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In their meta-analysis of magnetic resonance imaging studies of schizophrenia, Nelson et al¹ report that schizophrenia was significantly associated with bilaterally reduced hippocampal volumes and that no laterality differences were seen. The absence of laterality differences is important in providing evidence against those theories of the pathogenesis of schizophrenia that postulate that brain changes are predominantly left-sided.²

The appropriate statistical method for testing laterality differences depends on whether the left-sided and right-sided measurements (within individuals) are independent. Nelson et al used Cohen q as the measure for testing for laterality differences. Following their terminology, q=z_r1 - z_r2, where z_r1 represents the Fisher z transformation (formula 5 in their paper) of the correlation (r₁) between the vector indicating the schizophrenia and control group left hippocampal volumes and that indicating group membership (eg, 0 for patients and 1 for controls). These vectors are of dimension N, . . . [Full Text of this Article]