Memory is a complex concept that can be distilled into three steps: encoding, storage, and retrieval. In 2015, neuroscientists at MIT proposed a revision of this theory after finding that there are certain stored memories in the brain that cannot be retrieved using natural cues. These memories were dubbed “silent engrams”.

It was previously believed that the storage of memories relied on the strengthening of connections between cells in the brain. However, the discovery of silent engrams shows that a specific pattern of connections along an anatomical pathway between neurons is enough for the storage of a memory. Tonegawa and his team of researchers observed the formation of these silent memories, even while blocking the protein synthesis necessary for strengthening connections between neurons. The researchers also found that, despite the difficulty in retrieving these silent memories, they could be retrieved artificially using optogenetics, a method that uses light to control the activity of neurons. 

Tonegawa and his colleagues later conducted a study in 2017 that demonstrated how fear training in mice was altered using the inhibition of protein synthesis. The mice only showed the appropriate conditioned fear reactions when the memory engrams were activated using optogenetics. Moreover, these cells could be activated up to eight days after training. The silent engrams could even be reactivated when new connections were grown between engram cells in the mice. Their ability to naturally recall the memory was no longer blocked, and the conditioning developed during training was no weaker than normal mice who had undergone training.

The implications of these findings extend to Alzheimer’s patients who may be able to recover some memory through the reactivation of connections. There is some promise for patients suffering from the early stages of the disease. Although the results are preliminary, this shift in the theory could potentially save precious memories.

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This post was written by McKenzie Cline