The technique, called optigenics, involved splitting the mice into two groups: one got a drug that inhibits memory making, while the others got a saline solution. The mice that had been drugged no longer froze by being in the room that had previously shocked them, but once blue light was applied, the mice froze once again. Next, researchers then went to those engram cells, and saw that cells in the hippocampus and been strengthened due to the training and labelling, making them easier to be retrieved once the blue light reacted with them. The researchers still couldn't quite put their finger on where these memories were stored in the meantime.

However, they then noticed that it was a degree of connectivity between these engram cells within the hippocampus, the memory part of the brain, and it was this connectivity that remained even after the drug had apparently knocked out the memories. Tomas Ryan, MIT neuroscientist at MIT and co-author of the study said:

"We have strong reason now to believe that memory storage — that is, the storage of the memory information itself — is encoded through connectivity patterns of engram cells throughout the brain."

Moving towards helping humans with memory issues, like Alzheimer's, remains distant: "It's very difficult to be doing this in humans, partly for the ethical reasons — the work is invasive — but also because we tag the memories in the brain before they're learned," Ryan says. Like the experiment, memories would have to be tagged ahead of time in order to be recovered.