Sleep offers a master key to the mystery of consciousness


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Summary: During sleep, the brain analyzes the acoustic input but is unable to focus on the sound or identify the sound; therefore, no conscious perception of the stimuli occurs.

Source: Tel Aviv University

A new discovery from Tel Aviv University could provide a key to a major scientific puzzle: How does the waking brain convert sensory input into a conscious experience?

The groundbreaking study relied on data collected from electrodes implanted deep in the human brain for medical purposes.

The information was used to examine differences between the cerebral cortex’s response to sounds during sleep and while awake at single neuron resolution.

The researchers were surprised to discover that the brain’s response to sound during sleep remains strong in all parameters except one: the level of alpha-beta waves, which is associated with attention to auditory input and associated expectations are.

This means that during sleep the brain analyzes the acoustic input but cannot focus on or identify the sound and therefore no conscious perception occurs.

The study was conducted by Dr. Hanna Hayat and with significant contribution from Dr. Amit Marmelshtein in the lab of Prof. Yuval Nir of the School of Medicine, Sagol School of Neuroscience and Department of Biomedical Engineering, and co-supervised by Prof. Itzhak Fried of UCLA Medical Center. Other participants were: Dr. Aaron Krom and Dr. Yaniv Sela from the group of Prof. Nir and Dr. Ido Strauss and Dr. Firas Fahoum from Tel Aviv Sourasky Medical Center (Ichilov).

The paper was published in the prestigious journal nature neuroscience.

Prof. Nir: “This study is unique in that it builds on rare data from electrodes implanted deep in the human brain, allowing for high-resolution monitoring of the brain’s electrical activity down to the level of individual neurons.

“For understandable reasons, electrodes cannot be implanted in living human brains just for the purpose of scientific research. But in this study, we were able to use a special medical procedure that involved implanting electrodes into the brains of epilepsy patients to monitor activity in different parts of their brains for diagnosis and treatment purposes.

“The patients volunteered to help study the brain’s response to acoustic stimulation during wakefulness and sleep. “

The researchers placed speakers emitting different sounds at the patients’ bedsides and compared data from the implanted electrodes – neural activity and electrical waves in different areas of the brain – during wakefulness with different stages of sleep. In total, the team collected data from over 700 neurons over a period of 8 years, about 50 neurons in each patient.

dr Hayat: “After tones are received in the ear, the signals are passed from one station to the next in the brain. Until recently, it was believed that these signals decay rapidly during sleep once they reach the cerebral cortex. But when we looked at the data from the electrodes, we were surprised to find that the brain’s response during sleep was much stronger and more comprehensive than we had anticipated.

“Moreover, this strong response spread to many regions of the cerebral cortex. The magnitude of the brain response during sleep was similar to that observed during wakefulness, except for one specific feature where a dramatic difference was noted: the level of alpha-beta wave activity.”

The researchers explain that alpha-beta waves (10-30 Hz) are linked to attentional and expectancy processes that are controlled by feedback from higher regions in the brain.

When signals travel “bottom-up” from the sense organs to higher regions, there is also a “top-down” movement: the higher regions, relying on prior information accumulated in the brain, act as guides and send signals downward instructing the sensory regions on which inputs to focus, which to ignore, etc.

So, for example, when a particular sound is received in the ear, the higher regions can tell if it’s new or familiar and if it deserves attention or not. This type of brain activity manifests itself in the suppression of alpha-beta waves, and indeed previous studies have shown high levels of these waves in states of rest and anesthesia.

According to the current study, the strength of alpha-beta waves is the main difference between the brain’s response to acoustic input while awake and during sleep.

Prof. Nir concludes: “Our results have far-reaching implications beyond this particular experiment. First, they provide an important key to an ancient, intriguing mystery: what is the secret of consciousness? What is the “X Factor,” the brain activity that is unique to consciousness and allows us to be aware of things happening around us when we are awake and disappears when we are asleep?

“In this study, we discovered a new clue, and in future research we intend to further explore the mechanisms responsible for this difference.

This shows a drawing of a woman sleeping in the water
This means that during sleep the brain analyzes the acoustic input but cannot focus on or identify the sound and therefore no conscious perception occurs. Photo credit: Ana Yael

“Additionally, having identified a specific brain feature that distinguishes between conscious and unconscious states, we now have a clear quantitative measure – the first of its kind – to assess a person’s perception 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 different situations: checking whether patients remain unconscious during a surgery, monitoring the consciousness of people with dementia, or determining whether a supposedly comatose person who is unable to communicate is really unaware of their surroundings.

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“In such cases, low levels of alpha-beta waves in response to sound could indicate that a person presumed to be unconscious is actually sensing and understanding the words being said around them.

“We hope that our results will serve as a basis for the development of effective new methods for measuring the level of consciousness of people allegedly in different states of unconsciousness. “

About this news from consciousness research

Author: Noga Shahar
Source: Tel Aviv University
Contact: Noga Shahar – Tel Aviv University
Picture: The picture is attributed to Ana Yael

Original research: Open access.
“Reduced neural feedback signals despite robust neuron and gamma auditory responses during human sleep” by Hanna Hayat et al. nature neuroscience


Reduced neural feedback signals despite robust neuron and gamma auditory responses during human sleep

During sleep, sensory stimuli rarely elicit a behavioral response or conscious awareness. However, it remains unclear whether sleep inhibits certain aspects of sensory processing, such as feedforward or feedback signaling.

Here, we presented auditory stimuli (e.g., click-trains, words, music) during wakefulness and sleep in patients with epilepsy while recording neuronal spiking, microwire local field potentials, intracranial electroencephalogram, and polysomnography.

Acoustic stimuli induced robust and selective spiking and high-gamma (80–200 Hz) power responses across the lateral temporal lobe during both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Sleep moderately dampened response magnitudes, mainly affecting late responses beyond the early auditory cortex and entrainment to fast click-trains in NREM sleep.

In contrast, the auditory-induced alpha-beta (10–30 Hz) desynchronization (i.e., reduced power) that was prevalent during wakefulness was greatly reduced during sleep. Therefore, extensive auditory responses persist during sleep, while the decrease in alpha-beta output, which probably reflects neural feedback processes, is insufficient.

More broadly, our results suggest that feedback signaling is key to conscious sensory processing.

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