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Dyslexia
and related disorders
Education · Neuropsychology
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Auditory Processing Disorder
Dyscalculia · Dysgraphia
Dyslexia · Dyspraxia
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Perceptual noise exclusion
Phonological deficit
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This article is about developmental dyslexia. For acquired dyslexia, see Alexia (disorder).
Dyslexia is a type of reading disability usually manifested as a difficulty with written language, particularly with reading and spelling. A person diagnosed with dyslexia is called a dyslexic; and a dyslexic by definition has adequate intelligence. Evidence suggests that it is a result of a difference in how the brain processes written and/or verbal language. It is separate and distinct from reading difficulties resulting from other causes, such as deficiencies in intelligence, non-neurological deficiency with vision or hearing, or from poor or inadequate reading instruction.[1]
The word dyslexia comes from the Greek words δυσ- dys- ("impaired") and λέξις lexis ("word"). People with dyslexia are called dyslexic or dyslectic.
Contents [hide]
1 Overview
2 History
3 Dyslexia definitions
3.1 General definitions
4 Subtypes of dyslexia
5 Variations and related conditions
6 Scientific research
6.1 Theories of developmental dyslexia
6.1.1 The phonological hypothesis
6.1.2 The rapid auditory processing theory
6.1.3 The visual theory
6.1.4 The cerebellar theory
6.1.5 The magnocellular theory
6.1.6 Perceptual visual-noise exclusion hypothesis
6.2 Research also indicates
6.2.1 Genetic factors
6.2.2 Physiology
6.2.3 Effect of language orthography
7 Characteristics
7.1 General
7.2 Speech, hearing and listening
7.3 Reading and spelling
7.4 Writing and motor skills
7.5 Mathematical abilities
8 Remedial programs and technologies
9 Facts and statistics
10 Legal and educational support issues
11 Controversy
12 See also
13 External links
13.1 Historical
13.2 Research papers, articles and media
13.3 Regional associations and organizations
13.4 Support groups and organizations
14 References
[edit] Overview
Dyslexia
Classification & external resources ICD-10 F81.0, R48.0
ICD-9 315.02, 784.61
OMIM 127700 604254 606896 606616 608995 300509
DiseasesDB 4016
MeSH D004410
Dyslexia is most commonly characterized by difficulties with learning how to decode at the word level, to spell, and to read accurately and fluently. Dyslexic individuals often have difficulty "breaking the code" of sound-letter association (the alphabetic principle), and they may also reverse or transpose letters when writing or confuse letters such as b, d, p, q, especially in childhood ( A Pictorial Example[1] ). However, dyslexia is not a visual problem that involves reading letters or words backwards or upside down, nor are such reversals a defining characteristic of dyslexia.
Many individuals with dyslexic symptoms involving reading, writing, and spelling also exhibit symptoms in other domains such as poor short-term memory skills, poor personal organizational skills, problems processing spoken language, left-right confusion, difficulties with numeracy or arithmetic, and issues with balance and co-ordination.[2] These symptoms may coexist with or overlap with characteristics of Attention-Deficit/Hyperactivity Disorder, Auditory Processing Disorder.[3], Developmental Dyspraxia, dyscalculia, and/or dysgraphia.
Evidence that dyslexia is a neurological syndrome is substantial. Research also suggests an association with biochemical and genetic markers.[4][5][6] However, experts disagree over the precise definition and criteria for diagnosis, and some advocate that the term dyslexia be dropped altogether and replaced with the term reading disorder or reading disability (RD). Because reading skills occur on a continuum with no clear distinction between typical readers and dyslexic readers, some experts assert that the term dyslexia should be reserved for the two to five percent with the most severe reading deficits. [7]
Dyslexia is a lifelong disorder, and its persistence across the lifespan is a distinguishing characteristic. Although there is no cure for dyslexia, appropriate remedial treatment and compensatory strategies can mitigate its effects. [8]
[edit] History
The term 'dyslexia' was coined in 1887 by Rudolf Berlin, an ophthalmologist practicing in Stuttgart, Germany.[9] He used the term to refer to a case of a young boy who had a severe impairment in learning to read and write in spite of showing typical intellectual and physical abilities in all other respects.
