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This article is about developmental dyslexia. For acquired dyslexia, see Alexia (neurology)
Dyslexia is a learning disorder that manifests itself as a difficulty with reading, spelling and in some cases mathematics. It is separate and distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or from poor or inadequate reading instruction. It is estimated that dyslexia affects between 5% and 17% of the U.S. population.
Dyslexia is thought to be the result of a neurological defect/difference, and though not an intellectual disability, it is variously considered a learning disability, a language disability, and a reading disability, among others. Dyslexia is diagnosed in people of all levels of intelligence (normal to above).
There are many definitions of the disorder called dyslexia but no consensus. The World Federation of Neurology defined dyslexia as follows:
- Specific developmental dyslexia is a disorder manifested by difficulty learning to read despite conventional instruction, adequate intelligence, and adequate sociocultural opportunity. It is dependent upon fundamental cognitive disabilities that are frequently of constitutional origin.
Some of the other published definitions are purely descriptive, while still others embody causal theories. From the varying definitions used by dyslexia researchers and organizations around the world, it appears that dyslexia is not one thing but many, insofar as it serves as a conceptual clearing-house for a number of reading skills deficits and difficulties, with a number of causes.
Castles and Coltheart, 1993, described phonological and surface types of developmental dyslexia by analogy to classical subtypes of acquired dyslexia (alexia) which are classified according to the rate of errors in reading non-words. However the distinction between surface and phonological dyslexia has not replaced the old empirical terminology of dysphonetic versus dyseidetic types of dyslexia. The surface/phonological distinction is only descriptive, and devoid of any aetiological assumption as to the underlying brain mechanisms, in contrast the dysphonetic/dyseidetic distinction refers to two different mechanisms:— one relates to a speech discrimination deficit, and the other to a visual perception impairment.
Most people with dyslexia who have Boder's Dysiedetic type, have attentional and spatial difficulties which interfere with the reading acquisition process.
Signs and symptoms
Dyslexia symptoms vary according to the severity of the disorder as well as the age of the individual.
Pre-school age children
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 have a higher risk of being diagnosed as dyslexic than other children. Some of these symptoms are:
- Delays in speech
- Learns new words slowly
- Has difficulty rhyming words, as in nursery rhymes
- Low letter knowledge
- Letter reversal, ex: e b f p (normal)
Early primary school-age children
- Difficulty learning the alphabet or in order
- Difficulty with associating sounds with the letters that represent them (sound-symbol correspondence)
- Difficulty identifying or generating rhyming words, or counting syllables in words (phonological awareness)
- Difficulty segmenting words into individual sounds, or blending sounds to make words (phonemic awareness)
- Difficulty with word retrieval or naming problems
- Difficulty learning to decode words
- Difficulty distinguishing between similar sounds in words; mixing up sounds in multisyllable words (auditory discrimination) (for example, "aminal" for animal, "bisghetti" for spaghetti)
Older primary school children
- Slow or inaccurate reading, although these individuals can read to an extent.
- Very poor spelling
- Difficulty reading out loud, reads word in the wrong order, skips words and sometimes says a word similar to another word
- Difficulty associating individual words with their correct meanings
- Difficulty with time keeping and concept of time, when doing a certain task
- Difficulty with organization skills
- Due to fear of speaking incorrectly, some children become withdrawn and shy or become bullies out of their inability to understand the social cues in their environment
- Difficulty comprehending rapid instructions, following more than one command at a time or remembering the sequence of things
- Children with dyslexia may fail to see (and occasionally to hear) similarities and differences in letters and words, may not recognize the spacing that organizes letters into separate words, and may be unable to sound out the pronunciation of an unfamiliar word.
One major misconceptions of dyslexia is writing words backwards or letters move when reading. This is a very small population of dyslexics. Teachers/instructors can better identify dyslexic students when their writing doesn't seem to match their level of intelligence from prior observations. Common signs: Sentences often containing a similar-looking, but unrelated word in place of the one intended (what/want, say/saw, help/held, run/fun, fell/fall, to/too, etc.)
Effect of language orthography
The complexity of a language's orthography, or writing and spelling system, has a direct impact on how difficult it is to learn to read in that language; formally, this is the orthographic depth. Although English has an alphabetic orthography, it is a complex or deep orthography that employs spelling patterns at several levels. The major structural categories that make up English spelling are letter-sound correspondences, syllables, and morphemes. Some 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 in languages like Spanish; it is much more difficult to learn to read in languages that have more complex orthographies, as in English. Logographic writing systems, notably Chinese characters, pose additional difficulties.
