What is dyslexia?

Overview

There are different ways of defining dyslexia. Some definitions are used in research, others in medical diagnosis, others in civil rights law, and still others in educational evaluations or intervention plans. 

Despite differences in wording or emphasis, they are the same at heart. Understanding models of reading and sight words helps put dyslexia, whatever definition you use, in perspective. 

As a way to demystify dyslexia, Hugh Catts (2018) suggests that a basic definition of dyslexia is unexpected trouble recognizing words while reading, focusing on measurable skills rather than underlying causes.

IDA and NIH Definitions of Dyslexia

One of the most widely used definitions comes from the International Dyslexia Association (IDA Editorial Contributors, 2018), Wisconsin 2019 Act 86 and Wisconsin's Informational Guidebook on Dyslexia and Related Conditions: 

“Dyslexia is a specific learning disability that is neurobiological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge.”

Another comes from the National Institutes of Health (National Institute of Neurological Disorders and Stroke, 2019):

“Dyslexia is a brain-based type of learning disability that specifically impairs a person's ability to read. These individuals typically read at levels significantly lower than expected despite having normal intelligence. Although the disorder varies from person to person, common characteristics among people with dyslexia are difficulty with phonological processing (the manipulation of sounds), spelling, and/or rapid visual-verbal responding. In individuals with adult onset of dyslexia, it usually occurs as a result of brain injury or in the context of dementia; this contrasts with individuals with dyslexia who simply were never identified as children or adolescents. Dyslexia can be inherited in some families, and recent studies have identified a number of genes that may predispose an individual to developing dyslexia.”

The Diagnostic and Statistical Manual (DSM-5 315.00) from the American Psychiatric Association discusses dyslexia under the Specific Learning Disorders, saying:

“Dyslexia is an alternative term used to refer to a pattern of learning difficulties characterized by problems with accurate or fluent word recognition, poor decoding, and poor spelling abilities.” 

Dyslexia is also defined in federal law. Public Law 115-391, "The First Step Act," provides the following definition:

"The term 'dyslexia' means an unexpected difficulty in reading for an individual who has the intelligence to be a much better reader, most commonly caused by a difficulty in the phonological processing (the application of the individual sounds of spoken language), which affects the ability of an individual to speak, read, and spell."

A closer look at each part of the definition from the IDA highlights similarities among definitions and clarifies understanding of dyslexia:

Specific learning disability (SLD) 

• SLD is one of the categories of disability listed in the federal Individuals with Disabilities Education Act (IDEA), and dyslexia is one of the disabilities included under the umbrella of SLD.

• SLD is a term used by Wisconsin schools to determine eligibility for special education.

• IDEA and Wisconsin define SLD as “a disorder in one or more of the basic psychological processes involved in understanding or in using language, spoken or written, that may manifest itself in the imperfect ability to listen, think, speak, read, write, spell, or to do mathematical calculations.  . . . Such term includes such conditions as perceptual disabilities, brain injury, minimal brain dysfunction, dyslexia, and developmental aphasia.  . . . Such term does not include a learning problem that is primarily the result of visual, hearing, or motor disabilities, of mental retardation, of emotional disturbance, or of environmental, cultural, or economic disadvantage" (20 U.S.C. §1401(30), 34 CFR 300.8©910); PI 11.36(6) (a)).

Neurobiological in origin

• Dyslexia originates in the brain.

• Brain imaging shows differences in the development, structure, and functioning of the brain in people with dyslexia.

Difficulties with accurate and/or fluent word recognition

• Difficulty reading words accurately

• Lack of automaticity and effortlessness in reading 

Deficit in spelling and decoding abilities

• Trouble sounding out and spelling words

• Can include difficulty perceiving individual sounds in words, blending them together, or segmenting them

• Difficulty learning and remembering the sound-letter correspondences, and grasping and applying the alphabetic principle (the idea that the letters of our written language represent speech sounds)

Deficit in the phonological component of language 

• Weaknesses in one or more phonological processing skills including phonemic awareness (PA), phonological memory (PM), and rapid automatized naming (RAN) (Wagner et al., 2013)

• PA includes the understanding that words are made up of speech sounds and the ability to blend, segment, and manipulate (delete, substitute, or reverse) the smallest units of sound: the phonemes. A key research discovery several decades ago was that PA at the phoneme level is essential to reading, and PA performance in the early grades is more closely correlated with reading success than is intelligence or any other factor (Ehri, 1984; Stanovich 1986, 1994; Adams, 1990; Griffin and Olson, 1992; Scarborough, 1998). PM and RAN can be thought of as underlying brain processes that are necessary for strong reading skills. PM is the ability to temporarily store and retrieve verbal information from working memory, which impacts ease and accuracy of reading. Phonological memory lets a reader hold the beginning sounds of a word in mind while they decode and blend the rest of the sounds to figure out the word (Wagner et al., 2013). PM problems can often be detected in difficulty repeating real and nonsense words with accuracy (Hulme and Snowling, 2009). RAN is the ability to quickly retrieve the name of a letter, number, object, or other stimulus from long-term memory. RAN is a predictor of reading fluency. It is important to note that some students with dyslexia do not have a deficit in PA, but may have weaknesses in PM and/or RAN (Pennington et al., 2012; Fuchs et al., 2008). Though weakness in the phonological component (especially phonemic awareness) is the hallmark of dyslexia, there are some individuals who have trouble linking graphemes with phonemes, but whose cognitive profile does not include any phonological deficits (Catts et al., 2017; Pennington et al., 2012; Ring and Black, 2018; Elliott and Grigorenko, 2014; Seidenberg, 2017, Chapter 8).

