We sleep on average a third of our time. But what does the brain do during those long hours? Using an artificial intelligence approach capable of decoding brain activity during sleep, scientists at the University of Geneva, Switzerland, were able to catch a glimpse of what we think when we sleep.
By combining functional magnetic resonance imaging and electroencephalography, the Geneva team provided unprecedented evidence that the job of separating the thousands of pieces of information processed during the day takes place during deep sleep. In fact, at this point, the brain, which no longer receives external stimuli, can evaluate all these memories to retain only the most useful ones.
For this, it establishes an internal dialogue between its different regions. Furthermore, associating a reward with specific information encourages the brain to memorize it for the long term.
+ In a worldwide study, the 3rd dose of the vaccine causes controversy in Brazil: know what is true
In the absence of tools capable of translating brain activity, the content of our dormant thoughts remains inaccessible. However, we know that sleep plays an important role in memory consolidation and emotional control: when we sleep, our brain reactivates the memory trace built during the day and helps us regulate our emotions.
“To find out which brain regions are activated during sleep and to decipher how these regions allow us to consolidate our memory, we developed a decoder capable of deciphering the brain’s activity in deep sleep and what it corresponds to”, explains Virginie Sterpenich, researcher at the laboratory by Professor Sophie Schwartz in the Department of Basic Neurosciences, Faculty of Medicine, University of Geneva and principal investigator of this study.
“In particular, during deep sleep, the hippocampus – a temporal lobe structure that stores temporary traces of recent events – sends back to the cerebral cortex the information it has stored during the day. A dialogue is established that allows the consolidation of memory by repeating the events of the day and, therefore, reinforces the bond between the neurons”, he added.
To conduct the experiment, the scientists put volunteers on an MRI scan early in the evening and had them play two video games – a facial recognition game similar to ‘Guess Who?’ and a 3D maze from which the exit is to be found. These games were chosen because they activate very different brain regions and are therefore easier to distinguish on MRI images. In addition, the games were manipulated without the volunteers’ knowledge, so that only one of the two games could be won (half the volunteers won one and the other half won the second), so that the brain would associate the won game with a positive emotion.
The volunteers then slept on the MRI scan for an hour or two – the duration of a sleep cycle – and their brain activity was recorded again. Comparing MRI images of the waking and sleeping phases, the scientists observed that, during deep sleep, patterns of brain activation were very similar to those recorded during the game phase. “And very clearly, the brain revived the game won and not the game lost by reactivating the regions used during wakefulness. As soon as you go to sleep, brain activity changes. Gradually, our volunteers started to ‘think’ about the two games again, and almost exclusively about the game they won when they fell into a deep sleep,” says Virginie Sterpenich.
Two days later, the volunteers performed a memory test: recognizing all the faces of the game, on the one hand, and finding the starting point of the maze, on the other. Here again, the more the game-related brain regions were activated during sleep, the better the memory performance. Thus, the memory associated with the reward is greater when it is spontaneously reactivated during sleep.
With this work, the Geneva team opens up a new perspective on the study of the sleeping brain and the incredible work it does every night.
+ Learn about the effectiveness of each vaccine against Covid-19