Researchers have just demonstrated that our brain can track the sounds in its environment while we sleep, and favor the most relevant ones. They found this by exposing sleepers to complex sounds. The study was published in journal Nature Human Behaviour on January 14, 2019.
The research was conducted by researchers from the CNRS and ENS Paris1, in collaboration with Monash University (Australia).
The human brain may relax when asleep. But it doesn’t lose all responsiveness and awareness. According to the researchers, this ability could be one of the mechanisms which allow us to sleep in safety and then wake up at the right moment. Therefore, it can be tempting to take a nap on a train or on a bus.
But how to make certain that you are not going to miss your stop? Sleep seems to be accompanied by a loss in capacity to observe and interact with environment.
Yet former experiments have revealed that certain sounds are perceived even during sleep. For example, a sleeping person tends to wake up more easily at the sound of his first name instead of the name of any other person. Until present, research had focused on the capacity of sleeping brain to process inaccessible sounds.
However, this situation is not typical of everyday life as we often sleep, both during the day and at night, in rich auditory environments where various sounds are mixed with one another. Upon waking, automatically individuals tend to concentrate on the source which makes sense.
Brain activity during sleep
In this study, scientists identified the intellectual responses made while sleeping by a number of participants. They were simultaneously exposed to two voices which were very similar in their aural properties.
But highly different in terms of meaning. Like, one pronounced quotes from dialogues or articles, while the other one pronounced some words resembling French, but devoid of any meaning.
Scientists then used a method which can recreate what the sleepers hear on the basis of their brain activity. Then, they were able to approve that during light sleep, participants preferred the meaningful messages.
Thus, even while sleeping and unconscious, the brain records nearby sounds. It splits various acoustic sources and selects those that are the most logical and understandable.
This capacity to concentrate on relevant things is temporary, as it includes only slow-wave and light sleep. During this sleep stage, the brain seems proficient of processing information from the outside world. But only for short periods of time. Sleeping on a bus or train is not so awkward if you do it with just one ear closed.
The activity of different parts of the brain during sleep can give a sign to the functions of sleep.
Researchers have found that mental activity is existing during all stages of sleep, though from different brain regions. So, the brain never totally shuts down during sleep. Also, sleep intensity of a specific region is homeostatically related to the corresponding amount of activity before sleeping.
The use of imaging modalities such as PET and fMRI, along with EEG recordings, identifies the brain regions which participate in creating the typical wave signals and what their roles might be.