Understanding the Brain’s Default Mode Network

There is evidence of large-scale plasticity in musicians who, compared to non-musicians, have much larger neural structures that represent their hands and fingers in the motor cortex. But presumably these changes take place over many years of training. The same is true of bilinguals: they have extra neural structures for languages in the temporal regions of the brain. London cabbies famously have large hippocampuses, specifically in the regions that help us navigate and remember spatial locations.

Using MRI data, the signal that neuroscientists use to measure the activity of a certain brain region is called the Blood-Oxygen-Level-Dependent (BOLD) contrast. Without going into the complicated details, this signal tells you roughly how much blood and oxygen is flowing to an active brain region. When neurons increase their activity, they use more blood and oxygen (just like your muscles). A rise in the BOLD signal indicates an increase in brain activity.

Even though the network your brain uses to actively pay attention only requires a small fraction of your brain’s total energy, when this attention network activates, your default mode network reduces its activity. This is what is meant by anticorrelated: when your attention network activates, your default mode deactivates.

As you lie there letting your mind wander–or in the awkward language of neuroscientific writing, having Stimulus Independent Thoughts- your brain becomes more organized than if you are trying to concentrate on some task like color coding your Outlook calendar.

In the back of your brain (posterior) sits the precuneus. The precuneus is a hidden brain structure because it is close to the division between your brain’s hemispheres and parts off are deep in your brain. The precuneus is one of the regions that show the highest resting metabolic rate of any region in the brain. This means that at rest the precuneus starts devouring glucose like a crazed hummingbird. The precuneus seems to be involved in self-reflection. One of the best ways to get to know yourself is to find a quiet or comfortably noisy place, stare at the sky, space out for a while and see what the precuneus gets up to.

Like the precuneus, the parietal cortex is also involved in representing you to yourself, sometimes called “meta-cognition.” The ability to think about this question and to have some kind of answer comes partly from our lateral parietal cortex. Life would be pretty meaningless if you lacked any awareness of yourself.

Once you have a piece of information in your awareness you can do stuff with the information-e.g., think about it, make a decision, or let it simply pass from awareness back to your unconscious, such as in meditation. The amount of information you can hold in awareness at any moment also depends on how well your prefrontal cortex is working. There seems to be a trade-off in the brain between being able to store a lot of information in your working memory and cognitive flexibility or creativity.

As part of the default mode network, the medial prefrontal cortex typically starts to shut down when you are an effective person: when you are going from your early morning gym ses- sion to the office, reviewing your PowerPoint slides, getting to the meeting etc.

It is only when your thoughts drift inward does the medial prefrontal cortex start to light up and talk to its partners in idleness- the precuneus, the anterior cingulate cortex, and the lateral parietal cortex. The medial prefrontal cortex also seems to be involved in a kind of surveillance of your brain’s internal operations, so that when you stop doing stuff and become idle this region of the brain can report what’s going on in the deepest recesses of your mind.

In a nutshell, when you are being lazy, a huge and widespread network in your brain forms and starts sending information back and forth between these regions.

The default mode network supports self-knowledge, autobiographical memories, social and emotional processes, and creativity. It persists as long as you can relax.

What appears to us as a single memory has to be reassembled from multiple brain regions every time we recall that memory. The short path-lengths through network hubs help this process to be so fast and automatic that we take it for granted.

In fact, recent evidence indicates that in neurodegenerative diseases such as Alzheimer’s, the default node network is disrupted and shows less activation. This could be one reason why it becomes so difficult for Alzheimer’s patients to recall memories: the information stored in their brains cannot make its way through the network.

Conversely, people with schizophrenia show hyperactivity and hyperconnectivity in their default mode networks. If your default mode network is too active and its nodes have too many connections, you can have problems differentiating between reality and fantasy.

Source : Autopilot: The Art & Science of Doing Nothing by Andrew Smart

Goodreads : https://www.goodreads.com/book/show/18053732-autopilot

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