dyslexia

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(dĭs-lĕk'sē-ə) pronunciation
n.
A learning disorder marked by impairment of the ability to recognize and comprehend written words.

[New Latin : DYS- + Greek lexis, speech (from legein, to speak).]



Chronic neurological disorder causing inability or great difficulty in learning to read or spell, despite normal intelligence. It inhibits recognition and processing of graphic symbols, particularly those pertaining to language. Symptoms, including very poor reading skills, reversed word and letter sequences, and illegible handwriting, usually become evident in the early school years. With early recognition and specialized approaches to teaching reading, most dyslexics can learn to read. Anomalies have recently been found in reading-related pathways in the brains of dyslexic persons.

For more information on dyslexia, visit Britannica.com.

Definition

Dyslexia is a learning disability characterized by problems in reading, spelling, writing, speaking, or listening. It results from the inability to process graphic symbols. In many cases, dyslexia appears to be inherited.

Description

The word dyslexia is derived from the Greek word, dys (meaning poor or inadequate) and the word lexis (meaning words or language). Dyslexic children seem to have trouble learning early reading skills, problems hearing individual sounds in words, analyzing whole words in parts, and blending sounds into words. Letters such as "d" and "b" may be confused. Often a child with dyslexia has a problem translating language into thought (such as in listening or reading), or translating thought into language (such as in writing or speaking). Dyslexia is also referred to as developmental reading disorder (DRD).

Dyslexia is a problem involving higher (cortical) processing of symbols in the brain. Most children with dyslexia are of normal intelligence; many have above-average intelligence. However, when a child is dyslexic, there is often an unexpected difference between achievement and aptitude. Each child with dyslexia has different strengths and weaknesses, although many have unusual talents in art, athletics, architecture, graphics, drama, music, or engineering. These special talents are often in areas that require the ability to integrate sight, spatial skills, and coordination.

Common characteristics of a child with dyslexia include problems with:

  • identifying single words
  • understanding sounds in words, sound order, or rhymes
  • spelling
  • transposing letters in words
  • handwriting
  • reading comprehension
  • the spoken language
  • understanding directions
  • understanding opposites, such as up/down or early/late

Social and emotional difficulties often accompany this disorder, as children are unable to meet expectations of parents and teachers and feel frustrated at their inability to achieve their goals. They may have a negative self-image and become angry, anxious, and depressed.

Demographics

About 15–20 percent of the population of the United States has a language-based learning disability. Of students with specific learning disabilities receiving special education services, 70–80 percent have deficits in reading. With such a high incidence, there is a question as to whether this is really a difference in learning style rather than a true "disability." The condition affects males more than females, and appears in all ages, races, and income levels.

Causes and Symptoms

The underlying cause of dyslexia is not known, although research suggests the condition is often inherited. In 1999, The Centre for Reading Research in Norway presented the first research to study the largest family with reading problems ever known. By studying the reading and writing abilities of close to 80 family members across four generations, the researchers reported, for the first time, that chromosome 2 can be involved in the inheritability of dyslexia. When a fault occurs on this gene, it leads to difficulties in processing written language. Previous studies have pointed out linkages of other potential dyslexia genes to chromosome 1, chromosome 15 (DYX1 gene), and to chromosome 6 (DYX2 gene). The researchers who pinpointed the localized gene on chromosome 2 (DYX3) hope that this finding will lead to earlier and more precise diagnoses of dyslexia.

Research suggests a possible link with a subtle visual problem that affects the speed with which affected people can read. Anatomical and brain imagery studies show differences in the way the brain of a dyslexic child develops and functions.

Indicators of dyslexia include:

  • possible family history of learning disorders
  • difficulty learning to recognize written words
  • difficulty rhyming
  • difficulty determining the meaning (idea content) of a single sentence
  • writing or arithmetic learning problems

When to Call the Doctor

The doctor should be called if a child appears to have difficulty learning to read or exhibits any symptoms of dyslexia.

Diagnosis

Anyone who is suspected to have dyslexia should have a comprehensive evaluation, including medical, psychological, behavioral, hearing, vision, and intelligence testing. The test should include all areas of learning and learning processes, not only reading. Other causes of learning disabilities, such as attention deficit hyperactivity disorder (ADHD), affective disorders (e.g. depression or anxiety), central auditory processing dysfunction, pervasive developmental disorders, and physical or sensory impairments, must be ruled out before the diagnosis of dyslexia can be confirmed. A child of any age may be evaluated for dyslexia using an age-appropriate battery of tests.

