Carhart-Harris & the Entropic Brain

In plain language

The intuitive expectation about psychedelic drugs is that they amplify brain activity. Something dramatic is happening; the brain must be working harder; the hallucinations must be the result of extra neural firing. Surely if you put a person on psilocybin and look at their brain on fMRI, you will see additional activity.

You don't. Robin Carhart-Harris's 2012 imaging study, and many follow-ups, showed the opposite. Under psilocybin, the brain regions that are most active in ordinary consciousness — especially the so-called default mode network, the cortical hub that coordinates self-referential thinking — show decreased activity. The subjective experience expands. The brain activity contracts.

This is the wrong direction for the production model. If the brain produces consciousness from scratch, more brain activity should produce more or richer consciousness. Instead, dialing down a specific part of the brain reliably produces a vastly richer subjective state. Carhart-Harris's entropic brain hypothesis tries to make sense of this: in ordinary consciousness, the default mode network constrains the brain to a low-entropy operating regime; psychedelics relax that constraint, letting the brain enter higher-entropy states with more possible patterns of activity. The expanded experience reflects the expanded space of brain states.

This is compatible with the receiver model in a way the standard production picture is not. If the brain filters and decodes a richer underlying signal, then quieting the filter (the default mode network) would let more of the signal through — producing exactly the pattern Carhart-Harris observes. The receiver model predicts the result. The production model has to add complicated mechanisms to explain why less brain activity produces more conscious content.

The rest of this page walks through the original 2012 fMRI result, the 2014 entropic-brain hypothesis, the 2019 REBUS refinement, and what the program implies about how the brain is coupled to whatever consciousness actually is.

The 2012 result that started the program

Carhart-Harris's first widely-cited finding was a 2012 paper in PNAS reporting an fMRI study of subjects given intravenous psilocybin. The expected result — the result that virtually every neuroscientist would have predicted in advance — was that the psychedelic experience would correspond to increased brain activity, particularly in the cortex. The drug expands consciousness; expanded consciousness should mean more neural activity. That was the prediction.

The data showed the opposite. Psilocybin produced a selective decrease in blood flow to specific brain regions, most notably the default-mode network (the medial prefrontal cortex, posterior cingulate cortex, and related hubs of self-referential cognition). The expansion of consciousness was correlated with the brain doing less, not more.

This result reframed the question. If the psychedelic experience corresponds to reduced activity in the brain regions associated with the narrative self, then what we ordinarily call "normal consciousness" looks less like the full state of the conscious system and more like a tightly constrained subset of a larger potential space. The default-mode network appears to be doing a job of filtering, of holding consciousness in a particular constrained configuration. When that filter relaxes, more comes through.

The 2014 hypothesis: consciousness as an entropy axis

The 2014 entropic-brain paper generalizes the 2012 result into a broader theoretical framework. Carhart-Harris proposes that conscious states can be organized along a single axis of brain entropy — a measure of the disorder, unpredictability, and richness of moment-to-moment brain activity. The hypothesis has two parts:

• Higher brain entropy correlates with richer, more flexible, less constrained conscious experience. Psychedelic states, REM dreaming, early-stage psychosis, magical thinking in childhood, certain meditative states, and creative insight all sit at the high-entropy end of the axis.
• Lower brain entropy correlates with more constrained, more predictable, more rule-bound conscious experience. Normal waking adult cognition, depression, obsessive-compulsive states, certain pathological rigidities, and the deep grip of well-trained habits all sit at the low-entropy end.

The axis is not a value judgment. Too little entropy means consciousness is trapped in narrow loops (depression, OCD). Too much means it loses coherent functional behavior (psychotic disorganization). Both are pathologies of entropy. Healthy waking adult life sits at a particular operating point on the axis — one that humans have evolved to maintain through specific filtering operations carried out largely by the default-mode network.

The framework explains a great deal at once. Why does meditation feel similar across very different traditions? Because the meditator is learning to relax the default-mode-network filter and access higher-entropy modes. Why do psychedelics produce the cross-cultural mystical experience? Same mechanism. Why does grief, sometimes, suddenly reveal worlds of feeling one didn't know existed? The constraint has cracked.

