REBUS and the Entropic Brain: How Psychedelics Rewrite Reality

Language: en

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Overview

In 2019, Robin Carhart-Harris and Karl Friston published what has become the most influential theoretical paper in psychedelic science: “REBUS and the Anarchic Brain: Toward a Unified Model of the Brain Action of Psychedelics” in Pharmacological Reviews. The paper synthesizes two frameworks — Carhart-Harris’s entropic brain hypothesis and Friston’s free energy principle — into a unified model that explains how psychedelics alter consciousness, why they produce therapeutic benefit, and what they reveal about the fundamental architecture of the mind.

REBUS stands for Relaxed Beliefs Under Psychedelics. The core thesis: psychedelics work by reducing the precision weighting of high-level predictive models (beliefs, expectations, habitual interpretations) in the brain’s hierarchical processing architecture. When these top-down constraints relax, bottom-up sensory information and suppressed emotional content flow into consciousness with unprecedented force. The brain enters a state of enhanced entropy — more disordered, more flexible, more open to reconfiguration — in which entrenched pathological patterns (depression, addiction, PTSD) become temporarily malleable and available for revision.

If the brain is a corporate hierarchy, ordinary consciousness is the CEO issuing directives from the executive suite. Psychedelics send the CEO on vacation. The workers (sensory systems, emotional circuits, memory networks) suddenly speak for themselves, and the organization temporarily reorganizes from the bottom up. The result can be chaotic — but it can also be liberating, especially when the CEO had been running a dysfunctional operation.

The Predictive Processing Foundation

The Brain as a Prediction Machine

REBUS is built on the predictive processing framework — the theory that the brain is fundamentally a prediction machine. Rather than passively receiving and processing sensory information, the brain actively generates predictions about what sensory input it expects to receive, then compares these predictions against actual input. The difference between prediction and reality — prediction error — is the signal that drives learning and belief updating.

This framework, developed by Karl Friston (free energy principle), Andy Clark (predictive processing), and Jakob Hohwy (prediction error minimization), proposes that conscious perception is not a direct readout of sensory reality but a “controlled hallucination” — a model constructed by the brain and constrained (but not determined) by sensory input. What you see is not the world; it is the brain’s best guess about the world, updated by sensory evidence.

The brain’s predictive models are organized hierarchically. At the lowest levels, predictions concern simple sensory features (edges, colors, frequencies). At higher levels, predictions concern objects, scenes, social situations, and abstract concepts. At the highest levels, predictions concern the self — who you are, what you value, what the world means to you. These high-level predictions are what philosophers call “beliefs” and what psychologists call “schemas.”

Precision Weighting

Not all predictions are created equal. The brain assigns a “precision weight” to each prediction — a measure of confidence. High-precision predictions dominate perception and cognition; the brain treats them as reliable and gives them priority. Low-precision predictions are easily overridden by sensory evidence.

Precision weighting is the brain’s gain control system. It determines which predictions constrain perception and which are open to revision. In healthy function, precision weighting is flexible — the brain increases precision on reliable predictions and decreases it on unreliable ones. In psychiatric pathology, precision weighting becomes rigid — certain predictions become pathologically precise, dominating perception and behavior regardless of contrary evidence.

Depression can be modeled as pathologically precise negative self-predictions: “I am worthless,” “Nothing will ever improve,” “I don’t deserve love.” These predictions have become so precise, so dominant, that no amount of positive evidence can override them. The prediction error signal (“But your friends do care about you”) is registered but dismissed because the prior (“I am worthless”) has such high precision that it overwhelms the evidence.

The REBUS Model

The Core Mechanism

REBUS proposes that psychedelics work by reducing the precision weighting of high-level priors — particularly the self-related priors that constitute the ego and its habitual beliefs. The mechanism is pharmacological: 5-HT2A receptor agonism on cortical pyramidal neurons disrupts the hierarchical organization of predictive processing by reducing the gain of top-down predictions relative to bottom-up sensory signals.

The result: the brain’s predictive hierarchy temporarily flattens. High-level beliefs lose their dominance. Sensory information, emotional content, and associative connections that are normally filtered out by top-down predictions emerge into consciousness. The rigid patterns of thought and perception that characterize both ordinary consciousness and psychiatric pathology become temporarily malleable.

