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Historical Perspective
Simple Definition
Revised definition from the International Dyslexia Association
Research definition used by the National Institutes of Health
Definition of Learning Disability
Cause of Dyslexia
Phonemic Awareness
Phonological Processing and Phonics


[What is Dyslexia]

What Is Dyslexia?

Historical Perspective

Before the National Institutes of Health began their research in the 1980's, the only definition of dyslexia was an exclusionary one. If a child's difficulty with reading could not be explained by low intelligence, poor eye sight, poor hearing, inadequate educational opportunities, or any other problem, then the child must be dyslexic.

That definition was not satisfactory to parents, teachers, or researchers. So here are three different definitions in use today.

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Simple Definition

Dyslexia is an inherited condition that makes it extremely difficult to read, write, and spell in your native language—despite at least average intelligence.

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Revised definition from the International Dyslexia Association

Dyslexia is a neurologically-based, often familial, disorder which interferes with the acquisition and processing of language. Varying in degrees of severity, it is manifested by difficulties in receptive and expressive language, including phonological processing, in reading, writing, spelling, handwriting, and sometimes in arithmetic.

Dyslexia is not the result of lack of motivation, sensory impairment, inadequate instructional or environmental opportunities, or other limiting conditions, but may occur together with these conditions.

Although dyslexia is lifelong, individuals with dyslexia frequently respond successfully to timely and appropriate intervention.

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Research definition used by the National Institutes of Health

Dyslexia is a specific learning disability that is neurological 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.
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Learning Disability

Learning Disability is not a specific term; it is a category containing many specific disabilities, all of which cause learning to be difficult. The following definition of "learning disability" is used for legislative, financial, and educational purposes only. It is NOT a definition of dyslexia, which is one specific learning disability.

The term 'learning disability' means a disorder in one or more of the basic processes involved in understanding spoken or written language. It may show up as a problem in listening, thinking, speaking, reading, writing, or spelling or in a person's ability to do math, despite at least average intelligence.

The term does not include children who have learning problems which are primarily the result of visual, hearing, or physical handicaps, or mental retardation, or emotional disturbance, or of environmental, cultural, or economic disadvantage.

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Cause of Dyslexia

Dyslexia is an inherited condition. Researchers have determined that a gene on the short arm of chromosome #6 is responsible for dyslexia. That gene is dominant, making dyslexia highly heritable. It definitely runs in families.

Dyslexia results from a neurological difference; that is, a brain difference. People with dyslexia have a larger right-hemisphere in their brains than those of normal readers. That may be one reason people with dyslexia often have significant strengths in areas controlled by the right-side of the brain, such as artistic, athletic, and mechanical gifts; 3-D visualization ability; musical talent; creative problem solving skills; and intuitive people skills.

In addition to unique brain architecture, people with dyslexia have unusual "wiring". Neurons are found in unusual places in the brain, and are not as neatly ordered as in non-dyslexic brains.

In addition to unique brain architecture and unusual wiring, f/MRI studies have shown that people with dyslexia do not use the same part of their brain when reading as other people. Regular readers consistently use the same part of their brain when they read. People with dyslexia do not use that part of their brain, and there appears to be no consistent part used among dyslexic readers.

It is therefore assumed that people with dyslexia are not using the most efficient part of their brain when they read. A different part of their brain has taken over that function.

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Phonemic Awareness

Quotes from prominent NIH researchers:

"The lack of phonemic awareness is the most powerful determinant of the likelihood of failure to learn to read."

"Phonemic awareness is more highly related to learning to read . . . than tests of general intelligence, reading readiness, and listening comprehension."

"Phonemic awareness is the most important core and causal factor separating normal and disabled readers."

NIH research has repeatedly demonstrated that lack of phonemic awareness is the root cause of reading failure. Phonemes are the smallest unit of SPOKEN language, not written language.

Children who lack phonemic awareness are unable to distinguish or manipulate SOUNDS within SPOKEN words or syllables. They would be unable to do the following tasks:

  • Phoneme Segmentation: what sounds do you hear in the word hot? What's the last sound in the word map?
  • Phoneme Deletion: what word would be left if the /k/ sound were taken away from cat?
  • Phoneme Matching: do pen and pipe start with the same sound?
  • Phoneme Counting: how many sounds do you hear in the word cake?
  • Phoneme Substitution: what word would you have if you changed the /h/ in hot to /p/?
  • Blending: what word would you have if you put these sounds together? /s/ /a/ /t/
  • Rhyming: tell me as many words as you can that rhyme with the word eat.

If a child lacks phonemic awareness, they will have difficulty learning the relationship between letters and the sounds they represent in words, as well as applying those letter/sound correspondences to help them "sound out" unknown words.

So children who perform poorly on phonemic awareness tasks via oral language in kindergarten are very likely to experience difficulties acquiring the early word reading skills that provide the foundation for growth of reading ability throughout elementary school.

Phonemic awareness skills can and must be directly and explicitly taught to children who lack this awareness.

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Phonological Processing and Phonics

Phonemic awareness must exist or be explicitly and directly taught BEFORE phonics instruction begins. Otherwise, the phonics instruction will not make sense to the dyslexic child.

Phonological processing refers to understanding of sounds used in our language, ranging from big chunks of sound (words), to smaller chunks (syllables) and eventually to phonemic awareness (every sound within a syllable). Both phonemic awareness and phonological processing are auditory processing skills. Therefore, they can (and should) be taught before letters are introduced.

The goal of teaching phonics is to link the individual sounds to letters, and to make that process fluent and automatic, for both reading and spelling. In other words, phonics teaches students symbol-to-sound and sound-to-symbol.

But for phonics to work, a student must first have solid phonological processing and phonemic awareness.

To see how these different items are taught, take a look at our Teaching that Works page.

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