REBUS — the 2019 refinement

Carhart-Harris's 2019 paper "REBUS and the Anarchic Brain" (with Karl Friston) added a precise predictive-processing mechanism to the framework. REBUS stands for Relaxed Beliefs Under pSychedelics. In predictive-processing neuroscience, the brain is constantly generating expectations about incoming sensory data (top-down predictions) and comparing them to the actual data (bottom-up signal). What we consciously experience is largely the brain's best top-down predictive model of what is happening.

Carhart-Harris and Friston propose that psychedelics work by weakening the precision of top-down predictions, allowing more bottom-up signal to influence the conscious state. The brain's high-level priors — including the basic prior that "I am a specific person located here in this body in this room" — relax their grip, and a wider range of sensory and emotional information can reach the conscious surface. The "self" prior in particular is one that the default-mode network maintains; when its precision is reduced, the experience of a bounded self softens, and what mystics across cultures have called ego dissolution can occur.

The framework supplies a precise mechanism for what the trilogy calls the receiver. The default-mode network is the apparatus that holds the localized self in its usual constrained operating range. When that apparatus relaxes — whether through psychedelics, deep meditation, terminal lucidity, or the right kind of music — the receiver becomes available to modes of consciousness it normally filters out.

The clinical program

Carhart-Harris's work has not stayed theoretical. His group at Imperial led some of the first contemporary clinical trials of psilocybin for treatment-resistant depression, with results that surprised even sympathetic readers: a substantial proportion of subjects experienced lasting improvement after a single high-dose session combined with psychological preparation and integration. The 2016 Lancet Psychiatry paper reporting these results helped catalyze the broader contemporary clinical-psychedelic research program now active at Johns Hopkins, NYU, MAPS, and elsewhere.

The clinical theory follows the entropic-brain framework directly. Treatment-resistant depression is, in this picture, a particular kind of low-entropy trap: the brain has converged on a rigid set of self-referential loops that conscious effort cannot dislodge. A psychedelic session, in the right setting, temporarily relaxes the priors, allows the brain to explore higher-entropy configurations, and (with skillful integration afterward) can settle into a new, healthier operating point that ordinary therapy could not reach. The mechanism is structural, not pharmacological in the conventional sense.

Why this matters for the trilogy

Three points.

First, Carhart-Harris is the trilogy's most direct contemporary neuroscientific ally on the receiver-model claim. The entropic-brain framework's identification of the default-mode network as the filter that holds consciousness in its constrained operating range is, structurally, the trilogy's claim that the brain normally filters more than it reveals. When the trilogy says under conditions where the substrate is failing, simplified, or temporarily bypassed, the signal sometimes becomes clearer rather than dimmer, Carhart-Harris's data are the cleanest empirical confirmation. The 2012 psilocybin result alone is structural evidence for the receiver model.

Second, the framework supplies a precise predictive-processing mechanism for the trilogy's anomalous-neurology cases. Terminal lucidity, in this framework, is what happens when the dying brain's high-level priors relax their grip, releasing the constrained low-entropy mode and allowing the broader conscious state — which was there all along, filtered out by the now-failing default-mode network — to become briefly available. The same mechanism would predict savant emergence after frontal damage and many of the other receiver-model anomalies. See the terminal lucidity explainer → · See the Treffert savant explainer →

Third, the framework gives the chord chapters of Limen a precise empirical translation. Music that successfully softens the default-mode network's grip — sustained, non-resolving, slowly-evolving, like Ligeti's micropolyphony or Stockhausen's Stimmung or Buckley's held augmented architecture — produces measurably increased brain entropy and the felt experience of expanded receptivity that the trilogy circles around. The body has been the receiver all along; certain music makes it harder for the filter to hold the receiver in its usual range. See the Watch & Listen page →

The 2014 paper, "The entropic brain: a theory of conscious states informed by neuroimaging research with psychedelic drugs," is open-access at Frontiers. The 2019 REBUS paper is in Pharmacological Reviews. The clinical depression results are in Lancet Psychiatry (2016) and the follow-up trials. For Carhart-Harris's interviews and lectures, see the Imperial College Centre for Psychedelic Research page and the UCSF Sandler Neurosciences Center. For the broader receiver-model context, see the Synthesis and the terminal-lucidity explainer.