In 1896, W. Pringle Morgan, a British physician, from Seaford, East Sussex, England published a description of a reading-specific learning disorder in a report to the British Medical Journal titled "Congenital Word Blindness". This described the case of a boy named Percy who, at age 14, had not yet learned to read, yet showed normal intelligence and was generally adept at other activities typical of children of that age.[10]
During the 1890s and early 1900s, James Hinshelwood, a Scottish ophthalmologist, published a series of articles in medical journals describing similar cases of congenital word blindness, which he defined as "a congenital defect occurring in children with otherwise normal and undamaged brains characterised by a difficulty in learning to read." In his 1917 book Congenital Word Blindness, Hinshelwood asserted that the primary disability was in visual memory for words and letters, and described symptoms including letter reversals, and difficulties with spelling and reading comprehension.[11]
A key early researcher in dyslexia was Samuel T. Orton, a neurologist who worked primarily with stroke victims. In 1925 Orton met a boy who could not read and who exhibited symptoms similar to stroke victims who had lost the ability to read. Orton began studying reading difficulties and determined that there was a syndrome unrelated to brain damage that made learning to read difficult. Orton called the condition strephosymbolia (meaning 'twisted signs') to describe his theory that individuals with dyslexia had difficulty associating the visual forms of words with their spoken forms.[12] Orton observed that reading deficits in dyslexia did not seem to stem from strictly visual deficits.[13] He believed the condition was caused by the failure to establish hemispheric dominance in the brain.[14] He also observed that the children he worked with were disproportionately left- or mixed-handed, although this finding has been difficult to replicate.[15] Orton's hypothesis concerning hemispheric specialization was borne out by postmortem studies in the 1980s and 1990s establishing that the left planum temporale, a brain area associated with language processing, is physically larger than the corresponding right area in the brains of non-dyslexic subjects, but that these brain areas are symmetrical or slightly larger on the right for dyslexic subjects.[16] FMRI imaging studies of children and young adults reported in 2003 provide further support, demonstrating that increases in age and reading level are associated with a suppression of right hemispheric activity.[17] [18]
Influenced by the kinesthetic work of Helen Keller and Grace Fernald, and looking for a way to teach reading using both left and right brain functions,[19] Orton later worked with psychologist and educator Anna Gillingham to develop an educational intervention that pioneered the use of simultaneous multisensory instruction. The Orton-Gillingham approach to remedial reading instruction is still widely used and forms the basis of many reading intervention programs. [20]
In the 1970s, a new hypothesis, based in part on Orton's theories, emerged that dyslexia stems from a deficit in phonological processing or difficulty in recognizing that spoken words are formed by discrete phonemes (for example, that the word CAT comes from the sounds [k], [æ], and [t]). As a result, affected individuals have difficulty associating these sounds with the visual letters that make up written words. Key studies of the phonological deficit hypothesis include the finding that the strongest predictor of reading success in school age children is phonological awareness,[21] and that phonological awareness instruction can improve decoding skills for children with reading difficulties.[22]
The advent of neuroimaging techniques to study brain structure and function enhanced the research in the 1980s and 1990s. Since then, interest in the neurologically based causes has persisted. Current models of the relation between the brain and dyslexia generally focus on some form of defective or delayed brain maturation. More recently, genetic research has provided increasingly accumulating evidence supporting a genetic origin of dyslexia [23].
Researchers are currently searching a link between the neurological and genetic findings, and the reading disorder. There are many previous and current theories of dyslexia, but the one that has the most support from research is that, whatever the biological cause, dyslexia is a matter of reduced phonogical awareness, the ability to analyze and link the units of spoken and written language. [24].
[edit] Dyslexia definitions
Dyslexia is widely accepted to be a specific learning disability. That is, dyslexia has biological traits that differentiate it from other learning disabilities. However, there is no consensus on the definition of dyslexia, and the specific definition and diagnostic criteria has evolved over time as well as being subject to considerable disagreement and debate.
[edit] General definitions
Several national and international organizations have set out definitions of dyslexia based both on research and policy considerations as follows:
The World Health Organization (WHO) and World Federation of Neurology
A disorder manifested by difficulty learning to read, despite conventional instruction, adequate intelligence and sociocultural opportunity. It is dependent upon fundamental cognitive disabilities which are frequently of constitutional origin.