Test results are used to determine eligibility for special education services in many states as well as eligibility for programs in colleges and universities. They provide a basis for making educational recommendations, and determine the baseline for evaluation of improvement in the child's performance. In the United States, the Individuals with Disabilities Education Act (IDEA), Section 504 of the Rehabilitation Act of 1973 and the Americans with Disabilities Act (ADA) define the rights of children with dyslexia and other learning disabilities. These children are legally entitled to special services to help them overcome and accommodate their learning problems, including education programs to meet their needs. The Acts also protect people with dyslexia against unfair and illegal discrimination.

As further research pinpoints the genes responsible for some cases of dyslexia, there is a possibility that earlier testing will be established to allow for timely interventions to prevent the onset of the condition and treat it when it does occur. Unfortunately, in many schools, a child is not identified as having dyslexia until after repeated failures.

Treatment

Dyslexia is a life-long condition, but with proper intervention, a child can learn to read and/or write well. When a child is diagnosed with dyslexia, the parents should find out from the school or the diagnostician exactly what the problem is, what method of teaching is recommended, and why a particular method is suggested.

The primary focus of treatment is aimed at solving the specific learning problem of each affected child. Most often, this may include modifying teaching methods and the educational environment, since traditional educational methods will not always be effective with a dyslexic child. An Individual Education Plan (IEP) should be created for each child, reflecting his or her specific requirements. Special education services may include specialist help by an instructor specifically trained to teach dyslexic students through individualized tutoring or special day classes. It is important to teach these students using all the senses—hearing, touching, writing, and speaking—through a multi-sensory program.

People with dyslexia need a structured language program, with direct instruction in the letter-sound system. Teachers must provide the rules governing written language. Most experts agree that the teacher should emphasize the association between simple phonetic units with letters or letter groups, rather than an approach that stresses memorizing whole words.

To assist with associated social and emotional difficulties, teachers must use strategies that will help the child find success in academics and personal relationships. Such strategies include rewarding efforts and not just the results, helping the child set realistic goals, and encouraging the child to do volunteer work that requires empathy and a social conscience (for example, a child with dyslexia who does well in science or math could serve as a peer tutor in those subjects or could tutor a younger child with dyslexia). Psychological counseling may also be helpful.

Prognosis

There is a great deal of variation among different people with dyslexia, producing different symptoms and degrees of severity. The prognosis depends on the severity of the disability, but is usually good if the condition is diagnosed early, the intervention used is effective and appropriate for the specific child, and if the child has a strong self-image and supportive family, friends, and teachers. However, difficulties with reading may persist throughout adulthood, which may result in occupational problems in certain careers. However, many successful people, such as Erin Brockovich and Whoopi Goldberg, have dyslexia.

Prevention

Since learning disorders often run in families, affected families should try to recognize learning disability problems early. For families without a previous history of learning disabilities, an intervention can begin as early as preschool or kindergarten if teachers detect early signs.

Parental Concerns

There are many resources available to aid parents in helping their children. For example, the International Dyslexia Association () provides extensive information for parents, teachers, and children. Parents are encouraged to utilize these resources to ensure their child's success in school and in interactions with their peers and later as working adults. They must also guard against feeling that the child is lazy or not trying hard. Instead, they should provide a supportive and loving environment.

Dyslexia may have an impact upon the child's family. Non-dyslexic siblings may be jealous of the attention, time, and money the dyslexic child receives from the parents. Since dyslexia runs in families, one or both parents may have had similar school problems. The child's problems may bring back feelings of frustration and failure for parents, which may interfere with their parenting skills.

See also Language delay; Language disorders.

Resources

Books

Reid, Gavin.Dyslexia—A Complete Guide for Parents. Hoboken, NJ: John Wiley & Sons, 2004.

Shaywitz, Sally. Overcoming Dyslexia: A New and Complete Science-Based Program for Overcoming Reading Problems at Any Level. New York: Knopf, 2003.

Stowe, Cynthia. How to Reach and Teach Children with Dyslexia: A Parent and Teacher Guide to Helping Students of All Ages Academically, Socially, and Emotionally. San Francisco, CA: Jossey-Bass, 2002.

Organizations

International Dyslexia Association. Suite 382, Chester Bldg., 8600 LaSalle Rd., Ste. 382, Baltimore, MD 21286-2044. (800) ABC-D123 or (410) 296-0232.

Learning Disabilities Association. 4156 Library Rd., Pittsburgh, PA 15234-1349. (412) 341-1515; Fax: (412) 344-0224. www.ldanatl.org

[Article by: Judith Sims Beth A. Kapes]



Patrick Dempsey  
Patrick Dempsey
What is dyslexia?

