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WikiProject Dyslexia |
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WikiProject Dyslexia/Reorganization 2010 |
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Dyslexia is a learning disability that impairs a person's ability to read, [1] and which can manifest itself as a difficulty with phonological awareness, phonological decoding, orthographic coding, auditory short-term memory, and/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 estimated that dyslexia affects between 5 and 17 percent of the population. [6] [7] [8]
There are three proposed cognitive subtypes of dyslexia: auditory, visual and attentional. [9] [7] [10] [11] [12] [13] Although dyslexia is not an intellectual disability, it is considered both a learning disability [14] [15] and a reading disability. [14] [16] Dyslexia and IQ are not interrelated, since reading and cognition develop independently in individuals who have dyslexia. [17]
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. [18]
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". [19]
MedlinePlus and the National Institutes of Health define dyslexia as "a reading disability resulting from the inability to process graphic symbols". [20]
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". [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. [21] [22]
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. [23] [24] However, the distinction between surface and phonological dyslexia has not replaced the old empirical terminology of dysphonetic versus dyseidetic types of dyslexia. [22] [24] [25] The surface/phonological distinction is only descriptive, and devoid of any aetiological assumption as to the underlying brain mechanisms. [26] 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.
The symptoms of dyslexia vary according to the severity of the disorder as well as the age of the individual.
It is difficult to obtain a certain diagnosis of dyslexia before a child begins school, but many dyslexic individuals have a history of difficulties that began well before kindergarten. Children who exhibit these symptoms early in life have a higher likelihood of being diagnosed as dyslexic than other children. These symptoms include:
One common misconception about dyslexia is that dyslexic readers write words backwards or move letters around when reading. In fact, this only occurs in a very small population of dyslexic readers. Dyslexic people are better identified by writing that does not seem to match their level of intelligence from prior observations. Additionally, dyslexic people often substitute similar-looking, but unrelated, words in place of the ones intended (what/want, say/saw, help/held, run/fun, fell/fall, to/too, etc.) citation needed.
Several learning disabilities often occur with dyslexia, but it is unclear whether these learning disabilities share underlying neurological causes with dyslexia. [30] These disabilities include, but are not limited to:
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 background. [32] [33]
One view is represented by the automaticity/cerebellar theory of dyslexia. Here the biological claim is that the cerebellum of people with dyslexia is mildly dysfunctional and that a number of cognitive difficulties ensue. [32] [34]
This theory posits that reading is an unnatural act carried out by humans for an exceedingly brief period in our evolutionary history. It has been less than a hundred years that western societies promoted reading to the mass population and therefore the forces that shape our reading behavior have been weak. Many areas of the world still do not even have access to reading for the majority of the population. [35]
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). [32] [36]
The speed with which an individual can engage in the rapid automatized naming of familiar objects or letters is a strong predictor of dyslexia. [37] Slow naming speed can be identified as early as kindergarten and persists in adults with dyslexia.
A deficit in naming speed is hypothesized to represent a deficit that is separate from phonological processing deficit. Wolf identified four types of readers: readers with no deficits, readers with phonological processing deficit, readers with naming speed deficit, and readers with double deficit (that is, problems both with phonological processing and naming speed). Students with double deficits are most likely to have some sort of severe reading impairment.
Distinguishing among these deficits has important implications for instructional intervention. If students with double deficits receive instruction only in phonological processing, they are only receiving part of what they need. [38]
The concept of a perceptual noise exclusion (impaired filtering of behaviorally irrelevant visual information in dyslexia or visual-noise) deficit is an emerging hypothesis, supported by research showing that subjects with dyslexia 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. [39] [40] 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. [41]
The phonological deficit theory postulates that people with dyslexia have a specific impairment in the representation, storage and/or retrieval of speech sounds. It explains the reading impairment of dyslexic persons 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. [32]
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 people with dyslexia show poor performance on a number of auditory tasks, including frequency discrimination and temporal order judgment. [32]
The visual theory reflects another long standing 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. [32]
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 alphabetic orthographies that employ only 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. [42] Logographic writing systems, notably Japanese and Chinese characters, have a purer direct relationship between the sound of a word and the representative visual symbols, which pose a different type of dyslexic difficulty. [13] [43] [44] [45]
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. [43] [44] [45]
Dyslexia is attributed to neurological factors that influence the individual's ability to read, write, and spell written language. [24]
The following conditions may be contributory or overlapping factors, as they can lead to difficulties reading:
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, [56] [57] and may be observed as stuttering, cluttering or hesitant speech. [22]
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. [58]
The best approach is determined by the underlying neurological cause(s) of the dyslexic symptoms.
Context sensitive spell checkers combined with text-to-speech systems and aimed for dyslexia assistance, such as Ghotit Dyslexia Spellchecker, is the new direction in helping to adults and kids with dyslexia to write successfully. [59]
There are many different national legal statutes and different national special education support structures with regard to special education provision which relate to the management of dyslexia.
There have been a number of films, television programs, and works of fiction which focus on the topic of dyslexia.
