Psilocybin and Default Mode Network Changes in Depression

Psilocybin & Depression

3× greater network disruption than stimulants

Psilocybin caused more than threefold greater change in brain connectivity than methylphenidate in a 2024 Nature study, establishing that the scale of DMN disruption is pharmacologically driven, not just experiential.

Siegel et al., Nature, 2024

Two-phase effect

acute DMN suppression during dosing followed by post-treatment reorganization — confirmed across multiple studies in Gattuso's 2023 systematic review

43 patients

in Daws 2022 (Nature Medicine): post-treatment global brain integration correlated with antidepressant response; the escitalopram group showed no comparable change

REBUS model

Carhart-Harris and Friston's 2019 framework explains DMN hyperactivity in depression as overly rigid predictive priors — psilocybin loosens them temporarily

Siegel et al., Nature, 2024

View as image ↗

Most coverage of psilocybin's brain effects focuses on a single event: the suppression of the default mode network during the psychedelic session. That finding is real, well-documented, and consistent across studies. But the research has built toward a more precise claim, one that changes what the phrase "psilocybin and the default mode network" actually means for depression.

The default mode network, or DMN, is a cluster of brain regions that activate together during self-referential thinking and rumination. In a healthy brain, the DMN switches off when attention turns outward. In depression, it stays overactive — the circuitry behind repetitive negative thought, rigid self-narrative, and the kind of rumination that makes depression resistant to simple effort or logic. Psilocybin suppresses the DMN's connectivity during the dosing session. After the session, the DMN's relationship with other brain networks reorganizes. That post-treatment reorganization is what the evidence links to antidepressant outcomes.^[1]

A 2023 systematic review by Gattuso and colleagues documented this two-phase pattern across multiple imaging studies. Psilocybin showed the strongest depression-specific DMN effect of any psychedelic studied, and the post-treatment changes represented genuine network reorganization rather than a return to the prior depressed state.^[1] This article traces both phases and uses the REBUS model — the most complete theoretical framework for why the imaging patterns look the way they do — to explain what the brain scans mean for depression treatment. The broader clinical evidence is covered in the guide to psilocybin for major depression.

What the Default Mode Network Is — and Why Depression Locks It in the On Position

The default mode network is not a single brain region. It is a set of regions that activate together when the mind is not engaged with an external task. The medial prefrontal cortex, positioned behind the forehead, generates and maintains the narrative sense of self. The posterior cingulate cortex, toward the back and center of the brain, integrates past experience and memory with current emotional tone. The hippocampus and angular gyrus support the autobiographical and episodic dimensions of self-reflection. When these regions fire in concert, the result is the stream of self-referential thought: daydreaming, reviewing the past, imagining the future, thinking about how others perceive you.

In a healthy brain, the DMN is not always on. When attention shifts to an external task, the DMN deactivates and task-focused networks take over. The brain moves cleanly between modes.

In depression, that switch degrades. The DMN stays active even when the person tries to focus outward. Its output, in a depressed brain, is not neutral self-reflection. It is rumination. The same circuits that generate identity and self-awareness keep generating negative predictions about the self and the future — resistant to counter-evidence, resistant to the person's own attempts to redirect attention.

The REBUS model, proposed by Carhart-Harris and Friston in a 2019 review in Pharmacological Reviews, gives this a precise computational description.^[2] In that framework, the brain is a prediction-generating machine. It maintains internal models of the world and the self, and updates those models when new information contradicts them. The DMN implements the brain's model of the self — its running narrative about identity, expectations, and meaning.

In depression, that narrative becomes what Carhart-Harris and Friston call an overly strong top-down predictive prior. The brain's story about itself is weighted so heavily that new information cannot revise it. Positive experiences fail to update the model. Social connection fails to register as meaningful. Therapeutic insights fail to stick. The brain is, in computational terms, too confident in its depressed narrative to be corrected by experience.^[2]

This is why behavioral approaches and purely cognitive techniques often fail in severe depression. Not because they are wrong, but because the brain's architecture at that moment is not built for updating. The DMN is functioning exactly as designed, weighting the self-narrative very strongly. The problem is that the narrative is wrong, and the brain will not correct it.

