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The Sleeping Brain Yields Clues to the Conscious Mind

For release: Monday, July 27, 2009

Two gray fMRI scans are shown of a person's brain during rest and deep sleep. Activated regions of the DMN are shown in yellow. During deep sleep, these activated regions become desynchronized.

Recent studies have used brain imaging to identify the parts of our brains that underlie emotions from love to disgust, and behaviors from solving math problems to solving moral problems.

Now, NIH scientists are probing the very basis of conscious thought by examining differences in brain activity between wakefulness and sleep.

Suppose that a researcher wants to probe the neural basis of a specific behavior like gambling.  In a typical experiment, the researcher might ask subjects to play a betting game while their brain activity is recorded by functional magnetic resonance imaging (fMRI).  That kind of study has been used to demonstrate what researchers call task-related activity in a variety of different brain regions.

Several years ago, researchers made the intriguing discovery that some brain regions consistently become less active during task performance and more active when people are resting quietly.  These brain regions comprise a circuit now known as the default mode network (DMN).

The significance of the DMN is unclear, but one theory holds that it is active while people are reminiscing about the past, imagining the future, or just letting their minds wander.  In other words, DMN activity might represent conscious thought.  In the new study, scientists at National Institute of Neurological Disorders and Stroke (NINDS) tested that idea by looking for DMN activity in people who had fallen into a deep sleep.

“We wanted to test whether the DMN would change when the level of consciousness changes,” said Silvina Horovitz, Ph.D., a staff scientist in the NINDS Motor Control section.  At the time of the study, Dr. Horovitz was a research fellow in the NINDS Advanced MRI section, headed by Jeff Duyn, Ph.D.

The results, published in the Proceedings of the National Academy of Science*, show that during deep sleep, the DMN loses the signature patterns of activity that it has during rest.  The study supports the idea that the DMN plays a role in conscious experience, the researchers say.

The study involved monitoring the brains of sleeping subjects using both fMRI and electroencephalography (EEG), a technique in which surface electrodes are placed on the scalp.  The fMRI was used to monitor DMN activity, and EEG was used to look for electrical signals in the brain that mark the descent into deep sleep.  The study took place at the NIH in vivo Nuclear Magnetic Resonance facility in Bethesda, Maryland.

Deep sleep does not come easy inside a noisy MRI scanner.  So the scans were done between 2:00-6:00 a.m., after the subjects had stayed awake for 44 consecutive hours.  To develop a safe sleep deprivation protocol, Drs. Horovitz and Duyn collaborated with Thomas Balkin, Ph.D., chief of the Department of Behavioral Biology at the Walter Reed Army Institute of Research in Silver Spring, Maryland, and Allen Braun, M.D., chief of the language section at NIH’s National Institute on Deafness and Other Communication Disorders (NIDCD).  Dr. Balkin is an expert on sleep loss and insomnia; Dr. Braun studies the effects of sleep deprivation on language skills.

The researchers also gave the subjects ear plugs (which is standard practice for MRI scans), reduced the noise level of the scanner and put an extra mattress inside it.  Under these conditions, seven of 18 subjects fell into a deep sleep lasting more than 15 minutes.  The researchers observed that from wakefulness to deep sleep, the functional connections within the DMN tended to change.  Some parts of the DMN became more strongly synchronized with each other while others fell out of sync.

As a possible window into consciousness, the DMN touches on age-old, fundamental questions about the nature of “self.”  It also has relevance for understanding disorders in which conscious experience is altered or lost.  Along those lines, some research groups have begun probing DMN activity in comatose patients.

-By Daniel Stimson, Ph.D.

*Horovitz SG, Braun AR, Carr WS, Picchioni D, Balkin TJ, Fukunaga M and Duyn J.  “Decoupling of the brain’s default mode network during deep sleep.”  Proceedings of the National Academy of Science, Vol. 106, pp. 11376-11381, July 7, 2009.

Two gray fMRI scans are shown of a person's brain during rest and deep sleep. Activated regions of the DMN are shown in yellow. During deep sleep, these activated regions become desynchronized.
During rest (top), parts of the brain known as the default mode network (DMN) show a characteristic pattern of activity. During deep sleep (bottom), part of the DMN located in the front of the brain falls out of sync with other parts of the DMN. Courtesy of Silvina Horovitz, Ph.D., NINDS.

Last Modified February 9, 2011