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. [26] [86] [87] [88] [89]
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. [90]
That dyslexia is neurobiological in origin is supported by what Lyon et al. proclaimed as "overwhelming and converging data from functional brain imaging investigations" (2003, p. 3). The results of these studies suggest that there are observable differences in how the dyslexic brain functions when compared to the brain of a typical reader. Using fMRI, Shaywitz found that good readers show a consistent pattern of strong activation in the back of the brain with weaker activation in the front of the brain during reading tasks. In contrast, the brain activation pattern in dyslexics is the opposite during reading tasks—the frontal part of the brain becomes overactive with weaker activation in the back. Shaywitz points out "It is as if these struggling readers are using the systems in the front of the brain to try to compensate for the disruption in the back of the brain." [91]
Brain activation studies using PET to study language have produced a breakthrough in our 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. [92] with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural) [93]
A University of Hong Kong study argues that dyslexia affects different structural parts of children's brains depending on the language which the children read. [44] The study focused on comparing children that were raised reading English and children raised reading Chinese. This is supported in a review by T. Hadzibeganovic et al. (2010). [85]
A University of Maastricht (Netherlands) study revealed that adult dyslexic readers underactivate superior temporal cortex for the integration of letters and speech sounds. [94]
Molecular studies have linked several forms of dyslexia to genetic markers for dyslexia. [95] Several candidate genes have been identified, including at the two regions first related to dyslexia: DCDC2 [96] and KIAA0319 [97] on chromosome 6 [18], [98] and DYX1C1 on chromosome 15 [18].
A 2007 review reported that no specific cognitive processes are known to be influenced by the proposed susceptibility genes. [99]
A unifying theoretical framework of three working memory components provides a systems perspective for discussing past and new findings in a 12-year research program that point to heterogeneity in the genetic and brain basis and behavioral expression of dyslexia. [100]
In recent years there has been significant debate on the categorization of dyslexia. In particular, Elliot and Gibbs argue that "attempts to distinguish between categories of 'dyslexia' and 'poor reader' or 'reading disabled' are scientifically unsupportable, arbitrary and thus potentially discriminatory". [101]
While acknowledging that reading disability is a valid scientific curiosity, and that "seeking greater understanding of the relationship between visual symbols and spoken language is crucial" and that while there was "potential of genetics and neuroscience for guiding assessment and educational practice at some stage in the future", they conclude that "there is a mistaken belief that current knowledge in these fields is sufficient to justify a category of dyslexia as a subset of those who encounter reading difficulties".
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![]() | This is not a Wikipedia article: This is a workpage, a collection of material and work in progress that may or may not be incorporated into an article. It should not necessarily be considered factual or authoritative. |
WikiProject Dyslexia |
---|
![]() |
General information |
Project pages |
Resources |
WikiProject Dyslexia/Reorganization 2010 |
---|
Dyslexia is a learning disability that impairs a person's ability to read, [1] and which can manifest itself as a difficulty with phonological awareness, phonological decoding, orthographic coding, auditory short-term memory, and/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 estimated that dyslexia affects between 5 and 17 percent of the population. [6] [7] [8]
There are three proposed cognitive subtypes of dyslexia: auditory, visual and attentional. [9] [7] [10] [11] [12] [13] Although dyslexia is not an intellectual disability, it is considered both a learning disability [14] [15] and a reading disability. [14] [16] Dyslexia and IQ are not interrelated, since reading and cognition develop independently in individuals who have dyslexia. [17]
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. [18]
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". [19]
MedlinePlus and the National Institutes of Health define dyslexia as "a reading disability resulting from the inability to process graphic symbols". [20]
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". [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. [21] [22]
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. [23] [24] However, the distinction between surface and phonological dyslexia has not replaced the old empirical terminology of dysphonetic versus dyseidetic types of dyslexia. [22] [24] [25] The surface/phonological distinction is only descriptive, and devoid of any aetiological assumption as to the underlying brain mechanisms. [26] 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.
The symptoms of dyslexia vary according to the severity of the disorder as well as the age of the individual.
It is difficult to obtain a certain diagnosis of dyslexia before a child begins school, but many dyslexic individuals have a history of difficulties that began well before kindergarten. Children who exhibit these symptoms early in life have a higher likelihood of being diagnosed as dyslexic than other children. These symptoms include:
One common misconception about dyslexia is that dyslexic readers write words backwards or move letters around when reading. In fact, this only occurs in a very small population of dyslexic readers. Dyslexic people are better identified by writing that does not seem to match their level of intelligence from prior observations. Additionally, dyslexic people often substitute similar-looking, but unrelated, words in place of the ones intended (what/want, say/saw, help/held, run/fun, fell/fall, to/too, etc.) citation needed.
