The Default Mode Network: Rumination & Depression
Are those with high levels of self-awareness and introspection more prone to developing depression?
The brain is a beautiful thing. A physical store of abstract characteristics that create our identity, both as humans and as individuals. Although the brain's true colours are still an enigma, our understanding of how it processes and stores "us" is improving. An interesting concept within this topic is the discovery of the default mode network (henceforth, the DMN).
The history of this brain network dates back to the 1970s, when researcher David Ingvar identified increased blood flow, and therefore brain activity, in specific parts of the brain, even when the participants were at rest, i.e. not performing any mental tasks. Later, these findings correlated with neuroimaging data demonstrating increased activity, particularly in the frontal lobes - a brain area connected to our identity and consciousness. Hence, the name 'default' emerged - brain areas active at a default rest state, regardless of external stimulation.
In short, it's a collection of interconnected brain regions which show low activity when we are engaged in a specific task that requires attention. It becomes active when we are at rest; daydreaming, monitoring our environment, thinking without a specific goal in mind. It is thought to include many high functioning brain regions; including the medial prefrontal cortex, posterior cingulate cortex, lateral temporal cortex, and the hippocampal formation.
But what is its significance nowadays? An overactive default mode network appears to be linked with mental illness, like depression, anxiety, and even schizophrenia. Interestingly, the DMN has been reported to show some overlap in activity patterns across different mental health conditions, but with some distinguishing features. There are even grounds to suggest that this network has clinical significance for other psychological conditions, such as Alzheimer’s, autism, schizophrenia and PTSD (Buckner et al., 2008).
Depression is becoming an increasingly common mental illness, particularly in young adults. Insight into how our neurophysiology gives rise to these symptoms could contribute greatly to our mission of improving diagnosis and treatment. Research suggests an association between DMN dominating over other brain areas while at rest and depressive symptoms, including negative rumination and self-directed thoughts (Berman et al., 2011). A meta-analysis, conducted by Kaiser and colleagues in 2015, found that hyper-connections within the network, as well as between the DMN and frontal lobe, play a role in producing self-directed judgements and thoughts. This correlation could be a plausible explanation for heightened rumination style of thinking, characterised by obsessive overthinking, seen in depressed individuals. Thus, there is a possibility that abnormal connections within the DMN changes the way people view themselves and events in their life, making them more susceptible to developing depressive symptoms. Moreover, a study comparing connectivity strength of the DMN between individuals with major depression and healthy controls found more unstable connections in the former group (Wise et al., 2017).
An intriguing finding is that an overactive DMN can be regulated by a number of seemingly simple interventions, like acupuncture and meditation, thanks to their soothing effect. However, it can also be influenced by more extreme practices like deep brain stimulation and psychedelic drugs, which may help balance these unstable connections (Carhart-Harris et al., 2016). These discoveries can prove useful as a form of treatment in conjunction with therapy and medication.
The existence of this network is still debated and remains quite a controversial topic. Even those that have endorsed it admit uncertainty and demand further research - a cliché in psychology. But whatever evidence we possess is a huge step in understanding our identity and well-being on a deeper level. Some criticise the concept of the default mode for its inutility in explaining brain function. Most commonly, it is argued that a resting brain does more processing than a brain engaged with a demanding task. Therefore, there cannot be a unique pattern in brain activity during rest. Another argument is that it is difficult to define “resting” as a unique form of wakefulness. Although promising, the boundaries of this network seem hazy and undefined.
While these criticisms should be taken into account, the truth is that it is impossible to explain mental health issues using a single mechanism with any amount of certainty. Even if the DMN in isolation does not explain depression, at minimum it has sparked significant interest in comprehensively understanding brain function and our identity, even when not involved in a specific task. At best, it provides hope for those seeking support and answers for their mental state.