Occurrence and phenomenology of hallucinations in the general population: A large online survey

Abstract

Although epidemiological studies report that hallucinations occur in 6-15% of the general population, little is known about their phenomenology. To overcome this paucity, this study investigates the phenomenological characteristics of hallucinations in the general population, by using a nationally promoted online survey to assess hallucination phenomenology in four sensory modalities, through a self-report version of the Questionnaire for Psychotic Experiences (QPE), in 10,448 participants (aged 14-88 years). The phenomenology of hallucinations was assessed if hallucinations reportedly occurred in the past month. In the past month, auditory hallucinations were reported most frequently (29.5%), followed by visual (21.5%), tactile (19.9%), and olfactory hallucinations (17.3%); hallucinations in two or more modalities were reported by 47.6%. Substantial numbers of participants rated their hallucinations as severe, due to negative content (16.0-31.6%), previous bothersome experiences (14.8-20.2%), ensuing distress (10.5-16.8%), and/or ensuing disfunctioning (12.7-17.3%). Decreased insight was found in 10.2-11.4%. Hypnagogia was reported by 9.0-10.6%, and bereavement hallucinations by 2.8%. Despite a low prevalence of delusions (7.0%), these phenomena were significantly associated with recent hallucinations, observed in up to 13.4% of the participants with hallucinations during the past week (p < 0.001). Our results indicate a wide variety of the phenomenology of hallucinations in the general population and support the existence of a phenomenological continuum.

Fig. 1. Occurrence of hallucinations.

Based on response to screening items from the QPE in the entire study sample (n = 10,448), as measured for the auditory (green; b), visual (yellow; c), tactile (blue; d), and olfactory (red; e) sensory modalities, as well as an overall category combining the occurrence of hallucinations in all four sensory modalities (gray; a).

Fig. 2. Phenomenology of auditory hallucinations in the past month.

Total n (AH) = 3086. Additional phenomenological items are listed in Supplementary Fig. 3. Data are listed per QPE item, as obtained through multiple choice items with one answer possibility (a–f) and an open question, asking participants to describe the experienced content of AH (g, total percentage exceeds 100%). Invalid entries AH (g): likely external source (n = 4); experienced while asleep (n = 5); incomplete or incomprehensible information (n = 57).

Fig. 3. Phenomenology of visual hallucinations in the past month.

Total n (VH) = 2248. Additional phenomenological items are listed in Supplementary Fig. 4. Data are listed per QPE item, as obtained through multiple choice items withone answer possibility (a–f) and an open question, asking participants to describe the experienced content of VH (g; total percentage exceeds 100%). Invalid entries VH (g): incomplete or incomprehensible information (n = 44).

Fig. 4. Phenomenology of tactile and olfactory hallucinations in the past month.

Phenomenology of tactile hallucinations (a, b) and olfactory hallucinations (c, d) in the past month. Total n (TH) = 2077; total n (OH) = 1807. Additional phenomenological items are listed in Supplementary Fig. 5. Data are listed per QPE item, as obtained through multiple choice items with one answer possibility (a, c) and open questions, asking participants to describe the experienced content of TH (b) or OH (d). Some descriptions of TH contained terms that may be difficult to distinguish from regular physiological reactions to internal or external stimuli, such as itch, pain, shivers, or goosebumps, and were therefore categorized as ‘physiological’. Invalid entries TH: likely external source (n = 5); incomplete or incomprehensible information (n = 47). Invalid entries OH (d): likely external source (n = 24); incomplete or incomprehensible information (n = 42).

Fig. 5. Overlap between four sensory modalities of hallucinations experienced during the past month.

Total n = 5335. Each colored area represents the sample of participants with hallucinations in the past month in the following sensory modalities: auditory (AH; green), visual (VH; yellow), tactile (TH; blue), and olfactory (OH; red). Overlapping areas indicate participants that experience phenomena in overlapping modalities, the accompanying number of participants indicated within that area. Area sizes are not scaled to proportion.

Fig. 6. Presence of delusions in the past week, plotted against the most recent occurrence of hallucinations.

Total n = 6523. The distribution was statistically significant (χ² 228.3, p < .001, df 3). The presence of hallucinations was categorized as follows: Never, reported having never experienced hallucinations (n_delusions = 23); more than 1 month ago, reported having ever experienced hallucinations, but not during the past month (n_delusions = 66); past month but not week, reported having experienced hallucinations during the past month, but not the past week (n_delusions = 72); past week, reported having experienced hallucinations in the past week (n_delusions = 293).