Dyslexia is a learning disorder that manifests itself as a difficulty with reading, spelling and in some cases mathematics. It 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% of the U.S. population.
Dyslexia is thought to be the result of a neurological defect/difference, and though not an intellectual disability, a language disability, among others.
There are many definitions of the disorder called ''dyslexia'' but no consensus.
The World Federation of Neurology defined dyslexia as follows:
- ''Specific developmental dyslexia is a disorder manifested by difficulty learning to read despite conventional instruction, adequate intelligence, and adequate sociocultural opportunity. It is dependent upon fundamental cognitive disabilities that are frequently of constitutional origin.''
Some of the other published definitions are purely descriptive, while still others embody causal theories. From the varying definitions used by dyslexia researchers and organizations around the world, it appears that dyslexia is not one thing but many, insofar as it serves as a conceptual clearing-house for a number of reading skills deficits and difficulties, with a number of causes.
Castles and Coltheart, 1993, described phonological and surface types of developmental dyslexia by analogy to classical subtypes of acquired dyslexia (alexia) 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 relates to a speech discrimination deficit, and the other to a visual perception impairment.
Most people with dyslexia who have Boder's Dysiedetic type, have attentional and spatial difficulties which interfere with the reading acquisition process.
There is no cure for dyslexia, but dyslexic individuals can learn to read and write with appropriate educational support.
For alphabet writing systems, the fundamental aim is to increase a child's awareness of correspondences between graphemes and phonemes, and to relate these to reading and spelling. It has been found that training focused towards visual language and orthographic issues yields longer-lasting gains than mere oral phonological training.
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