– ICD-10, The International Statistical Classification of Diseases and Related Health Problems, tenth revision ICIDH-2, The International Classification of Impairments, Activities, and Participation
The British Dyslexia Association[2]
Dyslexia is a specific learning difficulty which is neurobiological in origin and persists across the lifespan. It is characterised by difficulties with phonological processing, rapid naming, working memory, processing speed and the automatic development of skills that are unexpected in relation to an individual’s other cognitive abilities.
International Dyslexia Association[3]
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.
Dyslexia Association of Singapore[4]
Dyslexia is a neurologically based specific learning difficulty that is characterised by difficulties in one or more of reading, spelling and writing. Accompanying weaknesses may be identified in areas of language acquisition, phonological processing, working memory, and sequencing. Some factors that are associated with, but do not cause, dyslexia are poor motivation, impaired attention and academic frustration.
Canadian Government
The Government of Canada’s Health Portal links its description to the BC HealthGuide web site using their definition.
Dyslexia is a common learning disability that hinders the development of reading skills. Reading is not a natural human act; it has to be learned. Having dyslexia does not mean that you or your child has difficulty learning subjects other than reading or is below average in intelligence. In fact, many people with dyslexia are above average in intelligence. However, not being able to read fluently or quickly can make many areas of learning more challenging.
[edit] Subtypes of dyslexia
In addition to varying definitions of dyslexia, educators and clinicians have also developed varying subtypes or classifications of dyslexia based on differing patterns of underlying symptoms. Such categories are useful in choosing among remediation strategies, and also for purposes of defining population groups for purposes of research. Large-scale data from comparative studies of reading patterns in dyslexic and normal readers supports a finding of at least two prevalent and distinct varieties of developmental dyslexia. [25] One common approach has been to differentiate a speech discrimination deficit from a visual perception impairment.[26]
Dysphonetic or Auditory Dyslexia
The most predominant form of dyslexia identified by researchers is the dysphonetic or auditory subtype, which is associated with difficulty connecting sounds to symbols, and attendant difficulty with sounding out words.[27] The American physician Elena Boder, who developed the Boder Test of Reading-Spelling Patterns (1973), reported this pattern among approximately 60% of the children she surveyed.[28]
Dyseidetic or visual dyslexia
The dyseidetic (visual or surface) subtype is associated with inability to develop a sight word vocabulary, slow and laborious reading as familiar words must be repeatedly deciphered, and unconventional but highly phonetic spelling. Boder reported that approximately 10% of her subjects exhibited this pattern, with an additional 22% showing a "mixed" type with elements of both the dyseidetic and dysphonetic forms.[29]
Rapid automatic naming or Double Deficit
Other researchers have identified a deficit related to "naming speed", which relates to the ability of students to rapidly verbalize the names of symbols such as letters and numbers when tested. A deficit in "rapid automatic naming" is seen as related to an impaired mental timing system.[30][31] When such difficulties exist in conjunction with a phonological deficit, it is characterized as double deficit dyslexia.[32]
[edit] Variations and related conditions
Dyslexia is a learning disability. It has many underlying causes that are believed to be a brain-based condition that influences the ability to read written language. It is identified in individuals who fail to learn to read in the absence of a verbal or nonverbal intellectual impairment, sensory deficit (e.g., a visual deficit or hearing loss), pervasive developmental deficit or a frank neurological impairment.
The following conditions may also be contributory or overlapping factors, or underlying cause of the dyslexic symptoms as they can lead to difficulty reading:
Auditory processing disorder is a condition that affects the ability to encode auditory information. It can lead to problems with auditory working memory and auditory sequencing. Many dyslexics have auditory processing problems including history of auditory reversals. Auditory processing disorder is recognized as one of the major causes of dyslexia.
Cluttering is a speech fluency disorder involving both the rate and rhythm of speech, and resulting in impaired speech intelligibility. Speech is erratic and dysrhythmic, consulting of rapid and jerky spurts that usually involve faulty phrasing. The personality of the clutterer bears striking resemblance to the personalities of those with learning disabilities.[33]
Dyspraxia is a neurological condition characterized by a marked difficulty in carrying out routine tasks involving balance, fine-motor control, and kinesthetic coordination. Problems with short term memory and organization are typical of dyspraxics. This is most common in dyslexics who also have attention deficit disorder.
Verbal dyspraxia is a neurological condition characterized by marked difficulty in the use of speech sounds, which is the result of an immaturity in the speech production area of the brain.
