Psychedelic Phenomenology¶

Psychedelics globally increase the permeability of the implicit-explicit boundary, exposing intermediate processing stages in hierarchical order from simple phosphenes to full dream-like scenes.

Psychedelic substances -- LSD, psilocybin, DMT, mescaline, salvia divinorum -- produce a remarkably consistent phenomenological profile: visual intensification, geometric patterns, synesthesia, altered time perception, emotional amplification, and at higher doses, ego dissolution and radical identity alteration. The Four-Model Theory accounts for this entire profile through a single principle: variable permeability of the boundary between implicit models (IWM, ISM) and explicit models (EWM, ESM).

The Permeability Mechanism¶

Under normal conditions, the implicit-explicit boundary acts as a selective filter. The vast computational activity of the substrate -- early visual processing, proprioceptive calibration, motor planning -- remains below the threshold of conscious access. Psychedelics weaken this filter globally, allowing intermediate processing stages to leak through to the conscious simulation.

The key insight is that this leakage is not random. It follows the visual processing hierarchy in a predictable, dose-dependent order:

Low dose / early onset: V1-level processing becomes accessible -- simple phosphenes, enhanced contrast, breathing and movement in static patterns.
Increasing dose: V2/V3-level processing becomes accessible -- geometric patterns, fractals, tessellations. These correspond to Kluver's (1966) form constants and are mathematically modeled by Bressloff et al. (2002).
Higher dose: Higher visual area processing becomes accessible -- faces, figures, complex scenes.
Very high dose: Full intermediate processing accessible -- narrative dream-like visions with emotional depth.

This ordered progression is a direct consequence of the permeability gradient: lower-level (earlier, simpler) processing stages become accessible before higher-level (later, more complex) ones, because the permeability increase propagates up the hierarchy.

Figure¶

graph TD
    subgraph "Dose-Dependent Permeability Increase"
        D1["Low Dose"]
        D2["Moderate Dose"]
        D3["High Dose"]
        D4["Very High Dose"]
    end

    subgraph "Hierarchical Content Exposure"
        V1["V1: Phosphenes\nEnhanced contrast, breathing patterns"]
        V23["V2/V3: Geometric Patterns\nFractals, tessellations, form constants"]
        HV["Higher Visual Areas:\nFaces, figures, scenes"]
        FULL["Full Access:\nDream-like narrative visions"]
    end

    D1 -->|"permeability ↑"| V1
    D2 -->|"permeability ↑↑"| V23
    D3 -->|"permeability ↑↑↑"| HV
    D4 -->|"permeability ↑↑↑↑"| FULL

    V1 --- V23
    V23 --- HV
    HV --- FULL

    style D1 fill:#E8F5E9,stroke:#333
    style D2 fill:#C8E6C9,stroke:#333
    style D3 fill:#A5D6A7,stroke:#333
    style D4 fill:#66BB6A,stroke:#333,color:#fff
    style V1 fill:#E3F2FD,stroke:#333
    style V23 fill:#BBDEFB,stroke:#333
    style HV fill:#90CAF9,stroke:#333
    style FULL fill:#42A5F5,stroke:#333,color:#fff

As dose increases, permeability rises through the processing hierarchy. Each level of the visual system becomes accessible in order, from V1 phosphenes through to full dream-like scenes.

REBUS Alignment¶

The independently developed REBUS (Relaxed Beliefs Under Psychedelics) model of Carhart-Harris and Friston (2019) arrives at a compatible account through the predictive processing framework: psychedelics relax top-down priors, allowing bottom-up prediction errors to propagate more freely. The Four-Model Theory and REBUS converge on the same phenomenological prediction -- hierarchical content exposure -- but from different theoretical starting points. REBUS explains that the hierarchy relaxes; the Four-Model Theory specifies where the relaxation occurs in the architecture (the implicit-explicit boundary) and what the exposed content represents (intermediate processing stages of the substrate that are normally kept implicit).

Intensity as Novelty¶

The characteristic sense of profundity during psychedelic experiences reflects not increased consciousness level but increased novel content. The permeability increase floods the simulation with information that the substrate has been processing all along but has never included in the conscious model. The conscious self encounters this content for the first time, producing the intense sense of revelation -- even though the substrate has been computing it continuously.

Key Takeaway¶

Psychedelics globally increase implicit-explicit permeability, producing a dose-dependent, hierarchically ordered exposure of normally unconscious processing stages -- from V1 phosphenes through geometric patterns to full dream-like visions. The progression is predictable because it follows the visual processing hierarchy.