Dyslexia is a developmental disorder in the brain that causes a disability in reading, spelling and writing. Dyslexics may see letters reversed both in shape and in words, e.g., a "p" may look like a "q" or "mat" may be read as "tam." Although dyslexics don't "grow out" of their condition, they can learn to adapt to it, learning varied techniques in reading and improving language development. Famous dyslexics include Tom Cruise, Cher, Albert Einstein, Walt Disney and Magic Johnson. Patrick Dempsey, who plays Dr. Derek Shepherd on Grey's Anatomy, has even credited his dyslexia with helping him to get where he is today. In a 2008 interview, he told Barbara Walters, "It's given me a perspective of — you have to keep working. I have never given up." He memorizes all his lines when he performs. Happy birthday to Patrick Dempsey, who turns 44 today.

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From our Archives: Today's Highlights, January 13, 2010

dyslexia (dĭslĕk'sēə), in psychology, a developmental disability in reading or spelling, generally becoming evident in early schooling. To a dyslexic, letters and words may appear reversed, e.g., d seen as b or was seen as saw. Many dyslexics never learn to read or write effectively, although they tend to show above average intelligence in other areas. With the aid of computerized brain scans such as positron emission tomography (PET), recent studies have offered strong evidence that dyslexia is located in the brain. Damage to the brain can cause a reading disability similar to dyslexia, known as acquired dyslexia or alexia.


(dis-lek-see-uh)

Difficulty in reading when experienced by persons with normal vision and normal or above-normal intelligence. A common example of dyslexia is reading words with the letters in reverse order, as in fyl for fly.

At the end of the 19th century (1896), a case study of a boy with congenital word-blindness was reported in the British Medical Journal by W. Pringle Morgan. Percy F was described as a bright, intelligent boy, quick at games, as good as others of his age except for an inability to learn to read. In spite of laborious and persistent training, he could only spell out words of one syllable with great difficulty. This report served as a stimulus for research and debate on word-blindness. Half a century later, the term 'dyslexia' was adopted for the condition. 'Dyslexia' is derived from the Greek 'dys' (difficulty) and 'lexicos' (words). Today, developmental dyslexia is seen as a complex neurological condition occurring in about 4 per cent of the population and is a legally recognized disability.

The criteria for recognizing the existence of a condition are that it should have a distinct aetiology, identifying characteristics, and a prognosis, and should respond to intervention. There has been a lot of controversy about the extent to which dyslexia meets these requirements and in the UK the term 'specific learning difficulties' was preferred for use in legislation ('specific learning disabilities' in the USA). The word 'dyslexia', however, has continued to be used in cognitive research and has entered everyday parlance to such an extent that the legal term in the UK has been changed to 'specific learning difficulties (for example, dyslexia)'. The definition of dyslexia and its subtypes, and programmes for remediation, continue to evolve with developments in research and understanding of the condition.

In the 1980s and 1990s, the dominant theoretical framework for dyslexia research focused on phonological deficits. These include difficulty segmenting words into phonemes, in keeping strings of sounds or letters in short-term memory, in repeating long non-words and in reading and writing short ones, and slowness in naming colours, numbers, letters, or objects in pictures. The association between poor phonological awareness and later reading difficulty has been demonstrated across different languages, ages, and tasks. The emphasis on phonological difficulty is seen in the Orton Society (now the International Dyslexia Association) Research Committee's definition of 1994. In this, dyslexia is seen as a specific language-based disorder of constitutional origin characterized by difficulties in single word coding, usually reflecting insufficient phonological processing abilities. Such difficulties are unexpected in relation to age and other cognitive and academic abilities and are not the result of generalized developmental disability or sensory impairment. Dyslexia is seen not only as a problem with learning to read but also in acquiring proficiency in writing and spelling.

Much remedial help for dyslexic children has focused largely on the provision of multisensory phonic programmes involving the systematic teaching of letter–sound correspondences. Many dyslexic children learn to read fairly accurately but they read slowly. The positive effects of phonological support are largely on reading accuracy with relatively little effect on reading fluency. Research on speed of processing has shown a strong relationship between early letter-naming speed and later reading fluency. A 'double deficit' hypothesis for dyslexia led to the suggestion that effective remedial programmes should contain not only phonologically based instruction but also a systematic and comprehensive support directed at developing reading fluency and comprehension.

