The participants who napped after their learning tasks showed increased connectivity between the hippocampus and two other areas of the brain involved in memory consolidation and reward processing.
The team, from the University of Geneva in Switzerland, reports the findings in the journal eLife.
The authors suggest that receiving rewards as we learn can help reinforce new information in memory, and daytime naps boost this effect.
Lead author Kinga Igloi, a postdoctoral researcher from the Department of Neuroscience, says:
"We already knew that sleep helps strengthens memories, but we now also know that it helps us select and retain those that have a rewarding value."
She explains that it makes "adaptive sense" to consolidate memory in a way that prioritizes information that is important for our success and survival.For the study, the team recruited 31 healthy volunteers and randomly assigned them to either a "sleep" group or a "wake" group. Both groups were invited to look at and remember eight pairs of pictures.
They were also told that for four of the eight pairs of pictures, there would be a higher reward for remembering them.
When the learning phase was finished, both groups had a 90-minute break. The sleep group had a daytime nap during the break, while the wake group just rested.
After the break, both groups were tested on their memory of the picture pairs and were also invited to score how confident they felt about giving a correct answer.
Then, after 3 months, there was a surprise test where both groups were again asked to remember the picture pairs and give their confidence score.
On recruiting the volunteers, the researchers had established that they all had the same sensitivity to reward, so they could rule this out as a possible influencer in the results.
Nap boosted long-term recall of rewarded learning
The results of the memory tests conducted after the 90-minute break showed that overall, the sleep group performed better. But in the case of the more highly rewarded picture pairs, both groups performed equally as well in the memory tests.
However, there was a striking difference in the surprise test 3 months later - the sleep group showed better memory performance for the more highly rewarded picture pairs than the wake group.
Also, the participants in the sleep group showed higher rates of confidence of getting the right answer, even 3 months later.
During the tests, participants in both groups underwent magnetic resonance imaging (MRI) scans of their brains. These showed that the sleep group participants had more activity in the hippocampus - a small area of the brain critical for forming memories. This was tied to more bursts of a type of brain activity called "slow spindles."
Spindles are a type of brain oscillation that is involved in consolidation of memory. In a recently reported study, researchers showed how a three-step brain oscillation process that includes spindles occurs when short-term memory traces in the hippocampus are relocated to more outer parts of the brain to consolidate long-term memory during sleep.
After 3 months, the sleep group also showed increased connectivity between the hippocampus and two other areas of the brain involved in memory consolidation and reward processing: the medial prefrontal cortex and the striatum.
Dr. Igloi concludes:
"Rewards may act as a kind of tag, sealing information in the brain during learning. During sleep, that information is favorably consolidated over information associated with a low reward and is transferred to areas of the brain associated with long-term memory."
She notes that the findings may also be "relevant for understanding the devastating effects that lack of sleep can have on achievement."
Meanwhile, Medical News Today recently learned that deep sleep boosts immunological memory. It appears that just as psychological memory stores information from environmental and social learning, the immune system stores immunological information for future use.