How Our Memories Evolve Through Memory Engrams and Neurogenesis

 This semester at Loyola University Chicago, I had the opportunity to hear Dr. Stephanie Grella talk about memory engrams, one of the most fascinating ideas I have encountered in neuroscience. An engram is essentially the physical trace of a memory in the brain. They are a specific set of neurons that light up together when you experience something and they light up again when you remember it later. For a long time scientists were only able to guess what these engrams looked like, but now thanks to new technologies, researchers can, not only find them but also manipulate them.

Dr. Grella shared with us a paper published in 2020 by authors Sheena Josselyn and Susumu Tonegawa called “Memory Engrams: Recalling the Past and Imagining the Future.” The paper described how researchers have used optogenetics, a technique that uses light to control neurons to tag the neurons involved in forming a memory. In one experiment, the researchers activated the same neurons that fired during a fearful experience and the animal acted as if it was reliving that fear. They were even able to create false memories by tricking the brain into linking two unrelated experiences. This research was able to reveal something groundbreaking in neuroscience. Memories aren’t just some abstract thoughts, instead they are physically stored in specific networks of neurons. These groups of cells are called engrams and they can be turned on, turned off, and even reshaped. And they are not just isolated to one part of the brain, they are spread throughout the brain. Different pieces of a single memory live across multiple brain regions. 

In a recent study published in 2025 by Ali Golbabaei along with Sheena Josselyn and fellow researchers in Current Biology, titled “Neurogenesis-Dependent Transformation of Hippocampal Memory Traces During Systems Consolidation” explores the idea of how engrams change over time through neurogenesis. The researchers discovered that when mice formed new memories, the hippocampus, which basically is the brain’s memory storage unit, first stored them in very high detail. However as time passed, those detailed memories lost resolution as the cortex took over, holding a fuzzier more generalized version of the memory. Interestingly, this fading of detail depended on neurogenesis which is the birth of new neurons in the hippocampus. When the researchers stopped neurogenesis from happening, the hippocampal memories stayed sharp instead of blurring over time. This new discovery revealed that the new neurons that form essentially rewrite or simplify our old memories. 

Dr. Grella’s talk tied all of this together beautifully. She explained in the talk how these discoveries are reshaping the way we think about memory as not something fixed but as something that is constantly evolving. In reality memories aren’t snapshots of experiences but are patterns of activity in the brain that can change and fade over time and can even be reawakened. Our brains keep editing, retelling, and refining our memories over time. 

References:

Golbabaei, Ali, Cesar A.O. Coelho, Mitchell L. de Snoo, Antonietta De Cristofaro, Sheena A. Josselyn, and Paul W. Frankland. “Neurogenesis-Dependent Transformation of Hippocampal Memory Traces during Systems Consolidation.” Current Biology, September 2025. https://doi.org/10.1016/j.cub.2025.09.005. 

Josselyn, Sheena A., and Susumu Tonegawa. “Memory Engrams: Recalling the Past and Imagining the Future.” Science367, no. 6473 (January 3, 2020). https://doi.org/10.1126/science.aaw4325.