This is the “relaxation” in REBUS — the relaxation of the brain’s belief system. It is not relaxation in the colloquial sense (calm, peaceful) but in the technical sense (reduced constraint, increased degrees of freedom). The psychedelic state is often intensely active and emotionally demanding; the relaxation is of cognitive rigidity, not of arousal.

The Entropic Brain

The entropic brain hypothesis, first published by Carhart-Harris in 2014, provides the quantitative framework for REBUS. Entropy — the mathematical measure of disorder, randomness, and information content — is proposed as the key variable that tracks the quality of conscious experience.

The entropic brain hypothesis places conscious states on a spectrum:

Sub-critical states (low entropy): Deep sleep, anesthesia, coma. Too little entropy for rich conscious experience. The brain is ordered but empty — a silent, crystalized system.

Critical states (medium entropy): Normal waking consciousness. The brain operates near a phase transition — the “edge of chaos” — where information processing capacity is maximized. Enough order for coherent experience, enough disorder for flexible responsiveness.

Super-critical states (high entropy): The psychedelic state, dream states, early psychosis. Too much entropy for ordinary stable consciousness, but rich in information content, associative connections, and novel experiences. The brain is exploring its full dynamical repertoire.

The entropic brain hypothesis predicts that psychedelics increase neural entropy above the normal waking baseline. This has been empirically confirmed: Lempel-Ziv complexity analysis of MEG data during psilocybin, LSD, and ketamine states all show increased neural signal diversity compared to placebo — the first pharmacological demonstration that brain entropy can be pushed above the normal waking level.

Psychiatric Pathology as Excessive Order

REBUS offers a unifying model of psychiatric pathology: conditions like depression, addiction, OCD, and PTSD are characterized by excessive order — pathologically precise, rigid, self-reinforcing predictive models that resist updating.

Depression: Rigid negative self-model (“I am worthless”) with pathologically high precision, overwhelming positive evidence.

Addiction: Rigid reward predictions (the substance/behavior will produce satisfaction) with pathologically high precision, overwhelming evidence that the addiction is destructive.

OCD: Rigid threat predictions (contamination, danger, incompleteness) with pathologically high precision, driving compulsive behaviors despite rational understanding that the threat is minimal.

PTSD: Rigid threat memories replaying with pathologically high precision, as if the trauma is recurring rather than remembered.

In all cases, the problem is not the content of the beliefs but their precision — their rigidity, their resistance to updating, their dominance over conflicting evidence. Psychedelics address this common mechanism by reducing precision across the hierarchy, allowing all of these rigid patterns to become temporarily malleable.

The Therapeutic Window

REBUS explains why psychedelics produce lasting therapeutic benefit rather than merely temporary symptom relief. The acute psychedelic experience creates a window of enhanced neural plasticity — both at the psychological level (relaxed beliefs, emotional openness, new perspectives) and at the biological level (psychoplastogenic dendritic growth, critical period reopening, epigenetic modification).

During this window, the brain can:

1. Disrupt entrenched pathological priors (the rigid beliefs that maintain the psychiatric condition)
2. Access suppressed emotional content (the unprocessed trauma, grief, or insight that the rigid priors had been keeping out of consciousness)
3. Revise the belief system (form new, healthier priors based on the emotional processing and new perspectives gained during the experience)
4. Consolidate the revised beliefs through enhanced neuroplasticity (the new synaptic connections that physically instantiate the revised belief system)

The therapeutic context — preparation, skilled therapist presence, and integration — is essential because it determines the direction of revision during the plasticity window. Without therapeutic scaffolding, the brain might revise its beliefs in unhelpful or harmful directions.

Empirical Support

Neuroimaging Evidence

Multiple neuroimaging studies have confirmed REBUS predictions:

Reduced hierarchical processing: Directed information flow analysis (using Granger causality and transfer entropy) shows that psilocybin reduces the directed flow of information from higher cortical areas (prefrontal cortex) to lower areas (sensory cortices) while increasing the flow from lower to higher areas. This is the neural signature of relaxed top-down prediction and enhanced bottom-up signaling.

DMN disruption: The default mode network — the neural substrate of the narrative self and its habitual predictions — shows dramatically reduced within-network connectivity and reduced activity during psychedelic states. The degree of DMN disruption correlates with the subjective intensity of the experience and with therapeutic outcomes.