Often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction

• Dyslexia is often a surprise, given the other strengths of the individual, as shown in Figure 2.

• Other intellectual strengths, motivation, and instruction are all sufficient for reading.

Secondary consequences

• A negative impact on reading comprehension caused by weakness in accurate decoding and automatic word recognition

• Lagging development of vocabulary, background knowledge, and other aspects of language comprehension caused by lack of reading experience, leading to further difficulties with reading comprehension

What Does it Mean To Be “At Risk for Dyslexia?”

Common definitions of dyslexia focus on a child having trouble with word recognition and spelling. Therefore, younger children who aren't expected to be able to read and spell yet would not fit the definition even if brain imaging showed a structure or pattern typical of a dyslexic brain.

If a child aged 3 to 5 has a family history of dyslexia, or if early screening shows weakness in pre-reading skills, such as age-appropriate phonemic awareness, phonological memory, automatized rapid naming, or letter recognition, the child should be considered “at risk” of developing a reading or spelling problem that could later be classified as dyslexia.

A child aged three to five is in the critical stage for intervention and prevention. Intervention should never be delayed until reading failure occurs. Early evidence-based intervention is an opportunity to prevent or significantly lessen future reading and spelling difficulties.

Causes of Dyslexia

Dyslexia is brain-based and has a genetic component. Dyslexia runs in families. About 40% of siblings of children with dyslexia will also have the condition, and almost 50% of the children of a parent with dyslexia will be affected. If both parents have dyslexia, 75% of their children likely will be impacted. 

Tracking the genetics of dyslexia is complicated because more than one gene is involved, and environment also plays a role in the development of the disability. Studies on twins have shown that about 30% of the risk for dyslexia comes from a child’s early environment, with poverty and parent education being risk factors. For a child with a genetic predisposition, the early literacy environment and quality of instruction are especially critical (Dorta, 2021).

The inherited genes impact both the structure and functioning of the areas and pathways of the brain that are used for reading. With the development of brain imaging technology in the 1990s, scientists have been able to demonstrate that dyslexia is brain-based. The brain models below represent the normal activation patterns of the reading brain compared to the “neural signature” of the dyslexic brain engaged in the same task. 

Myths About Dyslexia

• 7 common myths about dyslexia, Understood.org

• Dyslexia Myths, Dr. Nadine Gaab

• Dyslexia: Myths and Facts, Scholastic

• Myths and Misunderstandings about Dyslexia Explained, Wisconsin's Informational Guidebook on Dyslexia and Related Conditions

The efficient processors of written language are found in the left hemisphere of the typically-developing brain. These parts of the brain include areas that process the visual aspects of written symbols, areas that connect symbols with speech sounds, and areas that connect words with meaning. 

The back (red and yellow) areas are more heavily activated in reading than the front (green) area. Beginning readers slowly analyze words in the red area with some assistance from the green area. After a word is analyzed correctly several times, its spelling, pronunciation, and meaning are stored permanently in the yellow “word form” area, where maturing readers can recognize it instantly. A skilled reader can effortlessly and quickly recognize most words in the word form area (Shaywitz, 2003).

On the other hand, individuals with dyslexia experience disruption of the left rear pathways, and show under-activation of the red and yellow areas. Over time, they rely more and more heavily on the green area, along with some less efficient right-hemisphere areas, to figure out words. This pattern of reading, though it may provide some degree of accuracy, does not lead to the effortless and instantaneous word recognition that is so important for reading comprehension and motivation (Shaywitz, 2003).

The good news is that a group of studies has presented strong evidence that early, evidence-based intervention can improve reading performance and also change dyslexic brain activation patterns to more closely resemble non-impaired functioning. Typical of this line of research is a 2003 study by Shaywitz et al., in which struggling readers were provided with a phonologically-based reading intervention over the course of a year. At the end of the intervention, fMRI imaging showed increased activation in the red and green areas of the left brain, and students were reading more accurately and fluently. Moreover, a year after the intervention ended, the same students showed activation in the critical yellow word form area of the brain, and less activation in the right hemisphere. Struggling readers who had received a non-phonologically-based intervention did not show these brain changes. Study authors concluded: 

[T]hese data demonstrate that an intensive evidence-based (phonologic) reading intervention brings about significant and durable changes in brain organization, so that brain activation patterns resemble those of typical readers, with the appearance of the left occipitotemporal area and improvement in reading fluency. These data have important implications for public policy regarding teaching children to read: the provision of an evidence-based reading intervention at an early age improves reading fluency and facilitates the development of those neural systems that underlie skilled reading.

