Researchers have proposed that schizophrenia is a disease related to abnormal cerebral lateralisation following findings of increased "schizophrenia-like" symptoms in left-hemisphere epileptics. Theories regarding abnormal brain structural asymmetries in schizophrenia suggest either ambiguous or extreme motor asymmetry. These theories are conceptually similar to ones proposed to explain non-right-handedness in normal subjects. In this thesis I objectively evaluate these hypotheses.
Firstly, I critically survey the neuropsychological literature and find the evidence for lateralised cognitive deficit to be inconclusive. Next, a meta-analysis of studies reporting the finding of ventricular enlargement in schizophrenia is carried out and it is found that findings are highly influenced by methodological factors. A review of the literature concerning lateralised neuropathologies from brain imaging and postmortem studies similarly finds the evidence to be hindered by differences in experimental methodology. Furthermore, there is much disagreement between researchers regarding which asymmetries are empirically or theoretically meaningful.
The next chapter concentrates exclusively with the experimental measurement of hand performance. The Annett pegboard, the Tapley and Bryden circle marking, and the Bishop square tracing tasks of hand performance are extended and used to test hand performance in normal subjects as a function of increasing task difficulty. Pursuit tracking is used to consider the Fourier spectrum and sub-components of relative hand performance. The differences between the hands on the conventional and tracking tasks are then subjected to factor analyses. Surprising results are obtained in which performance tasks show moderate-to-high internal reliability but correlate poorly with one another. Their relevance to handedness and motor research is then discussed.
Schizophrenic hand preference is investigated in a meta-analytic assessment of studies reporting an increased incidence of non-dextral hand preference in schizophrenia. This is examined with respect to the definition and methods of measurement in these studies. Finally, the hand performance of schizophrenics is investigated. Testing hand performance, in conjunction with hand preference measures, allows for greater reliability in the evaluation of the notion of abnormal handedness in schizophrenia. Patients show poorer overall performance on all of the tasks, but show no significant differences in their degree of handedness as compared to normals. Conclusions are drawn that associations between abnormal handedness and disorders of brain structural asymmetry in schizophrenic patients are unlikely. Further implications for abnormalities of cerebral dominance and schizophrenia are considered
