Scientists already know that sleep consolidates learning of new information that we acquire during wakefulness.
Now, researchers at the University of Bern in Switzerland suggest that learning can also take place during deep, or slow-wave sleep.
In a study that features in the journal Current Biology, they show how associations with new foreign words can occur at certain phases of slow-wave sleep.
Much sleep research concerns the processes that stabilize and consolidate memories that form during periods of wakefulness.
There is now considerable evidence that replay during sleep strengthens memories and embeds them in the previously acquired knowledge store in the brain.
The study authors note that many deem it impossible that learning can take place during sleep because “sleep lacks the conscious awareness” and the necessary brain chemistry and activity.
In addition, studies that have examined sleep learning in humans have yielded conflicting results.
Learning during daytime naps
The researchers were intrigued by the question: If the sleep state strengthens a “memory trace” that forms during wakefulness, then why can’t the sleep state itself form a memory trace that endures into wakefulness?
Using electroencephalograms (EEGs), they recorded brainwave activity in 41 healthy male and female volunteers as they took a daytime nap and while they underwent subsequent memory tests.
During the nap, the volunteers also wore in-ear headphones through which the researchers played recordings of numerous verbal word pairs.
They devised each word pair so that one word was a familiar, native-language word while the other was a made-up “pseudoword.”
For example, they paired the word “house” with the pseudoword “tofer.” In another pair, the familiar word was “cork,” and the pseudoword was “aryl.”
After the nap, the volunteers underwent a test of their “sleep-formed associations.”
The test presented them with random samples of the pseudowords. At each presentation, they had to say whether the object the word described could fit inside a shoebox or not.
The results showed that the size classification of the pseudowords was better than chance if the “acoustic presentation of the second word of a pair during sleep repeatedly hit an ongoing slow-wave peak.”
Timing of encoding is key
Slow-wave, or deep sleep is the most beneficial stage for consolidating memories that form in the period of wakefulness that precedes it.
As the brain enters slow-wave sleep, its cells gradually synchronize their activity. They fall into a pattern that alternates every 0.5 seconds between brief periods of universal activity and inactivity. Periods of activity appear as peaks on EEGs.
The researchers found that the volunteers only encoded the association between a sleep-played, familiar, native-language word and its pseudoword under two conditions.
The first condition was repetition of the word pair, and the second condition was that the acoustic presentation of the second word had to coincide with an active phase of slow-wave sleep.
In other words, the volunteers were better able to correctly classify “tofer” as being too large to fit into a shoebox if they had heard the word pair “house-tofer” several times, and the second word had occurred while their brain cells were in an active phase of slow-wave sleep.
Co-first study author Marc Züst, Ph.D., says that they also observed that retrieval of sleep-learned words during the test coincided with activity in the hippocampus and language areas of the brain. The hippocampus plays a key role in memory and learning.
These are the same brain areas that are active when learning occurs during wakefulness.
“These brain structures appear to mediate memory formation independently of the prevailing state of consciousness — unconscious during deep sleep, conscious during wakefulness.”
Marc Züst, Ph.D.
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