Brain Networks: Blog 1 – The Default Mode Network

August 8, 2020 at 2:49 PM
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Brain Networks: Blog 1 – The Default Mode Network

By Brandon Ascencio UA ‘23, Brandy Eggan, and Jim Stellar

When someone says the word “daydreaming”, what do you think? If you take a moment to pause and look away from the screen, where does your mind go? It’s not uncommon for someone to just relax and take a breather from their daily lives, to get lost in the past, future, or present. As co-author BA says, “Sometimes I get lost in and explore the cobwebs of my mind in this sort of mindless trance. Whether I am zoning out of the show that’s on television or my fingers are methodically darting across the keys of a remote control of a video game I’ve played a thousand times, I find my brain doesn’t have a chance to just dedicate itself to thinking, strategy, or brainstorming. Rather, it is calm, cool and running effortlessly.”

What exactly is this sort of “absent-minded” thinking? Does it mean that the brain is moving as fast as ever in seemingly silent circuits or did we take those active thought circuits and slow them down or put them on hold for the time being? If we were to put someone in a brain scanner to see what’s going on within the brain and ask them to think of “nothing”, to just lie there, what would we see? Luckily, scientists have done this experiment and surprisingly have shown that there is robust brain activity in a network of clearly defined and interconnected regions even when the subject was supposedly doing “nothing.”  This pattern of brain activity is what we now call the default mode network (DMN).

The story begins in the front of the brain.

The Frontal Cortex and the DMN

The frontal lobes are classically involved in planning activities or what is called executive function, conclusions made from observations of brain damaged patients going back to the mid 1800s and extended by modern brain activity scanners.  At the start of the modern brain scan world, one of us co-authors (JS) even had the privilege in his postgraduate training in the late 1970s at the University of Pennsylvania of attending a scientific talk by Dr. David Invgar on an initial experiment with the then new regional cerebral blood flow brain activity scanners.  In what now also seems like the first observation of a piece of the DMN (although we did not call it that at the time), Dr. Invgar’s observation was when normal control patients were asked to rest quietly in the scanner, they developed pronounced activity in the frontal lobes, something that did not happen in the schizophrenic patients they also studied. When the researchers asked these normal control subjects what they were doing, they replied that they were thinking. When the researchers asked the schizophrenic patients if they were thinking, they responded with what the researchers characterized as a quizzical response …  thinking?

Before approaching that question, we need to look at the basic front-back of the brain structure of the DMN.

Frontal Connection to Posterior Cortex in the DMN

Accompanying the frontal lobe activity in the DMN is simultaneous activity in the posterior lateral cortex or what is sometimes called the temporal-parietal-occipital junction. The posterior cortex and this area in particular deals with sensory input from many sources, e.g. from auditory processing starting in the temporal lobe, from visual processing starting in the occipital lobe, and from somatosensory processing starting in the parietal lobe. While this characterization is an oversimplification, it makes the point as in the fMRI image below from a Menon 2015 summary paper on brain networks, that the DMN is an interconnection of planning functions from the front of the brain with sensory functions in the back of the brain.

 

 

Thus the DMN activity may not be just related to daydreaming, it may be that daydreaming in a context is really planning about objects that are represented in the posterior cortex. We do this all the time, when we have a free moment. Something simple, like planning a brief shopping trip or perhaps we are “thinking” about something more complex, having to do with our interactions with different people.

Dorsal and Ventral Medial Prefrontal Cortex in DMN: Two Aspects of Planning

Following the above diagram the DMN frontal regions feature two prominent subdivisions, the first of which is the Dorsal Medial Prefrontal Cortex (dmPFC). Briefly, the dmPFC has been associated with theory of mind, that is the ability to recognize others as having thoughts and feelings similar to one’s own and to put yourself in their place. It is also more generally associated with responsiveness to social interactions. The dmPFC also seems to be involved with another nearby brain area the anterior cingulate cortex (ACC) in tracking errors and therefore correcting for them when they are made, maybe especially as in regard to emotions. It also contains spindle neurons we wrote about in the previous blog.  Through the ACC, the dmPFC is involved with processing personal information, autobiographical memories, future goals and events, and decision making regarding those who are personally close to us.