Are Word Recognition Problems Always a Sign of Dyslexia?

Word recognition problems do not all emerge from dyslexia. Children may struggle to acquire the foundational skills of reading for many other reasons, both personal (poor vision or hearing, short attention span, low global intelligence, slow cognitive development, lack of motivation, etc.) and environmental (low literacy home environment, erratic school attendance, nutritional status, trauma, poor instruction, etc.). What distinguishes dyslexia from these related conditions are the language-related aspects of the reading problem and its genetic and neurobiological roots. 

ALL children with reading difficulties deserve effective intervention. Children who struggle with automatic word recognition, regardless of the reason, need the explicit and systematic instruction in the alphabetic code recommended by the Report of the National Reading Panel in 2000 and the DSM-5 manual of the American Psychiatric Association (Elliott and Grigorenko, 2014). 

Uncovering the underlying causes of the word recognition problem in dyslexia is not necessary to begin effective intervention, but may be helpful in forecasting the need for a more intensive and extensive intervention than might be needed for other children who are exhibiting similar symptoms.

What More Do We Know About Dyslexia?

• Early identification and effective treatment of pre-literacy skills and emerging decoding/encoding deficits are essential in allowing age-appropriate development of reading accuracy and automaticity, building comprehension skills through reading, achieving academic success, maintaining interest in school, promoting self-esteem, and helping prevent anxiety, depression, and anger. Although dyslexia is often diagnosed around 3rd or 4th grade, the optimum window for intervention is kindergarten to 1st grade. 

• Dyslexia has a genetic component and runs in families. Family history is important in early screening of children for reading risk factors. 

• It is the most common reading disability. 

• Dyslexia occurs along a continuum from mild to severe.

• It occurs in 5-20% of the population, depending on how inclusive your definition is and where you set the cutoff on the continuum. Regardless of where the cutoff is set, many students who fall above that benchmark undoubtedly are in need of the same kind of explicit, sequential, and systematic instruction as those who fall below. 

• As noted by reading researcher Susan Brady (2019), “Restricting access to intervention may satisfy the aim to limit school resources allocated for these purposes, but is not the kind of equitable and adequate system one would want." Solutions should be available in general education as well as special education.

• Dyslexia frequently occurs along with other conditions such as ADHD, dysgraphia, dyscalculia, executive functioning weakness, and/or oral language impairment. A diagnosis of one of these disorders should trigger a comprehensive evaluation to determine if other disorders are present. 

• It affects people of every gender, economic class, and ethnicity nearly equally.

• Dyslexia can impact an individual of any intelligence level.

• It can be found in people who speak any world language, though its symptoms may vary depending on the structure of the language.

• Reading failure frequently leads to inadequate academic, social, and emotional development, with an elevated incidence of anxiety, depression, school dropout rates, involvement with the juvenile justice system, incarceration, and suicide risk.

• Dyslexia is a lifelong condition without a “cure,” but effective intervention can mitigate symptoms.

• The International Dyslexia Association recommends Structured Literacy instruction for students with dyslexia. Having children listen to books, watch educational television, and engage in a wide variety of life experiences can help language comprehension development while the child is being taught to read. Dyslexia may make it difficult to acquire written proficiency in a second language.

• Dyslexia is not the result of laziness or lack of intelligence.

• As noted in the Joint Statement: Learning Disabilities, Dyslexia, and Vision from the American Academy of Ophthalmology, the American Academy of Pediatrics, and the American Academy for Pediatric Ophthalmology and Strabismus, dyslexia is not a vision problem, such as seeing letters backwards. It is a language-based disability. There is no evidence that colored overlays, special lenses, vision therapy, vitamins, antibiotics, mid-line crossing exercises, or other non-language-based interventions help dyslexia. 

Characteristics of Dyslexia at Different Ages

Additional Resources About Dyslexia

• In Defense of Facts: A Reply to 57 Reading Voices on the Issue of Dyslexia, International Dyslexia Association, Steven P. Dykstra, PhD

• Neurobiology of dyslexia: Areas of the brain involved in reading, International Dyslexia Association - Ontario.

Images

Figure 1. Catts, H. W. (2018). Decoding and language comprehension [Infographic]. Adapted from The Simple View of Reading: Advancements and false impressions. Remedial and special education, 39(5), 317–323.

Figure 2. Shaywitz, S. (2003). Sea of strengths model of dyslexia [Infographic]. From Overcoming dyslexia: A new and complete science-based program for reading problems at any level. Alfred A. Knopf, p. 83.

Figure 3. The Yale Center for Dyslexia & Creativity. (2017). Neural signature for dyslexia: Disruption of posterior reading systems [Image]. From Comments by Drs. Sally & Bennett Shaywitz on dyslexia & ADAAA. Retrieved January 7, 2022, from https://dyslexia.yale.edu/comments-by-drs-sally-bennett-shaywtiz-on-dyslexia-adaaa/