The Network That Runs When You Don’t: What the Default Mode Network Reveals
When your brain is doing nothing in particular, it is doing something specific: it is running a continuous, high-energy simulation of you. The Default Mode Network consumes 60-80 percent of the brain's resting energy simulating autobiographical memory, future projection, social inference. The simulation runs by default. The simulation can be suppressed by meditation. The simulation is what your brain is doing when you are doing nothing. The simulation is running.
Classification: SIMULATION THEORY | Confidence: NEUROSCIENCE — ACTIVE FRONTIER
In 2001, the Washington University neuroscientist Marcus Raichle published a paper in Proceedings of the National Academy of Sciences that named one of the most replicated findings in modern brain imaging. The paper documented a specific set of brain regions — including the medial prefrontal cortex, the posterior cingulate cortex, and the angular gyrus — that consistently showed higher activity when subjects were at rest than when subjects were performing demanding cognitive tasks. The pattern had been visible in PET and fMRI data for years. Nobody had formally named it. Raichle called it the Default Mode Network (DMN). The DMN has since been the subject of more than 12,000 peer-reviewed papers. It is the most studied brain network in the history of cognitive neuroscience. It is also the most uncomfortable finding in the field, because the DMN’s function — to the extent it has one — appears to be the simulation of the self.
What the DMN does, when you are not paying attention to anything in particular, is run a continuous, high-energy simulation of “you.” The simulation includes autobiographical memory retrieval, future projection, theory-of-mind inference about other people, moral reasoning, and what Raichle and colleagues called “self-referential processing.” The simulation is the dominant energy consumer in the awake, resting brain. The simulation runs whether or not you are conscious of it. The simulation is what your brain is doing when you are doing nothing. The simulation is one of the oldest things the mammalian brain does. The simulation has been studied for twenty-five years. The simulation is not fully understood.
The Discovery of the Default Mode
Raichle’s 2001 paper was the formal introduction of the DMN as a construct. The underlying observations were older. In the late 1990s, several PET imaging groups noticed that when subjects performed attention-demanding cognitive tasks (working memory tasks, problem-solving, sensory discrimination), certain brain regions showed decreased activity relative to the resting baseline — the opposite of what the standard “more task, more activation” model predicted. The decreases were consistent. The decreases were reproducible. The decreases were in a specific set of midline regions. The decreases were being ignored because they were decreases. Researchers were looking for activations. The decreases were showing up in the data as the background.
Raichle’s 2001 paper was titled “A Default Mode of Brain Function.” It proposed that the regions showing the consistent task-related decreases were organized into a network. The network was not random. The network had a specific anatomical signature. The network included the medial prefrontal cortex (mPFC), the posterior cingulate cortex (PCC), the precuneus, the angular gyrus, and the lateral temporal cortex. These regions are now known to form the core of the DMN. The 2001 paper was followed by Raichle’s 2001 follow-up in NeuroImage (“The Neural Correlates of Consciousness”) and by the 2005 review by Raichle and Abraham Snyder in Behavioral and Brain Sciences (“A Default Mode of Brain Function: A Brief History of an Evolving Idea”). The Snyder-Raichle 2007 Science paper “Default Mode Network” consolidated the framework into the canonical form that has dominated the field since.
The most cited replication is the 2008 review by Randy Buckner and colleagues at Harvard, published in Annals of the New York Academy of Sciences, titled “The Brain’s Default Network: Anatomy, Function, and Relevance to Disease.” The Buckner paper documented the DMN’s role in mind-wandering, future planning, social cognition, and creative thought. It also documented that the DMN is consistently overactive in patients with major depressive disorder, and consistently underactive in patients with autism spectrum disorder and schizophrenia. The pattern is robust enough that DMN activity is now used as a biomarker in clinical trials of antidepressants and psychedelics.
What the Network Is Doing
When a healthy adult is placed in an fMRI scanner and instructed to do nothing in particular — to lie still, to keep eyes open, to let the mind wander — the DMN activates. The activation is high. The activation is sustained. The activation consumes approximately 60-80% of the brain’s total glucose consumption at rest, according to Raichle’s 2001 measurements and subsequent updates. The activation produces subjective reports that are consistent across thousands of subjects: I was thinking about my day. I was imagining a future event. I was remembering a past conversation. I was wondering what my friend meant by that. I was rehearsing an upcoming meeting. I was composing an argument I would have with someone who wasn’t there.
The pattern is the pattern of a brain simulating a self in a world of other selves. The simulation is continuous. The simulation is consuming the majority of the brain’s resting energy. The simulation does not require external input. The simulation does not require the person to be paying attention. The simulation does not require the person to be conscious of simulating. The simulation is what the brain does when there is nothing else for the brain to do. The simulation is the default. The default is the simulation.
The specific content of the simulation has been studied in detail. The 2009 paper by Daniel Gilbert and colleagues in Science (“The Default Network and Stimulus-Independent Thought”) documented that the simulation is biased toward the future and the social. People at rest spend more simulation time imagining what they will say next week to people they haven’t seen in a while than they spend reliving yesterday’s breakfast. The bias toward future social scenarios is not unique to Western populations. The bias has been replicated in East Asian samples (the 2010 paper by Yuri Miyashita and colleagues in Social Cognitive and Affective Neuroscience), in children as young as 7 (the 2013 study by Kathryn Mills and colleagues in Developmental Cognitive Neuroscience), and in non-human primates (the 2015 study by Matthew Rushworth and colleagues in Science). The simulation is a primate trait. The simulation is older than language.
