In the last quarter of the twentieth century, two brain imaging techniques were developed that would dramatically change our understanding of how the brain works. Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have allowed scientists to look at brain activity while people are performing a whole range of mental tasks and create detailed 3-D maps of that brain activity. Using these new techniques, we have learned which brain areas are “turned on” during all sorts of mental activity—from staring at geometric patterns to viewing emotional pictures, reading, and memorizing word lists to having out-of-body experiences.
To produce these maps of brain activity, individuals are rolled into the center of a huge doughnut-shaped machine. Pictures of their brain activity are then taken, first while they’re just lying there resting and again while they’re doing some task. If we subtract the activity patterns seen while individuals are resting from the patterns seen when they’re performing the task, we get a picture of which brain regions are activated by the task—a map, if you will, of which parts of the brain actually perform the task. The use of fMRI to map brain activity in real time was an amazing breakthrough, and scientists quickly began mapping out dozens of brain functions.
As more and more brain imaging studies were published, it slowly became clear that something strange was happening. While turning on its own specific set of brain regions, each mental task also turned off other regions. At first this seemed quite reasonable. But as time went on, it also became clear that the regions being turned off were the same, no matter what task the participants were performing. And this made no sense.
Over time, however, Marcus Raichle, along with his colleagues at Washington University in St. Louis, realized what they were seeing.4 Scientists had been assuming that the activity pattern seen during quiet rest reflected the activity of a brain not doing anything. In retrospect, this was obviously a foolish assumption. Our brains are always thinking about something. Because of this, the brain areas that turn off whenever we start to carry out a mental task are the regions that do whatever the brain does when we’re “not doing anything.” Together these regions make up the default mode network (DMN), whose discovery has helped us appreciate just how true it is that the brain never rests.
When we look at the brain regions that make up the DMN, we find a sub-network that monitors the environment for important changes, watching out for any danger. Keeping us safe is probably one function of the DMN. But we also find a sub-network that helps us recall past events and imagine future ones, another that helps us navigate through space, and yet another that helps us interpret the words and actions of others. And these are the mental functions associated with mind wandering. Much of mind wandering involves hashing over the events of the day or anticipating and planning future events. Indeed, such planning has been proposed as a function of mind wandering.5 So it’s perhaps not surprising that mind wandering is associated with increased activity in the DMN.6 This appears to be a second function of the DMN.
The DMN is not a static structure, however. It changes based on what you’ve been doing earlier. Bob and his colleague Dara Manoach looked at how activity in the DMN changed after doing one of Bob’s favorite tasks: his finger-tapping task, which we saw in Chapter 5 involves learning to type the sequence 4–1–3–2–4 as quickly and accurately as possible.7 Young participants get a lot better in just a couple of minutes of practice, but then they plateau. A period of rest in the same day doesn’t make them any faster, but if they get a night of sleep and then try again, they become 15 to 20 percent faster. It’s another example of sleep-dependent memory evolution.
When Bob and Dara had participants learn the task while having their brains scanned, with periods of quiet rest before and after the training, they found that brain regions involved in performing the task were talking to each other more during the quiet rest after training than during the quiet rest before training. The DMN, which is normally measured during such periods of quiet rest, was altered by performing the task. And more important, the more the DMN was altered, the more improvement participants showed the next day. It was as if this new DMN activity was telling the brain what to work on once it fell asleep.
Indeed, much of the DMN is also activated during REM sleep, suggesting that the term daydreaming may be more appropriate than we thought. William Domhoff and his colleague Kieran Fox have gone so far as to suggest that dreaming, or at least REM sleep dreaming, constitutes a brain state of “enhanced mind wandering.”8 More recently, Domhoff has proposed that the neural substrate of dreaming lies within the DMN.9 When you put it all together, you get an exciting extension of our NEXTUP model. Whenever the waking brain doesn’t have to focus on some specific task, it activates the default mode network, identifies ongoing, incomplete mental processes—those needing further attention—and tries to imagine ways to complete them. Sometimes it completes the process shortly after the problem arises, making decisions without our ever realizing it. But at other times it sets the problem aside after tagging it for later sleep-dependent processing, either within or without dreaming. Several dream theories have suggested something like this—that dreaming helps us address areas of concern in our lives. The DMN might provide the mechanism for identifying these concerns, thereby determining what’s NEXTUP.
(Excerpted with permission from When Brains Dream: Exploring the Science and Mystery of Sleep by Antonio Zadra and Robert Stickgold; WW Norton & Company, 2021)