In a related set of presumptive planning activities, the ventral medial prefrontal cortex (vmPFC) seems to be involved in positive emotional information and internally valued reward outcomes as decisions are made. This operation is seen in a specific to neuroeconomics-like games that studied people making charitable donations in a brain scanner. The vmPFC also connects to other areas like the ACC (mentioned above) in determining the value of a potential option or the choice between two options with different values. These kinds of in-principle comparisons, particularly when emotions are involved, are thought to guide people in making decisions rather than executing them first and seeing what happens or simply remembering what happened last time. This PFC (both dm and vm) interaction and with the ACC shows the network operation within the planning side of the DMN.

Finally, the vmPFC interacts with the another brain area known to be involved with reward, the nucleus accumbens, an area studied in rats for cues to cocaine craving and relapse and the subject of many previous blogs. That interaction illustrates a principle beyond the scope of this blog of how cortical networks might reach down into older evolutionaldy subcortical networks of natural basic body maintenance such as the reward of eating when hungry.

Lateral and Medial Posterior Cortex in DMN: Two Aspects of Things in the World about which to Plan

If the prefrontal cortex is involved in DMN planning, the posterior cortex seems to be involved with the representation of things in the world such as objects or people, their locations/movements, polysensory integration, etc. To make another over simplification, here is where the auditory inputs from the temporal lobe meet the visual inputs from the occipital lobe and the somatic inputs from the parietal lobe. As stated, These objects and entities are what planning is often about – what one is thinking when planning – and it is represented in the posterior part of the brain. When the DMN is activated these two regions get activated together. For example, consider that in a DMN moment you might be planning to go to the grocery store to pick up milk. Presumably, it is in these posterior brain areas that you have a representation of a carton of milk and its weight, temperature, and maybe even its location in the store.

The lateral portion of this area in the diagram above, sits at the intersection where the cross-modal sensory properties of an object exist and is involved in multiple functions. While it is likely your pet cat represents you as independent streams of sound, sight, and touch, you integrate these streams in a full representation of your cat. That integration occurs in this region. It is also where language understanding occurs and control over speech production happens when you want to talk about something.  Interestingly, activating the  language network turns off the DMN. On the other hand, the medial portion of the posterior cortex represents more emotional, social, and internal functions and includes the posterior cingulate cortex (particularly in its ventral regions) and the precuneus cortex which you can find almost directly above the PCC in the brain. Of course, these brain areas play multiple roles in overlapping networks in the brain.

Conclusion

So to oversimplify what we’ve said so far the DMN is all about connections between planning and and the objects or subjects of that planning from their synchronous activity, but there is also evidence that these areas all function in other ways as well. Our point is that this connectivity is the foundation for a network, specifically the DMN which dominates brain activity when we are in a resting state as well as during a trance in a daydream. With mapping the active regions in this DMN it becomes easier to see the importance of this interconnection and unconscious processing occurring while our mind is wandering and this has major implications for the process of learning and transfer of knowledge from an active task to a mindless process. With having this sort of unconscious skill (which we believe is influenced by experiential learning) and our college students can better prepare themselves for that post-graduation job as the skill-based tasks become effortless and they can better focus on the critical thinking task at hand. How exactly does this information combine between skill and fact-and-theories knowledge? Maybe it is through the DMN.  But, it is important to note that this is not the only network in the brain and these not only do networks communicate within themselves, they communicate with other networks. It is therefore obvious that the connectivity is important and we will discuss that in Blog 2 of this series.

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Reflection and the Brain: A potential Connection between Limbic and Cortical processing circuitry
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