The finding shows that the brain handles memories differently during sleep than while awake, says Sara Mednick, a cognitive neuroscientist at the University of California, San Diego who was not involved in the research. Armed with this new knowledge, she says, therapists may be able to destabilize traumatic memories and overwrite the bad memories with good ones, then solidify the new memory with a nap.

In the new study, volunteers played a Concentration-type game in which they had to remember the locations of pairs of cards. Meanwhile, a mask wafted a slightly unpleasant odor into the volunteers’ nostrils. Once the volunteers had mastered the game, some stayed awake while others took about a 40-minute nap. Researchers reactivated the memory in some volunteers by releasing the odor again. After the nappers woke up, the volunteers played a slightly different version of the card game and were tested to see how well they recalled the locations of the original cards.

Both sleeping and awake volunteers who didn’t have their memories jogged by the odor remembered about 60 percent of the pairs. When researchers triggered memory reactivation while volunteers were awake, recall of the correct locations dropped to about 41 percent. The researchers had expected that result. Previous studies have shown that replaying a memory while awake makes it vulnerable to interference from new material, such as from the second card game.

But the real surprise came when the team replayed memories in the sleeping volunteers and checked how that affected their waking performance. “With odor reactivation, they were almost perfect,” says coauthor Susanne Diekelmann, a psychologist and neuroscientist at the University of Lübeck in Germany. Volunteers correctly picked out about 84 percent of the original card pairs when the memory replayed during a nap that consisted mostly of deep slow-wave sleep (volunteers were woken up before they entered rapid eye movement, or REM, sleep).

Brain scans also revealed that different areas of the brain were involved during memory replay depending on whether the volunteers were awake or asleep. While awake, replaying the memory triggered activity mostly in the right lateral prefrontal cortex, a part of the brain involved in memory recall. But during sleep, memory replay was associated with strong activity in the hippocampus and parts of the cortex. The hippocampus is involved in memory formation, and memories are transferred from short-term memory in the hippocampus to long-term memory in the cortex. Reactivating memories during sleep may speed the transfer, Diekelmann says.

The researchers are now testing whether replaying memories during REM will also stabilize them. Brain activity during that sleep state is similar to that while awake, so the researchers suspect memories may become unstable during REM to allow for editing and reorganization.

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