Memory engrams refer to long-lasting, learning-induced physical or chemical changes that occur in brain networks. Researchers have shown that the reactivation of these engram cells, which are located in the dentate gyrus of the hippocampus, triggers the recollection of specific memories.

Researchers in the journal PNAS demonstrate that engram cells in amnesic mice can also exist in an enduring silent state, in which memory information is retained and can be retrieved not by natural recall cues, but by strong stimulation with optogenetics—a technique that uses light to control the activity of genetically defined neurons.

During the training session, the authors placed mice in a chamber with an electrified grid floor that delivered foot shocks. Immediately after fear learning, the authors injected the mice with either the amnesia-inducing molecule anisomycin or saline as a control. One day later, the control group froze in fear when reexposed to the shock chamber, which was filled with natural recall cues, whereas the anisomycin-treated mice showed significantly less freezing behavior.

However, optogenetic stimulation of the engram cells 2–8 days after training triggered similarly high levels of freezing behavior in both control and anisomycin-treated mice.

According to the authors, future studies on the properties of silent engrams could illuminate the formation and retrieval of memories. 

http://www.pnas.org/content/early/2017/10/18/1714248114