The default mode network and why it matters.

“The brain secretes thoughts like the liver secretes bile.”

Pierre Cabanis 19th century french physiologist.

The Default Mode Network (DMN) wasn’t discovered because scientists were looking for it. It was an accidental a discovery that came about because something annoying kept getting in the way of their research.

In the 1980s–1990s researchers were interested in using advanced scanning techniques to discover which brain pathways, areas, or networks were involved in everyday tasks and activities.

They used early brain imaging techniques such as Positron Emission Tomography (PET) scans and then functional Magnetic Resonance Imaging (fMRI) to examine how the brain worked and which bits did what. The technique was built around a simple idea.

If we measure a baseline resting scan to show brain activity at rest and then repeat the scan while the subject undertakes a task; doing maths, thinking of a loved one, etc. If we then process these scans, subtracting the resting state from the active state, we can get a map that shows which parts of the brain are involved in the task that was being examined.

Unfortunately things were not that simple and researchers noticed something odd.

When people were resting quietly in the scanner waiting for the next activity (eyes closed, no task to perform), far from being quiet, the brain was, if anything, even more active when it was meant to be at rest. The same brain regions lit up reliably in different people in this resting state; areas that became much less active when the subjects were focussed on a task. A phenomenon that was both reliable and universal.

Confusingly the brain also uses more energy in the resting state than when theoretically more active.

These results did not fit with the standard theories of the time so initially they were seen as an error, background noise, something wrong with the scanners, an interesting baseline finding for the brain at rest but completely unimportant.

However, every time the experiment was repeated, the same “error” turned up and was too consistent to be ignored. This lead to the key understanding that because the brain was at rest did not mean that it was idle.

Researchers had to find an explanation for these unexpected results and then work out what the implications were.

So, the brain at rest uses a much more energy for whatever it was doing at that time and surprisingly task-related increases in energy use were small by comparison. The implication of this is that something systematic, and vital to brain function, was happening during rest.

This resulted in their turning their research completely on its head and the research started to concentrate on discovering which parts of the brain turned OFF during task activity rather than which parts turned on.

This question marked a key turning point in brain research.

In 2001 an analysis of a large number of PET scans was undertaken and this revealed that the same brain areas were consistently switched off (or at least down) when task based activity was undertaken. These included the medial Prefrontal Cortex (mPFC), the Posterior Cingulate Cortex and Cuneus (PCC), the Inferior Parietal Cortex and others. This was seen as the default (resting) mode for the brain – not yet a network.

From 2000 onwards researchers investigate resting states using fMRIs by putting subjects in the scanner, allowing them to rest, and then undertaking a scan with no task required other than to rest.

The results showed that there was a high correlation between the activity in these regions, and that they formed a persistent, and coherent network of brain regions that worked together. That this was apparent in all adults, across all cultures, became gradually more coherent during childhood and adolescent development, and still functioned during sleep and light anaethesia.

The Default Mode Network (DMN) was born.

Up until these discoveries the brain was thought to be virtually entirely reactive to outside events and mental activity was believed to occur in response to outside events and to be driven by external stimuli.

With the discovery of the DMN it became clear that the brain and mind actively generate content and thoughts, that inner mentation is a primary activity of the brain and not a side effect of other activities, and that the majority of our mental life is independent of external events.

This has meant a major rethink around theories of consciousness and how it is generated, how we develop a sense of self, how symptoms of mental illness are generated, and how some of these signs might not be symptoms of mental illness but evidence of the DMN temporarily working differently, and the whole picture of human development as it relates to self, social interactions, wellness, illness etc.

This is having an impact on how mental health and illness is thought about.

Our DMN is all about self referential thinking, helping us to maintain our autobiographical memories, providing a narrative of our lives that provides stability over time, and permits time travel – allowing us to think about the past, and to make plans for our future. It also plays a key role in social cognition permitting us to make educated guesses about how others might think or behave in differing situations based on a share human experience which has come to be called theory of mind.

Where symptoms of mental illness health are concerned the DMN helps to explain negative rumination that pears in depression, anxious overthinking that is seen in anxiety disorders, the fragmented experience of life narrative in Trauma survivors, the misattribution of meaning in psychosis and the identity instability of adolescents and young adults.

The DMN provides a home for our subjective experience – the sense of a me that looks out of our eyes and experiences our world. It is based in brain function and when it does not develop along the more usual lines helps to explain many of the symptoms of mental illness and normal functioning.

It is trained/programmed by our growing up in our family, on our street, in our town, in our culture. It reflects a long history of how our ancestors did things and how we continue to do things around here in order to survive. It develops out of our lived experience and provides a way to explain individual differences and the development of more enduring patterns of behaviour due to differing, perhaps sub-optimal environments for our development.