The sleeping brain, somewhere between unconsciousness and lucidity

Sleep does not isolate us from the world as we have long thought.

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Better understanding the brain mechanisms and the intermediate states between wakefulness and sleep could allow us to better understand their disruption. (Archival illustration)

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Speech synthesis, based on artificial intelligence, makes it possible to generate spoken text from of a written text.

We are capable, while sleeping, of hearing and understanding words, shows the work of French neuroscientists from the Brain Institute and the Sleep Pathology Department of the Pitié-Salpêtrière hospital in Paris published in the journal Nature Neuroscience (New window) (in English).

Even if it seems familiar to us, because we indulge in it every night, sleep is a very complex phenomenon, explains neurologist Lionel Naccache of the Pitié-Salpêtrière hospital in a press release.

Our research teaches us that waking and sleeping are not stable states: they are both a mosaic of conscious moments… and moments that don't seem to be.

A quote from Lionel Naccache, neurologist at the Pitié-Salpêtrière hospital

For Nadia Gosselin, scientific director of the Center for Advanced Studies in Sleep Medicine (CÉAMS) who did not participate in the study, this work is part of a trend that has appeared in recent years in the study of sleep. We realize that not all regions of the brain are at the same stages at the same time during sleep, explains Nadia Gosselin, who is also a professor in the Department of Psychology at the University of Montreal.

One region of the brain may be in deep sleep, while another region may be awake. This is the appearance of the notion of “local sleep”, which means that not all regions of our brain are asleep at the same time… Which also means that there is #x27;information that we can process while our electroencephalographic activity tells us that we are asleep.

A quote from Nadia Gosselin, CÉAMS

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A man sleeps in a sleep clinic during a polysomnogram. (Archive photo)

To arrive at their findings, French neuroscientists used polysomnography, a technique which not only makes it possible to measure electrical activity in the brain, but also observe eye movement activity, and muscle activity in the chin.

It is from these three variables that we succeed in identifying the stages of sleep, notes Professor Gosselin.

What we measure in the head with an encephalogram is the sum of all the activity that happens in the brain. We can thus attribute a sleep stage to a certain activity.

A quote from Nadia Gosselin, CÉAMS

The technique thus made it possible to directly observe the behavioral reactivity of 49 participants:

Narcoleptics have the particularity of easily and quickly achieving paradoxical sleep ( the stage where lucid dreaming occurs in which the sleeper is aware that he is dreaming, while he is asleep) during the day, which makes them good candidates for studying consciousness during sleep in experimental conditions .

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A woman sleeps during a polysomnogram in a sleep laboratory. (File photo)

Participants in both groups were asked to take a nap during which a human voice spoke a series of real words and made-up words. Sleepers were invited to react by smiling or frowning to be classified into one or other of these categories.

This way of interacting with participants is new, explains Professor Gosselin. Usually, people are asked to press something during sleep, and then at some point they stop responding. Instead, they were asked to respond with facial muscles. They were also asked to cognitively process the information, observes the professor.

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During the entire duration of the experiment one hour and thirty minutes, the participants were followed by polysomnography.

Then, upon waking up, participants were asked to report whether or not they had had a lucid dream during their nap, and whether they remembered interacting with anyone.

The data collected in the study clearly show that sleepers can respond to auditory stimuli. Most of the participants, whether they were narcoleptic or not, managed to respond correctly to verbal stimuli while remaining asleep, notes Isabelle Arnulf, one of the authors of the work, in a press release.

These events were certainly more frequent during lucid dream episodes, characterized by a high level of consciousness, but we have them observed occasionally in both groups, during all phases of sleep.

A quote from Isabelle Arnulf, Pitié-Salpêtrière Hospital

The researchers believe the results of their experiment suggest that there are transient windows of responsiveness to external stimuli during true sleep.

This new knowledge could possibly contribute, the researchers believe, to revising the current definition of sleep, a state that is ultimately very active, perhaps more conscious than we imagined, and open to the world and to others.

< p class="StyledBodyHtmlParagraph-sc-48221190-4 hnvfyV">In addition, this work suggests that it is possible to develop standardized communication protocols with sleeping people to better understand how mental activity changes during sleep. They also suggest that it would be possible to have access to the cognitive processes on which normal sleep, but also several sleep disorders, are based.

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While 25% of Canadians experience symptoms of insomnia, approximately 10% have problems with chronic insomnia.

Better understanding of the brain mechanisms and the intermediate states between wakefulness and sleep could allow us to better understand their disruption and explain sleep disorders such as insomnia and sleepwalking.

Currently, to distinguish wakefulness and different stages of sleep, researchers use simple and imprecise physiological indicators, such as brain waves made visible thanks to electroencephalographies.

Neuroscientist Delphine Oudiette of the Brain Institute, one of the main authors of the work, explains that these indicators do not allow us to grasp in detail what is happening in the heads of sleepers, since they are sometimes in contradiction with their testimony.

We need finer physiological measurements that are aligned with the sleeper's feelings; this in order to better define one's level of vigilance.

A quote from Delphine Oudiette, Brain Institute

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This work also established that it is possible to anticipate the moments when sleepers are more inclined to respond to stimuli. Indeed, the pooling of different data (physiological, behavioral and participants' responses to a questionnaire) made it possible to establish that the opening of these connection windows is preceded by an acceleration of brain activity, and by certain physiological indicators associated with intense cognitive activity.

For Nadia Gosselin, the authors of the work would have benefited from questioning the participants about the quality of their sleep.< /p>

We didn't ask them if they slept well. Most people, when answering questions, don't feel like they're asleep. But in this study, we do not know if they had restful sleep.

A quote from Nadia Gosselin, CEAMS

Is this that we can really process information and have restful sleep? We cannot say it with this work, she summarizes.

An observation that the authors of the study will want to raise in future work, since they will try to determine if the multiplication of connection windows is linked to the quality of the sleep, and if they can be used to treat sleep disorders.

More advanced neuroimaging techniques, such as magnetoencephalography and intracranial recording of brain activity, will help us better understand the brain mechanisms that orchestrate sleeper behaviors.

A quote from Delphine Oudiette, Institut du brain

For Nadia Gosselin, this work will certainly stimulate research in a field which, in addition to being linked to many sleep disorders, is associated with several heart and neurodegenerative diseases, including Alzheimer's.

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