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Dyslexia, Developmental

This entry considers the cognitive and sensory characteristics of dyslexia as well as current neural and genetic associations with this disorder of learning. Developmental dyslexia is usually characterized as a specific problem with reading and spelling that cannot be accounted for by low intelligence, poor educational opportunities, or obvious sensory or neurological damage. Because children with developmental dyslexia frequently have good spoken language skills, the earliest theories of developmental dyslexia were visual rather than linguistic. Developmental dyslexia was conceived as a form of “word blindness.” Today, developmental dyslexia is recognized as a primarily linguistic disorder. Children with developmental dyslexia have difficulty in the neural specification of the sound structure of language—a specific difficulty in phonological representation that has been found in every language studied to date.

The Development of Phonological Representation

Child phonology has undergone dramatic theoretical revision in recent years. The traditional view of phonological development was that babies began to learn language by recognizing phonemes. Phonemes are the individual sound elements that appear to make up words in languages and that roughly correspond to alphabetic letters. It was believed that all languages drew on a universal phonetic inventory of consonant and vowel phonemes. Linguists such as Janet Pierrehumbert proposed that babies actually learn language-specific “phonotactic templates” or “prosodic structures.” These templates are essentially phonological patterns that vary in sound intensity, pitch, duration, and rhythm. A common template for English is a bisyllabic pattern, with stronger first-syllable stress (a strong—weak stress template). The strong first syllable is typically louder, longer and higher in pitch than the second syllable. Familiar words that follow this pattern are mummy, daddy, biscuit, and baby.

As demonstrated in the cross-language review of reading development and dyslexia by Jo Ziegler and Usha Goswami, phonological representation in a preliterate child is different from phonological representation in a child who has achieved literacy. This is because learning to read changes the brain. Once the alphabetic code has been acquired, spoken language processing changes fundamentally. The literate brain imposes phonemes onto the sound structure of speech. The preliterate brain does not. Adults who have never learned to read perform very poorly in phoneme awareness tasks such as phoneme deletion (e.g., deleting the phoneme /t/ in stop to leave sop). Both preliterate children and illiterate adults perform well in phonological awareness tasks based on larger phonological units, however, such as syllables and rhymes. Hence, phonological representation prior to the teaching of literacy appears to be based on syllables and the subsyllabic segments of onset (any sounds before the vowel) and rime (the vowel and any sounds that follow it). The onset-rime segmentation of sing, sting, and spring would be s-ing, st-ing and spr-ing, respectively.

The Dyslexic Phenotype

In developmental dyslexia, there is a brain-based difficulty in representing the sound structure of speech. This selective difficulty with phonology is usually characterized by extremely poor performance in three areas of phonological processing. The first is phonological awareness of syllables, rhymes, and phonemes. For example, children with developmental dyslexia have difficulty in counting the syllables in a word such as university; they have difficulty in selecting the odd word out from a triple such as man, pat, fan; and they have difficulty in substituting phonemes in Spoonerism tasks (e.g., Bob Dylan becomes Dob Bylan). The second is phonological memory skills. Children with dyslexia find it difficult to remember verbal sequences and show poor performance on standardized tests of phonological memory such as nonword repetition (recalling items such as loddernapish and thickery). The third is rapid automatized naming (RAN). Children with dyslexia find it difficult to rapidly produce highly familiar phonological forms such as color names, object names, digits, and letters. These three areas of phonological processing are usually all impaired in a child who has been identified as having developmental dyslexia.

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