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Why does our brain stay attentive even when we’re asleep?

We generally consider sleep as a loss of consciousness during which we stop interacting with our environment. But perhaps you’ve had the experience of falling asleep on a train or bus only to wake up just as your station is being announced. Researchers at the CNRS and ENS Paris, in collaboration with Monash University (Australia), recently showed that our brains remain attentive even during sleep. Read on to find out why.

It may not seem like we're able to perceive surrounding noises as we sleep. But previous experiments have shown that we are still able to discriminate certain sounds during sleep. For example, we’re more likely to wake up if someone says our own name rather than someone else's. Research so far has mainly focused on the sleeping brain’s ability to process isolated sounds. But these situations aren't representative of our typical diurnal or nocturnal environment. Day and night, we live in rich auditory environments where sounds are layered and mixed. This is what scientists have termed "the cocktail effect." On a daily basis, most of our listening takes place in a noisy environment. This listening is based both on hearing and binaural function, which provide better understanding, improved sound quality and sensation, and allow us to analyze these “auditory scenes.” This analysis involves clumping sounds coming from the same source into a single auditory stream, as well as separating various sound sources into multiple distinct auditory streams. This second process of separating sounds is particularly helpful for understanding speech against a noisy background. This is how we’re able to carry on a conversation at a party where several people are speaking at once (in addition to background music and other noises).

In this study, carried out by Guillaume Legendre and his colleagues at the Department of Cognitive Studies, aimed to develop a protocol that is closer to our ecological environment. The researchers studied the brain responses of participants as they slept, exposing them to two voices speaking simultaneously. The voices were very similar in terms of their acoustic properties, but the content of what was said varied considerably between the two. In one ear, the participants heard a story told in the form of a dialogue, while in the other ear, they heard a stream of meaningless, but familiar sounds.

The team of scientists then discovered that during light sleep (which accounts for almost half of the night), the brain remained attentive to the different sounds, continuing to perceive, process, and filter them in order to identify the most relevant information. The “sleeping” brain is indeed capable of amplifying meaningful speech amidst other irrelevant signals. This ability is very valuable because it allows us to sleep safely and wake up at the right time, whether it's on the bus or in case of danger. These results also illustrate that human beings can pay attention without even being aware of it...

Finally, note that this “standby” mode doesn’t operate during deep sleep, as the brain limits external interferences during this time in order to consolidate memories.

So there’s no need to fear the Sandman, your brain is watching out for you…
Source: Guillaume Legendre, Thomas Andrillon, Matthieu Koroma and Sid Kouider: “Sleepers track informative speech in a multitalker environment”, in Nature Human Behaviour, Janv. 2019 // CNRS website: http://www.cnrs.fr/fr/le-cerveau-endormi-reste-attentif-son-environnement

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