DMN...It's All Connected, Man: How the default mode network can help us understand consciousness

Art by Sarah Nasson

BY SARAH NASSON

Your brain activity never really stops. Unless you’re dead, that is. But even as ‘at rest’ activity declines in certain regions, there are areas belonging to resting state networks in which activity actually increases. One of these networks is the default mode network (DMN), comprised of three major cortical brain regions: the dorsal medial prefrontal cortex (DMPC), ventral medial prefrontal cortex (VMPC), and the posterior cingulate cortex (PCC). In 1997, researchers looking at the brain activity of people engaged in goal-directed tasks were surprised to find a decrease in the activity of this constellation of regions when attention was shifted from these tasks to selfreferential thought. This characteristic organization of the brain’s ongoing, intrinsic activity was given its name in a 2001 article by Raichle and colleagues: “A Default Mode of Brain Function”(1).

Since 2001, research surrounding the DMN has skyrocketed in neurophysiology, cell biology, functional connectivity processes, the relationship between the DMN and disease states, mind-wandering, and self-referential processing among a multitude of other applications to psychology and behavior. Regardless of the subject matter, functions associated with the DMN’s activity always appear to revolve around some aspect of consciousness. Its activity has been linked to future-thinking and mind-wandering, self-referential judgments and metacognitive awareness, introspection and autobiographical memory (2,3,4). These functions also support a link between the DMN and metacognition, a process of thinking about one’s own cognition which they are aware of themselves as engaging in (as opposed to consciousness, which occurs outside of our awareness and understanding). However, the awareness involved in metacognition lends itself to the ability to study ‘higherorder’ states, which allows researchers to examine bigger questions concerning mechanisms underlying conscious awareness. This research highlighting significant roles of the DMN in altered states of consciousness ultimately implies that digging deeper into the mechanisms underlying functional outcomes of its activity may give us a better understanding of the cognitive symptoms in a number of psychopathologies. In turn, we may better understand the mechanisms underlying effective treatments for them, particularly alternative approaches like psychedelics.

As the name “network” suggests, the harmony of the independent nodes, or specific brain regions, making up the DMN is critical to its neural function. The coupled activity of these nodes is often described in terms of functional connectivity, or the temporal correlation of changes in the activity of a specific region (6). As such, deficits in the functions linked to DMN activity occur in contexts where the coupling of activity of DMN nodes or those of networks the DMN connects to is altered. For example, DMN dysfunction has been implicated in a number of psychological disorders including depression, ADHD, and schizophrenia. The failure of other brain networks to suppress the DMN during task performance has been correlated to metacognitive deficits observed in schizophrenia symptomatology as well as deficits in task performance in individuals with ADHD (3,5). Similarly, asynchronous activation of DMN nodes has also been associated with rumination in individuals with depression (2).

ABOVE: Artist’s rendition of the neural network.

Because the DMN is related to both psychopathologies and metacognition/ consciousness, understanding this link could open doors to alternative or novel treatments for conditions like mood disorders. It is no surprise then that alternative methods for treating psychopathologies like depression include meditation and psychedelics, the effects of which are also closely tied to DMN function. Associations of DMN nodes with one’s subjective sense of self has led to its characterization as “the neurobiological seat of the ego”, a title reinforced by evidence demonstrating its effective modulation by psychedelics like psilocybin and ketamine (4). Ego dissolution is analogous to “ego death”, the experience of distortions to one’s subjective sense of “self” or “ego”, in an experimental context (6). Research has found that under the influence of psilocybin, ego dissolution is correlated with decreased functional connectivity of the DMN, but also that the day after psilocybin-assisted psychotherapy, connectivity of the DMN was actually enhanced (7, 8).

In light of the DMN’s association with introspection, the suggestion that these results might correspond to a restructuring of one’s perspective, beliefs, view of the world, and even life narrative to in turn facilitate self-actualization appears substantiated. In this sense, while mainstream treatments for mood disorders such as selective serotonin reuptake inhibitors (SSRIs) used as pharmaceutical antidepressants emphasize a mechanism of rebalancing neurotransmitters, psychedelics may offer the opportunity to figuratively and literally restructure your mind. The benefits of meditation and mindfulness practices are also tied to the promotion of metacognitive awareness (9). The DMN and its functional connections with areas of the prefrontal cortex (PFC) considered ‘higher order’, such as the anterior and dorsolateral PFC, have been found to be altered by these mindfulness-based practices. This suggests that an interplay between the mind-wandering associated with DMN activity and metacognitive awareness associated with higher order PFC regions, such as that of internal states of the body, might be engaged by such practices (10).

By continuing to uncover neurobiological facets of the DMN that appear to differ among states of consciousness, from those induced by psychedelics to those that characterize episodes of mood disorder symptoms, the use of alternative methods to target pathologies featuring altered DMN function may garner more support.