A new discovery from Tel Aviv University may provide a key to a great scientific enigma: How does the awake brain transform sensory input into a conscious experience? The study relied on data collected from electrodes implanted, for medical purposes, deep in the human brain. The information was utilized to examine differences between the response of the cerebral cortex to sounds in sleep vs. wakefulness, at a resolution of single neurons.
The researchers were surprised to discover that the brain’s response to sound remains powerful during sleep in all parameters but 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 paper was published in the journal Nature Neuroscience.
“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,” says study contributor Yuval Nir from the School of Medicine, the Sagol School of Neuroscience, and the Department of Biomedical Engineering, in a statement.
“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 utilize 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,” Nir says.
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 vs. various stages of sleep. Altogether, the team collected data from over 700 neurons, about 50 neurons in each patient, over the course of 8 years.
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. Thus, for example, 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 anesthesia. According to the current study, the strength of alpha-beta waves is the main difference between the brain’s response to auditory inputs in states of wakefulness vs. sleep.
Nir summarizes: “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.”
Image: Ana Yael