When your brain sleeps, the orchestra plays but the conductor is missing

We spend approximately one-third of our lives sleeping, but why sleep is important is a big unanswered question, one which science has only begun to answer recently. We now know, for example, that the brain cleans itself while we sleep, and that long-term memories form during the rapid eye movement (REM) stage of sleep.

Your brain is highly active during sleep

Sleep can be defined as a temporary state of unconsciousness, during which our responses to the outside world are reduced. Yet, we also know that the brain is active during sleep, and there is growing evidence that it remains highly responsive: For instance, your sleeping brain will respond to your name, categorize words and then prepare appropriate actions, and even learn new information.

Now, a new study by researchers at UCLA and Tel Aviv University shows that the human brain remains highly responsive to sound during sleep, but it does not receive feedback from higher order areas — sort of like an orchestra with “the conductor missing.” The findings could point to a better understanding of the extent to which the brain processes information in disorders of consciousness such as coma and vegetative states, and to the neural mechanisms of conscious awareness.

The missing conductor

Hanna Hayat and her colleagues had the rare opportunity to record the activity of cells directly from the brains of 13 patients with drug-resistant epilepsy, who were being evaluated for brain surgery and gave written consent to participate in the study during the evaluation. The researchers implanted depth electrodes in multiple regions of the patients’ brains, primarily to identify the source of their seizures, so that the abnormal tissue could be surgically removed. Over the course of eight overnight sessions and six daytime naps, they played various sounds — including words, sentences and music — to the patients through bedside loudspeakers. They also used standard electroencephalogram (EEG) to monitor the patients’ sleep stages and recorded their sleep behavior with video. 

Hayat and her colleagues report, in the journal Nature Neuroscience, that the patients’ brains responded to the sounds in much the same way during sleep and wakefulness. In both states, the sounds evoked rapid and robust electrical activity, as well as high frequency gamma waves (80-200 Hz, or cycles per second) across certain regions of the temporal lobe, which are associated with the processing of auditory information. These “high gamma power responses” were only slightly smaller to responses obtained to the same sounds when they were played to the patients while they were awake.

There was, however, one important difference. When the patients were awake, but not while they slept, the sounds also evoked a more widespread and later, lower frequency response (10-30 Hz) referred to as desynchronization, which is thought to be associated with neural feedback processing from “higher order” brain regions, in both the auditory and visual pathways.

This reduced neural feedback appears to be a characteristic feature of sleep. The source of these feedback signals is still unclear, but the researchers speculate that they may originate in the frontal lobe, parietal lobe, or the thalamus, which processes sensory information before relaying it to relevant areas of the cerebral cortex.