What is dyslexia? What causes dyslexia?

14th November 2023

Awareness of dyslexia has significantly increased but do you know what the definition of dyslexia is and what causes dyslexia?

How do you define dyslexia?

The two adjectives used in Rose’s report (2009)  to describe the measure of a competent reader are ‘accurate’ and ‘fluent.’  These are essential skills for a learner to be successful in our current education system. The BDA describes Dyslexia as a ‘learning difference’ rather than a ‘disability’ and includes auditory and visual processing differences in their definition. These inclusions stem from the results of research and studies which identify these cognitive factors as being  typical features of dyslexia. It is not a difficulty with eyesight or hearing, it is how the brain processes both senses.

Rose (2009)  reports that dyslexia is not confined to difficulties with reading and writing but extends beyond to ‘co-occurring’ factors. The BDA also acknowledges that dyslexia can present with other difficulties such as motor-coordination problems, language deficits, slow mental calculation and poor organisational skills. These other manifestations of dyslexia are not indicators of dyslexia if not accompanied by at least one of the three main defined areas ascribed by the Rose report.

The Rose report defines the cognitive difficulties for dyslexic students as being in Phonological awareness is an essential foundation for literacy. It is the ability to recognise sounds. It allows a learner to detect rhyme, to segment words into syllables and recognise the onset and rime of a word. Phonological awareness enables a learner to isolate sounds so that they can blend and segment- essential skills for reading and spelling. Verbal, or phonological, memory involves the ability to temporarily store auditory or phonological information. It is crucial as it helps learners to remember the sounds associated with letters and words.  Rapid naming processing speed is the ability to quickly name familiar items, whether it be letters, numbers or objects.

Phonological Deficit Theory

The phonological deficit theory begins at the cognitive level- difficulty with identifying and discriminating between the different sounds in words. In 1974, Liberman proposed that dyslexic learners found it difficult to recognise individual phonemes in words. Children begin to show phonological awareness from a very young age. Hagtvet (1997) and Lundberg (2002) theorised that a phonological deficit is a probable predictor for  future reading difficulties.

 Adams (1990) identified the following as phonemic skills and, as such, the deficits for dyslexia, as proposed by this theory: nursery rhymes, rhyme and alliteration, blending and segmenting. Goswami and Byrant (1990) emphasised the significance of rhyme as not being necessarily dependent on particular phonemes but by hearing and distinguishing the onset and rime in a word. This is where they suggest, there needs to be explicit teaching.

Goswami also identified that dyslexic learners struggle with the cadence and rhythm in spoken words- the beats, intonation and syllables. This hypothesis is further developed in the Temporal Sampling Theory.

Snowling is a well respected advocate for  the phonological deficit theory. She, like Gotswami, sees phonological awareness as a fundamental prerequisite to reading. The phonemes need to link to the alphabet before the task of blending. 

Rapid naming is a benchmark in year 1 when predicting reading success (Hulme and Snowling 2012) . Dyslexic learners often have a deficit in this area. This is obviously going to impact reading fluency and understanding.  Learners will take longer to retrieve language, or may say the wrong word which will cause frustration. Brenznitz (2008) ‘Asynchrony Phenomenon’ identified a gap with the ability to process when decoding a word caused by difficulty transferring information between hemispheres of the brain. He and his colleagues successfully implemented a program to train the brain to process more quickly-they saw a significant improvement and claimed that it was transferable to other skills. 

The Double Deficit Theory

This rapid naming deficit was, at one time, seen as separate from the phonology deficit. Wolf and Bowers(1999), however discovered that both deficits can occur with dyslexic learners and that this led to a more pronounced behavioural difficulty with literacy. This is known as the Double Deficit Theory. On top of struggles with phonological awareness, dyslexic learners might find it a challenge to retrieve words from their memory at a pace that is rapid enough to support fluent reading. (Reid, G 2009).  In the classroom, this could mean that the learner takes longer to answer the question- it might seem that they do not know the answer; they might struggle to recall either verbal or written instruction and might not finish the task in the given amount of time.

The Magnocellular Theory

The Magnocellular Theory accounts for the visual difficulties that many dyslexic learners experience. – they report that words, letters or digits jump around or they can mis-see letters, substituting them for similar ones. Stein and Walsh (1997) further developed Lovegrove’s theory which theorised that there is a magnocellular deficit in dyslexic individuals coupled with a more ‘efficient parvocellular system’ which impeded their reading (The misdiagnosis of Dyslexia, Philips). The magnocellular cells are responsible for processing moving objects. When we read, our eyes cover the words and sentences at speed. If the stability of the words are affected, this could account for and explain the blurring that dyslexic readers experience. Those affected might also experience difficulty tracking the lines and keeping place within the text, while the sensitivity to brightness might make white paper unsuitable. This visual difficulty has been identified in 30-40% of people with dyslexia. Stein’s later research (2000) found that the magnocellular cells  can not be attributed to ‘visual persistence’ though post-mortem studies show that this area of the brain is indeed comparatively smaller than non-dyslexic individuals.

The Magnocellular Theory also relates to the auditory processing of information which explains the phonological difficulties that dyslexic learners experience. As with Goswami’s research into phonological awareness, Stein and Walsh (1997) saw a correlation between the impaired neural auditory processing pathways and the inability to distinguish between the rhythm, cadence and pitch of sounds. This will particularly impact the skill of sounding out words for decoding and encoding.

