking over these two articles in addition to the engaging talk Dr. Grella gave, it is easy to see how memory is such a complicated subject. It is very interesting to see how many factors go into whether a memory is kept or lost and how flexible they are. As with silent engrams, because they can be recalled artificially, it is understood that somehow the memory is not loss forever. Then with reconsolidation memories can be easily altered with their fluid nature. Lastly, forgetting appears to be caused largely by unrelated factors to the memory itself.

References:

Sheena A. Josselyn, Susumu Tonegawa, Memory engrams: Recalling the past and imagining the future.Science367,eaaw4325(2020).DOI:10.1126/science.aaw4325

Zaki, Y., Cai, D.J. Memory engram stability and flexibility. Neuropsychopharmacol. 50, 285–293 (2025). https://doi.org/10.1038/s41386-024-01979-z