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Developmental dyslexia (dyslexia) is a heritable condition typically diagnosed in the first school years, characterised by an impairment of reading abilities in spite of normal intelligence and adequate educational opportunities.
While the exact neurobiological mechanisms underlying this condition remain obscure, the most convincing current aetiopathogenetic view of dyslexia is that impaired reading stems from a defective representation and manipulation of phonemes, —that is, the sounds that we combine to build words.1 There are at least two major complications when studying the ultimate causes of dyslexia. Firstly, the composite neuropsychological picture that often marks dyslexia may lead people to consider reading problems as a part of a more extended neurobiological syndrome whose phenotypic boundaries are blurred, and whose genetic determinants may be especially difficult to identify with certainty.2 Secondly, the leading criterion to diagnose dyslexia remains that of a reading performance below the population mean (typically, a reading score two standard deviations below the general population mean). While reading performance is distributed normally in the population,3 the prevalence of dyslexics will vary considerably across different cultures, because it depends on the complexity of orthographic rules specific to a given language to which a subject is exposed.4 Contradicting a culturally bound identity of dyslexia is a recent, functional brain imaging study of adult subjects with dyslexia from different cultures and languages (English, French, Italian) that showed the same abnormal patterns of brain activation during implicit and explicit reading.5 This suggests common neurobiological causes for dyslexia regardless of a person’s spoken language, while variation in prevalence estimates across different cultures could at least partially reflect local difficulties specific to each language, when homogenous diagnostic criteria are applied.5
While functional brain imaging findings suggest biological unity for dyslexia, evidence based on genetic analyses of common determinants of dyslexia …