What happens to our consciousness when we sleep? Scientists have unlocked part of this mystery in a groundbreaking study using brain implants to analyse people as they sleep.
The researchers from Tel Aviv University worked with epilepsy patients who had electrodes implanted deep in their brains.
The researchers analysed how brains responded to sounds when they are asleep – and found that the brain’s response to sound remains powerful even when asleep.
But the alpha-beta waves associated with the sound were ‘switched off’, according to scientists.
The researchers believe that during sleep, the brain analyses the auditory input but is unable to focus on the sound or identify it, and therefore no conscious awareness ensues.
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Professor Yuval Nir said: "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, of the brain's electrical activity.
“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 utilise 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 from the implanted electrodes – neural activity and electrical waves in different areas of the brain – during wakefulness versus various stages of sleep.
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Altogether, the team collected data from more than 700 neurons, about 50 neurons in each patient, over the course of either years.
Dr Hanna Hayat said: "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.
"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 researchers explain that alpha-beta waves (10-30Hz) are linked to processes of attention and expectation that are controlled by feedback from higher regions in the brain.
As signals travel 'bottom-up' from the sensory organs to higher regions, a 'top-down' motion also occurs: the higher regions, relying on prior information that had accumulated in the brain, act as a guide, sending down signals to instruct the sensory regions as to which input to focus on, which should be ignored, etc.
When a certain sound is received in the ear, the higher regions can tell whether it is new or familiar, and whether it deserves attention or not.
This kind of brain activity is manifested in the suppression of alpha-beta waves, and indeed, previous studies have shown a high level of these waves in states of rest and anaesthesia.
The new study shows that the strength of alpha-beta waves is the main difference between the brain's response to auditory inputs in states of wakefulness vs. sleep.
Prof Nir said: "Our findings have wide implications beyond this specific experiment. 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."
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