Chenyang (Leo) Lin grew up in a city along China’s south coast. It was far from any woods. So the large trees and darting squirrels he saw while hiking in New Hampshire, last year, awed him. “That was very new to me.” That hike is now crystallized in Lin’s memory. So, oddly, are the more familiar chickens he saw on his way to that hike.

Lin is a brain and behavior expert at Boston University in Massachusetts. His latest research helps explain why both those chickens and squirrels have lodged in his memory.

Events that pack an emotional punch help people remember moments they would otherwise forget, that research shows. Lin is part of a team that shared these findings September 24 in Science Advances.

The brain seems to tie together related objects and events. That’s probably why Lin remembers ordinary farm animals along with what he saw as super cool squirrels.

Understanding what people remember — and forget — could help students retain information. And, Lin adds, it might also help therapists better treat trauma patients.

‘Rewards’ sometimes affect recall

Pairing a weak memory with a strong one can make the weaker one stick, too. Known as tag and capture, this theory has been around since the 1990s. Researchers suspected everyday memories could create temporary marks — or tags — on neurons. These tags would disappear, however, within a few hours. Think of them like Post-it notes that can easily fall off.

If, however, a more powerful memory comes along shortly afterward, the weaker memory might be saved.

Here’s how it was thought to work. The powerful memory triggers a rush of proteins in the hippocampus and linked areas of the brain. These are areas where memories form. Tagged neurons would capture those proteins. And just like adding a strong glue to the Post-it note, they’d make the weak memories stick to the powerful ones.

Lin and his team decided to test that idea.

They asked more than 100 people to observe sequences of images on a computer. Sometimes people saw animals (such as aardvarks, hamsters or sea stars). Other times they saw everyday tools. Participants initially just viewed these images — dozens of them. Next, they viewed similar images. This time, they’d be rewarded for labeling them as animals or tools.

About half the participants, for instance, might receive 900 points for identifying a hamster as an animal and 1 point for identifying a hammer as a tool. The other half received the opposite rewards: maybe 900 points for the hammer and 1 point for the hamster.

The next day, the team gave their participants a pop quiz on images they’d seen the day before. And, it turns out, both reward size and type of image sometimes affected what these people remembered.

Receiving a big reward for identifying animals enhanced someone’s memories of animals. Those who received a small reward remembered about 45 percent of the animals they’d seen in the first round. Participants remembered more — about 50 percent — if they’d received a large reward for labeling the animals earlier.

This memory boost didn’t work for tools, though. Big rewards didn’t help people remember the tools they’d seen earlier. Lin suspects that’s because people formed strong memories of the tools from the beginning, so adding a reward didn’t provide much more of a benefit.

Rewards for animals also didn’t help people remember which tools they’d seen. It seems the brain links memories within a category, not across different categories.

Morphing from temporary to recalled memories

Everyday memories are known to be temporary, says Joseph Dunsmoor. This cognitive neuroscientist works at the University of Texas at Austin where he has done similar research. This new study, he notes, is one of the first to show the brain is more likely to recall weak memories when they relate to some other, strong memory. “The thing that occurred in the past suddenly takes on more meaning,” he concludes.

Lin’s team also showed that people were better at forming new memories after receiving a big reward. After a big event, the brain is alert. It’s open to new information. So now, Lin says, new memories are more likely to stick around.

Sorting out what’s going on in the brain while memories form will take more time.

In the meantime, therapists could use these findings to help people enhance the everyday memories that occurred before a trauma. This might dull the pain of what came after, Lin says. Teachers might also use the findings to help students retain more of what they learn. For instance, Lin suggests, if Susie surprisingly jumps up and down in a math word problem, perhaps the class should jump up and down, too.