People often think of sleep as a time absent of any thought, but new research shows that the brain carries on making connections, even during slumber.

Using artificial intelligence, scientists at the University of Geneva (UNIGE), Switzerland, were able to better understand what we think about when we are asleep.

By combining functional magnetic resonance imaging (fMRI) and electroencephalography (EEG), the Geneva team provides evidence that the work of sorting out the thousands of pieces of information processed during the day takes place during deep sleep.

During sleep, the brain is free evaluate memories in order to retain only the most useful ones. To do so, it establishes an internal dialogue between its different regions. Moreover, associating a reward with specific information encourages the brain to memorize it in the long term. These results, published in the journal Nature Communications, shed new light on the human mind during sleep.

In the absence of tools capable of translating brain activity, the content of our sleeping thoughts remains inaccessible. We however do know that sleep plays a major role in memory consolidation and emotional management: when we sleep, our brain reactivates the memory trace built during the day and helps us to 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 activity of the brain in deep sleep and what it corresponds to,” says Virginie Sterpenich, a researcher in the laboratory of professor Sophie Schwartz in the Department of Basic Neurosciences at UNIGE Faculty of Medicine, and the principal investigator of this study, in a statement.

“In particular, we wanted to see to what extent positive emotions play a role in this process,” she says.

To conduct their experiment, the scientists placed volunteers in an MRI in the early evening and had them play two video games. These games were chosen because they activate very different brain regions and are therefore easier to distinguish in the MRI images. In addition, the games were rigged without the volunteers’ knowledge so that only one of the two games could be won (half of the volunteers won one and the other half won the second), so that the brain would associate the game won with a positive emotion.

The volunteers then slept in the MRI for one or two hours, the length of a sleep cycle, and their brain activity was recorded again. “We combined EEG, which measures sleep states, and functional MRI, which takes a picture of brain activity every two seconds, and then used a ‘neuronal decoder’ to determine whether the brain activity observed during the play period reappeared spontaneously during sleep,” Schwartz says.

By comparing MRI scans of the waking and sleeping phases, the scientists observed that during deep sleep, the brain activation patterns were very similar to those recorded during the gaming phase. “And, very clearly, the brain relived the game won and not the game lost by reactivating the regions used during wakefulness. As soon as you go to sleep, the brain activity changes. Gradually, our volunteers started to ‘think’ about both games again, and then almost exclusively about the game they won when they went into deep sleep,” says Virginie Sterpenich.

Two days later, the volunteers performed a memory test: recognizing all the faces in the game, on the one hand, and finding the starting point of the maze, on the other. Here again, more the brain regions related to the game were activated during sleep, better were the memory performances. Thus, memory associated to reward is higher when it is spontaneously reactivated during sleep.

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