Over the last two decades, there has been a growing interest in visual processing problems in relation to text. In 1980, Olive Meares, a teacher in New Zealand, described the difficulties reported by some children of dealing with glare from the printed page when reading. At about the same time, Helen Irlen, in California, was exploring the use of tinted lenses to alleviate a problem which she called scotopic sensitivity syndrome. Irlen centres were set up in several countries to supply tinted lenses using her techniques. This triggered a great deal of interest in visual processing difficulties and poor reading, particularly as only some dyslexic children appear to benefit from using coloured overlays or tinted lenses.

In the early 1970s, Arnold Wilkins was interested in the 4 per cent of people with epilepsy who experience visually induced seizures. He noted that susceptibility to visual discomfort is most pronounced in people who suffer from frequent severe headaches. He went on to argue that reading can provoke 'pattern glare' which can result in eyestrain and headaches, and even visual illusions and seizures. Electric lighting changes in brightness twice during each cycle of the alternating electricity supply and fluorescent lighting pulsates even faster. Wilkins suggested that, although the pulsations from such lights are too fast to be seen as flicker, they affect the firing of the visual neurons in the eye. In 1991, he examined twenty volunteers with a history of reading difficulty who had been selected by the Irlen Centre in London as having benefited from the use of tinted glasses. Seventeen of them had a history of migraine in the family; nearly all of them reported a reduction in headaches when wearing their tinted glasses. To investigate this finding systematically, he built a colorimeter for the intuitive manipulation of colour (hue) and depth of colour (saturation), and developed a Rate of Reading Test to identify the poor readers who would benefit from using coloured overlays on the printed page. There is now good research evidence that some poor readers, particularly those with a family history of migraine, can be helped in this way, and that this is due neither to a placebo nor a Hawthorne effect (Wilkins 1995).

Poor readers sometimes display erratic eye movements which appear to be associated with blurring of print, letters jumping over other letters, and letter reversals. Stein and Fowler (1985) reported a study on the effect of monocular occlusion on reading in dyslexic children. They suggested that the experience of an unstable visual world when reading could be attributed to unstable vergence control of the eyes and that this could be improved by the occlusion of one eye. This was open to the objection, however, that a relationship between vergence control and the experiences reported had not been shown to be causal. In 2000, they attempted to set up a double-blind randomized controlled trial to test this. One hundred and forty-three children were selected as dyslexic with unstable binocular control from a sample of 300 7-to 11-years-olds who had been referred to a learning disabilities clinic. The children were randomly assigned to wear spectacles with pale yellow lenses with or without the left eye occluded for all reading and writing activities for nine months. They were tested without their spectacles every three months by researchers who did not know whether they belonged to the occluded or non-occluded group. At the end of nine months, there was no difference in the number of children who achieved stable binocular control but the occluded group achieved it earlier and had gained an advantage in reading performance which was maintained throughout the study. It was of course impossible for the children, parents, or teachers to be unaware of whether a child was in the occluded or non-occluded group.

Evidence has accumulated about differences in the symmetry of the cerebral hemispheres in dyslexics and controls and on the predominant location of language processing in the left hemisphere. Much detailed research has been done on the visual system and its responses to different types of visual stimuli.

In 1980, Bill Lovegrove and his colleagues suggested that dyslexic readers have low-level impairments of the transient visual system. The transient (magnocellular) visual system carries fast low-contrast information and is responsible for detecting moving stimuli but does so with poor acuity. The sustained (parvocellular) system transmits information more slowly but with high acuity. In 1991, Margaret Livingstone and her co-workers examined the functioning of the magnocellular and parvocellular divisions of the visual system by recording visually evoked potentials in response to high-and low-contrast stimuli in dyslexic and non-dyslexic people. There were no differences between the groups for the high-contrast stimuli but there were for the low. With Albert Galaburda, she and their co-workers went on to carry out a series of histological postmortem studies of the brains of people known to have been dyslexic and to compare them with non-dyslexic controls. They found that whilst the magnocellular and parvocellular layers of the lateral geniculate nucleus were clearly separated in the controls they were more merged together in the brains of the dyslexics. Moreover, the magnocellular neurons in the dyslexics were about 30 per cent smaller than in the controls. Both the magnocellular and parvocellular layers receive neuronal projections from the ganglion cells of the retina at the back of the eye. Ninety per cent of these are parvocells which signal fine detail and colour and the remainder are larger magnocells which signal the timing of visual events. Differences have since been found between dyslexics and controls in the magnocells of the auditory thalamic relay associated with the processing of sounds.

