How the brain chooses which memories to save during sleep

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Memory can be mysterious. Certain life events remain clear in our minds no matter how long ago they occurred, while episodes from the prior day may already be fuzzy and difficult to recall.

A study, published Thursday in Science, reveals why this happens. Scientists have found that the brains of humans and other mammals have a system for choosing which life experiences are important enough to be cemented into long-term memory — and which will be allowed to fade away.

Experiments in mice revealed that during waking hours, cells in the brain’s hippocampus spark in a specific pattern called “sharp-wave ripples,” which tag important experiences for movement into long-term memory storage during sleep. 

Although the research was performed in mice, certain brain processes have remained almost the same as mammals have evolved, so the findings can tell us a lot about ourselves, said the study’s senior author, Dr. György Buzsáki, Biggs Professor of Neuroscience at NYU Langone Health.

As part of the research, Buzsáki and his colleagues put mice through a maze that had a sugary reward at the end for those that successfully reached it. Meanwhile, the researchers were monitoring the activity of nerve cells through electrodes implanted in the rodent brains that fed data into computer programs.  

They observed that as the mice paused to eat their treats, their brains sparked sharp-wave ripples that were repeated as many as 20 times. The daytime pattern of sharp-wave ripples was replayed during the night, a process that moved the experience into long-term memory.

It’s during sleep when experiences from waking hours deemed to be important are converted into enduring memories. 

Events that were followed by very few or no sharp-wave ripples failed to form lasting memories, the researchers noted.

This tagging process during waking hours is totally unconscious, Buzsáki said. “The brain decides on its own, rather than us deciding voluntarily,” he added. 

Relaxation needed for long-term memory

Still, the research suggests there are things we can do to increase the likelihood of a memory being stored permanently.

If, like the mice, we pause after an experience, it may help cement the events into long-term memory, Buzsáki said. We need that period of relaxation, when we’re not paying close attention to anything, to allow sharp-wave ripples to spark. That wakeful process is an essential part of creating a permanent memory.

Practically speaking, this means if you like to binge TV series, you’re not likely to remember much of any episode except the last one you watched, Buzsáki said.

“If you watch a movie and would like to remember it, it’s better to go for a walk afterwards,” Buzsáki said. “No double features.”

An intriguing finding of the new research is the discovery that there could be bursts of activity — the sharp-wave ripples — when the mouse is standing still and its brain is essentially idling, said Daniela Schiller, a professor of neuroscience and psychiatry at the Icahn School of Medicine at Mount Sinai.

What’s amazing, Schiller said, is the pattern recorded close to the event was the same as what was seen during sleep. And both patterns mimicked the mouse’s real-life experience.

The study showed that events followed by a pause and electrical bursts in the brain are the ones that we will find in long-term memory, said Daphna Shohamy, director of Columbia University’s Zuckerman Institute. If you observe animals, you can see them pausing during the day after a novel or rewarding experience, she said.

“We did a study a few years ago in which we had humans navigate a maze with random objects along the way, looking for a treasure,” Shohamy said. “If they got the treasure, they were more likely to remember the random object they passed along the way.”

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