Unanswered Questions and Recommendations
Given the paucity of systematic evidence regarding MMHs, there are many unanswered questions and avenues for further research. First, despite some preliminary data, it is not clear whether and how the base modality of hallucinations (ie, the most prominent and frequent) changes the prevalence of experiencing hallucinations in other modalities. Second, longitudinal studies are necessary to ascertain if MMHs change over time. Third, the frequency of unusual experiences (eg, someone only having an MMH once a month but consistent unimodal ones daily) is important in understanding multimodality.
It also remains unclear how many senses one should take into account, since the established 5 sensory domains might not capture the whole range of hallucinatory experiences. Blom describes 14 "senses" in which hallucinations have been reported, thus widening the range of sensory modalities involved. This raises the question of whether hallucinations in less well-known sensory domains are equally understood by the experiencers themselves and whether they can be easily conveyed to others. Arguably, there might be general difficulties in conveying nonverbal experiences to others in general, which would be a significant challenge in the assessment of MMHs.
In addition, despite the large body of literature on culture and unimodal hallucinations (eg, in schizophrenia[28,88,89] and in the general population), there is no systematic evidence of its specific influence on MMHs. The underreporting of MMHs could be partially due to their lack of emphasis in standard psychiatric assessments (which may, in turn, be due to an overemphasis on auditory verbal hallucinations in Western psychiatry). Research should look at transcultural data and compare MMHs across groups, especially where there is a strong element of spirituality. It would be interesting to investigate whether some subcultures have stronger expectations that spiritual entities will manifest themselves in particular modalities over others, eg, vision and auditory for "spiritual jaguars" in the Amazonian Wari' shamans, and vision/auditory/tactile/olfactory components in the experience of Jinns in Islamic cultures.
Furthermore, although an overview of multisensory integration (MI—the ability to integrate information from different sensory sources) is beyond the scope of this paper, it is important to consider MMHs in light of recent evidence on multisensory processing. MI involves several brain areas and networks, starting as early as in the superior colliculus. A growing body of evidence attests to: (1) MI problems in schizophrenia both for low-level stimuli and more complex ones, indicating issues with faulty "binding" of stimuli in time and/or space, and (2) a link between these issues and hallucinations. Similarly, PDP patients with hallucinations have problems with the integration of perceptual and attentional processing.[99,100] Therefore, findings linking MI problems to hallucinations raise the question of whether the heterogeneity of MMHs could be traced to different areas underlying MI, thus giving rise to different types of MMHs. This could be a very important avenue for further research and warrants further attention.
Finally, most current theories are not able to explain (1) why the rates of MMHs across modalities vary within an individual and across patient groups, (2) why patients can show a combination of simultaneous MMHs, serial MMHs, and unimodal hallucinations, and (3) what might give rise to the relatedness and/or congruency of the content of both simultaneous and serial MMHs.
To conclude, in line with the recommendations of the 2017 International Consortium on Hallucination Research, this review shows the need to carry out a systematic investigation of MMHs. Overall, the evidence of the high prevalence and adverse prognostic outcomes of MMHs across disorders highlights the need to develop better assessment tools and theoretical models to systematically investigate these experiences and inform treatment strategies alongside the help of service users who experience MMHs and unimodal hallucinations.
Thank you to Daniel Williams and Dominic Ffytche for helpful discussion.
This work was supported by Wellcome Trust grants (WT108720 and WT209513 to C.F.), a Swiss National Science Foundation grant (IZK0Z3_173146 to P.U.), NIHR Newcastle Biomedical Research Centre (J.-P.T.) and NIHR Maudsley Biomedical Research Centre (M.M.). This paper represents independent research funded by the National Institute for Health Research Biomedical Research Centre at South London and Maudsley NHS Foundation Trust and King's College London. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care.
Schizophr Bull. 2021;47(1):237-248. © 2021 Oxford University Press