The brain’s default mode network – what does it mean to us?

Marcus Raichle interviewed by Svend Davanger

“We discovered the default mode network accidentally, without any preconceived notion of it,” says the brain researcher Marcus Raichle. “Nobody had thought of anything like the default mode network in our brain. It is different from the brain’s visual and movement systems.”

Marcus Raichle is a neurologist and radiologist, and is a professor at the department of Radiology at Washington University School of Medicine in St. Louis, Missouri. In 2014 he received the Kavli Prize in Neuroscience “for the discovery of specialized brain networks for memory and cognition”, together with Brenda Milner and John O’Keefe. Approaching 80 years, he is still an active scientist. He has successfully described how our common spontaneous thoughts during free moments, including our small, “non-significant” memories, play an important role in how we live. Here with an early PET brain scanner. Photo by R. Hornbeck.

Active when we are passive

“While we have known about the cerebral cortex for centuries, the recent finding that certain areas of the cortex are connected in a previously unknown kind of network was unexpected. It is a basic network with surprising functions. The default mode network is comprised of several areas of the cortex that are most active when no external tasks demand our attention,” says Raichle.

“We discovered the default mode network when we asked participants in a study to perform tasks that were so demanding that they had to be absorbed in what they were doing. It is well known that when involved in such tasks, we use the attention network of the cortex. We noticed that activity in certain areas of the cortex was reduced during the involvement in demanding tasks. It was really surprising that, after the demanding tasks were completed, activity in these areas of the cortex increased again. The brain seemed to revert back to a default activity level, which is there in the absence of a specific, ongoing, external task. So we decided to take a closer look at it. We now understand it as a special network in the brain that, paradoxically, is more active when we are not involved in a goal-directed task. We conventionally tend to think that when we are not busy doing such things, our brain is “free”, or more passive, and we automatically think that we primarily use the brain to solve difficult tasks, or control goal-directed activities.

“It hadn’t occurred to anyone that the brain is actually just as busy when we relax as when we focus on difficult tasks. When we relax, however, the default mode network is the most active area of the brain.”

Important for survival

What goes on in the default mode network – a substantial part of our cortex that spends every free moment on things other than being involved in external tasks?

“Many of the functions of the network are associated with our perception of our selves. The default mode network is involved in our memory, particularly autobiographic episodic memories. These memories are our personal experiences, associated with a certain time and place, such as what did I have for breakfast today? Where was I yesterday evening? Episodic memory is a special thing. It is very personal and very self-relevant. It integrates memories from our lives in a self-relevant way. This may be the essence of the default mode network.

Isn’t it strange that a large area of the cortex is allocated to dealing with what seems like insignificant, random memories from past events in your life, which you often don’t think are of any use?

Raichle is used to such questions. “You’re thinking about this in a non-scientific way. Neuroscience and psychology have been devoted to the study of memory: how much we recall, and in what form we recall it. I have often wondered why we reminisce… it certainly is not the case that we reminisce simply for pleasure. What it does is give us a way to predict the future. Our daily memories play a role in helping us make a model of the world in which we live, and predict the future. If we get up from a chair and walk across the room, the brain will make predictions about motor actions, such as which foot goes in front of the other one. When we encounter people, we can remember who they are.

“This is important for survival; it isn’t just fun, it’s important. I suspect that the default mode network provides the integration that makes this possible,” Raichle speculates. “It’s not only important to remember what’s important, but also to put a value on what’s important. The part of the default mode network up front, down almost between your eyes, just above your nose, has to do with deciding whether something is good, bad, or indifferent. In other words, input from our body and the external world passes through this area of the brain before continuing on to the amygdala and the hypothalamus and the brain stem, which produce the emotional responses we experience all the time in our daily lives – everything that has to do with us getting along in the world.”

Daydreaming helps creativity

Some neuroscientists maintain that random episodic memories are a kind of junk material, that we are unhappier when we have such thoughts, and that in some cases these thoughts may cause disease?