The critical factor behind fluent word reading appears to be the ability to recognize letters, spelling patterns, and whole words effortlessly and automatically. Rod Nicolson and Angela Fawcett (1990) explored the idea of automaticity suggesting that dyslexic children have difficulties in making some cognitive and motor skills automatic in spite of intensive practice. A conscious compensation hypothesis was invoked to explain why in spite of more limited automaticity many dyslexic children are able to perform at apparently normal levels in most areas, most of the time. There is growing evidence that there are differences between dyslexics and controls in motor coordination and balance. Motor skills training can improve performance on motor tasks and it was suggested in the early 1970s that motor skills intervention could help learning-disabled children. A longitudinal evaluation of a balance remediation exercise training programme for dyslexic children and matched controls is currently under way in the UK.

It has been shown recently, using positron emission tomography (PET), that in comparison with controls, dyslexics showed less activation of the cerebellum during a motor learning task. In addition, the metabolism of the cerebellum has been shown to be lower in dyslexics than controls. Research using magnetic resonance imaging (MRI) in the USA has provided evidence of the involvement and integration of seventeen regions in the brain, including the cerebellum, in reading. The cerebellum contains and receives considerable inputs from the magnocells and recent histological study of the cerebellum has confirmed cerebellar differences between dyslexics and controls.

John Stein, Joel Talcott, and Caroline Whitton (2001) suggested that the influence of polyunsaturated fatty acids (PUFAs) on the magnocell membrane may be under the control of a particular enzyme that is higher in dyslexics than controls. They think that the decline of eating fish in modern diets has led to lower levels of PUFAs. Research is under way on supplementing the diets of dyslexic children with particular omega-3 fatty acids found in some fish oils and on a breath test to identify children who may benefit from this. They also suggested that individual differences in magnocellular sensitivity may be under genetic control.

Familial clustering of dyslexia is well documented. Strong support for genetic influences rather than a shared family environment comes from studies which have found twice the rate of dyslexia in monozygotic twins who have almost identical genetic make-up, compared with dizygotic twins who have about half their genes in common. Techniques in molecular genetics have so far identified loci on chromosomes 2, 4, 6, 13, and 18 as being implicated in dyslexia. Several authors have pointed out that some of these genetic markers occur in the same regions as genes involved in some autoimmune diseases which have an increased prevalence in dyslexics. Although considerable advances have been made, understanding the biological basis of dyslexia is still at a relatively early stage, and significant questions remain about its genetic basis and the brain mechanisms involved.

Since the identification of pre-reading correlates of dyslexia, considerable efforts have been made to develop early screening programmes; this is difficult because early childhood development is so variable but a recent study compared the literacy skills of children at genetic risk of dyslexia with those with no such family history. The children at genetic risk were more likely to have slower speech and language development at 4 years of age and literacy problems at age 6. It appears that early childhood screening should be sequential to allow for differences in rates of development and treated as probabilistic. Developments in screening and assessing the difficulties of school children and adults now include work on computerized screening and assessment.

In describing the pattern of difficulties in dyslexia, Tim Miles (1993) pointed out that dyslexia involves more than a language and literacy problem. Certainly there can be considerable individual differences in schoolchildren and adults with dyslexia, not least because many develop individual coping and compensatory strategies. For some, the extent to which their dyslexic difficulties will impair performance at work will be minimal but for others it may result in working in occupations which do not make full use of their real abilities and potential. The stress of coping with dyslexia at school, college or university, and work is well documented (e.g. Miles and Varma 1995). Information on screening, assessment, intervention, and current legislation affecting children and adults is available on the websites of the British Dyslexia Association, the Dyslexia Institute, and the International Dyslexia Association.

(Published 2004)

— Mary N. Haslum

    Bibliography
  • Bradley, L., and Bryant, P. (1983). 'Categorizing sounds and learning to read'. Nature.
  • Livingstone, M. S., Rosen, G. D., Drislane, F. W., and Galaburda, A. M. (1991). 'Physiological and anatomical evidence for a magnocellular defect in developmental dyslexia'. Proceedings of the National Academy of Sciences of the USA, 88.
  • Lovegrove, W. J., Martin, F., Blackwood, M., and Badock, D. (1980). 'Specific reading difficulty: differences in contrast sensitivity as a function of spatial frequency'. Science, 210.
  • Miles, T. R. (1993). Dyslexia: The Pattern of Difficulties (2nd edn.).
  • — —  and Varma, V. (1995). Dyslexia and Stress.
  • Morgan, W. P. (1896). 'A case study of congenital word blindness'. British Medical Journal, 2.
  • Nicolson, R. I., and Fawcett, A. J. (1990). 'Automaticity: a new framework for dyslexia research'. Cognition, 35/2.
  • Orton Society Research Committee (1994). 'Dyslexia'. Perspectives, 20.
  • Stein, J. F., and Fowler, S. (1985). 'Effect of monocular occlusion on reading in dyslexic children'. Lancet, 13 July.
  • — —  Talcott, J., and Whitton, C. (2001). 'The senorimotor basis of developmental dyslexia'. In Fawcett, A. J. (ed.), Dyslexia Theory and Good Practice.
  • Wilkins, A. (1995). Visual Stress.
  • Websites:www.bda-dyslexia.org.uk
  • www.dyslexia-inst.org.uk
  • www.interdys.org



