Hallucinations Under Psychedelics and in the Schizophrenia Spectrum

An Interdisciplinary and Multiscale Comparison

Pantelis Leptourgos; Martin Fortier-Davy; Robin Carhart-Harris; Philip R. Corlett; David Dupuis; Adam L. Halberstadt; Michael Kometer; Eva Kozakova; Frank LarØi; Tehseen N. Noorani; Katrin H. Preller; Flavie Waters; Yuliya Zaytseva; Renaud Jardri


Schizophr Bull. 2020;46(6):1396-1408. 

In This Article


In terms of the sensory modalities involved, AH are the most common modality of hallucinations in SCZs, with a prevalence of around 79%.[62] AH are three times as frequent as VH, which have a mean prevalence of approximately 27%.[62,63] The exact prevalence of hallucinations in other modalities is largely unknown, with significant variation between studies. Estimates vary for olfactory hallucinations (6–26%), gustatory hallucinations (1–31%), and somatic or tactile hallucinations (4–19%).[62,64,65] AH occur alone approximately half of the time,[66,67] while hallucinations in other modalities almost never occur alone.[66,68] Some studies report that multimodal or "fused" hallucinations (MMH; eg, seeing a talking head)[69] are highly prevalent in SCZs,[65,70–72] whereas other reports suggest that these hallucinations are rare.[73] By contrast, hallucinations induced by 5-HT2A agonists occur primarily in the visual domain[74] (a shared feature with neurological disorders, such as Parkinson's disease and Lewy body dementia). Nevertheless, distortions of body image, tactile hallucinations, and auditory alterations are not uncommon, especially when hallucinogens such as DMT or psilocybin are administered at high doses.[75–77] Audio-visual experiences have frequently been reported, but whether they qualify as hallucinations (or synesthesias) is still debated.[77] Olfactory and gustatory hallucinations are very rare in comparison, but have occasionally been reported.[78] Synesthesia-like experiences are also very common with serotonergic hallucinogens[79] but are uncommon in SCZs.[63]

With respect to the content of VH, serotonergic hallucinogens induce both elementary (brightly colored geometric form constants such as lattices, cobwebs, tunnels, and spirals)[78] and complex hallucinations.[74,76,80] Complex hallucinations are images of scenes or landscapes, often containing "ordinary" (humans, animals, artifacts, etc.) and "extraordinary" entities (chimeras, spirits, aliens, monsters, etc.). The prevalence of complex hallucinations increases with drug dose[76,81] and as the psychedelic experience progresses over time.[82] In SCZs, VH more often includes life-size images of faces, people, objects, or events, which may be bizarre or frightening. Typically, the hallucinations experienced in SCZs are detailed, concrete, and well-anchored in space.[83]

A series of experiential changes often precede the onset of psychosis, including AH (for a review, see Refs.[84,85]). The occurrence of these prodromal hallucinations often provokes intense emotions; they may be attributed to a supernatural origin and viewed as a sign of a larger meaning or fate.[86] Similarly, the VH induced by 5-HT2A agonists are often very meaningful and can be imbued with strong existential, metaphysical, and religious overtones.[80,87–89]

Psychosis is often accompanied by very rich and detailed hallucinations that are experienced as vivid, real, and beyond volitional control.[83,90] There may be profound changes in attention, reality testing, and memory.[91] Although the hallucinations induced by 5-HT2A agonists can be extremely vivid and may even feel more real than everyday sensory experiences, insight about their etiology is typically preserved; in other words, reality testing is not impaired and subjects using hallucinogens can typically distinguish between drug effects and normal waking consciousness.[80,92,93] In contrast, in SCZs, hallucinations tend to be more difficult to discriminate from every-day perception. An important contributing factor is the contextual differences between the two states: while psychotic episodes in SCZs occur recurrently and unpredictably, the psychedelic state is transient (the nature and prevalence of chronic perceptual abnormalities, such as acid flashbacks and the hallucinogen persisting perceptual disorder are still debated[77,94]), purposeful and voluntarily initiated, thus marked by a special sense of agency (see Anthropology section).

As summarized in Table 3, psychotic and serotonergic hallucinations differ in many respects: most notably in the modalities involved in the types of hallucinatory objects, and in the reality status ascribed to hallucinations. Yet, some commonalities can also be identified, especially as regards the meaningfulness, the emotional significance, and the metaphysical/spiritual quality of hallucinations (cf. Ref.[95]).