2Chapter The Brain and Developmental Dyslexia: Genes, Anatomy, and Behavior
Dyslexia is a common reading disability, affecting 5%–10% of the population. The disorder is diagnosed usually during early school age through specialized test batteries that uncover specific behavioral profiles centered on reading, writing, and phonological awareness. The defining symptom of developmental dyslexia is a severe and specific difficulty in reading acquisition that is unexpected in relation to other cognitive abilities and educational circumstances (Lyon, Shaywitz, & Shaywitz, 2003). At the cognitive level, there is widespread agreement that a large majority of dyslexic children suffer from what is commonly termed a phonological deficit-a problem with some aspects of the mental representation and processing of speech sounds (Snowling, 2000). Evidence for this phonological deficit comes from three main behavioral symptoms: (a) poor phonological awareness, (b) poor verbal short-term memory, and (c) slow lexical retrieval (Ramus, 2004; Wagner & Torgesen, 1987). In addition to phonological awareness, dyslexics may also have difficulties in the speed of processing as diagnosed by their performance on tests of rapid naming (see Wolf & Bowers, 2000; Wolf, Gottwald, Galante, Norton, & Miller, this volume).