Dyslexia is a learning disability that impairs a person’s ability to read, and which can manifest itself as a difficulty with phonological awareness, phonological decoding, orthographic coding, auditory short-term memory, and/or rapid naming. Dyslexia is separate and distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or from poor or inadequate reading instruction. It is estimated that dyslexia affects between 5 and 17 percent of the population.
There are three proposed cognitive subtypes of dyslexia: auditory, visual and attentional. Although dyslexia is not an intellectual disability, it is considered both a learning disability and a reading disability. Dyslexia and IQ are not interrelated, since reading and cognition develop independently in individuals who have dyslexia.
Classification
Spoken language is a universal form of man made communication. The visual notation of speech, written language is not found in all cultures and is a recent development with regard to human evolution.
There are many definitions of dyslexia but no official consensus has been reached.
The World Federation of Neurology defines dyslexia as “a disorder manifested by difficulty in learning to read despite conventional instruction, adequate intelligence and sociocultural opportunity”.
MedlinePlus and the National Institutes of Health define dyslexia as “a reading disability resulting from the inability to process graphic symbols”.
The National Institute of Neurological Disorders and Stroke gives the following definition for dyslexia:
“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 spelling, phonological processing (the manipulation of sounds), and/or rapid visual-verbal responding. In adults, dyslexia usually occurs after a brain injury or in the context of dementia. It can also be inherited in some families, and recent studies have identified a number of genes that may predispose an individual to developing dyslexia”.
Other published definitions are purely descriptive or embody causal theories. Varying definitions are used for dyslexia from researchers and organizations around the world; it appears that this disorder encompasses a number of reading skills, deficits and difficulties with a number of causes rather than a single condition.
Castles and Coltheart describe phonological and surface types of developmental dyslexia by analogy to classical subtypes of alexia (acquired dyslexia) which are classified according to the rate of errors in reading non-words. However, the distinction between surface and phonological dyslexia has not replaced the old empirical terminology of dysphonetic versus dyseidetic types of dyslexia. The surface/phonological distinction is only descriptive, and devoid of any aetiological assumption as to the underlying brain mechanisms. In contrast, the dysphonetic/dyseidetic distinction refers to two different mechanisms; one that relates to a speech discrimination deficit, and another that relates to a visual perception impairment.