Even though sleep medicine clinics abound around the world, what happens during our slumber remains mysterious because we are not awake to tell about it. 

A new study at Tel Aviv University (TAU) has nevertheless found that the brain’s reaction to sound remains strong during sleep in all but one parameter – the level of alpha-beta waves known to be connected with attention. This means that in sleep the brain perceives auditory (hearing) input but is unable to focus on the sounds to consciously identify them. 

The findings provide a first-of-its-kind quantitative measure that is unique to people who are awake, listening and able to understand what they hear, and disappears during sleep. The paper was published in the prestigious journal Nature Neuroscience under the title “Reduced neural feedback signaling despite robust neuron and gamma auditory responses during human sleep.” 

The discovery may provide a key to a great scientific puzzle – How does the awake brain transform sensory input into a conscious experience? The groundbreaking study relied on data collected from electrodes implanted deep in the human brain. This information was used to examine differences between the response of the cerebral cortex to sounds in sleep vs. wakefulness, at a resolution of single neurons.

The study was led by Dr. Hanna Hayat and with major contribution from Dr. Amit Marmelshtein, at the lab of Prof. Yuval Nir from TAU’s School of Medicine and the Sagol School of Neuroscience,and the department of biomedical engineering and co-supervised by Prof. Itzhak Fried from the University of California at Los Angeles Medical Center. 

“This study is unique in that it builds upon rare data from electrodes implanted deep inside the human brain of the brain’s electrical activity,” said Nir. “For understandable reasons, electrodes cannot be implanted in the brain of living humans just for the sake of scientific research. But in this study, we were able to use a special medical procedure in which electrodes were implanted in the brains of epilepsy patients, monitoring activity in different parts of their brain for purposes of diagnosis and treatment. The patients volunteered to help examine the brain’s response to auditory stimulation in wakefulness vs. sleep.”

The researchers placed speakers emitting various sounds at the patients’ bedside and compared data – neural activity and electrical waves in different areas of the brain – from the implanted electrodes 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 eight years.

“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,” added 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.”

The findings have wide implications beyond this specific experiment. Not only do they provide the key to the secret of consciousness, but also to 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,” said Nir.

 “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 including 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.”