Where does our consciousness go in sleep?

What else do we notice in our sleep?

When we sleep, we hardly notice anything of the outside world - at least not consciously. But subconsciously, our brain registers what is going on around us, as an experiment has now confirmed. Using brain wave measurements, the researchers were able to observe that our brain muffles nonsensical pseudo-speech sounds during sleep, but prefers to process meaningful, informative language. This suggests that our thinking organ still “listens” and evaluates acoustic stimuli even while sleeping. The experiment also reveals, however, that this filter switches over in phases during dream sleep - and then just attenuates the informative stimuli. Why is still open.

We spend about a third of our lives in sleep. During this time our active consciousness is switched off. We are no longer in control of what is happening and the brain goes its own way. While the body is at rest, however, a lot is going on in our mind. The brain flushes out waste materials, trims synapses so that they can react to new stimuli and experiences during the day, and it sorts our memory contents. Studies show that sleep is even essential for learning. So that the brain is not disturbed during this time, we only perceive stimuli from the outside world to a limited extent during sleep. Only alarm signals such as the alarm clock or a scream get through. But so far it has only been partially clarified how this selective suppression of external stimuli works. Is there a filter that does not even send a large part of the acoustic signals to the higher-level regions of the cerebral cortex? Or are all incoming signals still evaluated and processed despite the apparent suppression?

Language in the sleep test

Matthieu Koroma from the Sorbonne University in Paris and his colleagues have now investigated this more closely in an experiment. The focus of her interest was REM sleep (REM = Rapid Eye Movement). In this sleep phase, also known as dream sleep, our muscles are completely paralyzed, only the eyes move quickly behind the lids in phases. Studies suggest that these movements are closely linked to dreaming: Similar to the waking state, the eyes follow the dreamed images and events. To find out whether and how strongly our brain reacts to acoustic stimuli during REM sleep, the researchers asked 18 overtired test subjects to take a morning nap. During the fall asleep phase and throughout sleep, however, there was a special background noise: the participants heard a voice in one ear that was talking about various things. The other ear, on the other hand, was exposed to senseless pseudo-language.

The scientists tracked whether and how these speech stimuli were processed by the brain using an electroencephalogram (EEG), which recorded the brain waves of the participants. A special algorithm had previously been trained to recognize and differentiate between the specific brain signals when processing pseudo-language and real acoustic information. The EEG recordings revealed that when falling asleep and when sleeping lightly, both signals continue to be registered and processed by the brain. As in the waking phase, the brain selectively amplified the meaningful and thus potentially relevant real language and rather dampened the nonsensical chatter of the pseudo-language. This selective response to acoustic stimuli was also retained in parts of REM sleep.

Selective perception even in dream sleep

“Our results initially confirm that auditory stimuli are still processed by the sleeping brain,” say Koroma and his colleagues. Instead of blocking or filtering out all acoustic signals at lower levels of processing, the brain apparently listens so closely even during sleep that it can distinguish meaningful from meaningless information. “This indicates that associative brain regions are also involved in this selective processing, which can sort out the stimuli based on their information content,” the researchers explain. “This refutes the assumption of a filter at the level of the thalamus or cortex.” However, as the EEG analysis revealed, the brain apparently changes its reaction to external speech stimuli during REM sleep. Because the meaningful language was treated preferentially and strengthened during a large part of this sleep phase. But that changed abruptly when the rapid eye movements began, as the scientists observed.

As soon as the eyes of the sleeping subjects began to move quickly, the meaningful language in the brain was no longer amplified and processed preferentially. Instead, the brain now specifically suppressed this acoustic information. The signals of the pseudo-speech, however, remained largely the same during the entire REM sleep phase. “These analyzes show that periods of sustained eye movement prevent the preferential processing of informative language,” report Koroma and his colleagues. “This is the first evidence of a selective stimulus suppression mechanism during eye movement.” It is still unclear why the brain keeps out information from outside in this phase of REM sleep. The researchers suspect, however, that this may help the brain to concentrate better on internal processes. Because it is precisely in these eye movement phases that dreaming is particularly intense, which suggests that the brain is very active.

"Such suppressive mechanisms of sensory stimuli could serve to protect sleep and ensure that informative signals do not disrupt internal activity," the scientists speculate. Further studies must now clarify whether this assumption is correct.

Source: Matthieu Koroma (Sorbonne Universit´2, Paris) et al., Current Biology, doi: 10.1016 / j.cub.2020.04.047

19th May 2020

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