Dysgraphia is a disorder which expresses itself primarily during writing or typing, although in some cases it may also affect eye-hand coordination in such direction or sequence oriented processes as tieing knots or carrying out a repetitive task. Dysgraphia is distinct from Dyspraxia in that the person may have the word to be written or the proper order of steps in mind clearly, but carries the sequence out in the wrong order.
Dyscalculia is a neurological condition characterized by a problem with learning fundamentals and one or more of the basic numerical skills. Often people with this condition can understand very complex mathematical concepts and principles but have difficulty processing formulas and even basic addition and subtraction.
Scotopic sensitivity syndrome, also known as Irlen Syndrome, is a term used to describe sensitivity to certain wavelengths of light which interfere with proper visual processing. See also Orthoscopics and asfedia.
[edit] Scientific research
[edit] Theories of developmental dyslexia
The following theories should not be viewed as competing, but viewed as theories trying to explain the underlying causes of a similar set of symptoms from a variety of research perspectives and backgrounds.:
[edit] The phonological hypothesis
The phonological hypothesis postulates that dyslexics have a specific impairment in the representation, storage and/or retrieval of speech sounds. It explains dyslexics' reading impairment on the basis that learning to read an alphabetic system requires learning the grapheme/phoneme correspondence, i.e. the correspondence between letters and constituent sounds of speech. If these sounds are poorly represented, stored or retrieved, the learning of grapheme/phoneme correspondences, the foundation of reading by phonic methods for alphabetic systems, will be affected accordingly.[34]
[edit] The rapid auditory processing theory
The rapid auditory processing theory is an alternative to the phonological deficit theory, which specifies that the primary deficit lies in the perception of short or rapidly varying sounds. Support for this theory arises from evidence that dyslexics show poor performance on a number of auditory tasks, including frequency discrimination and temporal order judgment. Backward masking tasks, in particular, demonstrate a 100-fold (40 dB) difference in sensitivity between normals and dyslexics. [35] Abnormal neurophysiological responses to various auditory stimuli have also been demonstrated. The failure to correctly represent short sounds and fast transitions would cause further difficulties in particular when such acoustic events are the cues to phonemic contrasts, as in /ba/ versus /da/. There is also evidence that dyslexics may have poorer categorical perception of certain contrasts.[34]
[edit] The visual theory
The visual theory (Lovegrove et al., 1980; Livingstone et al., 1991; Stein and Walsh, 1997) reflects another longstanding tradition in the study of dyslexia, that of considering it as a visual impairment giving rise to difficulties with the processing of letters and words on a page of text. This may take the form of unstable binocular fixations, poor vergence, or increased visual crowding. The visual theory does not exclude a phonological deficit, but emphasizes a visual contribution to reading problems, at least in some dyslexic individuals. At the biological level, the proposed aetiology of the visual dysfunction is based on the division of the visual system into two distinct pathways that have different roles and properties: the magnocellular and parvocellular pathways. The theory postulates that the magnocellular pathway is selectively disrupted in certain dyslexic individuals, leading to deficiencies in visual processing, and, via the posterior parietal cortex, to abnormal binocular control and visuospatial attention. Evidence for magnocellular dysfunction comes from anatomical studies showing abnormalities of the magnocellular layers of the lateral geniculate nucleus (Livingstone et al., 1991), psychophysical studies showing decreased sensitivity in the magnocellular range, i.e. low spatial frequencies and high temporal frequencies in dyslexics, and brain imaging studies.[34]
[edit] The cerebellar theory
Yet another view is represented by the automaticity/ cerebellar theory of dyslexia. Here the biological claim is that the dyslexic's cerebellum is mildly dysfunctional and that a number of cognitive difficulties ensue. First, the cerebellum plays a role in motor control and therefore in speech articulation. It is postulated that retarded or dysfunctional articulation would lead to deficient phonological representations. Secondly, the cerebellum plays a role in the automatization of overlearned tasks, such as driving, typing and reading. A weak capacity to automatize would affect, among other things, the learning of grapheme±phoneme correspondences. Support for the cerebellar theory comes from evidence of poor performance of dyslexics in a large number of motor tasks, in dual tasks demonstrating impaired automatization of balance, and in time estimation, a non-motor cerebellar task. Brain imaging studies have also shown anatomical, metabolic and activation differences in the cerebellum of dyslexics.[34]
[edit] The magnocellular theory
