The method used for conducting the systematic literature review follows the PRISMA guidelines (Liberati et al., 2009; Moher, Liberati, Tetzlaff, & Altman, 2009) originally developed for studies evaluating health care interventions. These guidelines provide a stepwise selection of relevant articles after a literature research in the relevant database for systematic literature reviews in general.
3.1.1 Search procedure
In order to investigate how AVHs are conceptualised in neuroscientific and cognitive psychological approaches to voice-hearing, PubMed and PubPsych was searched using three search strings10. The first referred to auditory verbal hallucinations carrying the following terms: ―auditory verbal hallucinat*―, ―auditory hallucinat*‖, ―verbal hallucinat*‖, ―hear* voice*‖, ―hallucinat* AND voice*‖. The second search string contained terms regarding neuroscience ―brain‖, ―cognition‖, ―neurobiolog*‖, ―neurophysiolog*‖, ―neuropsycholog*‖, ―neurocognit*‖. The third search string contained terms aiming at obtaining studies concerning the conceptualisation of AVHs: ―model‖, ―account‖, ―approach‖, ―perspective‖, ―definition‖, ―theory‖11. The search terms were connected with an OR-operator and the search sets with an AND-operator, respectively. Moreover, in PubMed the Mesh-Term ―Humans‖ was set in order to get back only studies concerning humans. Titles and abstracts of the resulting studies were scanned by the author, as well as her supervisor (A.P.P.) to extract relevant works. The shortlisted articles were then scanned in order to obtain the final list of included works.
10 These specific search strings were defined after testing various different combinations of search terms, as they turned out to
devolve a great variety of relevant studies.
11 At this point it may be useful to shortly consider the differences between the terms of this search string. A theory can be
considered as a set of concepts that are interrelated and aiming at presenting a systematic view of certain phenomena that also may be explained and predicted by the respective theory (Kerlinger, 1986). Models may be derived from theories and are less general than theories and aim at explaining a specific phenomenon (Wilkinson, 2014). Thereby, models usually focus on certain features of a phenomenon and, thus, represent it in a simplified manner (Bailer-Jones, 2009). A definition can be roughly defined as ―a statement expressing the essential nature of something‖ (Merriam-Webster‘s online dictionary, 2017a). The terms approach, account, and perspective were intended in the most general way.
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For the systematic search of relevant philosophical works an equivalent search was carried out using the database PhilPapers. For this search the ―fuzzy filter advanced‖ was applied. Moreover it was chosen that the search term ―hallucinat*‖ must appear, the search terms ―auditory‖, ―verbal‖, ―acoustico-verbal‖, and ―acoustic‖ are excellent indicators of relevance, and the search terms ―philosoph*‖, ―phenomen*‖ are good indicators of relevance. The search specified in that way resulted in n = 623 hits (18.03.17). The resulting studies were scanned equivalently to the aforementioned search. A large part of the works devolved by the PhilPapers-search did not belong to the area of philosophy. For that reason and because in the area of philosophy ―systematic review‖ is no common research strategy, a slightly different search strategy was adopted for the area of philosophy. In addition to the articles identified through the PhilPapers search, references of those were searched. Works of the authors of the selected studies that would help to clarify their position were also considered in the review.
3.1.2 Inclusion and exclusion criteria
Studies published in the form of journal articles in the English language from 2000 until inclusively 2016 (31.10.) were included based on the searches of PubMed and PubPsych. This decision was based on the fact that the majority of the studies about ―auditory verbal hallucination‖ devolved by PubMed and PubPsych were published after 2000. For this reason the results should not differ significant from the ones that would be obtained by including earlier work. Another reason for the inclusion of more recent studies is grounded on the aim of the present work, which is to analyse which (implicit) assumptions about AVHs are guiding its recent investigation. Studies not dealing (explicitly) with AVHs or not focusing on them, pure reviews, pure meta-analyses, pure case reports, as well as treatment studies and studies only probing healthy participants were excluded. Studies not belonging to the areas defined as relevant for the present work were excluded as well.
For the area of philosophy, in addition to work published in the form of articles, chapters of books were included if relevant. According to Romdenh-Romluc (2009, see also González, 2010), hallucinations are studied in two domains of philosophy: first, in what could be called clinical philosophy or philosophy of psychiatry, and second in philosophy of perception. Whereas the first focuses on hallucinations as ―real experiences‖, the latter is more concerned about the ―hypothetical hallucination‖, that is, the possibility of having hallucinations in the sense of having perceptual experiences lacking external stimulation. To put it differently, the latter ―address[es] the topic of hallucination, not for its own sake, but only in the context of some wider issues‖ (Dorsch, 2010, p. 172). This approach, thus, is
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usually not concerned with the question of what a hallucination is, but rather operates with the assumption that hallucinations are indistinguishable from veridical perceptions in order to further examine what that means for philosophy of perception. The studies of interest for the present work belong to the first category. As in the present work we are interested in AVHs how they really occur and that are experienced by individuals, articles that treat hallucinations in the area of philosophy of perception will be excluded. For details regarding the selection process, see Figure 5.