For a companion look at the DMN from a consciousness and ego-dissolution perspective, the piece on psilocybin and the default mode network covers the broader neuroscience angle. This article focuses specifically on what the imaging shows in the context of depression.

What Gattuso's 2023 Systematic Review Found — Both Phases, Not Just One

In 2023, Gattuso and colleagues published the first systematic review focused specifically on how psychedelics modulate the default mode network, drawing on imaging studies conducted across multiple compounds. The consistent finding across the literature: psychedelics suppress DMN connectivity acutely, and psilocybin showed the strongest depression-specific effect of any compound reviewed.^[1]

But Gattuso's review did not stop at the acute phase. The paper documented post-treatment connectivity changes that are distinct from what is measured during or immediately after the session. After psilocybin treatment in depression populations, within-DMN connectivity changed alongside reduced isolation between the DMN and other major brain networks. The brain's self-referential circuit was not simply quieted during the session and then restored to its prior depressed state. Something in its organizational connectivity had shifted.^[1]

This two-phase finding resolves a puzzle that single-study results left open. If acute DMN suppression were the complete therapeutic mechanism, the antidepressant effect should fade quickly after the session when DMN activity resumes. For many patients, it does not. The persistence of antidepressant effects — documented for weeks and in some studies for months — suggests that the session initiates a change that continues after the acute drug effects resolve. Gattuso's systematic review is the clearest documentation that such a post-treatment change exists in the imaging data.

The two phases are biologically distinct events. Acute hypoconnectivity during the session is the pharmacological disruption. Post-treatment reorganization is what that disruption leaves behind. Research that captures brain imaging only during the acute session — or reports only the acute phase — is measuring phase one and drawing conclusions about phase two.

The Scale of Phase One — What Siegel's 2024 Nature Study Quantified

How large is the network disruption during the psilocybin session? In 2024, Siegel and colleagues published a direct experimental comparison in Nature. Participants received either psilocybin or methylphenidate, an active stimulant that produces measurable brain changes and serves as a more demanding comparison than an inert placebo.^[3]

Psilocybin caused more than threefold greater change in brain connectivity than methylphenidate. The desynchronization of cortical and subcortical networks during the psilocybin session was not incrementally different from the stimulant. It was categorically larger.^[3]

The Siegel study also generated data relevant to a long-running debate: does the intensity of the subjective experience determine the therapeutic outcome? The 2024 findings complicated the view that a mystical experience is the required mechanism. The massive network disruption Siegel measured did not require participants to report ego dissolution or a peak psychedelic experience. The pharmacological effect on brain connectivity was large even in participants who did not experience the most intense subjective states.

This matters for understanding what psilocybin is doing during the session. The DMN suppression is not simply a byproduct of the psychological experience. The pharmacology produces the network disruption. Whether the subjective experience also contributes to therapeutic outcomes independently is a separate question, but the Siegel data establish that the brain-level disruption does not depend on it.^[3]

For a visual summary of what the connectivity changes measured in these studies look like before and after treatment, the companion piece on brain connectivity before and after psilocybin provides the imaging evidence side by side.

Phase Two Is the Therapeutic Mechanism — What Daws Found in 43 Patients

The question of which brain change actually explains the antidepressant effect got a direct answer in Daws et al.'s 2022 Nature Medicine study. Forty-three patients received either psilocybin therapy or a course of escitalopram, the most widely prescribed SSRI. Brain imaging measured changes in global network integration before and after treatment.^[4]

Global integration is a measure of how interconnected a brain's networks are with each other. A brain with high network modularity has tightly compartmentalized networks that do not communicate across system boundaries. Depression is associated with elevated modularity: the DMN dominates without being regulated by networks responsible for external attention, sensory processing, or flexible goal-directed behavior.^[4]

After psilocybin therapy, global brain network integration increased. Previously siloed networks began communicating across boundaries that had been rigid. Within the psilocybin group, this increase in global integration correlated with antidepressant response. Patients whose brains showed more integration gain did better clinically.^[4]

The escitalopram arm showed a different post-treatment connectivity pattern. The specific increase in global integration documented in the psilocybin group was absent in the SSRI group. Escitalopram produces measurable brain changes over its weeks of use, but those changes did not include the network-opening effect that Daws measured after psilocybin.^[4]

This repositions the global integration increase as the therapeutic mechanism, not just a correlate of recovery. The brain change that explains why psilocybin works for depression is not an amplification of what antidepressants do. It is a different category of change. Phase one — acute DMN suppression — may be the initiating event. Phase two — the global integration that follows — is what phase one leaves behind when it works.

