Dyslexia

Dyslexia is marked by unusually slow reading development in children who are otherwise normally [Page 371]developing. Although most children learn to read with little conscious effort, children with dyslexia lag well behind their peers in acquiring this fundamental skill. These children fail to learn to read adequately despite normal hearing, cognitive, neurological, and emotional abilities. The disorder is not uncommon, with an estimated 5 to 15% of all children having significant difficulties in learning to read. Despite early reports, there is no sex link, with an equal preponderance in boys and girls. The term dyslexia is occasionally also used to refer to individuals with reading disturbances following stroke, although the more common term in that case is alexia, which is not discussed in this entry.

Diagnosis of dyslexia usually precludes general sensory impairments such as severe loss of hearing or vision, or a broader cognitive deficit. Affected individuals show slow laborious reading marked by errors in single word identification and poor text comprehension. In addition, they tend to show poor generalization, such that they have difficulty reading unfamiliar words or nonsense words, indicating that they have failed to learn consistencies in the relationship between spelling and sound.

There is a relatively broad consensus that dyslexia stems from a deficit in processing phonology, or the basic sounds of words. Phonological abilities seem to play an important role in successful reading development because they are an inherent part of reading: Recognizing a word involves translating orthography (a word's visual form) into a phonological code (the sounds that the word is composed of). Thus, the phonological deficit hypothesis of dyslexia holds that affected individuals have difficulty learning to read because of a deficit in how they process phonological information. This does not necessarily imply a perceptual deficit, as is discussed further in this entry.

Consistent with this, affected individuals have difficulty with phonological awareness. Measures of this include rhyme awareness (judging whether two words rhyme or producing words that rhyme with a sample word); phoneme blending (assembling a set of phonemes into a single word); phoneme counting/tapping (counting or tapping a finger to indicate distinct phonemes in a word); and phoneme deletion/elision (repeating a word after removing one or more of its constituent sounds). Importantly, phonological awareness relates to processing spoken words, and performing such tasks does not hinge on the ability to read words. Listeners must understand that auditory words are not wholes, but instead can be broken down into constituent sounds.

Phonological awareness allows the beginning reader to analyze spoken words into phonemes, which can in turn be associated with orthography. It also allows the beginning reader to make analogies among similar sounding words, which typically share similar spellings, allowing beginning readers to learn new words quickly. Phonological difficulties in dyslexia are thus hypothesized to make it more difficult to process or identify the similarities and differences among sounds, ultimately making learning to read more arduous. Some of the earliest evidence supporting this view comes from the finding that phonological skills in pre-reading children are highly predictive of later reading achievement. Thus, preliterate children with phonological difficulties tend to develop reading deficits later on. Furthermore, phonological deficits tend to persist into adulthood, even in cases where reading difficulties have been effectively remediated.

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