Using techniques for labelling active neurons, for example, teams have located circuits associated with the memory of a painful stimulus in rodents and successfully reactivated those pathways to trigger the memory.
Such findings could one day help to reveal why memories fail in old age or disease, or how false memories creep into eyewitness testimony.
Only in the past decade have new techniques for labelling, activating and silencing specific neurons in animals allowed researchers to pinpoint which neurons make up a single memory.
As new techniques provide a glimpse of the engram, researchers can begin studying not only how individual memories form, but how memories interact with each other and change over time.
In a follow-up study, Preston has started to probe the mechanism behind memory linking, and has found that related memories can merge into a single representation, especially if the memories are acquired in close succession13.
The researchers showed that neurons encoding one memory remained more excitable for at least five hours after learning, creating a window in which a partially overlapping engram might form.
“We actually build concepts, and we link things together that have common threads between them.” The cost of this flexibility could be the formation of false or faulty memories: Silva’s mice became scared of a harmless cage because their memory of it was formed so close in time to a fearful memory of a different cage.