Scientists at Paris’s École Normale Supèrieure have shown that the brain can learn and form new memories while sleeping.
The findings provide insight into the functions of the sleeping brain and bring researchers one step closer to understanding sleep’s role in memory formation.
The study, authored by Sid Kouider, a senior fellow in CIFAR’s Azrieli Program in Brain, Mind & Consciousness, was published last month in Nature Communications. It found that depending on the stage of sleep, the brain either facilitates or suppresses the formation of new memories.
“The brain during sleep is much more active than what was previously believed and it’s not only more active in terms of internal mechanisms like neural plasticity or memory consolidation. The brain can also be active regarding how it’s processing information from the external world,” said Kouider.
He and his team used EEG (electroencephalography) and a white noise recall test to see whether 20 volunteers were able to learn and make new memories of sounds they were exposed to while they slept.
“The brain during sleep is much more active than what was previously believed…”
Before going to sleep volunteers were played random white noise. Within this white noise, the researchers embedded a shorter distinct clip of white noise five times. In this waking test, subjects need to hear these patterns of five repeating clips about 20 times in order for their auditory cortex to adapt and be able to distinguish it from the background white noise.
To measure whether sleeping brains could learn in the same way, Kouider’s team also conducted the white noise test (with new repeated clips) while volunteers were sleeping. When the volunteers awoke in the morning, they were immediately able to correctly distinguish the repeated noise they had been played during the night from the background white noise, at least when the noise had been played during REM and light non-REM sleep stages.
This type of learning, called perceptual learning, is a basic form of learning that mostly involves the sensory cortex and doesn’t require multiple cortical regions to work together integrating information. According to Kouider, most previous studies that have tried to induce learning during sleep might have failed because they tried to target learning at too high a level. However, previous studies in his lab had shown that simple perceptual learning happens unconsciously without the subject having to pay attention to or be aware of the stimulus, giving his team the idea that it could occur during sleep.
What the researchers hadn’t expected was that when the patterns were played to volunteers during deep non-REM sleep, they became worse at recognizing the patterns in the morning than if they had been exposed to a completely new pattern altogether.
Why this occurs was not explored in the research but Kouider thinks it could be because during deep non-REM sleep, the brain is consolidating the day’s memories or pruning unnecessary connections and therefore actively suppressing input from the external environment that might interfere with these processes.
EEG measurements taken during the experiment also showed neural markers of this learning during light non-REM and REM sleep. During deep non-REM sleep, the slow waves that are characteristic of this stage correlated with volunteers’ difficulty learning new auditory patterns once awake. The more slow waves that were present during deep non-REM sleep, the harder it was for volunteers to distinguish the white noise patterns they had been exposed to during that stage of sleep the following morning. This result allowed the researchers to established a clear link between slow waves and the suppressive effect on learning.
In the past people believed that the brain shut down when you were asleep.
Over the last 20 years people have shown that this is not the case and in fact, the brain is very active consolidating the memories that you acquire during the day. Even so, the prevailing thought remains that humans sleep to avoid any interference from the environment. What Kouider and his team show with these results is that the picture is more complicated than that.
“It’s probably the case during deep sleep that you are trying to avoid interference from the environment, but for the other sleep stages you continue tracking information,” he said. “This is very important because if something happens you want to be able to wake up extremely quickly. And because you can track auditory stuff in the environment, you can also adapt to it. If you can hear, you can learn.”
This awareness of the environment around us in lighter stages of sleep is a sort of “standby mode” that Kouider and his lab will explore in an upcoming paper. They are also looking further into the link between memory consolidation and learning during sleep to see if these processes use the same mechanisms and whether or not these processes can happen simultaneously.
Kouider is also working with fellow Azrieli Program in Brain, Mind & Consciousness member Adrian Owen (Owen is the program Co-Director and Koerner Fellow at CIFAR) to compare how the brain responds to the external environment during sleep versus in patients in a vegetative state. They will conduct the research thanks in part to CIFAR’s Catalyst funding program.