What Events Do We Recall Best?

By Jane Farrell

When it comes to memory, we’re likelier to recall the things that may help us in the future, according to new research from the University of California, Davis, Center for Neuroscience.

The finding was published in the journal Neuron.

“Rewards help you remember things, because you want future rewards,” said Professor Charan Ranganath, a UC Davis neuroscientist and senior author on the paper. “The brain prioritizes memories that are going to be useful for future decisions.”

It’s estimated that we retain detailed memories for only a small proportion of the events of each day, Ranganath said. People with very detailed memories become overwhelmed with information. So if the brain is going to filter information and decide what to remember, it makes sense to save those memories that might be most important for obtaining rewards in the future.

Ranganath and postdoctoral researcher Matthias Gruber put this to the test by scanning the brains of volunteers by functional magnetic resonance imaging as they answered simple yes-no questions on short series of objects — for example, “do these objects weigh more than a basketball?” Each series of objects was shown on a background image for context, and depending on the context, the volunteers were told they would either get a large (dollars) or small (cents) reward for giving correct answers. At the end of a series, participants were told how much money they just won.

Once participants completed this part of the experiment, the volunteers were scanned during a resting period. Afterward, outside of the scanner, there was a surprise memory test for all objects that were shown during scanning.

Although participants were not expecting the memory test outside the scanner, they were better at remembering objects that were associated with a high reward, said Gruber, first author of the paper.

“Also, when an object was associated with high reward, people remembered better the particular background scene that was on the screen during scanning,” Gruber said.

Even more interesting, participants’ memory performance was predicted by brain activity measured during rest. When the researchers looked at brain scans of subjects at rest after giving yes-no answers — neither learning nor actively recalling the memory — they found the same pattern of activity as when subjects were doing the high-reward task. The subjects were apparently replaying the rewarding memories, strengthening connections and helping to fix the memory in place.

Although this study did not measure it directly, these interactions were likely related to release of dopamine, a neurotransmitter that is released in the brain when we expect rewards. Conditions such as Parkinson’s disease or aging are linked to reduced dopamine and often involve memory defects.