Fact of Fiction? Dyslexia Is Caused by a Phonological Deficit

Since the beginning of the twentieth century, dyslexia research has been dominated by a search for the Holy Grail: the single cognitive deficit that is necessary and sufficient to cause all behavioural characteristics of dyslexia. Until the 1950s, the belief was that dyslexia is attributable to visual processing problems.

Samuel Orton, arguably the key figure in setting the stage for the study of reading disabilities in the United States, linked mixed dominance to the symptoms he frequently observed in the clinic where he was hired: (a) reversals of letters such as p and q and b and d; (b) reading from right to left, manifested by reversals of paired letters, syllables within words, or whole words within sentences; and (c) a propensity to mirror read and/or write. From these observations he proposed his “strephosymbolia,” or twisted symbols theory, and suggested that dyslexics had a deficient visual perception of letters caused by the inheritance of mixed cerebral dominance.

Phonological awareness becomes the Holy Grail

In 1957 Noam Chomsky published his seminal book, Syntactic Structures, which transformed the study of language and with it, reading. Any role for visual processing was abandoned and dyslexia became a linguistic, phonological problem. In an influential book, Dyslexia: Theory and Research, Vellutino (1979) argued that many of the apparent visual problems could actually be attributed to language difficulties — especially to deficient phonological awareness.

Phonological awareness refers to an individual’s awareness of the phonological structure, or sound structure, of language. It is a listening skill that includes the ability to distinguish units of speech, such as rhymes, syllables in words, and individual phonemes in syllables. Phonological awareness is often confused with phonics, but it is different. Phonics requires students to know and match letters or letter patterns with sounds, learn the rules of spelling, and use this information to decode (read) and encode (write) words. Phonological awareness relates only to speech sounds, not to alphabet letters or sound-spellings, so it is not necessary for students to have alphabet knowledge in order to develop a basic phonological awareness of language. Phonemic awareness is a subset of phonological awareness that focuses on recognising and manipulating phonemes, the smallest units of sound. The two most important phonemic awareness skills are segmenting and blending.

The phonological deficit theory became the most well-developed and supported of the theories of dyslexia. It has been widely researched, both in the UK (York group) and in the US, resulting in a remarkable degree of consensus concerning the causal role of phonological skills in young children’s reading progress. Children who have good phonological skills, or good “phonological awareness”, become good readers and good spellers. Children with poor phonological skills progress more poorly. In particular, those who have a specific phonological deficit are likely to be classified as dyslexic by the time they are nine or ten years old. The US researchers have united in adopting the phonological deficit hypothesis since the early 1980s, and this united front has led to the investment of more than $15 million annually by the US government, via the National Institute for Child Health and Human Development (NICHD). Unfortunately, the interventions proved to be ineffective (Nicolson & Fawcett, 2018).

From a single to a multiple cognitive deficit model

Some researchers now claim that phonological factors may be less important than is commonly accepted, while others point out that children with poor phonological abilities can nevertheless develop good reading skills. Some findings indicate that phoneme awareness may develop as a consequence of exposure to reading and writing, as opposed to a cause, while other findings support an intermediate view, that phonological awareness and alphabetic literacy learning influence each other reciprocally. Given that a single phonological deficit is not necessary or sufficient to cause a reading disability, current thinking sees this as only one of multiple cognitive deficits that are likely to interact to cause reading disability.

Menghini et al. compared 60 dyslexic children to 65 age-matched normally reading children. The frequency of occurrence of children with dyslexia who exhibited a phonological deficit only was 18.3%, while most of the children (76.6%) showed other deficits in addition to a phonological deficit: 16.6% displayed executive deficits, 13.3% visual-spatial perception deficits, attention and executive deficits, 8.3% attention and perceptual deficits and 8.3% attention and executive deficits. A study by Moura, Simões and Pereira reveals significant processing speed, shifting, and verbal fluency deficits in a sample of 50 Portuguese dyslexic 8- to 12-year-olds compared to 50 typically developing children.

Franceschini et al. showed that visual-spatial attention in pre-schoolers is an important predictor of reading development. Children who had poor reading abilities during the first or second year of reading education made significantly more errors on a visual-search task and a spatial-cueing task than normal readers had when they were in kindergarten. Moreover, several studies tend to show that a number of dyslexics present a visual-processing deficit independently of phonological skills.

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