For a broader look at how the imaging evidence maps onto clinical outcomes, including amygdala blood flow and network modularity data across multiple studies, the companion piece on what psilocybin does to the depressed brain covers the full fMRI picture.

Why the REBUS Model Explains Everything the Scans Show

The imaging findings from Gattuso, Siegel, and Daws describe what psilocybin does to the brain. The REBUS model explains why those changes would be therapeutic specifically in depression.

REBUS stands for Relaxed Beliefs Under Psychedelics. The model, proposed by Carhart-Harris and Friston in 2019, applies the predictive coding framework to explain what psychedelics do at the network level.^[2] In predictive coding, the brain does not passively receive sensory information and then react. It generates predictions about incoming information and responds primarily to the gap between its predictions and what it actually receives. The DMN is the brain's highest-level prediction system for the self: it generates and maintains the model of who you are, what to expect, and what things mean.

In depression, those top-down predictions from the DMN become too strongly weighted. The brain's self-model is, in effect, over-confident. It treats the depressed narrative — "nothing matters," "nothing will improve," "this is who I am" — as too certain to revise. Experiences that contradict the model are discounted. The brain is stuck in a local minimum of its own prediction landscape, and it lacks the flexibility to climb out.

Psilocybin, in the REBUS framework, works by suppressing the DMN's hierarchical influence over other networks. The strong top-down predictive priors flatten. Bottom-up signals from experience, sensation, and perception gain more weight relative to the top-down model. The brain becomes briefly revisable — open to updating its self-model in response to inputs it would have discounted before.^[2]

The REBUS model also explains the post-treatment reorganization that Gattuso and Daws documented. When the DMN's top-down influence relaxes during the session, other networks are temporarily freed from its dominance. In the period that follows, those networks establish new connectivity patterns — including the global integration that Daws measured — that reflect a less DMN-dominated brain organization.

The model is also consistent with Siegel's 2024 finding that the network disruption does not require a mystical experience. The loosening of predictive priors is pharmacologically produced. What the brain does with that window of revisability depends on many factors, including the therapeutic context. But the window itself is opened by the drug.

For the upstream pharmacological story — why 5-HT2A receptor binding is the specific mechanism through which psilocybin produces all of these network changes — the companion piece on psilocybin and 5-HT2A receptors covers the receptor-level evidence.

The Clinical Connection — When fMRI Changes Predicted Who Recovered

In 2017, Carhart-Harris and colleagues published the first fMRI study specifically in patients with treatment-resistant depression who received psilocybin. Nineteen patients who had not responded to at least two prior antidepressant treatments received psilocybin with psychological support, and their brains were imaged before and after treatment.^[5]

Two specific connectivity changes correlated with symptom improvement. First, cerebral blood flow to the amygdala decreased after treatment. The amygdala is the brain's primary center for emotional reactivity and threat detection; reduced blood flow corresponds with reduced reactivity to negative emotional stimuli. Second, connectivity between the parahippocampal region and the prefrontal cortex decreased after treatment. The parahippocampal-prefrontal connection supports the recycling of memory-linked emotional responses — the substrate for rumination. Less connectivity meant less rumination-circuit activity.^[5]

Carhart-Harris described the pattern as a "brain reset" — not a return to a previous state, but a temporary release of the frozen connectivity characteristic of treatment-resistant depression, allowing a different organizational pattern to emerge in its place. The reset framing maps directly onto the REBUS model: the overly confident predictive priors of the depressed brain temporarily lose their grip, and the brain reorganizes during the period that follows.

