External hallucinations

External hallucinations are perceptions of objects, entities, or events in the shared environment that have no physical source.

As a drug effect they are acute and strongly dose-dependent, easing as the substance wears off, which distinguishes them from hallucinations in a psychotic episode. They range from faint shapes and movement at the periphery of vision to fully formed figures occupying real space.

External hallucinations are uncommon and occur almost exclusively at high doses. They are most commonly induced under psychedelics such as LSD, psilocybin mushrooms, and DMT, and under dissociatives such as ketamine, dextromethorphan, and 3-MeO-PCP; deliriants such as diphenhydramine, Datura, and Amanita muscaria can produce them with particular vividness. They also arise, less frequently, under depressants such as eszopiclone and promethazine.

External hallucinations are felt as the appearance of perceptual objects within the outer world: shadows resolve into figures, surfaces grow faces, and empty space fills with movement or structure. Unlike closed-eye imagery, these percepts occupy real-world coordinates and can be strikingly convincing.

Fixating on them tends to sharpen them briefly before they dissolve or shift. At high doses, particularly under deliriants, they can achieve full scene-like presence — apparent depth, motion, and solidity — becoming difficult to distinguish from actual objects in the room.

External hallucinations arise through different pathways depending on the drug class:

• Psychedelics — act on 5-HT2A receptors, amplifying spontaneous cortical activity and weakening suppression of internally generated patterns during sensory processing.
• Dissociatives — block NMDA receptors, destabilizing the brain's predictive model of the environment so that perceptual gap-filling produces spurious objects.
• Deliriants — block muscarinic acetylcholine receptors, creating a confusional state in which vivid imagery is inserted directly into perception of real space.
• Depressants — produce hallucinatory imagery at high doses, particularly near sleep onset, through mechanisms that are not fully characterized.

How much each pathway contributes in a given case is not fully understood.

Who experiences external hallucinations, and how vividly, varies considerably. Dose is the primary determinant — the effect is rare at moderate doses and rises sharply in likelihood as doses increase.

Set and setting matter substantially, especially under psychedelics and dissociatives: a disordered or unfamiliar environment, sleep deprivation, and prior hallucinatory history all lower the threshold. Those with a tendency toward perceptual anomalies in everyday life may be more susceptible. A calm, familiar setting and a grounded mindset tend to limit their intensity and duration.