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Developmental dyslexia is one of the most common neurodevelopmental disorders, affecting up to 10% of children. Dyslexia can turn school into a frustrating experience and make every day reading tasks feel overwhelming. But what causes it? And how can we help those who struggle with it? Over the years, researchers have suggested conflicting theories to explain its origin and are not able to agree on its cause. This article breaks down the leading theories behind developmental dyslexia and explores some alternative interventions.
What is developmental dyslexia
Developmental dyslexia is classified as a neurodevelopmental disorder in the DSM-V. It is defined by persistent difficulties in reading accuracy, fluency or comprehension, while the overall intelligence is unaffected. These challenges can also lead to struggles with acquiring knowledge through written text and building a strong vocabulary (Erbeli et al., 2021).
Although developmental dyslexia has a strong genetic component – heritability is estimated around 70% (Erbeli et al. 2021) – environment also plays a big role. A longitudinal twin study by Boivin et al (2013) found that while genetics have a higher influence in early childhood, after the age of six, factors like education, reading habits, and interventions account for up to 50% of the difference between children. This means there’s an important window during early school years where well-structured environments and interventions can make a big difference.
The phonological deficit hypothesis and the double deficit hypothesis
One of the most widely discussed theories is the phonological deficit hypothesis, which suggests that dyslexia stems from problems in the processing of speech-sounds. Individuals with developmental dyslexia do not have typical underlying phonological representations, which leads to difficulties with phonological processing and decoding. Not having adequate links between sounds and letters makes it difficult to read.
However, there’s more to it than just phonological processing. Many children with dyslexia also struggle with something called Rapid Automatized Naming (RAN), basically, how quickly they can name familiar objects like letters or numbers. Norton and Wolf (2011) proposed the double deficit model, suggesting that developmental dyslexia may be a consequence of one or both of these issues. Since phonological deficits and RAN difficulties are relatively independent, this model explains why dyslexia can look so different from one child to the next.
The magnocellular theory
Critics of the phonological deficit theory argue that almost all poor readers tend to score low on phonological awareness tests. This suggests that phonological deficits may be a consequence of poor reading rather than its root cause (Stein 2022). The magnocellular theory offers an alternative explanation. It proposes a dysfunction in the magnocellular visual system—cells in the eyes that process motion, location, and light. These signals travel through the dorsal pathway to the frontal cortex, which handles attention, working memory, and eye movement control.
A deficit in this pathway can lead to problems with letter localization, attention, and eye coordination, often causing a crowding effect, where words appear blurred or jumbled (Stein, 2022). This theory has strong empirical support, with numerous experiments showing that dyslexic children struggle with visual attention and eye movement control (Facoetti et al. 2006).
The auditory temporal processing hypothesis
The auditory temporal processing hypothesis shifts the focus to sounds. As early as 1973, Tallal and Piercy showed that children with dyslexia had difficulties processing rapidly shifting sounds with short duration. A more recent study by Lohvansuu et al. (2018) shows that babies with family history of developmental dyslexia show changed neuronal responses to pseudowords. These early differences predicted 44% of reading speed at the age of 14. There is growing evidence that music training in infancy can be an effective tool to train the processing of sounds, with a lasting effect in both language development and reading ability (Virtala & Partanen, 2018).
Some researchers believe that difficulties in auditory processing may be a consequence of magnocellular processing. They believe that the magnocellular theory does not only explain visual attention, but also auditory attention when processing sounds (Stein, 2022).
Action video games as an intervention
Traditionally, interventions for children with developmental dyslexia usually focuses on training phonological representations. Recent studies however suggest an alternative intervention, with its roots in the magnocellular theory. Action video games may help children with dyslexia improve their reading skills as these games have a high perceptual and cognitive load. They require players to focus on relevant information while filtering out distractions, training the brain’s attention and processing systems. Pasqualotto et al (2022), found that just a few months of playing action games led to measurable improvements in reading abilities, even six months later. These findings open up a fun and engaging new path to help children with dyslexia, very different from the traditional interventions.
Developmental dyslexia, the most common learning disability, affects children all around the world. Despite decades of research, scientists are not able to agree on its cause. The most debated theories vary a lot, suggesting impairments ranging from mental representations of sounds, the ability to quickly name familiar objects, an attention deficit with its origins in the visual system, to an auditory deficit when processing rapidly changing sounds. It is important to further investigate the causes of this developmental disorder in order to structure new effective interventions.
By Vita Jamila Keil
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Sitograhpy
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A.P.E. 
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