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  • Writer's pictureNina Kumari

Unlocking how psychedelics affect the brain

Psychedelic states induce an altered state of consciousness. Modern studies using brain imaging have allowed us to explore what might actually be going on in the brain during this altered state. And whilst a lot of questions still remain, here is the low down on what is happening in the brain during the psychedelic experience.

The summary

In simplest terms, neuroimaging studies have supported the idea that psychedelics affect multiple key brain regions that form a network coined the default mode network (DMN). It appears that psychedelic compounds significantly reduce activity in the brain’s default mode network and function to ‘reboot' the brain. This reboot is thought to enable the more prolonged beneficial effects of these substances.

So what is the Default Mode Network?

The default mode network refers to a group of interconnected brain regions that are associated with internally directed thought processes like self-reflection or self-criticism. Typically at times where we find our ‘minds wandering’, ourselves empathising with others as well as projecting ourselves in the past or future - our DMN is in an increasingly active state. In general, the DMN tends to be more active at rest than when individuals are focused on the external environment or performing a task.

Interestingly, the current hypothesis is that evolutionary the DMN played a major role in our survival, helping us maintain a continual sense of self, whilst differentiating ourselves from the world around us.

Increased activity in the DMN and mental health

Recent research has identified the involvement of the default mode network in mental health disorders, particularly depression (1). These studies have suggested that increased DMN activity may be a marker of a ''depressed mode'', that relates to a person being hypercritical, holding rigid thought patterns and ruminating on negative thought loops about themself.

Psychedelics and the Default Mode Network

Psychedelics are interesting in that they have been shown to: (a)decrease activity in some of these key hub brain areas of the DMN, (b)reduce connectivity within the DMN (2,3) whilst (c) also increasing connectivity between other brain networks that allows for stronger connections and cross-talking.  Interestingly these brain network changes correspond in timing with the peak psychedelic state (the point where individuals report their ego dissolving (a loss of the sense of a separate self)).

These brain network changes are similar to changes noted whilst individuals are in a meditative state ( a technique thought to also induce an altered state of mind). Experienced meditators have been shown to have reduced activation of the DMN (4).  These links suggest that psychedelics, like meditation, help individuals think about things in the here and now, rather than ruminate in the past or future.

Brain wave changes and the psychedelic experience

Using a tool that measures brain wave activity in the brain, researchers have identified common themes of brain activity across different psychedelics.  Psilocybin, DMT and LSD (5) have all shown a drop in alpha rhythms across the DMN that correlates in timing when individuals report a loss of sense of self during the psychedelic experience. Interestingly alpha waves typically are the most prominent wave frequency in the brain and signature for high-level consciousness common to man.

DMT has been unique in also showing an increasing prominence in gamma and theta waves, (alongside these changes) as the individual reaches peak experiences. These waves, which are ordinarily increased during dreaming and REM sleeping, are thought to relate to the immersive altered consciousness perception of the DMT experience (6).

Psychedelics and the disordered brain

These changes in the brain (in terms of brain activity and network connections) have helped researchers build a hypothesis that psychedelic use leads to the brain becoming more ‘entropic’ - more disordered and unpredictable, enabling the breakdown of functional brain units and enhanced global connectivity. This can disrupt certain pathways whilst allowing new connections between brain regions that were previously kept separate.

It is thought that this helps to unlock the brain's ability to remodel itself ( known as plasticity). In one such study at Imperial, changes in DMN in patients with treatment-resistant depression were found to decrease straight after treatment but increase one-day post-dose. This process has been likened to a reset mechanism, where this sudden ‘shock’ that breaks down connections in e.g. DMN, helps to reset the brain and reorder the brain into a healthier mode (7), particularly when supported by psychotherapy work which can help individual learn new more positive thought patterns.

“Brain imaging studies suggest that when psychedelics are absorbed they decrease activity in the default mode network. As a result the sense of self appears to temporarily shut down, and thus ruminations may decrease. The brain states observed show similarities to deep meditative states, in which increased activity occurs in pathways that do not normally communicate. This process has been compared to defragmenting a computer. Following this, it appears that the default mode network becomes more cohesive. We think this could be one of the reasons levels of anxiety and depression appear to reduce.

Dr. Simon Ruffell, Psychiatrist and Senior Research Associate at King’s College London

So what are the next steps

The ideas suggested above all still remain a hypothesis and a greater understanding of the brain, in general, will be needed before we can truly understand what these changes in the brain actually mean. This in time will also help build our understanding of how these changes can establish psychological health benefits.


(1) Broyd, S. J. et al. Default-mode brain dysfunction in mental disorders: a systematic review. Neurosci Biobehav Rev 33, 279–296, (2009).

(2) R.L. Carhart-Harris et al., “Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin,” PNAS, 109:2138-43, 2012.

(3)R.L. Carhart-Harris et al., “Neural correlates of the LSD experience revealed by multimodal neuroimaging,” PNAS, 113:4853-58, 2016.

(4) K.A. Garrison et al., “Meditation leads to reduced default mode network activity beyond an active task,” Cogn Affect Behav Neurosci, 15:712-20, 2015.

(5) D. Muthukumaraswamy et al., “Broadband cortical desynchronization underlies the human psychedelic state,” J Neurosci, 33:15171-83, 2013.

(6) Timmermann, C., Roseman, L., Schartner, M. et al. Neural correlates of the DMT experience assessed with multivariate EEG, 2019, 10.1038/s41598-019-51974-4.

(7) Carhart-Harris et al. Psilocybin for treatment-resistant depression: fMRI-measured brain mechanisms, 2017. 10.1038/s41598-017-13282-7

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