Psilocybin and the Default Mode Network: What fMRI Actually Reveals
In the year 2012 a small team of researchers at Imperial College London put volunteers inside an MRI scanner and gave them a dose of psilocybin. The participants thought that the drug would increase activity across the brain.. The data showed almost the opposite. Certain regions of the brain, which are deeply connected and use a lot of energy became less active. The biggest decreases happened in a network called the Default Mode Network.
That paper and the many papers that followed did not prove that psilocybin expands consciousness. It did not validate any claims. What it did was give neuroscience a place to look. For the time the subjective reports of psychedelic experiences, such as the feeling of unity and the sense of timelessness had a possible neural connection. The story that came out is more complicated than the headlines. The limits of the evidence are just as important as the findings.
This article explains what the Default Mode Network is, what psilocybin appears to do to it and what the current research can and cannot tell us. It ends with a section on how to read this literature for yourself beyond the press releases.
What is the Default Mode Network?
The Default Mode Network, also called DMN is not one part of the brain. It is a group of regions that work together. It became famous because of an observation. In the 1990s neuroimaging researchers noticed that certain brain areas decreased their activity when a participant focused on something external. When the task ended and the person was just lying in the scanner those same regions became active again.
By 2001 the neuroscientist Marcus Raichle named this pattern the brains default mode. The regions involved including the prefrontal cortex, the posterior cingulate cortex and parts of the inferior parietal lobe were always doing something even when the rest of the brain was at rest. They were not idle. They were busy with a kind of activity.
Later research found that DMN activity is connected to processes that involve thinking about oneself. Mind-wandering, remembering the past thinking about the future and constructing a sense of self all show DMN activity. When you are not focused on a task your brain runs a kind of simulation of your identity in the background and the DMN is the part of the brain that makes this simulation happen.
This matters because much of what we call consciousness is built on top of this self-referential narrative. The DMN is where the feeling of being a I" gets generated. That is what makes what happens next so interesting.
How psilocybin changes the picture
Psilocybin is a drug that affects the brain. The imaging studies are where the DMN story actually begins.
The first major paper, from the Carhart-Harris group in 2012 found that intravenous psilocybin decreased blood flow in regions of the DMN. The decrease in DMN activity correlated with the intensity of the effects participants reported. Larger decreases meant reported changes in consciousness.
Later studies refined the picture. They suggest psilocybin does two things at once. First it reduces the connection within the DMN. Second it increases communication between networks that normally do not talk to each other. The brain becomes less segregated. This dual effect is now treated as a mechanism for the feeling of ego dissolution that participants report.
Pull quote: The DMN is, in some practical sense, where the feeling of being a continuous "I" gets generated. Psilocybin appears to loosen its coherence, and the self loosens with it, briefly.
What the evidence can. What it cannot
It is tempting to conclude that we have a complete neurobiological account of mystical experience. We do not. The evidence supports some claims but it does not rule out many alternatives.
What the evidence supports:
Psilocybin produces measurable, reproducible decreases in DMN activity in human subjects across multiple independent studies.
The magnitude of those decreases tracks, statistically, with the magnitude of subjective effects on the Mystical Experience Questionnaire and similar instruments.
The brain shifts toward a more globally integrated state under psilocybin, then returns to baseline as the drug clears.
These effects appear in healthy volunteers and in clinical populations alike.
What the evidence does not support — at least not yet:
That decreased DMN activity causes the subjective experience, rather than accompanying it. Correlation is not causation, and fMRI cannot establish causal direction.
That a single mechanism explains the wide variation in subjective reports. Two people with similar DMN patterns can describe wildly different experiences.
That the DMN changes explain the lasting clinical or psychological effects that some people report weeks or months later. The acute neural signature and the long-term outcomes are studied separately, and the connection between them is still hypothetical.
That what is seen in 30-person fMRI studies will generalize cleanly to the general population, or to specific clinical conditions.
A useful rule of thumb when reading any neuroscience paper: the strength of the claim about brain activity is usually well-supported. The strength of the claim about what that activity means subjectively is often weaker than the press release suggests.
The REBUS hypothesis, a frame
In 2019 Karl Friston and Robin Carhart-Harris published a theoretical paper proposing a framework called REBUS. The idea is that psychedelics temporarily relax the brains expectations. Decreased DMN activity corresponds to weakened down constraints on perception and self-modeling. With expectations new associations form and old patterns of self-narrative become revisable.
