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Dyslexia, Phonological Processing in

Developmental dyslexia is by definition a disorder of written language acquisition, despite adequate intelligence and opportunity, and in the absence of obvious sensory, neurological, or psychiatric disorder. It primarily affects the acquisition of reading, particularly word identification and, secondarily, the acquisition of conventional spelling.

Underlying Causes of Developmental Dyslexia

Two main proximal causes have been considered. Historically, the initial hypothesis was that of a visual deficit (“congenital word blindness,” coined by William Pringle-Morgan in 1896). In the 1970s, it became evident that what had been interpreted as visual letter confusions were better explained by phonological confusions. Over the last three decades, it has been well established that most cases of dyslexia can be attributed to a subtle disorder of oral language (the “phonological deficit”), with symptoms that happen to surface most prominently in reading acquisition. It remains likely that a minority of cases of dyslexia are due to disorders in the visual modality, although the precise nature of the deficit remains unclear. The present entry focuses on cases of dyslexia with a phonological deficit.

Another important theoretical debate is whether the phonological deficit in dyslexia is specific to the linguistic domain or is caused by an underlying auditory deficit. Although there is considerable evidence that a subset of dyslexic children have difficulties in a variety of auditory tasks, there have been important challenges to the view that this is the underlying cause of their phonological deficit, hence the cause of their reading disability. Again, given that both sides of the debate agree that the phonological deficit is central to understanding dyslexia, this issue will not be further discussed here.

Symptoms of the Phonological Deficit

There is wide agreement on the main symptoms of the phonological deficit in dyslexia: They include poor phonological awareness, poor verbal short-term memory, and slow lexical retrieval. Phonological awareness refers to the realization that words are made of a combination of smaller units (syllables and phonemes) and to the ability to pay attention to these units and explicitly manipulate them. Typical tasks include counting the number of syllables or phonemes in a word, detecting whether words rhyme, deleting the initial (or final) phoneme, or performing simple spoonerisms (swapping the initial phonemes of two words). Verbal short-term memory typically refers to the ability to retain and immediately repeat verbal material of increasing length: sequences of two to nine digits (digit span), nonwords of two to five syllables (nonword repetition), or even sequences of nonwords (nonword span). Finally, lexical retrieval refers to the ability to quickly retrieve the phonological forms of words from long-term memory. In the context of dyslexia research, this is tested by having participants name series of 50 objects, colors, or digits as fast as possible (rapid automatized naming). Dyslexic children are typically found to have poor phonological awareness (particularly phoneme awareness), reduced short-term memory span, and slow automatized naming, although individual profiles along those three dimensions of course vary, leading to the possibility of subtypes. Thus, the most prominent symptoms of developmental dyslexia are diverse but united by their involvement of phonological representations—hence, the consensus hypothesis of a phonological deficit.

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