Six dyslexia theories and models are discussed: the discrepancy model, phonological deficit theory, magnocellular theory, rapid automatized naming, the double deficit hypothesis, and the multiple deficit model, which views dyslexia as the result of multiple interacting factors.
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Table of contents:
- Introduction
- The discrepancy model
- Phonological deficit theory
- Magnocellular theory
- Rapid naming deficit
- Double-deficit hypothesis
- Multiple deficit model of dyslexia
- The bottom line
Introduction
Six blind men came across an elephant. The first ran into its sturdy side and declared the animal a wall. The second felt the elephant’s tusk—round, smooth, and sharp—and pronounced it a spear.
The third grasped the squirming trunk and proclaimed the elephant a snake. The remaining three described it as a tree trunk (based on the leg), a fan (based on the ear), and a rope (based on the tail).
Scholars in the field of learning disabilities often invoke the parable of the blind men and the elephant to describe the state of dyslexia research. For more than a century, researchers have proposed different explanations for why some children struggle to learn to read. Some have focused on intelligence, others on phonological processing, visual pathways, rapid naming, memory, or attention. Each theory has contributed valuable insights, yet none appears to explain dyslexia fully (Spear & Sternberg, 1986; MacConville, 2007; Waber, 2010; Kapur, 2011).
The term dyslexia comes from the Greek words dys, meaning difficult, and lexis, meaning word. It refers to a learning difficulty characterized primarily by problems with accurate and fluent word recognition, spelling, and decoding. Common signs include:
- Reversing letters such as b and d
- Putting letters in the wrong order (reading felt as left)
- Misreading small words (such as a for and)
- Losing one’s place on a line or page
- Reading aloud hesitantly, word by word
- Sounding out a word correctly but being unable to recognize it
- Reading with poor comprehension
Spelling and writing difficulties often accompany reading problems.
Perhaps much of the confusion surrounding dyslexia arises because researchers have been touching different parts of the same elephant. Learning to read is a complex process that depends on a wide range of cognitive skills, not just one or two (Pennington, 2006; Tamboer et al., 2016).
The theories discussed below illustrate how scientific thinking about dyslexia has evolved—from explanations based on a single deficit to models that recognize the complex interaction of multiple cognitive factors.
The discrepancy model
Traditionally, dyslexia was defined as a discrepancy between a child’s reading performance and what would be expected given the child’s intelligence. In this view, the “true dyslexic” was typically seen as a child who struggled to read despite being of average or above-average intelligence (Elliott, 2015). When children with lower IQ scores struggled to read, their difficulties were often attributed to general intellectual limitations rather than dyslexia.
This distinction is now widely questioned. Research has shown that the difference between the intellectually able dyslexic poor reader and the so-called “garden-variety” poor reader is not as clear as once believed. Using brain imaging, Tanaka et al. (2011) found no meaningful differences in reading-related brain activity between poor readers with and without an IQ-reading discrepancy. Poor readers across IQ levels showed reduced activity in important reading regions, including the left parietotemporal and occipitotemporal areas. These findings were largely replicated by Simos et al. (2014).
Despite strong scientific evidence against the discrepancy model, many clinicians and practitioners continue to use it when diagnosing dyslexia (Elliott, 2015). The model, therefore, remains important historically, but it is no longer considered a reliable way to distinguish dyslexia from other forms of reading difficulty.
Phonological deficit theory
The idea that phonological weaknesses underlie dyslexia to some extent is part of almost every major theory of dyslexia’s causes. In its strongest form, however, the phonological deficit theory proposes that a deficit in phonological processing is the primary cause of dyslexia. Children with good phonological skills, or good phonological awareness, tend to become good readers and spellers. Conversely, children with poor phonological awareness often struggle to remember, recall, and manipulate the sounds of language.
A phonological deficit may cause difficulties with identifying rhyming words, segmenting words into individual sounds, or blending sounds to form words. As children approach school age, these weaknesses can make learning to read and spell more difficult. Successful reading depends on mastering the relationship between graphemes (written symbols) and phonemes (the sounds of language).
The notion that phonological deficits can fully explain dyslexia continues to be challenged, however. While phonological awareness is undoubtedly important for learning to read, a growing body of evidence suggests that it is only one part of a much larger picture. Some studies have even shown that learning to read improves phonological awareness, raising questions about whether poor phonological skills are always the cause of reading difficulties or sometimes a partly consequential factor.
Magnocellular theory
An alternative to the phonological deficit theory is the magnocellular theory. Magnocellular cells are specialized neurons that form part of the brain’s visual processing system. These cells help us focus our eyes, track moving objects, and filter out visual “noise.” According to the magnocellular theory, reading difficulties such as dyslexia arise because these visual pathways are underdeveloped or function abnormally.
In some respects, this theory echoes earlier views of dyslexia as a primarily visual disorder. During the late nineteenth and early twentieth centuries, many researchers believed that reading difficulties stemmed from problems in visual perception. Over time, however, this view gave way to theories emphasizing phonological processing.
Although the magnocellular theory has attracted considerable interest, it is not widely accepted as a complete explanation for dyslexia and has received limited experimental support. Nevertheless, visual difficulties may contribute to reading problems in some individuals. Because vision plays a critical role in reading, it is advisable to assess a child’s eyesight before testing for dyslexia. An optometrist or physician can determine whether visual problems are contributing to the child’s reading difficulties.
Rapid naming deficit
Research on rapid naming, or rapid automatized naming (RAN), began with the work of Denckla and Rudel. RAN refers to the speed with which an individual can retrieve the names of familiar symbols—such as letters, numbers, colors, or pictured objects—from long-term memory. Individuals with dyslexia typically perform more poorly on rapid naming tasks than their peers who read normally (Elliott & Grigorenko, 2014).
