December 11, 2018
Whether it’s the pleasant experience of returning to one’s childhood home over the holidays or the unease of revisiting a site that proved unpleasant, we often find that when we return to a context where an episode first happened, specific and vivid memories can come flooding back. In a new study in Neuron, scientists in MIT’s Picower Institute for Learning and Memory report the discovery of a mechanism the brain may be employing to make that phenomenon occur.
“Suppose you are driving home in the evening and encounter a beautiful orange twilight in the sky, which reminds you of the great vacation you had a few summers ago at a Caribbean island,” said study senior author Susumu Tonegawa, Picower Professor of Neuroscience at MIT. “This initial recall could be a general recall of the vacation. But moments later, you may get reminded of details of some specific events or situations that took place during the vacation which you had not been thinking about.”
At the heart of that second stage of recall, where specific details are suddenly vividly available, is a change in the electrical excitability of “engram cells,” or the ensemble of neurons that together encode a memory through the specific pattern of their connection. In the new study Tonegawa’s lab, led by postdoc Michele Pignatelli and former member Tomas Ryan, now at Trinity College Dublin, showed that after mice formed a memory in a context, the engram cells encoding that memory in a brain region called the hippocampus would temporarily become much more electrically excitable if the mice were placed back in the same context again. So for instance, if they were given a little shock in a specific context one day, then the engram cells would be much more excitable for about an hour after they were put back in that same context the next day.
The specific change in the engram cells’ electrical properties has some direct implications for learning and behavior that hadn’t been appreciated before. Importantly, during that hour after returning to the initial context, because of the engrams’ elevated excitability, mice proved better able to learn from a shock in that context and better able to distinguish between that and distinct contexts even if they shared some similar cues. The increase in excitability therefore allowed them both to learn to avoid places where danger happened very recently and to continue to function normally in places that happen to have some irrelevant resemblance. And because the effect was short-lived, it didn’t oblige them to remain overly attuned for very long.
“The short-term reactivation increases the future recognition capability of specific cues,” Pignatelli and Tonegawa’s team wrote. “Engram cell excitability may be crucial for survival by facilitating rapid adaptive behavior without permanently altering the fundamental nature of the long-term engram.”
Tonegawa added that “while the survival interpretation may be an evolutionary origin of this multi-step episodic memory recall” it likely also applies to positive episodic memories, like the vacation sunset experience, just as much.