A dual-task paradigm involving concurrent finger tapping and line orientation judgment was used to investigate brain processing differences in early adolescent good readers/poor spellers (dysgraphia), poor readers/poor spellers (dyslexia) and good readers/good spellers. Whereas all groups were similarly affected during the left-hand tapping condition, in the right-hand tapping condition the good spelling group displayed significantly less tapping disruption than both poor spelling groups, who did not differ significantly from each other. From these results, it can be inferred that individuals with dyslexia and dysgraphia share a left-hemisphere processing limitation that is not confined to written language. In light of other relevant research findings, I suggest that this limitation is due to the absence of a disembedding scanning mechanism for converting spatial arrays (e.g., spelling patterns) to temporal form—an impairment putatively caused by attempting to teach written language to children who are late in establishing left-hemisphere motor dominance.
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