Whenever you’re actively performing a task — say, lifting weights at the gym or taking a hard exam — the parts of your brain required to carry it out become “active” when neurons step up their electrical activity. But is your brain active even when you’re zoning out on the couch?

The answer, researchers have found, is yes. Over the past two decades they’ve defined what’s known as the default mode network, a collection of seemingly unrelated areas of the brain that activate when you’re not doing much at all. Its discovery has offered insights into how the brain functions outside of well-defined tasks and has also prompted research into the role of brain networks — not just brain regions — in managing our internal experience.

In the late 20th century, neuroscientists began using new techniques to take images of people’s brains as they performed tasks in scanning machines. As expected, activity in certain brain areas increased during tasks — and to the researchers’ surprise, activity in other brain areas declined simultaneously. The neuroscientists were intrigued that during a wide variety of tasks, the very same brain areas consistently dialed back their activity.

It was as if these areas had been active when the person wasn’t doing anything, and then turned off when the mind had to concentrate on something external.

Researchers called these areas “task negative.” When they were first identified, Marcus Raichle, a neurologist at the Washington University School of Medicine in St. Louis, suspected that these task-negative areas play an important role in the resting mind. “This raised the question of ‘What’s baseline brain activity?’” Raichle recalled. In an experiment, he asked people in scanners to close their eyes and simply let their minds wander while he measured their brain activity.

He found that during rest, when we turn mentally inward, task-negative areas use more energy than the rest of the brain. In a 2001 paper, he dubbed this activity “a default mode of brain function.” Two years later, after generating higher-resolution data, a team from the Stanford University School of Medicine discovered that this task-negative activity defines a coherent network of interacting brain regions, which they called the default mode network.

The discovery of the default mode network ignited curiosity among neuroscientists about what the brain is doing in the absence of an outward-focused task. Although some researchers believed that the network’s main function was to generate our experience of mind wandering or daydreaming, there were plenty of other conjectures. Maybe it controlled streams of consciousness or activated memories of past experiences. And dysfunction in the default mode network was floated as a potential feature of nearly every psychiatric and neurological disorder, including depression, schizophrenia and Alzheimer’s disease.

Since then, a flurry of research into the default mode has complicated that initial understanding. “It’s been very interesting to see the types of different tasks and paradigms that engage the default mode network in the last 20 years,” said Lucina Uddin, a neuroscientist at the University of California, Los Angeles.

The default mode was one of the first brain networks characterized by science. It consists of a handful of brain regions, including a few at the front of the brain, like the dorsal and ventral medial prefrontal cortices, and others scattered throughout the organ, like the posterior cingulate cortex, the precuneus and the angular gyrus. These regions are associated with memory, experience replay, prediction, action consideration, reward/punishment and information integration. (The colored highlighting in the following figure indicates some of the outer brain areas that become more active when the default network engages.)

Since its discovery, neuroscientists have loosely identified a handful of additional distinct networks that each activate seemingly disparate areas of the brain. These activated areas don’t act independently, but rather harmonize in synchrony with each other. “You can’t think about a symphony orchestra as just the violins or the oboes,” Raichle said. Similarly, in a brain network, the individual parts interact to bring about effects that they can only produce together.

According to research, the effects of the default mode network include mind wandering, remembering past experiences, thinking about others’ mental states, envisioning the future and processing language. While this may seem like a grab bag of unrelated aspects of cognition, Vinod Menon, the director of the Stanford Cognitive & Systems Neuroscience Laboratory, recently theorized that all of these functions may be helpful in constructing an internal narrative. In his view, the default mode network helps you think about who you are in relation to others, recall your past experiences and then wrap up all of that into a coherent self-narrative.