NORTHWESTERN (US)—Even in deep sleep, sounds make their way into our minds, researchers have found, and enhance associated memories.
“The research strongly suggests that we don’t shut down our minds during deep sleep,” says John Rudoy, lead author of the study and a neuroscience PhD student at Northwestern University. “Rather this is an important time for consolidating memories.”
For the study, participants were taught to associate each of 50 images with a random location on a computer screen before taking a nap. Each object—such as a shattering wine glass—was paired with a corresponding sound—such as the sound of breaking glass—delivered over a speaker.
Locations were learned by repeating trials until study participants were able to place all the objects in their assigned places. Approximately 45 minutes after learning, each participant reclined in a quiet, darkened room.
Electrodes attached to their scalp measured their brain activity, indicating when they were asleep, and, without waking anyone up, sleep sounds were then presented.
When asked later, none of the participants thought sounds had been played during the naps. Yet, memory testing showed that placements of the objects were more accurate for those cued by their associated sounds during sleep than for those not cued. Details were published in the journal Science.
“While asleep, people might process anything that happened during the day, what they ate for breakfast, television shows they watched, anything,” explains Ken Paller, senior author of the study and professor of psychology.
“But we decided which memories our volunteers would activate, guiding them to rehearse some of the locations they had learned an hour earlier.”
Whether or not memories are processed during sleep has been a subject of controversy, with most of the research on the topic focusing on REM, a normal stage of sleep characterized by rapid movement of the eyes. Vividly recalled dreams mostly occur during REM sleep.
Recent research, including the new Northwestern study, however, focuses on memory processing during deep sleep, rather than during REM sleep.
“We are beginning to see that deep sleep actually is a key time for memory processing,” Paller says. “Our little experiment opens the door to many questions.”
For example, would high-school students do better on SAT tests if daytime studying was supplemented with sleep sounds at night? Would students learning foreign vocabulary words or other facts do better in the morning after listening to related information as they slept?
Infants spend an inordinate amount of time sleeping, while their brains work over their recent experiences. Could an infant learn a first language more quickly if stimulation occurred during naps or overnight?
The study opens avenues for discovering boundaries of what can happen to memories during sleep, says coauthor Joel Voss. “Can memories be distorted as well as strengthened? Can people be guided to forget unwanted memories?”
Much work remains to determine whether the results of the new research translate to these and other contexts, Paller emphasizes. “We don’t know the answers at this point,” he says, “but more experiments about memory processing during sleep are certain to follow.”
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