Increased global connectivity: Psychedelics increase functional connectivity between brain regions that do not normally communicate, particularly between sensory networks and the DMN. This inter-network connectivity increase reflects the dissolution of the normally sharp boundaries between processing modules — the hierarchical barriers that top-down predictions maintain are broken down.

Increased neural entropy: Lempel-Ziv complexity, sample entropy, and other entropy measures are consistently elevated during the psychedelic state, confirming the entropic brain hypothesis.

Clinical Correlations

REBUS predictions about the relationship between acute experience quality and therapeutic outcome have been confirmed:

Mystical experience predicts outcome: The degree of mystical experience (unity, transcendence, noetic quality, sacredness) during psilocybin sessions predicts therapeutic outcome in depression, addiction, and end-of-life distress. REBUS interprets this: mystical experience represents maximal relaxation of self-related priors, creating the deepest possible window for belief revision.

Emotional breakthrough predicts outcome: The degree of emotional processing (accessing and working through difficult emotional content) during the psychedelic session predicts therapeutic outcome. REBUS interprets this: emotional breakthrough occurs when the priors that normally suppress emotional content (defense mechanisms, avoidance patterns) are relaxed.

Challenging experiences are not harmful: Contrary to common assumption, challenging experiences during psychedelic sessions (anxiety, fear, paranoia, confusion) do not predict worse outcomes and may predict better outcomes. REBUS explains this: challenging experiences indicate that deeply entrenched priors are being disrupted — the difficulty is the medicine.

The Free Energy Principle Connection

Friston’s Framework

REBUS gains its mathematical foundation from Karl Friston’s free energy principle, which proposes that all biological systems minimize variational free energy — a mathematical quantity related to prediction error. Minimizing free energy means minimizing surprise — maintaining the organism within its expected states.

In Friston’s framework, the brain’s predictive models serve to minimize free energy by generating accurate predictions that reduce prediction error. High-precision priors are models that have been successful at reducing prediction error in the past and have been reinforced by experience.

Psychedelics, by relaxing the precision of these priors, temporarily increase free energy — the brain enters a state where it is further from its expected states, where prediction error is elevated, where more possibilities are available. This is metabolically costly and psychologically intense, but it creates the conditions for model revision: the brain can explore a wider landscape of possible belief configurations and settle on a new configuration that better fits the evidence.

The Landscape Metaphor

Friston’s mathematics describes the brain’s belief system as a landscape — an energy surface with valleys (attractors, corresponding to stable belief configurations) and hills (unstable states the system avoids). Pathological beliefs correspond to deep, narrow valleys — highly stable but poorly matched to reality. The brain is trapped in a local minimum.

Psychedelics heat up the landscape — adding energy that allows the system to escape local minima and explore the broader landscape. This is mathematically analogous to simulated annealing in optimization: temporarily increasing temperature (entropy) to escape suboptimal solutions and find better ones.

The therapeutic process is then: (1) heat up the landscape (psychedelic administration), (2) explore the landscape (the psychedelic experience, with therapeutic guidance), (3) settle into a new, better minimum (integration and consolidation of new beliefs).

Limitations and Criticisms

The Unfalsifiability Concern

Critics argue that REBUS, because it is built on the extremely general free energy principle, may be unfalsifiable. If any outcome can be explained post hoc as “precision relaxation” or “free energy minimization,” the model cannot be meaningfully tested. Carhart-Harris and colleagues have responded by specifying concrete, quantitative predictions (entropy measures, directed information flow, DMN connectivity) that can be measured and potentially disconfirmed.

Individual Variability

REBUS does not fully account for the enormous variability in psychedelic responses. The same dose of the same compound can produce dramatically different experiences in different individuals, and these differences are not well-predicted by the model’s variables. Genetic factors (5-HT2A receptor density and polymorphisms), personality factors (openness, absorption, suggestibility), and contextual factors (set and setting) all modulate the response in ways that REBUS acknowledges but does not precisely model.

