Old memories interfere with remembering new ones

Scans reveal how the brain juggles outdated and fresh information

As anyone who has typed an outdated e-mail password before finally dredging up the new one knows, it’s easy to remember the wrong thing. Now, by capturing the battle between right and wrong memories in the brain, scientists have found that the struggle can get messy.

The results, published March 7 in the Journal of Neuroscience, bring scientists closer to understanding how people usually manage to pull up the right memory, and what goes wrong when this process fails. “To me, one of the most remarkable things isn’t how much we store in memory, but how well we’re able to find a memory,” says study coauthor Brice Kuhl of Yale University.

To study this battle of new versus old memories in the brain, Kuhl and his team had 24 undergraduates learn a picture-word pair, then learn a different one and finally describe the more recent pair. To create the original memory, participants were twice shown a word above an unrelated picture of a face, an object or a scene. For instance, the word “swim” would appear over a picture of Al Gore.

The researchers then shuffled the association, replacing Gore with an image of the Grand Canyon, and showed the participants the new pair. While in a brain scanner, participants were then shown the word “swim” and asked what kind of picture went underneath it in the newer memory.

Because specific parts of the brain respond strongly to different categories of pictures, Kuhl and his team could tell from the scan data whether the person was thinking about a face or a scene or an object. That distinction allowed the team to gauge the strength of the new and old memories separately.

When the pattern of brain activity for the older memory was stronger, the participants were more likely to fail to correctly recall the newer memory, the team reports. Even when a person successfully reported the new memory, traces of the old memory were called to mind. What’s more, when the new and old memory strengths were evenly matched, the participants took longer to answer. “We’re seeing evidence of things people are trying to not remember,” Kuhl says.

Scientists have long suspected that an unwelcome intrusion from older memories can cause memory problems, says neuroscientist David Badre of Brown University in Providence, R.I. “The problem with that is that it’s hard to see,” he says. This new approach allows the researchers to size up the struggle, Badre says: “It’s not much of a competition if you have a giant wrestling a dwarf.”

The results are based on healthy undergraduates, people who generally don’t have memory problems. A similar approach could be used to study what happens in the brains of people who have trouble remembering, Kuhl says.

Laura Sanders

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

From the Nature Index

Paid Content