From relaxed beliefs under psychedelics (REBUS) to revised beliefs after psychedelics (REBAS)
In a small within‑subject study (N = 11) administering 1 mg and 25 mg psilocybin, the 25 mg dose acutely reduced confidence in negative self‑beliefs with reductions persisting at four weeks and strongly correlating with improved well‑being. These belief changes were linked to increased EEG entropy and subjective effects, providing preliminary empirical support for the REBUS-to‑REBAS hypothesis that relaxation and revision of belief confidence mediate psychedelic therapeutic effects and introducing a new measure to test this mechanism.
Authors
- Robin Carhart-Harris
- David Erritzoe
- Hannes Kettner
Published
Abstract
The Relaxed Beliefs Under pSychedelics (REBUS) model proposes that serotonergic psychedelics decrease the precision weighting of neurobiologically-encoded beliefs. We conducted a preliminary examination of two psychological assumptions of REBUS: (a) psychedelics foster acute relaxation and post-acute revision of confidence in mental-health-relevant beliefs; which (b) facilitate positive therapeutic outcomes and are associated with the entropy of EEG signals. Healthy individuals (N = 11) were administered 1 mg and 25 mg psilocybin 4-weeks apart. Confidence ratings for personally held beliefs were obtained before, during, and 4-weeks post-psilocybin. Acute entropy and subjective experiences were measured, as was well-being (before and 4-weeks post-psilocybin). Confidence in negative self-beliefs decreased following 25 mg psilocybin. Entropy and subjective effects under 25 mg psilocybin correlated with decreases in negative self-belief confidence (acutely and at 4-weeks). Particularly strong evidence was seen for a relationship between decreases in negative self-belief confidence and increases in well-being. We report the first empirical evidence that the relaxation and revision of negative self-belief confidence mediates psilocybin's positive psychological outcomes, and provide tentative evidence for a neuronal mechanism, namely, increased neuronal entropy. Replication within larger and clinical samples is necessary. We also introduce a new measure for examining the robustness of these preliminary findings and the utility of the REBUS model.
Research Summary of 'From relaxed beliefs under psychedelics (REBUS) to revised beliefs after psychedelics (REBAS)'
Introduction
Psychedelic therapy has attracted growing interest because of its reported transdiagnostic effects on mental health, including reductions in anxiety, depression and suicidal ideation and increases in well-being. The Relaxed Beliefs Under pSychedelics (REBUS) model frames these effects within a Bayesian predictive-processing account: serotonergic psychedelics induce an ‘‘entropic’’ brain state that relaxes the precision-weighting of high-level predictive beliefs, which should be experienced subjectively as reduced confidence in beliefs or assumptions. Within this transient state of enhanced neural and cognitive plasticity, the model proposes that maladaptive, overweighted beliefs (for example, negative self-perceptions) become amenable to revision, potentially yielding therapeutic benefit. This study set out to test two psychological assumptions derived from REBUS. First, the investigators examined whether psilocybin acutely reduces confidence in personally held negative self-beliefs (belief relaxation) and whether such reductions persist 4 weeks later (belief revision, here termed REBAS). Second, they tested whether any relaxation and revision in negative self-belief confidence is associated with (a) neurobiological markers of REBUS—specifically increases in neural entropy measured with EEG, (b) the intensity of acute unitive experiences, and (c) changes in well-being four weeks after dosing. The study focused on healthy, psychedelic-naïve volunteers and used a within-subjects dosing contrast between a subthreshold (1 mg) and an active (25 mg) psilocybin dose.
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Study Details
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- APA Citation
Zeifman, R. J., Spriggs, M. J., Kettner, H., Lyons, T., Rosas, F. E., Mediano, P. A. M., Erritzoe, D., & Carhart-Harris, R. L. (2025). From relaxed beliefs under psychedelics (REBUS) to revised beliefs after psychedelics (REBAS). Scientific Reports, 15(1). https://doi.org/10.1038/s41598-023-28111-3
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