Engrams and Infantile Amnesia

This semester Dr. Stephanie Grella discussed the paper, “Memory engrams: Recalling the past and imagining the future.” In this discussion, Dr. Grella introduced the topic of engrams which are neural substrates that are used for storing and recalling memories. For a neuron to become part of an engram, synaptic plasticity and intrinsic excitability play a huge role. The neurons that experience higher excitability are more likely to be added into the engram when the memory formation occurs. Once the memory is formed, it can then be retrieved when retrieval cues reactivate the engram in which that memory was created. A key part of this paper that I found intriguing was the mention of silent engrams. Silent engrams are engrams that cannot be naturally recalled with retrieval cues, but they still exist in the brain. In some studies, on mice, scientists have used direct optogenetic stimulation to retrieve memories that were once thought to be lost by artificially reactivating the engram. Therefore, it is believed that there are many memories in the brain that aren’t lost forever, we just don’t have the correct retrieval cues to reactivate them.

              A recent paper titled, “Hippocampal encoding of memories in human infants,” discusses a study in which Yates and colleagues explored infantile amnesia and an infant’s ability to encode memories during infancy. In this experiment, they used fMRI in awake infants from 4 months to 2 years old that were performing a memory task to measure hippocampal activity. Their study showed that infants have the capability to encode memories, especially those 12 months or older. Therefore, this means that in infancy, we can create and store memories, but as an adult, we do not possess the correct retrieval cues to reactivate the engram that encoded our infancy memories. Some ideas as to why infantile amnesia occurs could be from differences in the encoding process or postencoding mechanisms that have to do with retrieval.

              This study shows similarities with the silent engrams that Dr. Grella spoke about in our seminar. Through Dr. Grella’s presentation we learned about the existence of silent engrams and how some memories may be lost but not actually gone forever. The missing piece is the retrieval cues that align with the memory. This could also be the case for the memories that are lost during infancy. Yates’ study showed that infants can and do encode memories, as seen through the infant’s hippocampal activity during the memory tasks. Therefore, memories of our early years do exist somewhere in our brains. However, the issue with memory retrieval lies in the process of recall, similar to silent engrams. This connection highlights the importance of understanding the mechanisms behind engram reactivation. The more we know about the process of retrieval and reactivation, the more we may be able to discover our lost memories from adulthood as well as infancy. These findings lead to an intriguing idea that one day we may be able to reactivate our childhood memories that we once thought were lost forever. 

References:

Josselyn, S. A., & Tonegawa, S. (2020). Memory engrams: Recalling the past and imagining the future. Science, 367(6473). https://doi.org/10.1126/science.aaw4325

Simms, C. (2025). Babies do make memories — so why can’t we recall our earliest years? Nature. https://doi.org/10.1038/d41586-025-00855-0

Yates, T. S., Fel, J., Choi, D., Trach, J. E., Behm, L., Ellis, C. T., & Turk-Browne, N. B. (2025). Hippocampal encoding of memories in human infants. Science, 387(6740), 1316–1320. https://doi.org/10.1126/science.adt7570