Is mixed-handedness more frequent among patients with schizophrenia than in the general population? – A meta-analysis
The primary purpose of the present study is to examine the relationship between handedness and schizophrenia. I will approach the question by using meta-analytical techniques; testing the hypothesis that mixed-handedness is more frequent among schizophrenic patients then healthy subjects (controls). Firstly, background information within the frames of schizophrenia and handedness will be reported. Secondly, the restrictions of the current study will be described in the subsections of study selection, search strategy, data extraction and data analyses in the methods section. Thereafter, in the results section the meta-analytical findings will be revealed. Lastly, in the discussion section, the results will be interpreted and analysed. The current definition of schizophrenia according to the American Psychiatric Association (2000), DSM-IV-TR, is that it is a chronic incapacitating sickness characterized by the bizarre changes in cognition, behaviour and affect. Some common typical symptoms are bizarre delusions and behaviours, hallucinations (generally auditory) and disorganised speech. Based on the symptoms present, schizophrenic patients may be labelled into one of many subtypes, which is the paranoid, catatonic, hebephrenic (“disorganized”) and simple subtypes. If the patient does not fit into any subtype, then the patient will be characterized as having “undifferentiated” schizophrenia. An important focus in the study of schizophrenia has been the abnormal lateralization; since, cerebral asymmetries reflect normal developmental processes (Crow, T.J., Colter, N., Frith, CD., Johnstone, E.C. & Owens, D.G.C., 1989) there have been lesser anatomical and functional asymmetries in patients with schizophrenic symptoms than in the general population (Crow, T.J., 1997). It has been suggested that the relationship between schizophrenia and handedness is due to an irregularity of the cerebral dominance gene (Crow et al., 1989), which is thought to control cerebral asymmetries and handedness (Cannon, M. Byrne, M., Cassidy, B., Larkin, C., Horgan, R., Sheppard, N.P. & O’Callaghan, E., 1995). Therefore, hand dominance has been considered as an expression of cerebral dominance and as a result strongly correlated with anatomical asymmetry. Subsequently, hand preference has been widely used as an easy-to-measure proxy of brain asymmetry (Sommer, I., Ramsey, N., Kahn, R., Aleman, A. & Bouma, A., 2001). Lastly, it is important to emphasize the fact that patients with schizophrenia has been reported being more non-right-handed than the healthy population (Deep-Soboslay, A., Hyde, T.M., Callicott, J.P., Lener, M.S., Verchinski, B.A., Apud, J.A., Weinberger, D.R. & Elvevåg, B., 2010). Thus, that is the reason for selection mix-handedness as a variable for testing the hypothesis. Method
The inclusion criteria for the study were (a) the patients diagnosed with schizophrenia had to meet DSM (at least DSM III, DSM III-R, DSM-IV or DSM-IV-TR) criteria; (b) the method used for measuring mix-handedness should either be Annett Hand Preference Questionnaire (AHPQ) or Edinburgh Handedness Inventory (EHI). Data from non-clinical samples were excluded. Search strategy
The literature search of research papers were made on three databases: web of knowledge, OVID MEDLINE and Google Scholar. The search was limited to the years 1995-2010. Within the individual databases two keywords were used in order to identify relevant papers, “mixed-handedness” and “schizophrenia” (limited to the “title” and not “anywhere in the article”), with reference to the inclusion and exclusion criteria. Over 20 studies were identified by the search strategy and the inclusion and exclusion criteria of which four contributed to the meta-analysis. Data extraction
Following was extracted from each study:...
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