Why Some Memories Stay Clear While Others Don’t

When you go down memory lane with old friends, you’ll probably find some events from decades ago are crystal clear, while others – even if they occurred more recently– are fuzzy and difficult to recall.

Why is this? Some people joke that their brains are like Swiss Cheese, and some memories just fall through the holes. Believe it or not, this is not far from the truth!

Scientists have discovered the real reason why you remember some things and not others, and what they’ve found can help you sharpen your memory even if it’s already lagging.

Your Brain Prioritizes Information For You

Researchers at NYU Grossman School of Medicine have discovered that the brain is intentionally selective. In other words, the brain chooses which memories are important enough to save and which to let fade away. While this might sound as simple as the Swiss Cheese “theory,” it’s quite complex and occurs through a brain process neurologists call sharp wave ripples.

We’ll explain more about these ripples in a moment, but first, let’s unpack some facts about your ability to retain new information. To successfully remember information science shows that you need both sufficient sleep to convert some of each day’s experiences into permanent memories and rest periods between learning.

Rest periods activate an area of the brain called the hippocampus. When scientists first discovered this, they were surprised. That’s because although the hippocampus is a key memory area, brain regions involved with converting events into permanent memories during sleep - at least in the type of skills that are learned and retained through practice and repetition - occur in the sensorimotor regions which control movement. Movement, of course, is not really happening when you’re asleep!

So, the discovery of the hippocampus’s role during rest periods left scientists wanting answers to two questions:

1. How does the brain determine which memories are tagged as important enough to hold on to until sleep secures them into long-term memory?
2. How does the hippocampus bring this about?

Researchers took these questions into the laboratory with a group of mice since mice have a memory recall process that works in a similar way to all mammals, including humans.

The NYU team implanted electrodes in the mice's brains to monitor nerve cell activity in an area of the hippocampus. Then, the mice explored a maze, and if successful, researchers rewarded them with a tasty treat.

The data revealed that when these mice took a well-earned rest to savor their reward, it sparked the firing of distinct sharp wave ripples repeated up to twenty times. Sharp wave ripples are sequences of synchronized rhythmic patterns of high-frequency electrical impulses that encode memories.

Waking Patterns Repeated During Sleep

While the researchers knew that these ripples were involved with memory formation during sleep, what was really interesting was that when the mice were asleep, the same pattern of sharp wave ripples produced during their waking hours was repeated thousands of times, and it was this repetitive pattern of recordings that moved the experience into long-term memory.

When few or no sharp wave ripples are produced following an event, lasting memories aren’t formed, and no sharp wave ripples occur when actively exploring sensory information or moving.

György Buzsáki, MD, PhD, senior author of the study published in the journal Science in March, said: “Our study finds that sharp wave ripples are the physiological mechanism used by the brain to ‘decide’ what to keep and what to discard.”

Future Treatment: Riding The Waves

First author Winnie Yang was optimistic that “future research may reveal devices or therapies that can adjust sharp wave ripples to improve memory, or even lessen recall of traumatic events.”¹

While this is certainly exciting, you don’t have to wait for a new treatment. To improve your recall of events, you can take advantage of these findings right now.

The authors write that the incidence of sharp wave ripples and replays will increase when the experience includes novelty, repetition, and reward.² So, practically, when you have an experience, one you want to remember, you need to pause, relax, savor it; don’t pay close attention to anything, then experience a little more, then pause again.

These pauses allow the brain to go into an “idle” reassessment mode, which encodes the memory during sleep. This knowledge can also be used to learn new information, a language, a musical instrument, or any new skill.

It can also be used to remember enjoyable experiences.

Dr. Buzsáki gave a few examples of increasing your memory power by saying that if you watch one TV episode straight after another, you’re not likely to remember much of any episode except the last. If you want to remember them all, watch them one at a time and reflect on each one for a short while. Similarly, if you want to remember the movie you just saw, go for a walk afterwards. “No double features.”³