Day or night — your brain is always listening

You're fast asleep. But your brain isn't taking the night off, according to new research funded by the U.S. National Science Foundation.

Researchers studied activity in the human cerebral cortex in response to music and other sounds. They observed vigorous auditory responses in the sleeping brain, similar in many ways to responses in the wakeful state but differing in a key component. While the waking state is characterized by ongoing feedback signals in the brain as it attends to and interprets incoming sounds, researchers found that those signals are greatly reduced during sleep.

The findings suggest that these feedback brain signals are key to conscious sensory processing and could lead to a better understanding of how people process sensory information when they are unconscious.

"The neuronal orchestra is never shut off from the environment when the person is deep asleep," said Itzhak Fried, a co-author of the study. "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 researchers monitored the brain activity of epilepsy patients through implanted electrodes. The electrodes were placed to identify the areas in the brain where the patients' seizures were occurring before they underwent neurosurgery to cure their epilepsy. The patients who agreed to participate in the study had a nearby speaker that would play music and words and measured the response when the patients were awake and when they were asleep.

Brain cells in the primary auditory cortex responded to the stimulation during sleep but higher brain regions showed subdued responses.

"That's probably why we are still not conscious, although we are still processing the sensory information from the external world,” Fried said. “You're not completely shut from the environment in that sense.” The presence of auditory responses during sleep could also lead to ways to improve memory during sleep when the brain processes incoming and recent information, as shown in parallel studies also supported by NSF.

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