Our default network—a collection of interconnected brain areas that become active when we daydream or let our mind wander—may enable our most distinctively human abilities, such as imagining the future, discerning what is going on in another person’s head and experiencing a sense of self.
At the same time, our continuous use of the default network throughout life may contribute to the neural breakdown that leads to Alzheimer’s disease.
The default network, identified a dozen years ago by Marcus Raichle and colleagues at the Washington University School of Medicine, functions most vigorously when the brain has no pressing tasks to perform. The network links the medial prefrontal cortex in the front of the brain with the posterior cingulated in the rear, as well as other areas vital to the creation of memories.
New work reveals that the default network includes countless smaller links held together by “hubs” that apparently receive and redirect neural transmissions, according to Olaf Sporns of Indiana University. He and his colleagues used diffusion spectrum imaging, which reveals the long connective tracts in the brain, to construct a detailed map that includes the default network.
“The analogy I use is that of the transportation network,” Sporns says. “If you want to go from A to B, you will often have to travel through a hub node.”
By linking brain regions in such a manner, these hubs enable the brain to efficiently share large amounts of information very rapidly, which could form the basis for such formidable mental abilities as introspection, imagination and the “stream of consciousness” that emerges spontaneously when the mind is not engaged in a specific task.
However, Sporns suspects that these “hub nodes” also leave the brain vulnerable to damage or degeneration.
“When there’s a disturbance in a hub, it ripples through the network and disrupts things much more globally,” Sporns says. “Such phenomena have been found in transportation networks, social networks, the Internet and so on. In the brain it may be similar—when hub nodes are disrupted, the effect is widespread.”
Those hub nodes appear to be especially vulnerable to Alzheimer’s disease. The amyloid plaques characteristic of Alzheimer’s develop in a pattern that overlaps the default network, according to Randy Buckner of Harvard. He believes the chronically high metabolic activity in the default network may somehow promote this problem.
“Why some brain areas are particularly vulnerable to Alzheimer’s disease has been a long-standing mystery,” Buckner says. “The striking observation that the disease begins in the default network provides insight into a possible source of vulnerability. Heightened metabolism within the default network may accelerate the disease process.”
In addition, he has found parts of the default network disrupted even in older adults who don’t have Alzheimer’s. These disruptions result in poor cognitive performance in a variety of mental tests. And people who carry the gene for ApoE4, the strongest genetic marker for Alzheimer’s, show lower metabolism in their default networks.
“Pathology preferentially accumulates in the default network even before symptoms emerge,” Buckner and colleagues write in a recent paper in the Annals of the New York Academy of Sciences.
Buckner has proposed a “metabolism hypothesis” suggesting that chronically high activity in the default network throughout life results in the production of the amyloid that turns up in the plaques of Alzheimer’s disease.
Bolstering this idea are recent findings by John R. Cirrito and his colleagues at Washington University, who found that sluggish endocytosis—the process by which neurons get rid of waste and debris—leads to the release of toxic beta-amyloid from the cell, thereby contributing to the production of the plaques found in Alzheimer’s patients. This breakdown seems to be accelerated by high activity in the synapse caused by the electrical storms of epilepsy. Some patients with temporal lobe epilepsy, for example, develop amyloid deposits at an early age, Cirrito observes.
The default network, most active when the mind is disengaged, “runs hot,” according to Sporns, because it helps generate the self-reflective “stream of consciousness” that flows through the mind continuously whenever the brain is not engaged in a specific task that involves processing input from the external environment.
Might that explain why people who remain socially involved and mentally active in old age have a lower incidence of Alzheimer’s? People who keep their brain busy with specific tasks presumably would not let their minds wander as often, and therefore would use their default network less.
“It’s an interesting hypothesis,” Sporns says. “There is a lot of evidence that points to a special kind of neural activity going on in the default network.
Over time, that could contribute to changes in gene expression patterns and the biochemistry of that part of brain, and might precipitate the onset of Alzheimer’s.”