The brain can form new memories as you sleep
FRANCE: A sleeping brain can form fresh memories, according to a team of French neuroscientists.
The researchers played complex sounds to people while they were sleeping, and afterward the sleepers could recognise those sounds when they were awake.
The idea that humans can learn while asleep, a concept sometimes called hypnopedia, has a long and odd history. But in the new study, the neuroscientists demonstrated it is possible to teach acoustic lessons to sleeping people.
‘‘We proved that you can learn during sleep, which has been a topic debated for years,’’ said Thomas Andrillon, an author of the study and a neuroscientist at PSL Research University in Paris. Just don’t expect Andrillon’s experiments to make anyone fluent in French.
Researchers in the 1950s dismantled hypnopedia’s more outlandish claims. Sleepers cannot wake up with brains filled with new meaning or facts. Instead, test subjects who listened to trivia at night woke up with ‘‘non-recall’’.
Yet success is possible, if the subject is not trying to learn dictionary definitions or kung fu.
In recent years, scientists have trained sleepers to make subconscious associations. In a 2014 study, Israeli neuroscientists had 66 people smell cigarette smoke coupled with foul odours while they were asleep. The test subjects avoided smoking for two weeks after the experiment.
In the new research, Andrillon and his colleagues moved beyond association into pattern learning. While a group of 20 subjects was sleeping, the neuroscientists played clips of white noise. Most of the audio was ‘‘purely random’’, Andrillon said. But there were patterns occasionally embedded within the complex noise: sequences of a single clip of white noise, 200 milliseconds long, repeated five times.
The subjects remembered the patterns. The lack of meaning worked in their favour; sleepers can neither focus on what they’re hearing nor make explicit connections, Andrillon said. This is why nocturnal language tapes don’t quite work – the brain needs to register sound and semantics. But memorising acoustic patterns like white noise happens automatically.
‘‘The sleeping brain is including a lot of information that is happening outside,’’ Andrillon said, ‘‘and processing it to quite an impressive degree of complexity.’’
Once the sleepers awoke, the scientists played back the whitenoise recordings. The researchers asked the test subjects to identify patterns within the noise. It’s not an easy task, Andrillon said, and most people would struggle with unless they happened to remember the repetitions from a previous night’s sleep. The test subjects successfully detected the patterns far better than random chance would predict.
What’s more, the scientists discovered that memories of whitenoise pattern formed only during certain sleep stages. When the authors played the sounds during REM and light sleep, the test subjects could remember the pattern the next morning. During the deeper non-REM sleep, playing the recording hampered recall. Patterns ‘‘presented during nonREM sleep led to worse performance, as if there were a negative form of learning’’, Andrillon said.
This marked the first time that researchers had ‘‘evidence for the sleep stages involved in the formation of completely new memories’’, said Jan Born, a German neuroscientist.
In Andrillon’s view, the experiment helps to reconcile two competing theories about the role of sleep in new memories: In one idea, our sleeping brains replay memories from our waking lives. As they’re played back, the memories consolidate and grow stronger, written more firmly into our synapses. In the other hypothesis, sleep instead cuts away at older, weaker memories. But the ones that remain stand out, like lonely trees in a field.
The study indicates that the sleeping brain can do both, Andrillon said. They might simply occur at separate moments in the sleep cycle, strengthening fresh memories followed by culling.