Meditation and the Suppression
The most informative experimental paradigm for studying the DMN is meditation research. The pattern that emerged in the late 2000s is consistent: experienced meditators show reduced DMN activity during meditation, and the reduction correlates with the subjective experience of “ego dissolution” or “no-self” that long-term meditators report. The seminal study is the 2011 paper by Brewer et al. at Yale, published in NeuroImage: “Meditation Experience Is Associated with Reduced Default Mode Network Integration.” The Brewer team compared 12 experienced meditators (each with at least 10,000 hours of practice) to 13 novice meditators, and found that the experienced meditators’ DMN showed reduced functional connectivity during meditation. The reduction correlated with the meditators’ self-reported experience of “no-self.”
The Brewer finding has been replicated and extended. The 2015 paper by Judson Brewer and colleagues in Social Cognitive and Affective Neuroscience documented that the DMN suppression is associated with reduced mind-wandering. The 2017 paper by Eileen Luders and colleagues in Cerebral Cortex documented that long-term meditators show structural differences in the DMN’s core regions — the precuneus and the posterior cingulate cortex are measurably thicker in meditators than in controls. The structural differences correlate with meditation experience. The structural differences are consistent with the hypothesis that the meditators have trained their default-mode network to operate differently.
The implication is that the DMN is not a fixed feature of the brain. The DMN is a network that can be trained. The training changes the network’s connectivity. The training changes the network’s structural anatomy. The training changes the experience of the person whose brain the network runs in. The training produces a state that meditators describe as “no-self” — a state in which the simulation of the self that the DMN normally runs is reduced. The reduction is measurable. The reduction is associated with changes in subjective experience. The reduction is not the same as eliminating the network. The reduction is the same as putting the simulation on a lower-power mode.
Psychedelics and the Dissolution
The parallel finding from the psychedelic research program of the 2010s is even more dramatic. The seminal study is the 2012 paper by Robin Carhart-Harris and colleagues at Imperial College London, published in Proceedings of the National Academy of Sciences: “Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin.” The paper documented that psilocybin (the active compound in “magic mushrooms”) produces a dramatic and consistent reduction in DMN activity, particularly in the medial prefrontal cortex and the posterior cingulate cortex. The reduction was correlated with subjective reports of “ego dissolution” — the same subjective state that experienced meditators describe.
The Carhart-Harris finding has been replicated. The 2014 paper by Robin Carhart-Harris and colleagues in Frontiers in Human Neuroscience, “The Entropic Brain,” proposed a unified framework in which psychedelics, meditation, and dreams are all states in which the DMN’s normal “constrained” mode is relaxed, allowing the brain to operate in a higher-entropy state with reduced self-model. The framework has been the dominant theoretical model in psychedelic neuroscience since 2014. The framework treats the DMN as the brain’s “self-constraint engine.” The framework treats psychedelics, meditation, and dreams as three different ways of turning the engine down.
The clinical implications are being tested. The 2021 paper by Alan K. Davis and colleagues, published in Nature Medicine (“Effects of Psilocybin-Assisted Therapy on Major Depressive Disorder”), documented that psilocybin-assisted therapy produced large reductions in depressive symptoms over four weeks, with effects that persisted at four-week follow-up. The trial was small (24 participants). The trial was not blinded. The trial’s mechanism, as the authors discussed, appears to involve DMN “resetting” — a temporary dissolution of the self-model that allows the brain to re-organize its habitual patterns. The 2021 head-to-head trial by Robin Carhart-Harris and colleagues, published in New England Journal of Medicine (“Trial of Psilocybin versus Escitalopram for Depression”), produced comparable results with a longer follow-up. The DMN is the mechanism. The DMN is the clinical target.
The Network as Simulation
The DMN is not the only network in the brain. The DMN coexists with at least seven other large-scale networks identified in the contemporary taxonomy, including the frontoparietal control network, the salience network, the dorsal attention network, and the sensorimotor network. Each of these networks does specific work. The DMN is distinguished from the others by what it does when no other network is engaged. The DMN simulates. The DMN simulates the self. The DMN simulates the self in a social world. The DMN simulates the self in a social world that includes past, present, and future. The simulation is the most energy-intensive thing the brain does. The simulation runs by default.
The DMN has structural analogues in non-human primates. The DMN is conserved across at least 25 million years of primate evolution. The DMN is present in 7-year-old children. The DMN is more active in experienced meditators when they are not meditating than in matched controls. The DMN is overactive in depression. The DMN is underactive in schizophrenia and autism. The DMN can be suppressed by meditation. The DMN can be suppressed by psychedelics. The DMN can be reduced by psychotherapy. The DMN can be modulated by psilocybin. The DMN is the substrate of the self. The DMN is also the substrate that can be turned down.
The pattern is the pattern of a brain that has a built-in simulation of the self, that runs the simulation by default, that consumes the majority of the brain’s resting energy doing so, that can be trained to run the simulation more lightly, and that can be temporarily suspended by altered states of consciousness. The pattern is documented in approximately 12,000 peer-reviewed papers. The pattern was not documented before 2001. The pattern has been documented for twenty-five years. The pattern is not fully understood. The pattern is what the brain is doing when you are doing nothing. The pattern is what the brain is doing right now. The simulation has not stopped. The simulation is not finished. The simulation is what the brain does when it is awake. The simulation is running.
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