n

An impairment of the ability to read. Dyslexic persons often reverse letters and words, cannot adequately distinguish the letter sequences in written words, and have difficulty determining left from right.

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Dyslexia
Classification and external resources
ICD-10 R48.0
ICD-9 315.02
OMIM 127700
DiseasesDB 4016
MeSH D004410

Dyslexia is a very broad term defining a learning disability that impairs a person's fluency or comprehension accuracy in being able to read,[1] and which can manifest itself as a difficulty with phonological awareness, phonological decoding, orthographic coding, auditory short-term memory, or rapid naming.[2][3] 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.[4][5] It is believed that dyslexia can affect between 5 and 10 percent of a given population although there have been no studies to indicate an accurate percentage.[6][7][8]

There are three proposed cognitive subtypes of dyslexia: auditory, visual and attentional.[7][9][10][11][12][13] Reading disabilities, or dyslexia, is the most common learning disability, although in research literature it is considered to be a receptive language-based learning disability.[14] Researchers at MIT found that people with dyslexia exhibited impaired voice-recognition abilities.[15]

Accomplished adult dyslexics may be able to read with good comprehension, but they tend to read more slowly than non-dyslexics and may perform more poorly at nonsense word reading (a measure of phonological awareness) and spelling.[16] Dyslexia is not an intellectual disability, since dyslexia and IQ are not interrelated, as a result of cognition developing independently.[17]

Contents

Classification

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".[18]

MedlinePlus and the National Institutes of Health define dyslexia as "a reading disability resulting from the inability to process graphic symbols".[19]

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 so on, and recent studies have identified a number of genes that may predispose an individual to developing dyslexia".[1]

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.[20][21]

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.[22][23] However, the distinction between surface and phonological dyslexia has not replaced the old empirical terminology of dysphonetic versus dyseidetic types of dyslexia.[21][23][24] The surface/phonological distinction is only descriptive, and devoid of any etiological assumption as to the underlying brain mechanisms.[25] Studies have, however, alluded to potential differential underlying brain mechanisms in these populations given performance differences.[26][27][28] 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.

Signs and symptoms

Some early symptoms that correlate with a later diagnosis of dyslexia include delays in speech,[29] letter reversal or mirror writing,[30][31] and being easily distracted by background noise.[32] At later ages symptoms can include a difficulty identifying or generating rhyming words, or counting syllables in words (phonological awareness),[33] a difficulty segmenting words into individual sounds, or blending sounds to make words,[34] a difficulty with word retrieval or naming problems,[35][36][37] commonly very poor spelling[38] which has been called dysorthographia (orthographic coding), and tendencies to omit or add letters or words when writing and reading are considered classic signs.[39] A common misconception about dyslexia is that dyslexic readers write words backwards or move letters around when reading – this only occurs in a very small population of dyslexic readers.[40] Dyslexic people are better identified by writing that does not seem to match their level of intelligence from prior observations.

Cause

Since the symptoms of dyslexia were first identified by Oswald Berkhan in 1881,[41] and the term dyslexia coined in 1887 by Rudolf Berlin,[42][43] generations of researchers have been investigating what dyslexia is and trying to identify the biological causes. The theories of the etiology of dyslexia have and are evolving with each new generation of dyslexia researchers, and the more recent theories of dyslexia tend to enhance one or more of the older theories as understanding of the nature of dyslexia evolves. Theories should not be viewed as competing, but as attempting to explain the underlying causes of a similar set of symptoms from a variety of research perspectives and background.[44][45]

