Heterogeneity of deficits in developmental dyslexia and implications for methodology.

Most of the studies overviewed by Farmer and Klein (1995) in their review of temporal processing disorders in developmental dyslexia have taken a group study approach in which a group of dyslexic readers is compared with a group of normal readers on some task thought to be relevant to the reading disorder. Because of the acknowledged heterogeneity of deficits among developmental dyslexics, this group study methodology is inappropriate and is likely to lead to findings in one lab that cannot be replicated in another. The single case study methodology, which has been used successfully in the study of adult neuropsychological impairments, should be adopted in the study of developmental impairments as well. In the case study approach, each individual is studied thoroughly with tasks designed to tap the various components of the cognitive domain under study in order to determine which components are spared and which impaired. Data are not averaged, but reported separately for each case. Some recent findings from case studies on developmental dyslexia are reviewed and suggestions are made as to how the case study approach could be used in analyzing whether a temporal processing disorder, or any other hypothesized factor, is causal to the reading disorder.