“There are no ironclad arguments here,” says Raichle. “With a phenomenon like mind-wandering, it is difficult to maintain that it is of no use at all – just like dreaming. Dreaming is mind-wandering disconnected. Why do we dream? Although there is no clear scientific answer, we cannot claim that dreams are just an inconvenience. I don’t believe that. It is also true that in a number of psychiatric disorders, wandering thoughts are detrimental. Being able to dissociate is important. For a schizophrenic individual, some random thoughts may be perceived as the truth. That is an example of mind-wandering gone awry. On the other hand, normal mind-wandering serves important functions. Many researchers believe that creativity is associated with daydreaming or spontaneous thoughts about interesting problems. Personally, I do a bit of that; it is one way we work things out.”

The vulnerable part of the brain

Is the default mode network particularly vulnerable?

“It is well-documented how the metabolism in the default mode network is different from the rest of the brain. There is an association between neurodegenerative diseases, such as Alzheimer’s disease, and the network. With Alzheimer’s disease, there are accumulated plaques in the brain, and the most common of these are tangled specifically in the default mode network. There is something about the default mode network that carries a risk for such diseases. The metabolism and chemistry in the default mode network is different from other areas of the cortex. The general metabolism here is high. In the brain, a continuous renewal of synapses of nerve cells is taking place. There may be a higher speed of remodeling, or synaptic plasticity, in the default mode network. This might possibly cause a vulnerability of the nerve cells. The hippocampus, which is part of the default mode network, seems to be the most vulnerable part of the brain.”

Three networks working together

“Recent brain research has often focused on networks, rather than specific areas. We are used to talking about things like the ‘visual center’, ‘speech center’, and ‘motor center’ of the brain. The areas of the cortex that are located between these centers have traditionally been called ‘association areas’, with somewhat diffuse ideas about what is going on there – but it might be coordination or integration of information. We have developed a more sophisticated understanding of what goes on in the association areas. The default mode network is comprised of five to six cooperating areas of the cortex. The two largest are located near the midline of the brain, partly in the frontal lobe area, partly further back in the parietal lobe. Two other areas are located a bit above the ear, on the inner side, on each side of the brain.”

Two other networks in the association area are called the attention and salience networks – what is the difference between these and the default mode network?

“Attention networks – there are actually two of them – determine our capacity to attend to specific tasks we need to do. Studies indicate that certain areas of the cortex have increased activity when we perform tasks with the attention focused in a specific direction or on specific aspects of the task. These areas are active all the time, no matter what we are doing. But their activity increases, as does their impact, when we focus on distinct tasks. I like to think of it as losing yourself in your task or work, and you just focus on what you are doing.

“The salience network seems to be involved in determining how relevant a particular piece of information, or a thing you are looking at or thinking about, is to you. This brings us closer to the perception we have of how we integrate information from the world around us with ourselves. We must be capable of relating to the external world, as well as our bodies, with some sort of self-image. Something could be important to you because you like it, or because you don’t like it and it is potentially dangerous. Those are critical decisions for survival.

“To summarize the function of the three networks: the attention network makes it possible for us to relate directly to the world around us, i.e., here and now, and the default mode network makes it possible to relate to ourselves and our memories and previous experiences, i.e., the past and future. The salience network makes us switch between the two others according to our needs.”

How is the default mode network important?

Raichle’s discovery of the default mode network may have important implications. On the one hand, it may help us find ways of dealing with medical problems, such as depression, schizophrenia, and dementia. On the other hand, new knowledge about the default mode network and the self-reflecting thoughts that it stimulates may facilitate our understanding of how we function in our daily lives. We are more than intellect or the motor control of arms and legs, which is often the focus of brain researchers. Perhaps it may at times be good to know that our brain actually gives us room for our spontaneous thoughts and the associations and emotions that may at first seem a bit weird.

Translator: Anne Grete Hersoug
Copy-editor: Ann Kunish