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. 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," said first author Biyu He, a graduate student.
"What we've been finding is reorienting the way we think about how the brain works. 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," said senior author Marcus Raichle, M.D., professor of radiology, of neurology and of neurobiology.
For the study, researchers asked volunteers to perform specific mental tasks as their brains were 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. When the subjects perform these tasks, the scans reveal increases in blood flow to different parts of the brain indicating that the brain areas are contributing to the mental task.
While scientists know that deeper structures underlie goal-oriented mental processes, which 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. 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," said Raichle.
Thus, for a better understanding of this phenomenon, researchers used fMRI to conduct detailed analyses of brain activity in subjects asked to do nothing. However, scientists assumed that increased blood flow to a part of the brain indicates that part has contributed to a mental task, but they wanted more direct evidence linking increased blood flow to stepped-up activity in brain cells.
So, the researchers took fMRI scans of five patients with intractable epilepsy at St. Louis Children's Hospital. The scans, during which the subjects did nothing, were taken prior to the temporary installation of grids of electrodes on the surfaces of the patients' brains.
The level of detail provided by the grids is essential clinically for pinpointing the source of the seizures for possible surgical removal, a last resort employed only when other treatments failed.
The results confirmed that the fMRI data gathered earlier reflected changes in brain cell activity exhibited in the gamma frequency signal. But they also noticed the persistent low-frequency signal, which also corresponded to the fMRI data.
"What we've shown provides a bridge between the fMRI work many scientists are doing now and the earlier work involving electrical recordings from the brain that emphasized slow activity. Bringing those two fields together may give us some very interesting insights into the brain's organization and function," said He.
The study is appearing online this week in the Proceedings of the National Academy of Sciences.