A multidisciplinary working group reviewed evidence on the similarities and differences between hallucinations induced by psychedelics and those occurring in schizophrenia-spectrum disorders, examining data from pharmacology, brain imaging, phenomenology, and anthropology. The authors highlight both shared features and distinct characteristics across these scales, and attempt to integrate findings using computational approaches. They conclude with recommendations for future research, emphasizing the need for further study to clarify the relationship between these types of hallucinations.
Hallucinations in schizophrenia may be understood as part of self-disturbances, arising from non-conscious low-level sensory anomalies and a disrupted perception-action cycle, rather than solely from problems of phenomenal consciousness. Historical phenomenological analyses by the Early Heidelberg School (1909–1932), particularly Mayer-Gross, who studied hypnagogic experiences, mescaline-induced model psychoses, and detailed accounts from people with schizophrenia, shaped this view. Debates with contemporaries led to the conclusion that hallucinations in schizophrenia relate to shifts in the organization of consciousness, later influencing Schneider's First Rank Symptoms. The chapter assesses how these early contributions inform current phenomenological understanding of hallucinations.