A positron emission tomographic study of impaired word recognition and phonological processing in dyslexic men.

BACKGROUND Developmental dyslexia is characterized by impaired word recognition, which is thought to result from deficits in phonological processing. Improvements during the course of development are thought to disproportionately involve orthographic components of reading; phonological deficits persist into adulthood. OBJECTIVE To localize the neural correlates of impaired word recognition and phonological processing in men with developmental dyslexia. METHODS Regional cerebral blood flow was measured with oxygen 15 positron emission tomography in 17 men with dyslexia and in 14 matched controls during the performance of phonological and orthographic tasks--pronunciation (reading aloud) and lexical decision making--designed to activate posterior and anterior perisylvian cortices, respectively. RESULTS Altered patterns of activation (reduced activation, unusual deactivation) were seen in dyslexic men in mid- to posterior temporal cortex bilaterally and in inferior parietal cortex, predominantly on the left, during both pronunciation and decision making. In contrast, dyslexic men demonstrated essentially normal activation of left inferior frontal cortex during both phonological and orthographic decision making. CONCLUSION These, along with prior findings, are compatible with a hypothesis of bilateral involvement of posterior temporal and parietal cortices in dyslexia.

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