Effect of language orthography

The complexity of a language's orthography or spelling system – formally, its orthographic depth – has a direct impact on how difficult it is to learn to read that language. English has a comparatively deep orthography within the Latin alphabet writing system, with a complex orthographic structure that employs spelling patterns at several levels: principally, letter-sound correspondences, syllables, and morphemes. Other languages, such as Spanish, have mostly alphabetic orthographies that employ letter-sound correspondences, so-called shallow orthographies. It is relatively easy to learn to read languages like Spanish; it is much more difficult to learn to read languages with more complex orthographies, such as English.[46] Logographic writing systems, notably Japanese and Chinese characters, have graphemes that are not linked directly to their pronunciation, which pose a different type of dyslexic difficulty.[13][47][48][49]

From a neurological perspective, different types of writing system, for example alphabetic as compared to logographic writing systems, require different neurological pathways in order to read, write and spell. Because different writing systems require different parts of the brain to process the visual notation of speech, children with reading problems in one language might not have a reading problem in a language with a different orthography. The neurological skills required to perform the tasks of reading, writing, and spelling can vary between different writing systems and as a result different neurological deficits can cause dyslexic problems in relation to different orthographies.[47][48][49]

Cross-cultural prevalence

Cross-cultural study of the prevalence of dyslexia is difficult as different scholars and different countries often use different criteria to distinguish the cases of dyslexia in the continuum between the able and delayed readers at schools. According to the existing literature, the prevalence of dyslexia can vary widely between cultures. For example, Christall reports differences between 1% and 33%.[50] Some populations (for example, Japanese and Chinese) may have relatively small number of dyslexic schoolchildren,[51] and some populations (for example, European, and particularly sub-Saharan African populations) might have higher dyslexia prevalence. According to some researchers, despite the significant differences between the writing systems, Italian, German and English populations suffer similarly from dyslexia.[52]

Exacerbating conditions

Several learning disabilities often occur with dyslexia, but it is unclear whether these learning disabilities share underlying neurological causes with dyslexia.[53] These disabilities include, but are not limited to:

  • Dysgraphia – a disorder which expresses itself primarily through writing or typing, although in some cases it may also affect eye–hand coordination, direction- or sequence-oriented processes such as tying knots or carrying out a repetitive task. In dyslexia, dysgraphia is often multifactorial, due to impaired letter writing automaticity, finger motor sequencing challenges, organizational and elaborative difficulties, and impaired visual word form which makes it more difficult to retrieve the visual picture of words required for spelling. Dysgraphia is distinct from dyspraxia in that dyspraxia is simply related motor sequence impairment.
  • Dyscalculia – a neurological condition characterized by a problem with basic sense of number and quantity and difficulty retrieving rote math facts. Often people with this condition can understand very complex mathematical concepts and principles but have difficulty retrieving basic math facts involving addition and subtraction.
  • Attention Deficit Disorder[29][54][55] – a high degree of co-morbidity has been reported between ADD/ADHD and dyslexia/reading disorders,[56] it occurs in between 12% and 24% of those with dyslexia.[8]
  • Aphasia – neurologically based speech disorders, which can cause alexia (acquired dyslexia).
  • Auditory processing disorder – A condition that affects the ability to process auditory information. Auditory processing disorder is a listening disability.[57] It can lead to problems with auditory memory and auditory sequencing. Many people with dyslexia have auditory processing problems [58] and may develop their own logographic cues to compensate for this type of deficit. Auditory processing disorder is recognized as one of the major causes of dyslexia.[59][60][61] Some children can acquire auditory processing disorder as a result of experiencing otitis media with effusion (glue ear, sticky ear, grommets) and other severe ear conditions.[62]
  • Developmental dyspraxia[29] – A neurological condition characterized by a marked difficulty in carrying out routine tasks involving balance, fine-motor control, kinesthetic coordination, difficulty in the use of speech sounds, problems with short-term memory and organization are typical of dyspraxics.
  • Scotopic sensitivity syndrome, also known as Irlen Syndrome – A term used to describe sensitivity to certain wavelengths of light which interfere with visual processing.[63][64]
  • Specific language impairment (SLI) – A developmental language disorder that can affect both expressive and receptive language. SLI is defined as a "pure" language impairment, meaning that is not related to or caused by other developmental disorders, hearing loss or acquired brain injury. A study by the Universities of Maastricht and Utrecht examined speech perception and speech production in 3-year-old Dutch children at familial risk of developing dyslexia. Their performance in speech sound categorization and their production of words was compared to that of age-matched children with SLI and typically developing controls. The results of the at-risk and SLI-group were highly similar. Analysis of the individual data revealed that both groups contained subgroups with good and poorly performing children. Their impaired expressive phonology seemed to be related to a deficit in speech perception. The findings indicate that both dyslexia and SLI can be explained by a multi-risk model which includes cognitive processes as well as genetic factors.[29][65][non-primary source needed]

