Understanding the Default Mode Network
How TMS and Ketamine work together to reset the Default Mode Network
The Default Mode Network (DMN) is a brain network that is best known for being active when we are not focused on the outside world. It lights up on fMRI and PET scans when we are doing things such as daydreaming, ruminating, planning for the future, thinking about the past, thinking about ourselves and other inward thoughts. It is composed of the medial prefrontal cortex, posterior cingulate cortex/precuneus and angular gyrus.
Observations related to the activity level of the brain at rest date back to the late 1930s when the inventor of the electroencephalogram (EEG) first noticed electrical oscillations even when the brain wasn't doing an active task. The idea of a DMN was controversial but has gained increasing credibility with more advanced imaging techniques including the fMRI and PET scans.
Research into the DMN expanded in the 2000s as the DMN appeared to have more clinical significance in areas such as Alzheimer’s disease, autism, ADHD, chronic pain, trauma based disorders and Major Depressive Disorder. In particular to depression, rumination, a hallmark feature of Major Depressive Disorder, is associated with increased DMN connectivity and control over other networks during rest (3).
Fortunately, there are several interventions that can help to modulate the activity of the DMN including exercise, acupuncture, meditation, psychotherapy and antidepressant medications. TMS and psychedelic drugs such as ketamine also exert influence on the DMN.
Transcranial magnetic stimulation has been shown to help normalize depression-related hyperconnectivity in the DMN (1). TMS also helps to restore balance between the DMN and other areas of the brain such as the dorsolateral prefrontal cortex (DLPFC) (1).
There is also evidence that psychedelics help to facilitate disruption of the default mode network. Studies have shown that the degree of ego dissolution a patient experiences when treated with psilocybin correlates with decoupling of nodes within the DMN circuit as seen on fMRI. A sense of ego dissolution positively predicted improved psycho-social functioning four months following treatment (2).
It is hypothesized that this desynchronization within the DMN allows the brain to “reboot” following a treatment with ketamine or other psychedelic agents. It should be noted, however, that these treatments are being carried out in very controlled settings with patients who have received counseling on how to prepare themselves for the treatment. In the case of psychedelics, it seems that the mindset of the patient, along with the treatment setting, are just as important as the medication for the treatment to be effective.
When used together, TMS and ketamine can be very powerful treatments that work in conjunction with other interventions such as psychotherapy, meditation and exercise. The Default Mode Network helps us to conceptualize how these treatments are complementary to one another. It is likely to be an area of interest for researchers and clinicians as our understanding of the neuroanatomy of Psychiatric conditions continues to grow and develop.
Resources
Conor Liston et al, “Default Mode Network Mechanisms of Transcranial Magnetic Stimulation in Depression”,Biological Psychiatry; Volume 76, Issue 7, October 2014, Pages 517-526.
Lukasz Smigielski, Milan Scheidegger, Michael Kometer, Franz X. Vollenweider, “Psilocybin-assisted mindfulness training modulates self-consciousness and brain default mode network connectivity with lasting effects”,NeuroImage, Volume 196, 2019, Pages 207-215.
Yvette I. Sheline, Deanna M. Barch, Joseph L. Price, Melissa M. Rundle, S. Neil Vaishnavi, Abraham Z. Snyder, Mark A. Mintun, Suzhi Wang, Rebecca S.Coalson, Marcus E. Raichle, “The default mode network and self-referential processes in depression”, Proceedings of the National Academy of Sciences Feb 2009, 106 (6) 1942-1947; DOI:10.1073/pnas.0812686106
