REBUS (psychedelics)
Explanation
REBUS (Relaxed Beliefs Under Psychedelics) is a neuroscientific model proposed by Robin Carhart-Harris and Karl Friston in 2019 to explain how psychedelics produce their characteristic subjective effects. The thesis: these substances relax the high-level priors (prior beliefs) of the brain, allowing sensory information to be processed in non-habitual ways.
The theoretical framework is predictive processing. Normally, the brain generates hierarchical predictions: higher regions predict what lower ones should sense, and only prediction errors are passed up to correct beliefs. This conserves resources but introduces biases: we see what we expect to see, we feel what we are predisposed to feel.
Psychedelics (especially serotonergic ones such as psilocybin, LSD, DMT) act on 5-HT2A receptors that are particularly abundant in higher cortical regions. By activating these receptors, top-down predictions are relaxed, losing precision. This has several consequences: sensory information gains relative weight, rigid classifications dissolve, experiences become more fluid, multidimensional, connected.
Subjectively, REBUS explains the typical features of psychedelic states: ego dissolution (rigid models of the self relax), synaesthesias (previously separate sensory modalities communicate), mystical experiences (subject-object categories blur), emotional intensification (emotions are no longer filtered by habitual priors).
Therapeutic application: in pathologies such as depression, anxiety, addiction and PTSD, patients are trapped in rigid and negative cognitive priors. REBUS suggests that psychedelics, by relaxing these priors, open a plasticity window in which new beliefs can form. Psychotherapy integrated with the substance helps consolidate beneficial changes during this period.
REBUS is part of the broader entropic brain hypothesis, and connects with predictive-processing theories, IIT and other theories of consciousness. Its influence on clinical neuroscience is growing, especially with the resurgence of scientific research on psychedelics for psychiatric disorders.
Strengths
- An integrative theoretical framework.
- Foundation for assisted psychedelic therapies.
- Integrates phenomenology and neuroscience.
- Testable predictions.
Main critiques
- Still under construction: partial mechanistic detail.
- Generalisation of 'priors' to all experience disputed.
- Risk of pharmacological oversimplification.
- Clinical evidence under development.