The Role of the Subjective Experience

A fundamental question remains: is the subjective psychedelic experience necessary for therapeutic benefit, or is the biological effect (neuroplasticity, entropy increase, DMN disruption) sufficient? REBUS implies that the subjective experience is essential — it is the process by which beliefs are consciously accessed, processed, and revised. But non-hallucinogenic psychoplastogens (Olson’s tabernanthalog) may produce neuroplasticity without subjective experience, potentially dissociating the biological from the experiential. If TBG proves therapeutically effective, REBUS will need to explain why.

Four Directions Integration

• Serpent (Physical/Body): REBUS grounds the psychedelic experience in physical brain dynamics: 5-HT2A receptor activation, cortical pyramidal neuron excitability, thalamocortical circuit disruption, entropy increase measurable by information-theoretic analysis of neuroimaging data. The body’s neural hardware is the medium through which beliefs are relaxed and revised. Somatic experiences during psychedelic sessions — body heat, trembling, nausea, waves of sensation — are the physical correlates of the predictive hierarchy reorganizing itself.

• Jaguar (Emotional/Heart): REBUS explains why psychedelic therapy is emotionally intense: when the priors that suppress difficult emotions are relaxed, the emotions surge into consciousness. This emotional flooding is not a side effect but the mechanism. The grief that was blocked, the rage that was contained, the love that was feared — all emerge when the precision weighting that held them down is released. The jaguar’s fire is the emotional intensity of belief revision.

• Hummingbird (Soul/Mind): REBUS describes the liberation of the mind from its own habitual constraints. When high-level predictions relax, the mind enters a state of genuine openness — novelty, wonder, awe, the felt sense of seeing the world for the first time. This is the phenomenology of reduced precision: the world is no longer filtered through the lens of expectation but encountered directly, with all its strangeness and beauty. The hummingbird drinks from every flower because no prediction tells it which flowers to ignore.

• Eagle (Spirit): The mystical experience — the core predictor of therapeutic outcome — is the state of maximal prior relaxation. When all self-related predictions dissolve (ego dissolution), what remains is consciousness without content — pure awareness, unbounded by any belief about who you are or what the world is. The contemplative traditions call this liberation, awakening, satori. REBUS provides a neuroscientific framework for the most universal of spiritual experiences: the dissolution of the separate self and the encounter with something larger.

Key Takeaways

• REBUS (Relaxed Beliefs Under Psychedelics) proposes that psychedelics work by reducing the precision weighting of high-level predictive models (beliefs) in the brain’s hierarchical processing architecture.
• The entropic brain hypothesis provides the quantitative framework: psychedelics increase neural entropy above the normal waking baseline, expanding the brain’s repertoire of accessible states.
• Psychiatric conditions (depression, addiction, PTSD, OCD) are modeled as pathologically precise priors — rigid beliefs that resist updating. Psychedelics relax these priors, creating a window for belief revision.
• The therapeutic window involves both psychological (relaxed beliefs, emotional processing, new perspectives) and biological (neuroplasticity, critical period reopening) components.
• Empirical support includes confirmed predictions about reduced hierarchical processing, DMN disruption, increased between-network connectivity, and increased neural entropy.
• The mystical experience and emotional breakthrough during psychedelic sessions predict therapeutic outcome, consistent with REBUS’s emphasis on the experiential process of belief revision.

References and Further Reading

• Carhart-Harris, R. L., & Friston, K. J. (2019). REBUS and the anarchic brain: Toward a unified model of the brain action of psychedelics. Pharmacological Reviews, 71(3), 316-344.
• Carhart-Harris, R. L. (2018). The entropic brain — revisited. Neuropharmacology, 142, 167-178.
• Carhart-Harris, R. L., et al. (2014). The entropic brain: A theory of conscious states informed by neuroimaging research with psychedelic drugs. Frontiers in Human Neuroscience, 8, 20.
• Friston, K. (2010). The free-energy principle: A unified brain theory? Nature Reviews Neuroscience, 11, 127-138.
• Schartner, M. M., et al. (2017). Increased spontaneous MEG signal diversity for psychoactive doses of ketamine, LSD and psilocybin. Scientific Reports, 7, 46421.
• Clark, A. (2013). Whatever next? Predictive brains, situated agents, and the future of cognitive science. Behavioral and Brain Sciences, 36(3), 181-204.
• Seth, A. K. (2021). Being You: A New Science of Consciousness. Dutton.