We lose a crucial feature of consciousness while sleeping, an 8-year study reveals

When we dream, something mysterious happens in our brains – we experience something similar to being awake, and yet also very different from being awake, and scientists are still trying to reveal exactly what is going on in that intermediate state.

Now another clue has been discovered. A new study has found that a crucial feature of consciousness – the ability to pay attention to sounds or identify them – is actually turned off while we sleep, and it can help us figure out how our brains dream .

Mapping the brains of living people while both awake and asleep is not easy – few of us would have electrodes implanted in our skulls during our daily activities – but here the team took advantage of the medical research conducted on epilepsy patients.

“We were able to use a special medical procedure in which electrodes were implanted in the brains of epilepsy patients and monitored activity in different parts of their brain for diagnosis and treatment,” says neuroscientist Yuval Nir, from Tel Aviv University in Israel.

“Patients volunteered to help study the brain’s response to auditory stimulation in wakefulness versus sleep.”

The electrodes allowed the researchers to see the differences in the response of the cerebral cortex when patients were in different stages of sleep compared to when they were awake – all the way down to individual neurons.

For the purpose of the study, the researchers played a variety of sounds through speakers at the volunteers’ bedside tables. Data on over 700 neurons (approximately 50 per patient) were collected over eight years.

While the brain’s response to sound largely remained on during sleep, there was an increase in the level of alpha-beta waves – waves associated with attention and anticipation. It seems that incoming sounds are analyzed but not passed on to consciousness.

This contradicts previous thinking: that sound-related signals quickly decay in the brain during sleep. In fact, they remain stronger and richer than we thought, it’s just that there is a significant difference in the way they are treated while we sleep.

“The strength of the brain’s response during sleep was similar to the response observed during wakefulness, with the exception of one specific feature where a dramatic difference was detected: the activity level of alpha-beta waves,” says lead author neuroscientist Hanna Hayat , from Tel Aviv University.

These alpha-beta waves (10-30Hz) are controlled by feedback from higher up in the brain – this feedback (including whether sounds are new or not) helps our minds figure out which sounds are important and need to be listened to.

A similar form of upward shift in alpha-beta wave patterns has previously been observed in patients under anesthesia, but it has not been seen in people who sleep. Researchers describe it as a way to understand the “fascinating riddle” of how the conscious brain differs from the unconscious brain.

This also gives researchers a quantitative and reliable method of measuring whether someone is really unconscious or not: during hospital operations, in comatose people when checking for signs of dementia, and so on.

“Our results have broad implications beyond this specific experiment,” says Nir. “In future research, we intend to explore the mechanisms responsible for this difference.”

The research is published in The neuroscience of nature.

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