Marcus Raichle, a neurologist and radiologist at Washington University, got interested in daydreaming by accident. It was the early 1990s, and Raichle was studying the rudiments of visual perception. His experiments were straightforward: A subject performed a particular task, such as counting a collection of dots, in a brain scanner. Then he or she did nothing for thirty seconds. (“It was pretty boring for the subjects,” Raichle admits. “You always had to make sure people weren’t dozing off.”) Although the scanner was still collecting data in between the actual experiments, Raichle assumed that this information was worthless noise. “We told the subjects to not think about anything,” he says. “We wanted them to have a blank mind. I assumed that this would lead to a real drop in brain activity. But I was wrong.”
One day, Raichle decided to analyze the fMRI data collected when the subjects were just lying in the scanner waiting for the next task. (He needed a baseline of activity.) To his surprise, Raichle discovered that the brains of the subjects were not quiet or subdued. Instead, they were overflowing with thoughts, their cortices lit up like skyscrapers at night. “When you don’t use a muscle, that muscle isn’t doing much,” Raichle says. “But when your brain is supposedly doing nothing, it’s really doing a tremendous amount.”
Raichle was fascinated by the surge in brain activity between tasks. At first, he couldn’t figure out what was happening. But while sitting in his lab one afternoon, he came up with the answer: The subjects were daydreaming! (“I was probably daydreaming when the idea came to me,” Raichle says.) Because they were bored silly in the claustrophobic scanner, they were forced to entertain themselves. This insight immediately led Raichle to ask the next obvious question: Why did daydreaming consume so much energy? “The brain is a very efficient machine,” he says. “I knew that there must be a good reason for all this neural activity. I just didn’t know what the reason was.”
After several years of patient empiricism, Raichle began outlining a mental system that he called the default network, since it appears to be the default mode of thought. (We’re an absent-minded species, constantly disappearing down mental rabbit holes.) This network is most engaged when a person is performing a task that requires little conscious attention, such as routine driving on the highway or reading a tedious book. People had previously assumed that daydreaming was a lazy mental process, but Raichle’s fMRI studies demonstrated that the brain is extremely busy during the default state. There seems to be a particularly elaborate conversation between the front and back parts of the brain, with the prefrontal folds (locate just behind the eyes) firing in sync with the posterior cingulate, medial temporal lobe, and precuneus. These cortical areas don’t normally interact directly; they have different functions and are part of distinct neural pathways. It’s not until we start to daydream that they begin to work closely together.
All this mental activity comes with a very particular purpose. Instead of responding to the outside world, the brain starts to explore its inner database, searching for relationships in a more relaxed fashion.