From a neurological perspective, different types of writing, for example, alphabetic as compared to pictographic, 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 skill deficits can cause dyslexic problems in relation to different orthographies.
The following conditions often occur with dyslexia in the same individual. It is unclear whether these conditions share underlying neurological causes with dyslexia.
- 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 tying 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 characterised 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 formulae and even basic addition and subtraction.
- Developmental dyspraxia is 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 organisation are typical of dyspraxics.
- Specific Language Impairment is 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 categorisation and their production of words was compared to that of age-matched children with specific language impairment (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.
- 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, consisting 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.
Dyslexia is believed to be a neurological condition that influences the individual's ability to read and spell written language.
The following conditions may be contributory or overlapping factors, similar to dyslexia as they can lead to difficulty 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, and may be observed as stuttering, cluttering or hesitant speech.
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.
The best approach is determined by the underlying neurological cause(s) of the dyslexic symptom.
Society and culture
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.
Film, television, and literature
There have been a number of films, television programs, and works of fiction which focus on the topic of dyslexia.
In recent years there has been significant debate on the categorisation of dyslexia, in particular Elliot and Gibbs (2008) published a paper in which they argued,
(...)that attempts to distinguish between categories of 'dyslexia' and 'poor reader' or 'reading disabled' are scientifically unsupportable, arbitrary and thus potentially discriminatory.
While acknowledging that reading disability was 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, (...) 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.
The majority of currently available dyslexia research relates to the alphabetic writing system, and especially languages of European origin. However more research is becoming available regarding dyslexia in speakers of Hebrew and Chinese.
- Identified by Oswald Berkhan in 1881, the term 'dyslexia' was later coined in 1887 by Rudolf Berlin, an ophthalmologist practising in Stuttgart, Germany.
- In 1896, W. Pringle Morgan published a description of a reading-specific learning disorder in the British Medical Journal "Congenital Word Blindness".
- During the 1890s and early 1900s, James Hinshelwood published a series of articles in medical journals describing similar cases of congenital word blindness. 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.
- 1925 Samuel T. Orton determined that there was a syndrome unrelated to brain damage that made learning to read difficult. Orton's theory strephosymbolia described individuals with dyslexia having difficulty associating the visual forms of words with their spoken forms. Orton observed that reading deficits in dyslexia did not seem to stem from strictly visual deficits. He believed the condition was caused by the failure to establish hemispheric dominance in the brain. Orton later worked with psychologist and educator Anna Gillingham to develop an educational intervention that pioneered the use of simultaneous multisensory instruction.
- In contrast, Dearborn, Gates, Bennet and Blau considered a faulty guidance of the seeing mechanism to be the cause. They sought to discover if a conflict between spontaneous orientation of the scanning action of the eyes from right to left and training aimed at the acquisition of an opposite direction would allow an interpretation of the facts observed in the dyslexic disorder and especially of the ability to mirror-read.
- 1949 research conducted under (thesis G. Mahec Paris 1951) went further. The phenomenon is clearly linked to the dynamics of sight as it disappears when the space between letters is increased, transforming the reading into spelling. This experience also explains the ability to mirror-read.
- In the 1970s, a new hypothesis emerged: that dyslexia stems from a deficit in phonological processing or difficulty in recognizing that spoken words are formed by discrete phonemes. Affected individuals have difficulty associating these sounds with the visual letters that make up written words. Key studies suggested the importance of phonological awareness,
- 1979 Galaburda and Kemper, and Galaburda et al. 1985, reported observations from the examination of post autopsy brains of people with dyslexia. Their studies reporting observed anatomical differences in the language center in a dyslexic brain, taken with the similar work of Cohen et al. 1989, suggested abnormal cortical development, which was presumed to occur before or during the sixth month of foetal brain development.
- 1993 Castles and Coltheart describe developmental dyslexia as two prevalent and distinct varieties using the subtypes of Alexia, Surface and Phonological Dyslexia. Manis et al. 1996, concluded that there were probably more than two subtypes of dyslexia, which would be related to multiple underlying deficits.
- 1994 From post autopsy specimens Galaburda et al., reported : Abnormal auditory processing in people with dyslexia suggests that accompanying anatomical abnormalities might be present in the auditory system. Supported the reported behavioral findings of a left hemisphere-based phonological defect in dyslexic individuals.
