On the relationship of hemispheric specialization and developmental dyslexia

Hemispheric specialization for linguistic material was compared for normal and dyslexic subjects under dichotic listening (Experiment 1) and hemi-retinal presentation (Experiment 2) conditions. In both experiments, group data indicated that dyslexic subjects were, overall, less accurate in their performance than normals but that both groups showed similar right ear/visual field superiority. However, examination of individual subjects scores in both experiments indicated that the distribution of lateralization scores for dyslexic subjects was bimodal, whereas that for normal subjects was unimodal. These results suggest that the dyslexic 'population' is heterogeneous with regard to cerebral lateralization and that previous work treating it as homogeneous is most likely misleading. It appears important to both carefully examine individual subject data in such studies and to consider the consequences of there being different types of cerebral lateralization etiologies for what has been typically considered to be a homogeneous dyslexic population.