The Development of Early Orthographic Representations in Children : The Lexical Asymmetry Hypothesis and Its Implications for Children with Dyslexia

Dyslexia is a developmental word-reading and spelling disorder affecting anywhere from 6 to 17% of school-age children, with variability in prevalence estimates depending largely on the severity of the cutoff point adopted for diagnosis. While definitions vary, the vast majority contain a common set of key elements, including that (1) dyslexia is neurobiological in origin; (2) the dominant characteristics or symptoms are persistent and severe difficulties in the development of accurate and/or fluent word recognition, decoding, and spelling skills; and (3) the neurocognitive influences are multifactorial, primarily involving phonological processing deficits, as well as weaknesses in other oral language skills and processing speed. Developmentally, phonological processing deficits—impaired representation of, or access to, the abstract units of spoken language—have been implicated as the principal source of reading difficulties in children with dyslexia by disrupting the ability to establish various levels of spellingto-sound correspondence knowledge. This knowledge underlies accurate and fluent word recognition development through the process of phonological decoding, supporting self-teaching, and/or orthographic learning. As such, deficits in individuals with dyslexia are more likely to be observed when the phonological demands of the word-reading task are greater (e.g., decoding of pronounceable non-words), giving rise to the well-documented non-word reading deficit in children with developmental dyslexia (e.g., Harm & Seidenberg, 1999; Metsala, Stanovich, & Brown, 1998; Rack, Snowling, & Olson, 1992).

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