Dyslexia Is Caused by Phonological Deficits. Fact or Fiction?

Our fact or fiction series:

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 behavioral 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 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 students don’t need to have alphabet knowledge to develop a basic phonological awareness of language. Phonemic awareness is a subset of phonological awareness that focuses on recognizing 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. (2010) 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 et al. (2015) 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. (2012) showed that visual-spatial attention in preschoolers 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 dyslexics present a visual-processing deficit independently of phonological skills.

A study by Lipowska et al. (2011) concludes: “The presented research constitutes another piece of evidence showing that children with dyslexia are afflicted with both phonological problems and visuospatial deficits connected with the right posterior parietal cortex. Diagnosis limited only to the linguistic aspect may produce an incomplete picture of the actual pathogenic mechanisms. … omitting [visual perception and visuospatial deficits] in the process of diagnosis may lead to a distortion of the picture of a child with developmental dyslexia and thus diminish the chance for effective therapy.”

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Authored by Susan du Plessis (B.A. Hons Psychology; B.D.) who has 30+ years’ experience in the LD field.
Page last reviewed: May 26, 2021.
Next review due: May 26, 2023.

References and sources:

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Catts, H. W., & Adlof, S. (2011). Phonological and other language deficits associated with dyslexia. In S. A. Brady, D. Braze & C. A. Fowler (Eds.), Explaining individual differences in reading: Theory and evidence (pp. 137-51). New York: Psychology Press.

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Fawcett, A. (2001). Dyslexia at school: a review of research for the DfES. Unpublished review for the Department for Education and Skills, the British Dyslexia Association and the Dyslexia Institute.

Franceschini, S., Gori, S., Ruffino, M., Pedrolli, K., & Facoetti, A. (2012). A causal link between visual spatial attention and reading acquisition. Current Biology, 22, 814–819.

Guardiola, J. G. (2001). The evolution of research on dyslexia. Retrieved from http://ibgwww.colorado.edu/~gayan/ch1.pdf

Hallahan, D. P., & Mercer, C. D. (2002). Learning disabilities: Historical perspectives. In R. Bradley, L. Danielson, & D. P. Hallahan (Eds.). The LEA series on special education and disability. Identification of learning disabilities: Research to practice (pp. 1-67). Mahwah, NJ, US: Lawrence Erlbaum Associates Publishers.

Macdonald, S. J. (2009). Towards a sociology of dyslexiaExploring links between dyslexia, disability and social class. Germany: VDM Verlag Dr. Müller.

Manolitsis, G., & Tafa, E. (2011). Letter-name letter-sound and phonological awareness: Evidence from Greek-speaking kindergarten children. Reading and Writing: An Interdisciplinary Journal, 24(1): 27-53.

Menghini, D., Finzi, A., Benassi, M., Bolzani, R., Facoetti, A., Giovagnoli, S., … Vicari, S. (2010). Different underlying neurocognitive deficits in developmental dyslexia: A comparative study. Neuropsychologia, 48(4): 863–72.

Moura, O., Simões, M. R., & Pereira, M. (2015). Executive functioning in children with developmental dyslexia. The Clinical Neuropsychologist, 28(S1): 20-41. DOI: 10.1080/13854046.2014.964326

Lipowska, M., Czaplewska, E., & Wysocka, A. (2011). Visuospatial deficits of dyslexic children. Medical Science Monitor : International Medical Journal of Experimental and Clinical Research17(4), CR216–CR221. 

Nicolson, R. I., & Fawcett, A .J. (2018). Procedural learning, dyslexia and delayed neural commitment. In T. Lachmann & T. Weis (Eds.). Reading and dyslexia (pp. 229-57). Cham, Switzerland: Springer.

Pennington, B. F. (2006). From single to multiple deficit models of developmental disorders. Cognition, 101(2): 385-413.

Stein, J. (2018). The magnocellular theory of developmental dyslexia. In T. Lachmann & T. Weis (Eds.). Reading and dyslexia (pp. 97-128). Cham, Switzerland: Springer.

Valdois, S., Bidet-Ildei, C., Lassus-Sangosse, D., Reilhac, C., N’guyen-Morel, M. A., Guinet, E., & Orliaguet, J. P. (2011). A visual processing but no phonological disorder in a child with mixed dyslexia. Cortex, 47(10), 1197–1218.

Vellutino, F. R. (1979). Dyslexia: Theory and research. Cambridge, MA: MIT Press.