The importance of rapid naming has been demonstrated in numerous studies. A meta-analysis of 137 studies involving 28,826 participants found a moderate-to-strong relationship between RAN and reading performance. Further analyses revealed that RAN contributes to all four major aspects of reading: word reading, text reading, non-word reading, and reading comprehension. The strongest relationships were found for real-word reading and text reading (Araújo et al., 2015).
Despite these findings, researchers have not reached agreement on exactly why rapid naming is so closely related to reading ability. As Araújo et al. (2015, p. 869) noted, there is “still no consensus regarding the mechanisms responsible for this relationship.” Nevertheless, the evidence suggests that rapid naming plays an important role in reading development and may contribute to dyslexia independently of phonological awareness.
Double-deficit hypothesis
As the name suggests, the double deficit hypothesis points to weaknesses in two distinct areas. It extends the phonological deficit theory by acknowledging the important role that phonological awareness plays in learning to read, while proposing that phonological weaknesses alone cannot explain all cases of dyslexia.
According to this theory, deficits in phonological processing are not the only cause of reading difficulties. While many individuals with dyslexia struggle with phonological awareness, others do not. In these cases, researchers often observe weaknesses in rapid automatized naming (RAN)—the ability to quickly retrieve familiar symbols, letters, numbers, colors, or objects from long-term memory.
Although there is often overlap between phonological deficits and rapid naming deficits, they can occur independently. Some children struggle primarily with phonological processing, others primarily with rapid naming, and some with both.
The double deficit hypothesis also provides a framework for understanding differences in severity. Children with deficits in both phonological awareness and rapid automatized naming are expected to experience the most severe reading difficulties. Those with a deficit in only one area generally experience milder difficulties. By recognizing that more than a single cognitive weakness can contribute to dyslexia, the double-deficit hypothesis represented an important step beyond single-deficit explanations of reading failure.
Multiple deficit model of dyslexia
Recent developments in dyslexia research have led many researchers to conclude that models based on a single deficit are inadequate. One reason is that dyslexia frequently co-occurs with other learning and developmental disorders, including dysgraphia, dyscalculia, and ADHD. Building on these observations, the multiple deficit model explains dyslexia as the result of multiple interacting risk factors rather than a single underlying cause.
This theory proposes that reading depends on a wide range of cognitive skills. Consequently, dyslexia may emerge from complex interactions among these skills. Weaknesses in phonological awareness, memory, attention, processing speed, reasoning, and other cognitive abilities may all contribute to reading difficulties. In particular, working memory has received increasing attention as a skill that plays an important role in reading development. Rather than viewing dyslexia as the result of a single deficit, the multiple-deficit model treats it as a probabilistic outcome of multiple interacting influences.
Critics argue that the model is difficult to test and falsify. Nevertheless, its insistence on treating dyslexia as a multifaceted problem is probably the most realistic approach to the issue.
Unlike earlier theories that focused on a single part of the elephant, the multiple deficit model attempts to consider the whole animal: vocabulary; focused, sustained, divided, and visual-spatial attention; visual, auditory, and phonological processing; rapid naming and processing speed; verbal, visual, auditory, sequential, iconic, short-term, long-term, and working memory; logical thinking; and more.
The role that memory may play in creating the dyslexia elephant is perhaps still underestimated. Nevertheless, it should be considered a fundamental skill. Riding and Pugh found that visual sensory (iconic) memory is significantly related to reading fluency, accuracy, and comprehension. Guthrie and Goldberg identified significant positive associations between visual sequential memory and paragraph comprehension, oral reading, and word recognition. Stanley, Kaplan, and Poole compared 33 dyslexic children with 33 controls aged eight to twelve and found that the dyslexic children were inferior on tasks involving visual and auditory sequential memory.
Further evidence comes from a study published in Dyslexia, which compared 60 dyslexic children with 65 age-matched typical readers on verbal, visual-spatial, and visual-object tasks. The results documented a generalized impairment of long-term memory capacities in the dyslexic group. Importantly, the deficit was not limited to phonological material but also affected visual and spatial information, suggesting that dyslexia is associated with multiple cognitive deficits (Menghini et al., 2010).
Weiss et al. (2014) tested dyslexic and non-dyslexic musicians on auditory processing and auditory memory. While the dyslexic musicians performed as well as their non-dyslexic counterparts on auditory processing tasks—and better than the general population—they performed significantly worse on measures of auditory working memory, including memory for rhythm, melody, and speech sounds. Moreover, these abilities were highly correlated with reading accuracy.
One should also consider the role of mutualism. According to the mutualistic view, cognitive abilities support and reinforce one another as they develop. For example, better reasoning skills may help a child acquire vocabulary more rapidly, while a larger vocabulary may, in turn, support the development of reasoning skills (Kievit et al., 2017). Vocabulary size has also been shown to be closely related to phonological awareness. Likewise, nonverbal intelligence and working memory are associated with performance on phonological awareness tasks. Perhaps improving vocabulary, nonverbal intelligence, and working memory would improve phonological awareness—and vice versa.
The bottom line
The elephant is an enormous animal. Each blind man was touching only one part. To understand the elephant, they had to put all the parts together.
The same may be true of dyslexia. While each theory discussed in this article has contributed valuable insights, no single deficit appears sufficient to explain developmental dyslexia. Reading depends on a complex interaction of cognitive skills, and weaknesses in any combination of these may contribute to reading difficulties.
Has the time not come for learning disability practitioners to consider the whole elephant?
Edublox provides support for students with dyslexia, dyscalculia, and other learning difficulties. We help students worldwide overcome academic challenges and reach their full potential. Book a free consultation to discuss your child’s learning needs and discover how we can help.
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