Summary: A study of the cerebral cortex discovered there is a robust response to sound during sleep that largely mirrored the brain’s response during wakefulness. However, differences in brain waves that help the brain understand sound and anticipate what comes next is missing during sleep.
You’re fast asleep. But some regions of your brain tasked with hearing sound aren’t taking the night off, according to new research from scientists at UCLA and Tel Aviv University.
A unique study of brain activity in the cerebral cortex of epilepsy patients found there was a robust response to sound during sleep that largely mirrored the brain’s response during wakefulness. However, there was one key difference from wakefulness, namely in the level of alpha-beta waves.
The attenuation of these waves characterizes the awake state and indicate neural feedback from higher brain centers helping to understand sound and anticipate what may come next. This was the key factor lacking in sleep.
“The neuronal orchestra is never shut from the environment when the person is deep asleep,” said Dr. Itzhak Fried, a study co-author and director of UCLA’s Epilepsy Surgery Program.
“The neurons are like musicians playing Mozart, each one with great fidelity and volume. Only the conductor, the one who monitors performance and leads expectations, is missing.”
The study was published online Monday in the journal Nature Neuroscience.
Fried, who in previous research has extensively studied the brain’s activity during wake and sleep, said the findings could help us understand to what extent information is being processed by people in unconscious states, such as comatose patients or those under anesthesia. They may also point to ways, possibly by auditory stimulation, of enhancing memory during sleep, when the brain consolidates recent information.
Researchers had an unusually up-close view into the activity of single brain cells in patients with severe epilepsy through electrodes that were implanted in their brains to identify where seizures were occurring for potential curative surgery.
Patients at UCLA and Tel Aviv Sourasky Medical Center who agreed to participate in the study were set up with bedside speakers that played words and music when the patients were awake and listening, as well as sound asleep. Fittingly, one of the musical choices in the study was Mozart’s “Eine kleine Nachtmusik,” or “A Little Night Music.”
Over 7 years, the team collected data from over 700 neurons during wakefulness and different stages of sleeping, allowing them to compare neuronal activity and brain waves. Brain cells in the primary auditory cortex responded most vigorously during sleep, but there was a decline in the “top-down” neural feedback from higher brain regions that mediate attention and expectation.
“That’s probably why we are still not conscious, although we are still processing the sensory information from the external world. So you’re not completely shut from the environment in that sense,” Fried said.
Other study authors include lead author Dr. Hanna Hayat, Amit Marmelshtein, Dr. Aaron Krom and Dr. Yaniv Sela from the group of Dr. Yuval Nir, and Dr. Ido Strauss and Dr. Firas Fahoum from the Tel Aviv Sourasky Medical Center.
About this sleep and auditory neuroscience research news
Author: Jason Millman
Contact: Jason Millman – UCLA
Image: The image is in the public domain
Original Research: Open access.
“Reduced neural feedback signaling despite robust neuron and gamma auditory responses during human sleep” by Itzhak Fried et al. Nature Neuroscience
Reduced neural feedback signaling despite robust neuron and gamma auditory responses during human sleep
During sleep, sensory stimuli rarely trigger a behavioral response or conscious perception. However, it remains unclear whether sleep inhibits specific aspects of sensory processing, such as feedforward or feedback signaling.
Here, we presented auditory stimuli (for example, click-trains, words, music) during wakefulness and sleep in patients with epilepsy, while recording neuronal spiking, microwire local field potentials, intracranial electroencephalogram and polysomnography.
Auditory 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 only moderately attenuated response magnitudes, mainly affecting late responses beyond early auditory cortex and entrainment to rapid click-trains in NREM sleep. By contrast, auditory-induced alpha–beta (10–30 Hz) desynchronization (that is, decreased power), prevalent in wakefulness, was strongly reduced in sleep.
Thus, extensive auditory responses persist during sleep whereas alpha–beta power decrease, likely reflecting neural feedback processes, is deficient. More broadly, our findings suggest that feedback signaling is key to conscious sensory processing.