What do a dose of psilocybin and a night in a Parkinson's ward have in common? According to a major international review in Schizophrenia Bulletin, they share more than one might imagine. Both produce visual hallucinations - and, remarkably, both seem to activate and suppress the brain's visual system in strikingly similar ways.
Researchers from the International Consortium on Hallucinations Research set out to compare two seemingly distant realities: the visionary worlds induced by serotonergic psychedelics such as psilocybin and DMT, and the visual distortions that arise spontaneously in Lewy body diseases like Parkinson's and dementia with Lewy bodies (DLB). Their findings open a doorway between neurology and consciousness studies - a place where perception, imagination, and neurochemistry intertwine.
The Brain's Conversation With Itself
At first glance, the comparison seems paradoxical. Psychedelics amplify the brain's dynamic complexity; neurodegeneration, by contrast, degrades it. Yet both conditions produce vivid visual experiences that feel real even when logic says they are not.
The review shows that in both psychedelics and Lewy body diseases, the brain exhibits a dual signature:
- Hyperactivity in higher-order associative cortices (the regions that interpret and give meaning).
- Hypoactivity in early visual regions (the ones that simply register light and form).
This imbalance - too much interpretation, too little sensory data - creates a state where the brain begins to fill in the gaps. When bottom-up sensory information weakens, the mind's internal models take over, projecting imagery from memory, expectation, and emotional salience. In short, the brain stops receiving the world and starts predicting it.
This phenomenon, known as hallucinogenesis through sensory degradation, may be the common thread between mystical visions and neurodegenerative illusions.
Entities, Shadows, and the Architecture of Seeing
The researchers catalogued the strange continuity of hallucination types across these domains. Patients with Parkinson's disease often begin with metamorphopsias - distortions of color, size, and movement - and progress toward complex "entity" hallucinations, from fleeting presences in peripheral vision to fully formed people or animals. Psychedelic users report a similar evolution, often within a single session: from geometry and light to recognizable forms and encounters with sentient beings.
The same brain area appears to mediate these experiences: the middle temporal (MT) cortex, responsible for integrating visual motion. Both Parkinsonian patients and psychedelic subjects show reduced MT responsiveness. The brain compensates by overactivating higher visual regions, producing motion where there is none - an echo of the mind's desperate attempt to make sense of incomplete data.
From there, the hallucination becomes self-sustaining. The cortex feeds on its own feedback, turning imagination into perception. This feedback loop - the hyper-excitatory top-down takeover - may be one of the fundamental mechanics of hallucinatory consciousness.
Serotonin's Hidden Role
The biological bridge between Parkinsonian hallucinations and psychedelic visions lies in serotonin receptors, particularly 5-HT2A and 5-HT1A. Psychedelics like psilocybin and DMT activate these receptors, reshaping communication across cortical hierarchies. In Lewy body diseases, the same receptor (5-HT2A) becomes upregulated - more abundant than normal - and correlates with the onset of hallucinations. Remarkably, medications that block this receptor, such as pimavanserin, can reduce or eliminate these hallucinations without worsening motor symptoms.
This overlap suggests that the serotonergic system - long associated with mood, sleep, and perception - is also the regulator of visual precision. When finely tuned, it balances sensory input and imagination. When overstimulated or degraded, it blurs the boundary between the two.
Vision as a Predictive Model
The study proposes a fascinating inversion of how we think about seeing. Perception is not passive. The brain is not a camera - it is a predictive modeler. Each moment of vision results from a negotiation between external data (bottom-up) and internal expectations (top-down). Hallucinations, whether induced by psychedelics or disease, emerge when the balance tilts too far toward expectation. The mind begins to believe its own projections.
This model aligns with the predictive coding framework in modern neuroscience, which views consciousness as a continuous inference loop. When the signal from the outside world weakens - due to degeneration, sensory deprivation, or serotonergic disruption - the loop closes in on itself. Vision becomes imagination mistaken for reality.
The Shared Threshold Between Science and Mystery
For Seven Reflections, the implications reach beyond medicine. These findings blur the line between pathology and revelation. A hallucination, after all, is the brain's proof that imagination can override matter. Psychedelics temporarily open that door; neurodegeneration leaves it ajar. Both reveal that reality, as we experience it, is an active construction - fragile, adaptive, and deeply dependent on balance between chaos and control.
Perhaps what we call "seeing" is not a sensory act at all, but a conversation between neurons about what might be there. And when that dialogue becomes too one-sided - when the brain begins to speak louder than the world - we step across the border where biology meets meaning.
In that shared space between psilocybin and Parkinson's, the mind shows its most haunting truth: it does not merely witness reality - it creates it.
