Understanding the Default Mode Network

Functional magnetic resonance imaging (fMRI) permits us to make inferences on brain activity in living people based off visually compelling images. For one, it has allowed us to make some important comments on naturally occurring brain networks, including the default mode network. In order to understand such networks, however, some background in functional connectivity is needed first.

What Is Functional Connectivity MRI?

Many fMRI studies are done while the patient is actively doing some activity. For example, if they push a button with their right hand, you may later see part of the left hemisphere near the motor cortex light up at that time. 

Another approach is to look at the brain while the research volunteer is doing nothing at all in the scanner⁠—just lying there. This technique is sometimes called “resting state” fMRI. 

While we lay there, different areas of the brain have oscillatory activity, meaning waves of electricity that are associated with the MRI signal. Sometimes, these waves are in synchrony with each other, meaning they hit the highs and lows of the waveform at the same time. It’s a bit as if they were different members of an orchestra playing the same piece of music while following the same conductor. Two such areas are said to be functionally connected.

Functional connectivity doesn’t have to be measured at rest. Activities such as paying attention to something important can change patterns of functional connectivity across the brain.

Functional connectivity doesn’t necessarily mean that two areas of the brain are directly and physically connected. For example, two different brain areas may be quite far apart, but both receiving signals from a central brain region like the thalamus. These may still be functionally connected if their signals are in synchrony.

Introducing the Default Mode Network

Over the last decade or so, increasing attention has been paid towards this functional connectivity as a way of finding networks in the brain that are related to particular activities, including just resting. One of the most prominent networks to be discussed is the default mode network.

The term “default mode” was first used by Dr. Marcus Raichle in 2001 to describe resting brain function. It had previously been noted that a “resting” brain uses hardly less energy than a brain doing an “active” task, suggesting that perhaps the brain doesn’t “rest” so much as it changes the type of activity in which it is actively engaged. 

The default mode network (DMN) involves low-frequency oscillations of about one fluctuation per second. The network is most active when the brain is at rest. When the brain is directed towards a task or goal, the default network deactivates.

There may, in fact, be more than one default mode network⁠—what we have called the DMN may actually be a collection of smaller networks, each dedicated to something a bit different than the other. Nevertheless, certain brain areas are now commonly understood to be part of the DMN.

What Parts of the Brain Are in the DMN?

Areas of the brain included in the default mode network include the medial temporal lobe, the medial prefrontal cortex, and the posterior cingulate cortex, as well as the ventral precuneus and parts of the parietal cortex. All of these regions have been associated with some aspect of internal thought. For example, the medial temporal lobe is associated with memory. The medial prefrontal cortex has been associated with theory of mind, the ability to recognize others as having thoughts and feelings similar to one’s own. The posterior cingulate is thought to involve integrating different kinds of internal thoughts. Mirror neurons have also been posited to interact with the DMN.

What Does the DMN Do?

Because the default mode network is most active at rest and because of the structures involved, some people have posited that it is associated with introspective thought, including activities like daydreaming or retrieving memories. Others have suggested, however, that the activity may just be related to physiological processes unrelated to any particular activity⁠—even resting⁠—though this opinion seems to be falling out of favor.

Changes in the default mode network have been linked to a wide number of different diseases, including Alzheimer’s disease, autism, schizophrenia, bipolar disorder, post-traumatic stress disorder, depression, and more. Diseases may cause either too little activity or too much, and sometimes the data vary as to which is actually occurring. Whether this reflects a poor understanding of the disease, the technique, or both is often uncertain. 

One of the criticisms that have arisen regarding the DMN is that changes within it seem very nonspecific⁠—what use is a measurement if it doesn’t actually tell you what the problem is? Others have questioned if the network is even a viable concept, though as research piles up the biological actuality of the DMN becomes harder to question.

Other networks, such as those associated with attention, vision, and hearing, have also been described. While the medical benefits of these networks remain unclear, they may reflect an important change in how we think about the brain, and who can say where such thinking will take us in the future?