Illustration: Kieran Blakey

The Nuance

This Is What’s Happening in Your Brain When You’re Daydreaming

Unlocking the mysteries of the brain’s default mode network

WWhat does the human brain do with its free time? A team of neuroscientists at the Washington University School of Medicine in St. Louis wanted to find out, so they recruited a few dozen people and asked them to sit in PET scanners with their eyes closed, leaving their minds free to wander.

“Many suspect that left unconstrained, [the brain’s] activity will vary unpredictably,” those neuroscientists wrote in the 2001 study paper that detailed their findings. But their PET scan data argued otherwise. The researchers’ findings revealed that specific areas of the brain — including both the posterior cingulate cortex and the precuneus — reliably switched on when people’s minds were allowed to wander freely. Meanwhile, many other regions predictably switched off. This suggested that the brain, rather than engaging in random and uncoordinated activity, tended to slip into a kind of resting operational state.

“People are still debating all its functions, but it seems to help us imagine the future — kind of like mental time travel.”

While they acknowledged that identifying the “baseline state” of a system as complex as the human brain was a difficult task, the Washington University team more or less claimed to have done just that. They called this baseline state the brain’s default mode network (DMN). Since then, follow-up research efforts have attempted to elucidate the DMN’s function and utility. Those efforts have yielded some fascinating insights into the role the DMN plays in mental health — and also, possibly, in mental disorders. But experts say that the more they learn about the DMN, the more it defies tidy explanations.

“People are still debating all its functions, but one seems to be to help us imagine the future — kind of like mental time travel,” says Scott Barry Kaufman, a professor of psychology at Columbia University and host of The Psychology Podcast. This future-conceptualizing function may help with some planning and decision-making processes, but it may also contribute to anxiety, which tends to involve worries about what’s to come. Kaufman says that areas of the brain involved in memory recall also light up when the DMN is active, which suggests that in order to imagine the future, the brain relies on its past experiences.

“The default mode network is also very self-oriented,” Kaufman adds. “It draws heavily on areas relating to the construction of the sense of self.” Some researchers have postulated that the DMN is the system that houses and nurtures what Freud called the ego.

“It’s the ‘me’ network — that’s the easiest way to think about it,” says Judson Brewer, MD, PhD, an associate professor and director of research and innovation at Brown University’s Mindfulness Center. He says there’s been speculation that certain patterns of DMN activity are associated with the kind of ruminative, self-oriented thinking that fuels anxiety and other mental health disorders. And some research supports this theory. One 2016 study published in Brain Imaging Behavior linked increased connectivity in several DMN regions with symptoms of anxiety and depression. Last year, U.K. researchers published evidence that altered DMN activity is associated with negative thought patterns in people with depression.

Brewer says that the DMN seems to be highly active when our minds are wandering, daydreaming, or lost in thought. On the other hand, activities or experiences that firmly anchor the brain in the here and now seem to quiet the DMN. In 2011, Brewer and colleagues published a study in the Proceedings of the National Academy of Sciences that linked mindfulness meditation practice with the “deactivation” of certain regions of the DMN. Some of his follow-up work found that heightened activity within one DMN structure — the posterior cingulate cortex (PCC) — was associated with both poor attentional control and craving. Mindfulness meditation quiets activity in the PCC, Brewer says. This may partly explain how mindfulness helps people manage anxiety, addiction, and other disorders linked to craving and rumination. Some apps Brewer developed have been shown to help people manage these conditions. “When we get caught up in craving or in worrying about all the worst-case scenarios, that seems to be when the default mode network is activated,” he says.

A parallel avenue of research has found that hallucinogenic drugs seem to affect the DMN in ways that may improve mental health or treat addiction. Studies have shown that ketamine, LSD, and psilocybin — the psychoactive compound in magic mushrooms — all decrease elements of DMN activity, says Manoj Doss, a postdoctoral research fellow at the Johns Hopkins Center for Psychedelics and Consciousness Research. These findings have fueled speculation that deactivation of the DMN promotes the feeling of interconnectedness and ego evaporation that many people say they experience while taking these drugs. Research from Doss’s group at Hopkins has also found that psilocybin decreases symptoms of depression and anxiety in people with terminal cancer, and that the “mystical experiences” it occasions can help longtime smokers quit.

While all of this is fascinating and great fodder for speculation, Doss and others say everything to do with the DMN is messy. “Every drug people have studied” — from magic mushrooms to alcohol — “seems to be associated with decreased default mode network connectivity, so that doesn’t tell us much,” he says.

At the same time, Doss says that the studies linking depression and anxiety to patterns of heightened DMN activity are inconsistent and selectively include or ignore a diverse collection of brain structures. “The default mode network is involved in all kinds of things, and the hallucinogens we’ve studied don’t seem to impact it super selectively,” he says. “I think a lot of things you read about the default mode network are kind of picking up random bits that go with a certain narrative while completely disregarding a lot of the research.”

Others echo this take. “There’s this narrative that meditation quiets the default mode network, and that’s a great thing because then we’re not stuck in our own self-narrative, but that’s overly simplistic,” Kaufman says. Some research has found that elements of social interaction — specifically, intuiting what other people are thinking and feeling — seem to rely heavily on the DMN. While there are times when worrying or getting too caught up in self-referential thinking can be harmful, Kaufman says daydreaming can also be constructive. Research has linked mind-wandering with creative insights, learning, and other helpful mental processes.

“I’m pro mindfulness meditation,” Kaufman adds. “I think it gives us greater awareness of our thoughts and greater ability to decide where to direct our attention.” But he says that to label the DMN as all bad, or to point to it as the root of depression or other forms of mental disorder, is not supported by the existing science. While the DMN seems to be involved in mental processes that can become dysregulated, that doesn’t mean these processes are inherently pathological.

“I think all we can really say about the default mode network today is that it has a lot of functions, and that some are quite positive,” Kaufman says. “Not all minds that wander are lost.”

I write about health and science. I live in Detroit with my wife and kids. I’m trying to learn German, but my progress so far is nicht gut.

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