Two methodological problems often arise in dyslexia research. The first problem concerns the validity of experimental measures and the related problem of interpreting null results. The second problem involves the effects of sampling from a disabled population if the disorder under investigation has multiple unknown origins. Both problems were illustrated by a discussion of research aimed at testing the visual perceptual deficit hypothesis of dyslexia. Many studies cited as evidence against this hypothesis suffer from one or both problems. Data are presented in support of the visual perceptual deficit hypothesis. The authors suggest that visual perceptual dysfunction may account for some forms of dyslexia. Visual perceptual deficits must be ruled out as an explanation for poor performance by dyslexic children on tasks using visual stimuli before “higher order” deficits in cognitive processing can be reasonably postulated.
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