What Is Dyslexia?


Editor's Note: This article was originally written by Dr. Robb Mapou.

Dyslexia is probably the most misunderstood learning disability. When I ask people what they think dyslexia is, they typically respond, "Reversing letters and numbers." Even psychologists at my continuing education workshops have given this response. This shows the widespread nature of this misunderstanding. When the word dyslexia, is broken apart, it means difficulty (dys) reading (lexia). So, dyslexia, is, very simply, a learning disability that affects sounding out (decoding) and reading individual words. It is based in a fundamental deficit in phonological awareness, which is the ability to take in, take apart, and put together the sounds that make up words.

Dyslexia is the only learning disability for which there is a definition-based on research, or, what is referred to in clinical practice as an evidence-based definition.     This definition, established by the International Dyslexia Association, states that:

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.

What this means is that people with dyslexia have a difficult time recognizing that words are composed of individual sounds, or phonemes. Without this skill, they cannot decode words into sounds when reading or encode sounds into words when spelling. As a result, they read individual words very slowly. That is, they are not automatic or fluent readers of individual words. Because of this, they are also are not fluent, or quick, when reading text. This is why their reading is slow and inefficient. Difficulty encoding when spelling slows their speed when writing and, because of additional problems with spoken language, discussed below individuals with dyslexia, can struggle with expressive writing, as well as with reading comprehension.

Some people with dyslexia have an additional deficit in rapid naming, which is the ability to look at and name letters, digits, colors, or pictures of objects quickly. This is a second factor that slows reading, because after a word is decoded and read, rapid naming in rapid naming slows the speed at which the meaning of the word can be accessed.

Dyslexia is, at its core, a disorder that affects language more broadly. They may have difficulty understanding what they hear, which is a deficit in comprehension of spoken language, also called receptive language. In addition to affecting the ability to understand directions or information presented in lectures, this is a third factor can affect reading speed, because it slows comprehension of what is being read. However, for most people with dyslexia, spoken language comprehension is stronger than reading comprehension. And, for some, it can be very strong. This is why audio books from digital files or CDs (e.g., Learning Ally; Bookshare) or speech-to-text software (e.g., Kurzweil 3000) can be very helpful. Moreover, new speech-to-text features on e-readers (Kindle, iPad) make books even more accessible to individuals with dyslexia without additional assistive technology.

A fourth cognitive skill that can be affected by dyslexia is span for auditory-verbal information, or the amount of verbal information that can be held in mind and repeated back immediately after presentation. Many individuals with dyslexia have difficulty repeating a string of digits, a long word list, sentences, or brief stories. This weakness can leave a person prone to overloading when a large amount of verbal information is presented at once. In addition to affecting the ability to follow multistep directions, to attend to and take in information in lectures, or to take notes, overloading can affect reading. This is because it can cause one to lose track of the beginning of a sentence or paragraph by the time one reaches the end, making rereading necessary. This, too, slows reading speed.

Finally, people with dyslexia can have difficulty with word retrieval more broadly. They may know what they want to say, but may be unable to pull out the word that they want. This, in turn, can affect spoken language production, also called expressive language. All of these spoken language problems can affect verbal learning and memory. These are all skills stressed in school, which is why people with dyslexia typically have a hard time in school, if their problem is not identified early. Because these weaknesses can specifically affect learning a foreign language, people with dyslexia often have a very hard time learning a foreign language.

Because dyslexia is a specific disability that affects reading, people with dyslexia are of average or above average intelligence. In fact, what is often a mystery to teachers and parents of children dyslexia is the enormous problem these children have learning to read, despite being obviously intelligent. Of course, as Sally Shaywitz as eloquently stated in her book, Overcoming Dyslexia, this can be an enormous burden for very bright individuals who attend graduate school, law school, or medical school. Because they do not read quickly, timed tests, including entrance examinations (e.g., GRE, LSAT, MCAT, GMAT), are especially hard for them. They will also struggle with the reading load of advanced study, because they require far more time than their peers to complete their assigned reading.

All of these problems also indicate very clearly what dyslexia is not. It is not a visual disorder. In fact, most individuals with dyslexia whom I have evaluated have very strong visual skills. They are quick to perceive patterns, easily copy block designs or complex visual designs, do very well on visual tasks that stress planning, problem-solving, and organization, and learn and remember visual information very well. Often, they are exceptionally strong in math, which contrasts with their far weaker reading, spelling, and writing skills. Although some individuals with dyslexia may have visual processing problems when tested using experimental tasks, these subtle difficulties do not explain the reading problem. Rather, 20 years of research has shown that dyslexia is due to a weakness in the language skills needed for effective reading and in the areas of the brain that are specialized for language and for recognizing the printed word. In my next piece, I will talk about brain functioning in people with dyslexia, what dyslexia looks like in adults, evaluation of dyslexia, and interventions that can help improve reading in people with dyslexia.


Lyon, G. R., Shaywitz, S. E., & Shaywitz, B. A. (2003). A definition of dyslexia. Annals of Dyslexia, 53, 1-14.

Mapou, R. L. (2009). Adult learning disabilities and ADHD: Research-informed assessment. New York: Oxford University Press.

Shaywitz, S. (2003). Overcoming dyslexia. New York: Alfred A. Knopf.