Cerebrum Winter 2022

“The networkwas first foundwhen were at rest, so th

BILL GLOVIN: Why don’t we begin with a brief description of what the default network is, how and when it was discovered, and why it’s important. RANDY L. BUCKNER: In the 1990s, neuroscientists were just starting to do functional imaging studies. For the first time, we had brain scanners that could see the mind at work. We were like kids in a candy store in the sense that we no longer needed a scalpel to see the brain; the new technology allowed us to safely discern information out about what parts of the brain people used when given different tasks and different kinds of visual or auditory stimuli. I was a graduate student at the time at Washington University and one of my mentors, Marcus Raichle, was at the forefront of positron emission tomography (PET), an imaging technique that measures physiological changes in the brain and shows where blood flow is increasing due to brain activity. This is when many of us first became aware of the Dana Foundation, which was helping fund our work. I was a Dana fellow in those early days, and this was an exciting time in neuroscience. In early studies, we often asked participants to perform very simple tasks: read and say words, detect colors in pictures, or try to recognize whether a viewed word was on an earlier studied list. The imaging revealed the parts of the brain involved in their responses. But what jumped out at us was something unexpected: When people weren’t asked for a response or given a specific task, much of their brain still remained active. There were skeptics, but Raichle argued that the brain activity seen in the scanner that was not related to specific tasks or responses was—in and of itself—a groundbreaking phenomenon. At the time we didn’t know what to make of the activity pattern that occurred when people were intended to be at rest. The pattern was anatomically specific involving the highest-order association regions of the brain and recurred across many separate studies. Nancy Andreasen, the renown psychiatrist, also early noted the rest pattern of activity and suggested that the brain was at work—spontaneously thinking—when left undirected. The considerable irony was that when we stopped instructing people to do tasks in the scanner, we saw that the human brain imagines, constructs, and explores mentally. Look at your own task with this interview, for example. You have the difficult challenge of trying get content from a scientist so that you can communicate it to the public. You’re imagining my perspective, all the ways I’m

struggling to explain this, and thinking of a plan to engage me based on what you believe. You also have this sort of imagined audience in the reader and you’re able to listen and hear my words and then mentally construct a plan to deliver it clearly and effectively to a reader. The default network may be helping you do all this, which is extraordinary. How did the name “default network” come about? BUCKNER: The network was first found when people were at rest, so the idea caught on that the network was the default state of the brain. It is the idea that people default to their own processes and make their own mental explorations when not given a test or task in a scanner. However the name is also a bit misleading because we now know the network is active when people intentionally remember and plan. It’s not just used during rest. It’s a network that can be called upon to do many forms of focused mental exploration. How does the network part fit it? BUCKNER: The network refers to different parts of the brain—the hippocampus, specific regions along the midline of the cortex, for example—acting together, presumably because they are anatomically connected parts of a large, distributed brain system. As study of the default network has progressed, we’ve learned that there are likely multiple separate networks next to one another. There is one network that seems to be involved when people use memory systems to remember especially, if their memory involves constructing a mental scene. Another network physically next to it becomes active when people imagine what other people are thinking. How has your research evolved? BUCKNER: It’s amazing what’s happened over the last two decades. For the first time, we’re really starting to get a handle on the networks that humans use to mentally explore. When we began using neuroimaging to learn about the brain, we targeted seeing, hearing, and attending to the outside world. What has emerged is the beginning of an understanding of how the brain helps us to be mental explorers. We have an incredible capacity to imagine beyond the present—to think about possible futures, what ifs, and how others who are not ourselves might be perceiving a situation. Our research has evolved to study the networks of the brain responsible for these extraordinary human abilities.


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