Default Mode Network

The idea that the brain could be constantly busy is not new. An early proponent of that notion was Hans Berger, inventor of the familiar electroencephalogram, which records electrical activity in the brain with a set of wavy lines on a graph. In seminal papers on his findings, published in 1929, Berger deduced from the ceaseless electrical oscillations detected by the device that “we have to assume that the central nervous system is always, and not only during wakefulness, in a state of considerable activity".

Over the years, however, others became curious about what was happening when someone was simply resting and just letting the mind wander. This interest arose from a set of hints from various studies that suggested the extent of this behind-the-scenes activity. One clue came from mere visual inspections of the images. The pictures showed that areas in many regions of the brain were quite busy in both the test and the control conditions. In part because of this shared background 'noise', differentiating a task from the control state by looking at the separate raw images is difficult if not impossible and can be achieved only by applying sophisticated computerized image analysis. Further analyses indicated that performing a particular task increases the brain’s energy consumption by less than 5 percent of the underlying baseline activity. A large fraction of the overall activity—from 60 to 80 percent of all energy used by the brain—occurs in circuits unrelated to any external event. With a nod to our astronomer colleagues, our group came to call this intrinsic activity the brain’s dark energy, a reference to the unseen energy that also represents the mass of most of the universe. The question of the existence of neural dark energy also arose when observing just how little information from the senses actually reaches the brain’s internal processing areas. Visual information, for instance, degrades significantly as it passes from the eye to the visual cortex.

Of the virtually unlimited information available in the world around us, the equivalent of 10 billion bits per second arrives on the retina at the back of the eye. Because the optic nerve attached to the retina has only a million output connections, just six million bits per second can leave the retina, and only 10,000 bits per second make it to the visual cortex. After further processing, visual information feeds into the brain regions responsible for forming our conscious perception. Surprisingly, the amount of information constituting that conscious perception is less than 100 bits per second. Such a thin stream of data probably could not produce a perception if that were all the brain took into account; the intrinsic activity must play a role. Yet another indication of the brain’s intrinsic processing power comes from counting the number of synapses, the contact points between neurons. In the visual cortex, the number of synapses devoted to incoming visual information is less than 10 percent of those present. Thus, the vast majority must represent internal connections among neurons in that brain region.

Marcus E. Raichle is an American neurologist at the Washington University School of Medicine in Saint Louis, Missouri. He is a professor in the Department of Radiology with joint appointments in Neurology, Neurobiology and Biomedical Engineering. His research over the past 40 years has focused on the nature of functional brain imaging signals arising from PET and fMRI and the application of these techniques to the study of the human brain in health and disease. He received the Kavli Prize in Neuroscience 'for the discovery of specialized brain networks for memory and cognition', together with Brenda Milner and John O’Keefe in 2014.

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