Hallucinations, often studied in clinical contexts, are increasingly recognized as occurring in the general population (6–15% prevalence). Models of sensory perception suggest that misperceptions, including hallucinations, illusions, and distortions, are byproducts of physiological processes facilitating rapid perception. Studying hallucinations in non-clinical populations helps understand their underlying mechanisms without confounding factors like medication or comorbidity. Examining the phenomenology of hallucinations is crucial for understanding underlying mechanisms and guiding assessment and treatment. While prevalence studies exist, detailed phenomenological characteristics in the general population remain understudied. This study aims to reveal the phenomenology of hallucinations in a large sample of the general population (aged 14 and over) and assess the association with delusions.

Existing literature shows a prevalence of hallucinations in the general population ranging from 6% to 15%, based on large-scale epidemiological surveys. However, the phenomenological characteristics of these hallucinations remain largely unexplored, with most studies focusing on subpopulations (e.g., university students, elderly) or specific hallucination types. Previous research has indicated an association between hallucinations and delusions in the general population, suggesting a potential link to be explored further in this study.

This cross-sectional, observational study utilized a nationally promoted online survey, "Zie ik spoken?" ('Do I see ghosts?'), conducted between September 2016 and May 2017 in the Netherlands. The survey employed the Questionnaire for Psychotic Experiences (QPE), adapted for self-report, to assess hallucination phenomenology across four sensory modalities (auditory, visual, tactile, and olfactory). The survey included questions on hallucination frequency, duration, distress, impact on daily functioning, insight, interaction, content, and association with previous bothersome experiences. An optional section assessed delusions using QPE items. A total of 10,448 valid entries were included. Demographic data (age, gender, education, handedness, native language) were collected. Data analysis involved descriptive analyses, chi-square tests for categorical data, and ANOVA or Kruskal-Wallis tests for continuous data, depending on data distribution. The study was exempted from full ethical review due to its anonymous nature and low burden on participants.

The survey revealed high percentages of hallucination occurrence: auditory (29.4%), visual (21.5%), tactile (19.9%), and olfactory (17.3%). Many participants (47.6%) reported hallucinations across multiple modalities. A substantial number rated their hallucinations as severe, citing negative content (16.0–31.6%), bothersome experiences (14.8–20.2%), distress (10.5–16.8%), and/or dysfunction (12.7–17.3%). Decreased insight was found in 10.2–11.4%. Hypnagogic hallucinations were reported by 9.0–10.6%, and bereavement hallucinations by 2.8%. Despite a low prevalence of delusions (7.0%), these were significantly associated with recent hallucinations (up to 13.4% in participants with hallucinations in the past week). Detailed content analysis revealed common themes, such as voices (auditory), shadows and people (visual), being touched (tactile), and fire (olfactory). The findings also show the high percentages of participants in general population who show severe phenomenological characteristics of hallucinations.

The high reported occurrence of hallucinations in this study, compared to previous prevalence studies, may be due to the broad definition of hallucinations, the low-key profiling approach, and the low threshold for reporting in an online survey. This study offers detailed descriptions of the content of tactile, olfactory, and non-verbal auditory hallucinations, rarely reported in the general population. The high percentages of participants experiencing hallucinations in multiple modalities or with musical content, previously associated with pathological states, warrant further investigation. The association between hallucinations and delusions aligns with neurobiological models of their interaction in psychosis. The identified subgroups with severe phenomenological presentations (distress, dysfunction, negative content, dangerous commands, comorbid delusions, lack of insight) are clinically significant and could benefit from care. These findings support the notion of a hallucination continuum, ranging from fleeting illusions to severe, persistent hallucinations, reflecting the spectrum of severity in the general population. This aligns with neurobiological models of sensory perception involving bottom-up and top-down processes.

This large-scale online survey provides comprehensive phenomenological data on hallucinations in the general population. The high prevalence of hallucinations and their diverse phenomenology, including severe presentations, challenges traditional views and highlights the need for a broader understanding of these experiences. Further research should focus on the underlying mechanisms of these diverse presentations, and potential clinical interventions. The results suggest the importance of phenomenological assessment in both clinical and research settings.

Limitations include the anonymous nature of the online survey, potentially leading to unverifiable reliability and response bias. The sample's skewed distribution towards young, female respondents limits generalizability. Recall bias may affect certain items, and the self-report nature of the QPE may influence the reliability of delusion and insight assessments. The study did not assess all hallucination types, nor did it include clinical determinants (medical history, medication use).