In predictive coding theory, the brain constantly generates predictions about incoming sensory data. When predictions match, you experience an ordinary, expected world. When they mismatch, the brain updates the model. Over a lifetime, certain predictions — about self, identity, social roles, emotional reactions — become extremely entrenched. They are what predictive coding researchers call "priors." Priors save energy. They also constrain what experiences are possible.
REBUS is a hypothesis, not a confirmed mechanism. It fits subjective reports and clinical findings but it is not uniquely supported by the evidence. Other models. They may be describing different parts of the same phenomenon.
REBUS is a hypothesis, not a confirmed mechanism. It fits a wide range of subjective reports and clinical findings, but fitting evidence is different from being uniquely supported by evidence. Other models exist. The thalamic filter model, for example, emphasizes changes in how the thalamus gates sensory information. The neuroplasticity model focuses on the structural changes psilocybin may induce in dendritic spines. None of these models are mutually exclusive. They may be describing different parts of the same elephant.
The honest position is that we have multiple plausible frameworks, each supported by different lines of evidence, and no consensus on which is most complete.
Clinical implications cautious optimism
The DMN findings have generated interest in research particularly for treatment-resistant depression. The reasoning is straightforward. Many psychiatric conditions involve self-focused negative thinking, which is exactly the kind of activity the DMN seems to support. If psilocybin temporarily disrupts DMN coherence perhaps it offers patients a window in which negative self-narratives become more malleable.
Several trials have produced results with reductions in symptoms that persist weeks beyond a single dosing session. These trials are small though and not yet sufficient to recommend psilocybin as a treatment.
What critics get right
Not everyone in the field is sold on the DMN story. Several critiques deserve a fair hearing.
The first critique is methodological. Most psilocybin neuroimaging studies have sample sizes and effect sizes can be inflated. Replication has been better than in areas of neuroscience but it is still limited.
The second critique is about the tool itself. FMRI is a measure. It does not measure activity directly. Drawing conclusions about neural events, from that signal requires care.
The third thing to consider is the concept itself of the Default Mode Network. Some researchers think that what we call the Default Mode Network is actually overlapping subnetworks that do different things and that putting them all together might make things more confusing than clear.
The fourth thing is about language. The phrase ego dissolution has a lot of even religious meaning that might not match up with what the brain is actually doing. Reducing self- processing for 90 minutes is not necessarily the same thing as having a unitive mystical experience even if people say they had the latter after the former. None of these criticisms mean the research is bad. They just help us understand it better. The story of the Default Mode Network is one of the well-supported ideas in psychedelic neuroscience but it is still a young field.
What this does not prove
There are some things that people might claim. They are not actually supported by the current evidence. Psilocybin does not turn off the ego because the ego is not a thing that can be turned on or off. The Default Mode Network is not the place where consciousness happens. It is one of several big networks that are involved in self-related processing. Decreased Default Mode Network activity is not unique to psychedelics because things like meditation certain types of breathwork and even some stages of sleep show similar changes.. How intense an experience is does not always match up with how much it helps clinically because some people who have mild experiences get a lot better while some people who have intense experiences do not get better as much. These differences matter because they affect what we can responsibly say about the drug, the experience and the science.
How to read the literature yourself
If you want to learn about this research there are three things you can do. First try to read the research papers instead of summaries in the news because the Imperial group, the Johns Hopkins group, the NYU group and several European labs publish their papers openly and you can find most of the important papers on PubMed. Second pay attention to how many people were in the study what kind of study it. When the brain imaging was done, because these things can affect what the results mean.. Third look for the parts of the paper where the researchers talk about the limitations of their study because these parts are often more helpful than the summary at the beginning and researchers in this field have been very honest about what their data cannot show.
The larger frame
Psilocybin and the Default Mode Network is one of the popular ideas in contemporary neuroscience partly because it gives us a nice connection between the molecular and the experiential. We can point to a receptor, a network, a pattern of connections and a subjective report. Tell a story that connects them all. That connection is real. It is not as strong as people often make it out to be.
What we know is that psilocybin makes measurable reproducible changes in the way large-scale brain networks are organized and that these changes match up with the intensity of effects in a statistically meaningful way. What we are still trying to figure out is what these changes mean how they relate to long-term outcomes and whether the Default Mode Network idea will still be useful when we have advanced tools. The story is not finished yet.. That is okay. It is the way things are in an active scientific field and the best thing to do is to follow the evidence wherever it goes even if it takes us away from the most compelling story.