Experience of speech acquisition delays and speech and language problems can be due to problems processing and decoding auditory input prior to reproducing their own version of speech,[66][67] and may be observed as stuttering, cluttering or hesitant speech.[21]

Management

There is no cure for dyslexia, but dyslexic individuals can learn to read and write with appropriate educational support. [68] There are techniques, and technical aids, which manage or even conceal symptoms of the disorder. [69] Since stress and anxiety are contributors to a dyslexic's weaknesses in absorbing information, removing these can assist in improving understanding. [70]

For dyslexia intervention with 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.[71]

There is some evidence that the use of specially tailored fonts may provide some measure of assistance for those suffering from dyslexia; however scientific confirmation of this currently appears to be limited to a single master's thesis.[72][73] Intervention early on while language areas in the brain are still developing is most successful in reducing long term impacts of dyslexia.[74]

History

Dyslexia was identified by Oswald Berkhan in 1881,[75] but the term dyslexia was later coined in 1887 by Rudolf Berlin,[76] an ophthalmologist practicing in Stuttgart, Germany.[77] 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 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 14-year-old boy who had not yet learned to read, yet showed normal intelligence and was generally adept at other activities typical of children of that age.[78]

Research

The majority of currently available dyslexia research relates to the alphabetic writing system, and especially to languages of European origin. However, substantial research is also available regarding dyslexia for speakers of Arabic, Chinese, and Hebrew.[25][79][80][81][82]

Genetic research into dyslexia has its roots in the work of Galaburda and Kemper, 1979,[83] and Galaburda et al. 1985,[84] from the examination of post-autopsy brains of people with dyslexia. When they observed anatomical differences in the language center in a dyslexic brain, they showed microscopic cortical malformations known as ectopias and more rarely vascular micro-malformations, and in some instances these cortical malformations appeared as a microgyrus. These studies and those of Cohen et al. 1989[85] suggested abnormal cortical development which was presumed to occur before or during the sixth month of fetal brain development.[23]

In the area of neurological research into dyslexia, modern neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) have produced clear evidence of structural differences in the brains of children with reading difficulties. 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.[86]

Brain activation studies using PET to study language have produced a breakthrough in understanding of the neural basis of language over the past decade. A neural basis for the visual lexicon and for auditory verbal short-term memory components have been proposed,[87] with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural).[88]

Gene–environment interaction

Research has examined gene–environment interactions in reading disability through twin studies, which estimate the proportion of variance associated with environment and the proportion associated with heritability. Studies examining the influence of environmental factors such as parental education,[89] and teacher quality[90] have determined that genetics have more influence-supportive environments than less-optimal environments. Instead, it may just allow those genetic risk factors to account for more of the variance in outcome, because environmental risk factors that affect that outcome have been minimized.[91]

As environment plays a large role in learning and memory,[92] is likely that epigenetic modifications play an important role in reading ability. Animal models and measures of gene expression and methylation in the human periphery are used to study epigenetic processes, both of which have limitations in extrapolating to the human brain.

See also

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External links

Organizations

Translations:

Dyslexia

Top

Dansk (Danish)
n. - ordblindhed

Nederlands (Dutch)
dyslexie, leesblindheid

Français (French)
n. - dyslexie

Deutsch (German)
n. - Dyslexie, Lesestörung

Ελληνική (Greek)
n. - δυσλεξία

Italiano (Italian)
dislessia

Português (Portuguese)
n. - dislexia (f) (Med.)

Русский (Russian)
дислексия

Español (Spanish)
n. - dislexia

Svenska (Swedish)
n. - dyslexi, nedsättning av läs- o skrivförmågan beroende på hjärnskada

中文(简体)(Chinese (Simplified))
诵读困难

中文(繁體)(Chinese (Traditional))
n. - 誦讀困難

한국어 (Korean)
n. - 실독증, 읽기 장애, 난독증

日本語 (Japanese)
n. - 失読症

العربيه (Arabic)
‏(الاسم) خلل بسيط في الدماغ يسبب صعوبه في القراءة والكتابه‏

עברית (Hebrew)
n. - ‮קשיי קריאה וכתיבה, דיסלקציה‬


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Copyrights:

Mentioned in

Dyslexia: The Hidden Disability (1988 Education Film)
The Princess and the Cabbie (1981 Drama Film)
Dyslexia (1997 Health & Fitness Film)