Anomalous Bimanual Coordination Among Dyslexic Boys*

Most neuropsychological models of dyslexia have focused on left hemisphere dysfunctions; however, several recent studies also suggest difficulties with interhemispheric integration. We assessed that possibility by testing bimanual coordination in 18 dyslexic and 18 nondisabled dextral boys (9-14 years) of above average intelligence on an Etch-a-Sketch-like apparatus. The task measured various ratios and directions of hand rotation with and without visual feedback. Group performance was equivalent on trials requiring parallel movements of the hands (both clockwise). In contrast, on trials requiring mirror movements (clockwise + counterclockwise), dyslexics were slower and less accurate than controls, particularly with their left hands. Moreover, dyslexics had notable difficulty on the mirror movement trials without visual feedback, often unknowingly reverting to parallel hand movements. The dyslexics' deficits were largely consistent with previous observations of partial commissurotomy patients, supporting the hypothesis of impaired interhemispheric coordination. However, the inability to perform mirror movements seen among the dyslexics has never before been observed in any other population. We propose that a combination of deficient interhemispheric collaboration plus anomalous organization of the ipsilateral motor hand pathways may account for the dyslexics' performance.