During a drive through New Hampshire, Chenyang “Leo” Lin noticed a few squirrels darting through the trees. It was a simple, unremarkable moment. But he remembered it clearly, along with the farmland his group had passed earlier. It struck him as odd. Why did these ordinary scenes stick in his mind?
That question stuck with Lin, a doctoral student in neuroscience at Boston University. And it led to a sweeping new study that may help explain how our brains decide what to remember—and what to forget.
“Memory isn’t just a passive recording device: Our brains decide what matters, and emotional events can reach back in time to stabilize fragile memories,” said Dr. Robert M.G. Reinhart, a BU College of Arts & Sciences associate professor of psychological and brain sciences at Boston University.
Published this week in Science Advances, the study by Lin, Reinhart, and their colleagues sheds light on a long-debated mystery: how our brains sometimes pluck unremarkable moments from the blur of everyday life and store them as lasting memories—just because they brushed up against something emotionally powerful.
How the Brain Picks Favorites
Lin had grown up in a coastal city in southern China, far from any woods. He wasn’t used to big trees and he definitely wasn’t used to squirrels. It was something unusual for him.
Herein lies an important part of memory.
Think back to the last time you received life-changing news—good or bad. Do you remember what you were wearing or what song was playing? How about what you had for lunch? You may or may not remember these things, but you definitely have a better chance than remembering what you had for lunch some random day two years ago.
According to the Boston University team, emotional or “salient” experiences act like memory magnets. They can strengthen the memories of neutral events that happen just before or after them. But, as the study shows, this effect isn’t all-or-nothing. It’s guided by what the researchers call graded prioritization.
In other words, our memory is sensitive to similarity. But it doesn’t work the same for all memories.
Testing Memory’s Rules
To uncover this, the researchers ran ten separate experiments involving 648 participants. Subjects were shown images—some of animals, some of tools—and later exposed to stimuli that carried a “salient” punch: either a cash reward or, in some cases, a mild electric shock.
The day after, participants took a surprise memory test.
Here’s what the researchers found:
- Strong events improved memory for what came after them, especially if those later events were emotionally neutral. This is known as proactive memory enhancement (PME).
- But when it came to reaching backwards in time, or retroactive memory enhancement (RME), the brain seemed to work differently. It only strengthened prior memories if they shared high-level visual features—like shape or category—with the big emotional event.
For instance, seeing a bison on a Yellowstone trail might etch itself into memory. But it may also stabilize your recollection of a rabbit you glimpsed just moments earlier—if your brain saw the two animals as conceptually similar.
“For the first time, we show clear evidence that the brain rescues weak memories in a graded fashion, guided by their high-level similarity to emotional events,” Lin added.
The idea, Lin says, aligns with a longstanding theory in neuroscience known as behavioral tagging: a weak memory leaves a temporary “tag” in the brain. If a strong emotional experience happens nearby (either before or after) it can supply the molecular ingredients needed to stabilize that weak trace.
But as it turns out, this only works if the two memories share conceptual ground.
Not All Memories Are Equal
One of the most surprising findings was that what was being remembered mattered too.
Animal images, for instance, tend to be more emotionally evocative, but harder for participants to remember. These images benefited most from memory rescue. Tool images, by contrast, were more readily remembered even without emotional tagging.
This suggests the brain applies different rules based on the strength of the original memory. Weak memories, like those for animal images, are more likely to be rescued. Stronger memories may already be “safe” and don’t need extra help. That’s why emotional tagging doesn’t always work in the same way for everyone—or even for every memory.
The authors believe this selective enhancement offers an evolutionary edge. In the wild, remembering the fox that chased you—and the path you took just before—could mean the difference between safety and danger. And that path is only worth remembering if the brain flags it as relevant.
Applying the Findings
The researchers believe their work could inform real-world strategies, especially in education and healthcare.
“In education, pairing emotionally engaging material with fragile concepts could improve retention,” Reinhart said. “In a clinical setting, we could potentially rescue memories that are weak, way back in the recesses of our mind because of normal aging, for example. You can flip it, too, for people with trauma-related disorders—maybe you don’t want to rescue a distressing memory.”
Teachers, for example, could better help students by linking dry facts to emotional stories, games, or rewards. Meanwhile, caregivers for people with dementia might use meaningful music or images to anchor important, everyday memories.
Still, the research has limitations. The memory tests involved simple images and tightly controlled lab conditions. Real-life memory—like the kind that lets us recall a conversation or navigate a city—is far messier.
And the team didn’t directly measure the brain’s activity during memory formation. Lin said the team hopes to use brain imaging and other tools in future studies to observe how these memory mechanisms play out in real time.
This isn’t the first study to suggest this type of mechanism. But it is one of the largest The team pooled data from ten separate experiments, including those from other labs, to run a meta-analysis. They also used artificial intelligence—specifically a convolutional neural network—to measure the visual similarity between images.
That allowed them to track how far each mundane image was, in concept, from the emotional event—and how much that distance mattered for memory. The closer the match, the stronger the boost.
In the end, it’s about how our brains choose to connect the dots—moments made memorable because of what followed, or what they resembled.