Several learning disabilities often occur with dyslexia, but it is unclear whether these learning disabilities share underlying neurological causes with dyslexia. [30] These disabilities include, but are not limited to:
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 background. [32] [33]
One view is represented by the automaticity/cerebellar theory of dyslexia. Here the biological claim is that the cerebellum of people with dyslexia is mildly dysfunctional and that a number of cognitive difficulties ensue. [32] [34]
This theory posits that reading is an unnatural act carried out by humans for an exceedingly brief period in our evolutionary history. It has been less than a hundred years that western societies promoted reading to the mass population and therefore the forces that shape our reading behavior have been weak. Many areas of the world still do not even have access to reading for the majority of the population. [35]
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). [32] [36]
The speed with which an individual can engage in the rapid automatized naming of familiar objects or letters is a strong predictor of dyslexia. [37] Slow naming speed can be identified as early as kindergarten and persists in adults with dyslexia.
A deficit in naming speed is hypothesized to represent a deficit that is separate from phonological processing deficit. Wolf identified four types of readers: readers with no deficits, readers with phonological processing deficit, readers with naming speed deficit, and readers with double deficit (that is, problems both with phonological processing and naming speed). Students with double deficits are most likely to have some sort of severe reading impairment.
Distinguishing among these deficits has important implications for instructional intervention. If students with double deficits receive instruction only in phonological processing, they are only receiving part of what they need. [38]
The concept of a perceptual noise exclusion (impaired filtering of behaviorally irrelevant visual information in dyslexia or visual-noise) deficit is an emerging hypothesis, supported by research showing that subjects with dyslexia 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. [39] [40] 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. [41]
The phonological deficit theory postulates that people with dyslexia have a specific impairment in the representation, storage and/or retrieval of speech sounds. It explains the reading impairment of dyslexic persons 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. [32]
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 people with dyslexia show poor performance on a number of auditory tasks, including frequency discrimination and temporal order judgment. [32]
The visual theory reflects another long standing 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. [32]
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 alphabetic orthographies that employ only 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. [42] Logographic writing systems, notably Japanese and Chinese characters, have a purer direct relationship between the sound of a word and the representative visual symbols, which pose a different type of dyslexic difficulty. [13] [43] [44] [45]
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. [43] [44] [45]
Dyslexia is attributed to neurological factors that influence the individual's ability to read, write, and spell written language. [24]
The following conditions may be contributory or overlapping factors, as they can lead to difficulties reading:
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, [56] [57] and may be observed as stuttering, cluttering or hesitant speech. [22]
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. [58]
The best approach is determined by the underlying neurological cause(s) of the dyslexic symptoms.
Context sensitive spell checkers combined with text-to-speech systems and aimed for dyslexia assistance, such as Ghotit Dyslexia Spellchecker, is the new direction in helping to adults and kids with dyslexia to write successfully. [59]
There are many different national legal statutes and different national special education support structures with regard to special education provision which relate to the management of dyslexia.
There have been a number of films, television programs, and works of fiction which focus on the topic of dyslexia.
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. [26] [86] [87] [88] [89]
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. [90]
That dyslexia is neurobiological in origin is supported by what Lyon et al. proclaimed as "overwhelming and converging data from functional brain imaging investigations" (2003, p. 3). The results of these studies suggest that there are observable differences in how the dyslexic brain functions when compared to the brain of a typical reader. Using fMRI, Shaywitz found that good readers show a consistent pattern of strong activation in the back of the brain with weaker activation in the front of the brain during reading tasks. In contrast, the brain activation pattern in dyslexics is the opposite during reading tasks—the frontal part of the brain becomes overactive with weaker activation in the back. Shaywitz points out "It is as if these struggling readers are using the systems in the front of the brain to try to compensate for the disruption in the back of the brain." [91]
Brain activation studies using PET to study language have produced a breakthrough in our 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. [92] with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural) [93]
A University of Hong Kong study argues that dyslexia affects different structural parts of children's brains depending on the language which the children read. [44] The study focused on comparing children that were raised reading English and children raised reading Chinese. This is supported in a review by T. Hadzibeganovic et al. (2010). [85]
A University of Maastricht (Netherlands) study revealed that adult dyslexic readers underactivate superior temporal cortex for the integration of letters and speech sounds. [94]
Molecular studies have linked several forms of dyslexia to genetic markers for dyslexia. [95] Several candidate genes have been identified, including at the two regions first related to dyslexia: DCDC2 [96] and KIAA0319 [97] on chromosome 6 [18], [98] and DYX1C1 on chromosome 15 [18].
A 2007 review reported that no specific cognitive processes are known to be influenced by the proposed susceptibility genes. [99]
A unifying theoretical framework of three working memory components provides a systems perspective for discussing past and new findings in a 12-year research program that point to heterogeneity in the genetic and brain basis and behavioral expression of dyslexia. [100]
In recent years there has been significant debate on the categorization of dyslexia. In particular, Elliot and Gibbs argue that "attempts to distinguish between categories of 'dyslexia' and 'poor reader' or 'reading disabled' are scientifically unsupportable, arbitrary and thus potentially discriminatory". [101]
While acknowledging that reading disability is a valid scientific curiosity, and that "seeking greater understanding of the relationship between visual symbols and spoken language is crucial" and that while there was "potential of genetics and neuroscience for guiding assessment and educational practice at some stage in the future", they conclude that "there is a mistaken belief that current knowledge in these fields is sufficient to justify a category of dyslexia as a subset of those who encounter reading difficulties".
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