What is the brain's default mode network and why is it so important?

The name, default mode network, is not exactly catchy, but this is because it is based on a functional description rather than a marketing strategy. Arguably, it is the most important discovery made about the brain in the last two decades. So, what is it and why is it so important?

A brain network can be thought of as linked regions of activity that contain the same kind of mental signature: similar brainwave frequencies. This can be thought of as a harmonically tuned flow of information within the cranium, and we have about seven major ones. Some allow us to experience our senses; sound, images, smell, and the way our body moves.

What is the brain's default mode network and why is it so important?

Three major networks represent our overall thinking processes. These are the executive control network, the default mode network, and the salience network, which is a kind of gearbox for the brain that allows network switching.

Do you sometimes overhear conversations as you forage around for resources at a local shopping centre or when on public transport? Would you say these people are often involved in passing judgement? Criticising takes up a large part of our lives, but this is not necessarily bitchiness. Dealing with a column of numbers, stocktaking inventory, or editing an article requires that we not only pay attention to the task at hand but also evaluate, along the way. This is the domain of the brain's executive control network (ECN).

This ECN is active when we are goal-setting, solving problems in straight-forward and usual ways (convergent thinking), and managing threats. It underlies the development of self control, and even the mastering of what we might call the capitalist way. And since much of our education system fosters and enhances this kind of thinking it is well studied and understood. But it is also the kind of thinking most easily replicated by using robotics, algorithms and artificial intelligence: technologies currently replacing jobs.

By contrast, the default mode network (DMN) corresponds with qualities of the mind that we see as representing the best parts of being human: creativity, innovative thinking, introspection, understanding the thoughts of others, moral reasoning, mental planning, meditation, daydreaming, mental plasticity, and is necessary for the process of sleep.

The reason this network has remained hidden for so long from neuroscience, and was discovered almost by mistake, is because it is only seen in a quiet brain. This is when it comes out to play. And Marcus Raichle describes how it gained its name as a kind of factory setting for the brain; one that the organic matrix defaults to after all other tasks are dismissed. Truth be told though, the brain is never actually quiet; it is just busy in different ways at different times. But when other networks go off-line the components of the DMN bounce back up from their suppressed condition to a resting state of activity.

This DMN discovery has been hugely important, spawning more than 4000 published studies. Some of these have looked at the development of the network during childhood, since we are not born with these networks in full operation. Others have concentrated on what happens when the network breaks down or distorts, because when this happens a number of problematic states of mind emerge.

Let's look at how the DMN ties in with getting a good night's sleep. When there is a need to wake early or you are sleeping away from home do you find that you have a disturbed night of sleep, at least during the first few nights away? Research into brain activity when sleeping in an unfamiliar environment shows an absence of the clusters of activity characteristic of the DMN. The brain appears unable to get into the relaxed, self-reflective, trance-like state necessary for sleep, and non-REM sleep brain waves do not occur. But why is the switch-over to the DMN blocked in unfamiliar situations? The answer may lie with observations made on dolphins.

Dolphins can sleep in one half of the brain while they keep the other half (other hemisphere) alert. This means that if a threat comes along they can recognise it and react, fully waking the brain more quickly than is possible when both hemispheres are asleep. It seems that we humans are able to do the same thing.

This is our body’s way of balancing vigilance with the necessity of sleep in an uncertain world. From a neurophysiological perspective, even half a brain that remains alert means that the executive function (ECN) is still active in that hemisphere, locking out the DMN. To sleep really well we need to be able to switch all of the brain to the DMN; and this only occurs when we feel safe and are able to let go of the external world.

There is more on brain networks, what else they do for us, and ways to fix them when they 'break', in my book.

Till next time – B.W. Cribb

#DefaultModeNetwork #BrainNetworks #Sleep

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