Abstract and Introduction
Hallucinations can occur in different sensory modalities, both simultaneously and serially in time. They have typically been studied in clinical populations as phenomena occurring in a single sensory modality. Hallucinatory experiences occurring in multiple sensory systems—multimodal hallucinations (MMHs)—are more prevalent than previously thought and may have greater adverse impact than unimodal ones, but they remain relatively underresearched. Here, we review and discuss: (1) the definition and categorization of both serial and simultaneous MMHs, (2) available assessment tools and how they can be improved, and (3) the explanatory power that current hallucination theories have for MMHs. Overall, we suggest that current models need to be updated or developed to account for MMHs and to inform research into the underlying processes of such hallucinatory phenomena. We make recommendations for future research and for clinical practice, including the need for service user involvement and for better assessment tools that can reliably measure MMHs and distinguish them from other related phenomena.
Various definitions have been advanced for "hallucinations," but there is general consensus that a hallucination can be defined as a sensory experience that resembles veridical perception without having a corresponding sensory stimulation from the external environment. Hallucinations can occur in all senses, including auditory, visual, olfactory, kinesthetic, and more. Hallucinatory experiences span nosological categories and are a clinical manifestation of many psychiatric disorders (schizophrenia and bipolar), neurodegenerative diseases (dementia with Lewy bodies [DLB]), and Parkinson's disease psychosis [PDP]), as well as sensory disorders like hearing impairment or eye disease.[8,9]
Traditionally, hallucinations are often assumed to occur in one modality at a time (unimodal) and can be associated with different disorders—auditory hallucinations (AHs) in schizophrenia and visual hallucinations (VHs) in DLB. Where hallucinations do occur in different modalities, the predominant understanding is that they occur at different times (ie, they are not fused/simultaneous, like seeing and hearing a talking head; though see). Consequently, clinical assessments have had a focus on single modalities, thus biasing data collection toward unimodal hallucinations in potentially problematic ways. Nevertheless, growing recognition that hallucinations may occur in multiple modalities has shifted the attention to a systematic search for such multimodal phenomena.[3,6,13–17]
Despite the lack of in-depth scrutiny in the field, accounts of hallucinations across all senses can be traced through time. Historical examples include medieval descriptions of spiritual voice hearing, such as those by Margery Kempe, who did not just hear the "voice of God" but also had visions and other sensory experiences. A recent case study shows the experience of Mr T.A., a patient with schizophrenia who saw and heard humanoid creatures associated with a foul smell and who could go through his body, causing him unpleasant coenesthetic sensations (disorder of bodily perception). Such examples challenge the notion that unimodal hallucinations are the overwhelmingly prevalent clinical manifestations of psychiatric and organic disorders[13–17,23] and highlight the need for more accurate clinical assessment and management.
While hallucinations across multiple senses are starting to attract increasing research interest, several outstanding questions need to be addressed. This review, therefore, focuses on "multimodal hallucinations" (MMHs), ie, hallucinations that co-occur in different modalities, either in a simultaneous or in a sequential (serial) manner, with the overall objective of providing an overview of the field, highlight areas that require further scrutiny and identify issues of potential clinical importance.
Schizophr Bull. 2021;47(1):237-248. © 2021 Oxford University Press