What Is the Default Mode Network?

You might be the laziest person on the planet, but your brain never rests. So what does it get up to when you’re more or less checked out?

In the 1930s, Hans Berger, a German psychiatrist who had recently invented the electroencephalogram (EEG), suggested that our brains are always active, even when we don’t seem to be doing much with them. Few people took the idea seriously at the time (maybe because Berger had spent much of his career trying to prove telepathy, or perhaps because he was just ahead of his time). Some 40 years later, in the 1970s, researchers confirmed that blood flow to the brain, a useful proxy for brain activity, varies depending on what you’re doing.

Still, it was difficult to learn much about which regions of the brain are active during various activities until the advent of modern brain imaging technology, such as positron emission tomography (PET) scans and magnetic resonance imaging (MRI). However, by the turn of the century, scientists were using these technologies to study the brain. 

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Researchers noticed that when study subjects were resting in the scanner, not doing any particular task (perhaps they were in the control group or just waiting for the experiment to get underway), their brains showed interesting activity patterns. The researchers found that certain brain regions show lower activity levels when we’re paying attention to some task but jump into action when we’re not engaged in any specific mental task. Berger was right that the brain is always active; in fact, it turns out that it is very active indeed, even during those periods when it was previously thought to be resting.

In 2001, a team led by Marcus Raichle, a neuroscientist at Washington University, published the first of a series of papers describing the phenomenon and giving a name to this network of brain areas: the Default Mode Network (DMN).

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The brain areas involved in the DMN seem to be mostly: the medial prefrontal cortex, the posterior cingulate cortex, part of the inferior parietal lobe, and sometimes, maybe, the middle temporal lobe. Maybe? Yes, scientists are still working to pinpoint which parts of the brain are involved in the DMN. In fact, it may be that there are several DMNs and different combinations of areas that network for different purposes.

One thing all these brain areas have in common is that they play a role in the inner voice. The brain's DMN kicks in when we’re daydreaming, reminiscing, or thinking about, say, that text your sister sent this morning, what you’re having for dinner, noticing that it’s getting kind of hot in the room, wondering what your cat is thinking when it looks at you like that, wondering what your sister meant by that text she sent this morning — you know, all those seemingly random thoughts that meander through your mind when you’re not focused on anything else. Meanwhile, areas of the brain that are responsible for things like attention, working memory and decision-making take a breather.

Since the DMN was discovered, scientists have been working to better understand what it is, what it does, and why it’s important. Though it’s early days in understanding or even completely defining the DMN, the concept has already spawned some fascinating research. Some are focused on the role the DMN — and changes to its patterns — plays in dementia. Other research looks at the DMN in autism and certain types of psychosis. Some researchers are examining how addiction changes the DMN. The role of the DMN in post-traumatic stress disorder (PTSD) is yet another area of research.

Perhaps one of the most interesting ideas about the DMN is that it plays a role in the construction of the self. The brain regions involved in autobiographical memory and ideas about the self are part of the DMN. Many of the ideas that float around the mind during this “downtime” have to do with our personal lives and our sense of who we are.

So it’s possible that when your brain isn’t doing anything that seems important, it’s actually quite busy making all the connections that make you feel like you.

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