- The development of neuroimaging technologies during the 1980s and 1990s enabled dyslexia research to make significant advances. Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies have revealed the neural signature of adult normal reading (e.g. Fiez and Petersen, 1998; Turkeltaub et al., 2002) and phonological processing (e.g., Gelfand and Bookheimer, 2003; Poldrack et al., 1999). Employing various experimental approaches and paradigms (e.g., the detection or judgment of rhymes, nonword reading, and implicit reading), these studies have localized dysfunctional phonological processing in dyslexia to left-hemisphere perisylvian regions, especially for the alphabetic writing system (Paulesu et al., 2001; for review, see Eden and Zeffiro, 1998). However, it has been demonstrated that in nonalphabetic scripts, where reading places less demands on phonemic processing and the integration of visual-orthographic information is crucial, dyslexia is associated with under activity of the left middle frontal gyrus (Siok et al., 2004).
- 1999 Wydell and Butterworth reported the case study of an English-Japanese bilingual with monolingual dyslexia. Suggesting that any language where orthography-to-phonology mapping is transparent, or even opaque, or any language whose orthographic unit representing sound is coarse (i.e. at a whole character or word level) should not produce a high incidence of developmental phonological dyslexia, and that orthography can influence dyslexic symptoms
- 2003 A review by Collins and Rourke concluded that the current models of the relation between the brain and dyslexia generally focus on some form of defective or delayed brain maturation.
- 2007 Lyytinen et al. Researchers are seeking a link between the neurological and genetic findings, and the reading disorder.
- 2008 S Heim et al. in a paper "Cognitive subtypes of dyslexia" describe how they compared different sub-groups of dyslexics in comparison with a control group. This is one of the first studies not to just compare dyslexics with a non dyslexic control, but to go further and compared the different cognitive sub groups with a non dyslexic control group.
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 background.
- Cerebellar theory
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.
- Evolutionary hypothesis
This theory posits that reading is an unnatural act, and carried out by humans for an exceedingly brief period in our evolutionary history (Dalby, 1986). It has been less than a hundred years that most western societies promoted reading by the mass population and therefore the forces that shape our behavior have been weak. Many areas of the world still do not have access to reading for the majority of the population. There is no evidence that "pathology" underlies dyslexia but much evidence for cerebral variation or differences. It is these essential differences that are taxed with the artificial task of reading.
- 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).
- Naming speed deficit and double deficit theories
The speed with which an individual can engage in the rapid automatized naming of familiar objects or letters is a strong predictor of dyslexia. Slow naming speed can be identified as early as kindergarten; slow naming speed 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 severe reading impairments.
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.
- Perceptual visual-noise exclusion hypothesis
The concept of a perceptual noise exclusion (Impaired filtering of behaviourally 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. 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.
- Phonological deficit theory
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 people with dyslexia 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.
- 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 people with dyslexia show poor performance on a number of auditory tasks, including frequency discrimination and temporal order judgment.
- Visual theory
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.
Research using neuroimaging
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.
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."
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. with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural)
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. The study focused on comparing children that were raised reading English and children raised reading Chinese.
A University of Maastricht (Netherlands) study revealed that adult dyslexic readers underactivate superior temporal cortex for the integration of letters and speech sounds.
Molecular studies have linked several forms of dyslexia to genetic markers for dyslexia. Several candidate genes have been identified, including at the two regions first related to dyslexia: DCDC2 and KIAA0319 on chromosome 6, and DYX1C1 on chromosome 15.
A 2007 review reported that no specific cognitive processes are known to be influenced by the proposed susceptibility genes.
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.
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- Research papers, articles and media
|Symptoms and signs: Speech and voice / Symptoms involving head and neck (R47-R49, 784)
|Other speech disturbances
|psychology navs: mental processes, disorders, symptoms/signs, speech/voice, psychotherapy
mouth navs: anat/dev, noncongen/congen face/congen GI/neoplasia, symptoms, proc
|Mental and behavioral disorders (F · 290–319)
(acute alcohol intoxication
, alcohol dependence
, alcoholic hallucinosis
, Alcohol withdrawal
, delirium tremens
, Korsakoff's syndrome
, alcohol abuse
) · opioids
, opioid dependency
) · sedative/hypnotic
, benzodiazepine dependence
, benzodiazepine withdrawal
) · cocaine
, cocaine dependence
) · general
, Physical dependence
, Rebound effect
diagnosed in childhood
|psychology navs: mental processes, disorders, symptoms/signs, speech/voice, psychotherapy