‘What is the secret of consciousness?’ Critical feature of conscious attention disappears during sleep

Scientists from Tel Aviv University have unlocked a major key to the mystery of consciousness. A groundbreaking new study reveals a critical feature of conscious attention disappears when a person is sleeping.

Researchers analyzed data collected from electrodes implanted deep in the human brain of epilepsy patients. These electrodes were monitoring activity in different parts of the brain for diagnosis and treatment. The patients volunteered to help examine the brain’s response to sound stimulation when they were awake compared to when they were sleeping.

Inside Brain. CREDIT: Ana Yael

During the study, researchers discovered the human brain’s response to sound remains powerful during sleep in all parameters except 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,” the researchers explain.

“This study is unique in that it builds upon rare data from electrodes implanted deep inside the human brain, enabling high-resolution monitoring, down to the level of individual neurons,” says Professor Yuval Nir, from the School of Medicine, the Sagol School of Neuroscience, and the Department of Biomedical Engineering, in a statement.

To conduct the sleep study, speakers were placed at the patients’ bedside and emitted various sounds. Researchers collected data from over 700 neurons, about 50 neurons in each patient, over the course of eight years. They compared data of neural activity and electrical waves in different areas of the brain while patients were awake vs. various stages of sleep.

“After sounds are received in the ear, the signals are relayed from one station to the next within the brain. Until recently it was believed that during sleep these signals decay rapidly once they reach the cerebral cortex,” explains study author Dr. Hanna Hayat. “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.”

These alpha-beta waves are linked to processes of attention and expectation that are controlled by feedback from the brain’s higher regions. While signals travel “bottom-up” from sensory organs to higher regions, a “top-down” motion is also happening. The higher regions are sending down signals to instruct the sensory regions which sounds to focus on and which should be ignored. For example, when the ear hears a certain sound, the higher regions can tell whether it is new or familiar, and whether it deserves attention or not.

“Our findings have wide implications beyond this specific experiment,” says Professor Nir. “First, they provide an important key to an ancient, fascinating enigma: What is the secret of consciousness? What is the ‘X-factor,’ the brain activity that is unique to consciousness, allowing us to be aware of things happening around us when we are awake, and disappearing when we sleep? In this study we discovered a new lead, and in future research we intend to further explore the mechanisms responsible for this difference.”

Nir hopes their findings will help develop new methods for measuring the level awareness of individuals who are in various states of unconsciousness.

“In addition, having identified a specific brain feature that is different between states of consciousness and unconsciousness, we now have a distinct quantitative measure – the first of its kind – for assessing an individual’s awareness of incoming sounds. 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,” explains Nir. “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.”

The study is published in the journal Nature.

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