Summary: The brain tunes out information from the outside world, such as the sound of speech, during REM sleep. During light sleep, the brain prioritizes meaningful speech, just as it does during a wakeful state. Researchers believe the mechanism enables the brain to protect the dreaming phase, which is essential for memory consolidation.
Scientists from the CNRS and the ENS-PSL in France and Monash University in Australia have shown that the brain suppresses information from the outside world, such as the sound of a conversation, during the sleep phase linked to dreaming. This ability is one of the protective mechanisms of dreams. The study, carried out in collaboration with the Centre du Sommeil et de la Vigilance, Hôtel-Dieu, AP-HP—Université de Paris, is published in Current Biology on 14 May 2020.
While we dream, we invent worlds that bear no relation to the quietness of our bedroom. In fact, it is rather unusual for elements of our immediate environment to be incorporated into our dreams. To better understand how the brain protects itself from outside influences, researchers invited 18 participants to a morning nap in the lab. Morning sleep is rich in dreams. Dreams mostly occur during what is known as REM sleep, since the brain is somehow in a waking state during this phase of sleep, showing brain activity similar to that when a person is awake. The body, on the other hand, is paralyzed, although not entirely. During certain phases of REM sleep, the eyes continue to move. Research has shown that such movements are related to dreaming.
To study how the dreaming brain interacts with external sounds, the scientists got volunteer sleepers to listen to stories in French mixed with meaningless language. By combining the electroencephalogram with a machine learning technique, they confirmed that, even when the brain is asleep, it continues to record everything that goes on around it. They also showed that, during light sleep, the brain prioritizes meaningful speech, just as it does when in the waking state. However, such speech is actively filtered out during eye movement phases in REM sleep. In other words, our sleeping brain can select information from the outside world and flexibly amplify or suppress it, depending on whether or not it is immersed in a dream!
The team believes that this mechanism enables the brain to protect the dreaming phase, which is necessary for emotional balance and consolidation of the day’s learning. Although dreams are predominant during periods of eye movement, they can also occur during other phases of sleep. Are they then accompanied by a similar suppression of sensations from the outside world?
About this neuroscience research article
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Sleepers Selectively Suppress Informative Inputs during Rapid Eye Movements
Sleep leads to a disconnection from the external world. Even when sleepers regain consciousness during rapid eye movement (REM) sleep, little, if any, external information is incorporated into dream content. While gating mechanisms might be at play to avoid interference on dreaming activity, a total disconnection from an ever-changing environment may prevent the sleeper from promptly responding to informative events (e.g., threat signals). In fact, a whole range of neural responses to external events turns out to be preserved during REM sleep. Thus, it remains unclear whether external inputs are either processed or, conversely, gated during REM sleep. One way to resolve this issue is to consider the specific impact of eye movements (EMs) characterizing REM sleep. EMs are a reliable predictor of reporting a dream upon awakening, and their absence is associated with a lower arousal threshold to external stimuli. We thus hypothesized that the presence of EMs would selectively prevent the processing of informative stimuli, whereas periods of REM sleep devoid of EMs would be associated with the monitoring of external signals. By reconstructing speech in a multi-talker environment from electrophysiological responses, we show that informative speech is amplified over meaningless speech during REM sleep. Yet, at the precise timing of EMs, informative speech is, on the contrary, selectively suppressed. These results demonstrate the flexible amplification and suppression of sensory information during REM sleep and reveal the impact of EMs on the selective gating of informative stimuli during sleep.