The 19-patient cohort was too small for definitive conclusions on its own. But the fMRI patterns were directionally consistent with what Daws documented at 43 patients in 2022 and with what Gattuso synthesized across the broader literature. The clinical specificity of the changes — amygdala blood flow and parahippocampal connectivity, not diffuse changes across all circuits — provided early evidence that psilocybin targets the specific circuits most implicated in depression's biology.

For readers interested in whether the brain changes that correlate with recovery can be detected before the first session — turning brain imaging from an observation tool into a patient-selection tool — the companion piece on who responds best to psilocybin for depression covers the predictor research, including 2024 work on baseline fMRI biomarkers. The piece on how brain scans are proving psilocybin rewires depression covers the broader evidence for lasting network changes.

The Bottom Line

Six imaging studies and a 2023 systematic review describe a two-phase process: psilocybin suppresses default mode network connectivity massively during the session, then the DMN's relationship with other networks reorganizes in the days and weeks that follow. The acute suppression is pharmacologically driven and does not require a mystical experience to occur. The post-treatment reorganization — measured as increased global integration in 43 patients by Daws et al. in Nature Medicine — correlates with antidepressant response. The REBUS model provides the computational framework explaining why this mechanism is specifically therapeutic in depression.

Frequently Asked Questions

Does psilocybin suppress the default mode network?

Yes, psilocybin suppresses default mode network connectivity during the session — a finding confirmed across multiple studies and synthesized in Gattuso's 2023 systematic review. But the acute suppression and the post-treatment reorganization are two different biological events. The suppression resolves as the drug clears. What correlates with antidepressant outcomes is the reorganization that follows, measured as increased global brain network integration in Daws et al.'s 2022 Nature Medicine study of 43 patients. Stopping at "psilocybin suppresses the DMN" describes phase one and misses the phase that explains the clinical effect.

What is the default mode network and why does it matter for depression?

The default mode network is a cluster of brain regions — anchored by the medial prefrontal cortex and posterior cingulate cortex — that activate together during self-referential thinking. In a healthy brain, the DMN deactivates when attention turns to external tasks. In depression, it stays hyperactive, generating the rumination and rigid negative self-narrative that characterize the illness. The REBUS model (Carhart-Harris and Friston, 2019) describes this as the brain's self-referential model becoming too strongly weighted to update — the depressed brain is too confident in its own negative narrative to revise it in response to new experience.

How does psilocybin reduce rumination in the brain?

Psilocybin temporarily disrupts the default mode network's dominance over other brain networks, reducing the rumination circuit's grip on attention. In the REBUS framework, this works by flattening overly rigid top-down predictive priors — the brain's overconfident model of the self loosens, allowing bottom-up signals from experience more influence. After treatment, connectivity between the parahippocampal region and the prefrontal cortex decreased in 19 TRD patients (Carhart-Harris et al., 2017), and this decrease correlated specifically with reduced rumination and improved mood.

What happens to the DMN when you take psilocybin?

Two distinct changes occur at different timescales. During the session, DMN internal connectivity suppresses dramatically — Siegel et al.'s 2024 Nature study found this desynchronization was more than threefold greater than methylphenidate. After the session, over the following days and weeks, the DMN's relationship with other networks reorganizes. Gattuso's 2023 systematic review documented both phases, confirming post-treatment changes in DMN connectivity that represent genuine network reorganization rather than a return to the prior depressed state. The second phase is what correlates with antidepressant outcomes.

Does psilocybin quiet the inner critic in the brain?

The imaging data is consistent with that description. The medial prefrontal cortex — the brain's primary generator of self-referential thought and the source of the self-critical inner voice — is the central node of the DMN. Psilocybin reduces the DMN's influence both during the session and in the period that follows. The post-treatment decrease in parahippocampal-prefrontal connectivity documented by Carhart-Harris et al. in 19 TRD patients correlated specifically with reduced rumination. The change is not just a quieting during the acute experience — it shows up in the brain's connectivity patterns for weeks afterward.

Sources & References

1. 1. Source details for this reference are not yet available.
2. 2. Source details for this reference are not yet available.
3. 3. Source details for this reference are not yet available.
4. 4. Source details for this reference are not yet available.
5. 5. Source details for this reference are not yet available.