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Neuroscientists: We hear when we are asleep — but we don’t listen

The new finding may help determine whether an apparently unconscious or demented person can actually understand what is said to him

Earlier this week, we talked about the fact that the human nose is much more sensitive than we sometimes think. Our sense of smell gets ignored in favor of visual, auditory, or symbolic information — but it’s still there.

Tired man sitting in car, wearing in sleeping mask

The same goes with our hearing when we are asleep, researchers say:

The researchers were surprised to discover that the brain’s response to sound remains powerful during sleep in all parameters but one: the level of alpha-beta waves associated with attention to the auditory input and related expectations. This means that during sleep, the brain analyzes the auditory input but is unable to focus on the sound or identify it, and therefore no conscious awareness ensues.

Tel-Aviv University, “During sleep the brain’s reaction to sound remains strong, but one critical feature of conscious attention disappears” at ScienceDaily (July 12, 2022) The paper is open access.

The researcher had more than the usual trouble finding subjects because they needed people who getting surgery done for medical reasons anyway before they planted electrodes deep in the brain. And it took a while:

The researchers placed speakers emitting various sounds at the patients’ bedside and compared data from the implanted electrodes — neural activity and electrical waves in different areas of the brain — during wakefulness vs. various stages of sleep. Altogether, the team collected data from over 700 neurons, about 50 neurons in each patient, over the course of 8 years.

Tel-Aviv University, “During sleep the brain’s reaction to sound remains strong, but one critical feature of conscious attention disappears” at ScienceDaily (July 12, 2022) The paper is open access.

And their patience was rewarded:

Until recently it was believed that during sleep these signals decay rapidly once they reach the cerebral cortex. But looking at the data from the electrodes, we were surprised to discover that the brain’s response during sleep was much stronger and richer than we had expected. Moreover, this powerful response spread to many regions of the cerebral cortex. The strength of brain response during sleep was similar to the response observed during wakefulness, in all but one specific feature, where a dramatic difference was recorded: the level of activity of alpha-beta waves.”

Tel-Aviv University, “During sleep the brain’s reaction to sound remains strong, but one critical feature of conscious attention disappears” at ScienceDaily (July 12, 2022) The paper is open access.

Those are the brain waves associated with attention and expectation. Because the subjects’ brains were receiving information but not doing anything with it, the waves flowed freely. When we are paying attention, however, many such waves of them are suppressed. Think of listening to a caller on a cell tphone while standing on a noisy platform. We suppress our awareness of the surrounding din. If we had nothing else to think about, we would notice it much more.

The researchers hope that their find will help with determining — in doubtful cases — whether people are conscious or not:

We hope that in the future, with improved techniques for measuring alpha-beta brain waves, and non-invasive monitoring methods such as EEG, it will be possible to accurately assess a person’s state of consciousness in various situations: verifying that patients remain unconscious throughout a surgical procedure, monitoring the awareness of people with dementia, or determining whether an allegedly comatose individual, unable to communicate, is truly unaware of his/her surroundings. In such cases, low levels of alpha-beta waves in response to sound could suggest that a person considered unconscious may in fact perceive and understand the words being said around him.

Tel-Aviv University, “During sleep the brain’s reaction to sound remains strong, but one critical feature of conscious attention disappears” at ScienceDaily (July 12, 2022) The paper is open access.

Around the turn of the century, neuroscientist Adrian Owen began studying people who were in a persistent vegetative state (PVS). He discovered that, contrary to widespread belief, they could often understand and respond to questions.

The more we know about brain waves during periods of unconsciousness — actual or apparent — the more precisely we can pinpoint the problems. But the curious thing is, so much of neuroscience in these matters has been learning what we “know” that ain’t so. Keeps it fun though.

Note: The featured photo is courtesy volant on Unsplash.


You may also wish to read:

The nose really does know, it turns out… But we usually don’t notice. Our sense of smell may have declined in recent millennia but it is sharper than we think. Losing one’s sense of smell — as with COVID — is dangerous as well as traumatic but, relative to other senses, we don’t yet know much about how it works.


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Neuroscientists: We hear when we are asleep — but we don’t listen