Direct recording shows brain signal persists even in dreamless sleep

Neuroscientists at Washington University School of Medicine in St. Louis have taken one of the first direct looks at one of the human brain's most fundamental "foundations": a brain signal that never switches off and may support many cognitive functions.

The results, appearing online this week in the Proceedings of the National Academy of Sciences, are an important step forward for efforts to outline what neuroscientists call the functional architecture of the brain. Better understanding of this architecture will aid efforts to treat brain injury and mental disorders.

Although the brain's different specialized regions can be considered as a collection of physical structures, functional architecture instead focuses on metaphorical structures formed by brain processes and interactions among different brain regions. The "foundation" highlighted in the new study is a low-frequency signal created by neuronal activity throughout the brain. This signal doesn't switch off even in dreamless sleep, possibly to help maintain basic structure and facilitate offline housekeeping activities.

"A different, more labile and higher-frequency signal known as the gamma frequency activity has been the focus of much brain research in recent years," says first author Biyu He, a graduate student. "But we found that signal loses its large-scale structure in deep sleep, while the low-frequency signal does not, suggesting that the low-frequency signal may be more fundamental."

"What we've been finding is reorienting the way we think about how the brain works," says senior author Marcus Raichle, M.D., professor of radiology, of neurology and of neurobiology. "We're starting to see the brain as being in the prediction business, with ongoing, organized carrier frequencies within the systems of the brain that keep them prepared for the work they need to do to perform mental tasks."

Neurologists have already spent many years exploring the upper levels of the brain's functional architecture. In these studies, researchers typically ask volunteers to perform specific mental tasks as their brains are scanned using functional magnetic resonance imaging (fMRI). Such "goal-oriented" tasks might include looking for or studying a visual stimulus, moving an arm or leg, reading a word or listening for a sound. As the subjects perform these tasks, the scans reveal increases in blood flow to different parts of the brain, which researchers take as indications that the brain areas are contributing to the mental task.

In the past decade, though, scientists have realized that deeper structures underlie goal-oriented mental processes. These underlying brain processes continue to occur even when subjects aren't consciously using their brain to do anything, and the energies that the brain puts into them seem to be much greater than those used for goal-oriented tasks.

"The brain consumes a tremendous amount of the body's energy resources—it's only 2 percent of body weight, but it uses about 20 percent of the energy we take in," says Raichle. "When we started to ask where all those resources were being spent, we found that the goal-oriented tasks we had studied previously only accounted for a tiny portion of that energy budget. The rest appears to go into activities and processes that maintain a state of readiness in the brain."