There is a unifying theory that attempts to integrate all the findings mentioned above. A generalization of the visual theory, the magnocellular theory postulates that the magnocellular dysfunction is not restricted to the visual pathways but is generalized to all modalities (visual and auditory as well as tactile). Furthermore, as the cerebellum receives massive input from various magnocellular systems in the brain, it is also predicted to be affected by the general magnocellular defect (Stein et al., 2001). Through a single biological cause, this theory therefore manages to account for all known manifestations of dyslexia: visual, auditory, tactile, motor and, consequently, phonological. Beyond the evidence pertaining to each of the theories described previously, evidence specifically relevant to the magnocellular theory includes magnocellular abnormalities in the medial as well as the lateral geniculate nucleus of dyslexics' brains, poor performance of dyslexics in the tactile domain, and the co-occurrence of visual and auditory problems in certain dyslexics.[34]
[edit] Perceptual visual-noise exclusion hypothesis
The concept of a perceptual noise exclusion (Visual-Noise) deficit is an emerging hypothesis, supported by research showing that dyslexic subjects experience difficulty in performing visual tasks such as motion detection in the presence of perceptual distractions, but do not show the same impairment when the distracting factors are removed in an experimental setting.[36] The researchers have analogized their findings concerning visual discrimination tasks to findings in other research related to auditory discrimination tasks. They assert that dyslexic symptoms arise because of an impaired ability to filter out both visual and auditory distractions, and to categorize information so as to distinguish the important sensory data from the irrelevant.[37]
[edit] Research also indicates
[edit] Genetic factors
Developmental dyslexia also appears to have a genetic component, such that it can tend to occur in multiple members of the same family. Reading difficulties in dyslexia can vary in their severity. The condition is not restricted to childhood, and can persist through adulthood. In addition, while early reports suggested dyslexia is more prevalent in boys, more recent studies have indicated it is not sex-linked, and occurs both in boys and girls with equal frequency.
Studies have linked several forms of dyslexia to genetic markers.[38][39][40] One major genetic study identified a region on chromosome 6, DCDC2, as possibly linked to dyslexia.[5]
As of 2007, genetic research in families with dyslexia have identified nine chromosome regions that may be associated with susceptibility to dyslexia. However, several of the major studies have not been replicated.[41]
[edit] Physiology
Using functional Magnetic Resonance Imaging (fMRI), it has been found that people with dyslexia have a deficit in parts of the left hemisphere of the brain involved in reading, which includes the inferior frontal gyrus, inferior parietal lobule, and middle and ventral temporal cortex.[42][43]
In 1979 , anatomical differences in the brain of a young dyslexic were documented. Albert Galaburda of Harvard Medical School noticed that the language center in a dyslexic brain showed microscopic differences known as ectopias and microgyria. Both affect the typical six-layer structure of the cortex. An ectopia is a collection of neurons that have pushed up from the lower layers of the cortex into the outermost one. A microgyrus is an area of cortex that includes only four layers instead of six. These differences affect connectivity and functionality of the cortex in critical areas related to auditory processing and visual processing, which seems consistent with the hypothesis that dyslexia stems from a phonological awareness deficit. Others have reported from CAT scan studies that the brains of dyslexic children were symmetrical unlike the asymmetrical brains of non-dyslexic readers who had larger left hemispheres.[44]
[edit] Effect of language orthography
Some studies have concluded that speakers of languages whose orthography has a strong correspondence between letter and sound (e.g. Serbian, Croatian, Korean, Italian and Spanish) suffer less from effects of dyslexia than speakers of languages where the letter is less closely linked to the sound (e.g. English and French).[45]
In one of these studies, reported in Seymour et al.,[46] the word-reading accuracy of first-grade children of different European languages was measured. English children had an accuracy of just 40%, whereas among children of most other European languages accuracy was about 95%, with French and Danish children somewhere in the middle at about 75%; Danish and French are known to have an irregular pronunciation.
However, this does not mean that dyslexia is caused by orthography: instead, Ziegler et al.[47] claim that the dyslexia suffered by German or Italian dyslectics is of the same kind as the one suffered by the English ones, supporting the theory that the origin of dyslexia is biological. However, dyslexia has more pronounced effects on orthographically difficult languages.
[edit] Characteristics
Formal diagnosis of dyslexia is
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