The Cerebellar Deficit Theory

Another biological causal theory is the Cerebellar Deficit Hypothesis. The cerebellar is active when a skill is being learnt and less active once the skill has become automatic (Nicolson and Fawcett, 1990). If the cerebellar in the brain is impaired, the cognitive effects impact phonology and the speed at which an individual can process information (fulbright et al., 1999) which concurs with other theories. A cerebellar dysfunction can also impact the Broca’s area of the brain which will affect an individual’s ability to carry out tasks automatically, without conscious thought (Nicolson et al, 2001). Timing for sequencing information is also key to the autonomisation of that information. Learners need autonomy for reading, writing, typing and other motor skills. This will account for the reported examples of struggles with motor ability, balance and muti-tasking (Augur 1985). If a learner struggles with automatising skills, they will struggle with reading (Fawcett and Nicolson, 1992) as well as carrying out motor tasks such as balance, or writing. They will have to concentrate much harder, consciously thinking about each action, each skill. Overlearning will help improve automaticity as it will strengthen the pathways so that retrieval becomes less of a conscious behaviour. Repetition and constant recall is needed to help information to transfer to the long term memory. The ‘average’ learner will need to review information at least three times (Teaching Dyslexic Students, pg 129) while dyslexic learners will need significantly more repetition to effectively recall learnt information.

The Working Memory Model

Baddley and Hitch’s working memory model (1974) focuses on the often reported and acknowledged in the Rose report (2009), problems with the working memory of individuals with dyslexia. The short term memory temporarily stores information while the working memory ‘does something’ with that information. The central executive ‘coordinates’ the information that it then sends to either the phonological loop or the visual spatial sketch pad. To code, remember and eventually transfer information from short to long term, the loop rehearses the information. When reading, children will need to remember, or ‘store’ the preceding information to make sense of the information that is in front of them. If the ability to store in the short term is hampered, it  could affect learners at a word, sentence or paragraph level.  The visual spatial sketch pad is responsible for the short term memory of images. This would come into play for orthography as well as directions and spatial awareness. The central executive acts like a kind of triage by prioritising and ordering the information to be remembered. (Baddeley 1986). In 2000, Baddley addressed the missing link (that of the explanation for the transfer from short to long term memory) by identifying the episodic buffer, believed to be the conduit for both the phonological and  visual information from the working to the long term memory (Baddeley, 2000)

For dyslexic learners in the classroom, they may find it a struggle to remember what they have just heard (if teacher instruction) or just read (if written instruction) and this will make it difficult to keep the same pace as others in the classroom. It will take them longer to read a text as they may have to re-read information to help make sense of new information while copying off the board could also be a challenge as they may have to keep checking the spellings. Equally, a dyslexic child will find it a significant challenge if asked to solve mental calculations, or perform a task which requires them to follow a number of steps as they need to remember information at the same time as doing something with that information.

The Importance of Oracy for Dyslexia

Phonology is one of the 4 pillars for oral language success. Having discussed the difficulty that dyslexic children can have with phonological awareness, this, alongside the development of pragmatics, semantics and grammar can be addressed through good oral communication before a child has even started school. Oral language skills underpin reading skills and life-skills, such as communication and socialisation. Communication is key for children to express emotion and ideas. Receptive and expressive language is developed through exposure to oral communication at a very young age. It is as equally important to a child’s emotional wellbeing as it is to understanding tasks in the classroom and accessing the curriculum.

 ‘Reading develops from a foundation of oral language,’ (Nation and Angell (2006). Snowling (2003) emphasises the role of  pre-school oral communication as the foundation for literacy success. The foundation allows for semantic understanding of words that are decoded. If children listen to stories read aloud, without the complications of graphology or orthography, they are already being exposed to vocabulary, grammar, syntax and, therefore, understanding.  

Genetic Factors Contributing to Dyslexia

The above is focused very much on environmental factors relating to dyslexia but the hereditary factor must also be taken into account. There is a much higher risk of developing dyslexia if a parent has the chromosomes 1,2,3,6,11 and 25 (Scerri and Shulte-Korne, 2010), and an even higher likelihood if a sibling is also dyslexic.  The left side of the brain is affected by these chromosomes, hence the acquisition of language is affected. If a parent is dyslexic, it is especially important that they prioritise oral skills with their child as soon as possible.

Pennington (2006) adds another level to Morton and Frith’s three level framework which links with the genetic predisposition for dyslexia. His Multi Deficit model includes an etiological level which considers the risk factors for both the genetic and environmental causes of dyslexia- each cause serves as a ‘risk’ and ‘protective’ gauge. The model accounts for the range of cognitive and behavioural features of dyslexia. The etiological level also allows for the consideration of other co-occurring conditions which are prevalent with dyslexia. There are common cognitive and biological factors which dyslexia shares with ADHD, dyscalculia, DCD, ADD, ASD, DLD and Dyspraphia. This neurodiverse view allows assessors to see each learner as an individual- though deficits are shared, they are not shared in equal measure. Likewise, each will have different strengths in different areas though commonalities will also be found. This model is now the model of choice for many as the additional element allows for more personalised assessment and intervention.

There is much consensus about the behavioural aspects of dyslexia. All theories acknowledge the role that phonological impairment plays for dyslexia even though there may be some difference in agreement about the nature of the problems. Phonological deficit can be the single case of dyslexia, while other deficits serve to aggravate the condition. There is also a consensus that developmental dyslexia has a genetic origin. Brain imaging scans have shown that there are differences between the magnocellular and the cerebellar regions of the brain but how great a part they play in dyslexia is open to debate. For us, as educators, the main thing we need to focus on is the undisputed behavioural manifestations of the cognitive impairments. We need to use the information to assess, diagnose for the deficits for particular areas and put the tailored intervention in place that will ensure the progress of our dyslexic learners.

https://www.bdadyslexia.org.uk/dyslexia/about-dyslexia/what-is-dyslexia

British Dyslexia Association (2023).

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