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This article is about developmental dyslexia. For acquired dyslexia, see Alexia (disorder).
Dyslexia is a learning disability that manifests primarily as a difficulty with written language, particularly with reading and spelling. 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. Stanovich, KE. (1988) Explaining the differences between the dyslexic and the garden-variety poor reader: the phonological-core variable-difference model. Journal of Learning Disabilities, 21(10):590-604.
Evidence suggests that dyslexia results from differences in how the brain processes written and/or verbal language. Although dyslexia is the result of a neurological difference, it is not an intellectual disability. Dyslexia occurs at all levels of intelligence; sub-average, average, above average, and highly gifted. A Conversation with Sally Shaywitz, M.D., author of Overcoming Dyslexia. http://www.schwablearning.org/articles.aspx?r=718 retrieved October 15, 2007.
The word dyslexia comes from the Greek words δυσ- dys- ("impaired") and λέξις lexis ("word"). People with dyslexia are called dyslexic or dyslectic.
| Dyslexia Classification & external resources | |
| ICD-10 | F81.0, R48.0 |
|---|---|
| ICD-9 | 315.02, 784.61 |
| OMIM | 127700 604254 606896 606616 608995 300509 |
| DiseasesDB | 4016 |
| MeSH | D004410 |
Dyslexia is most commonly characterized by difficulties with learning how to decode at the word level, to spell, and to read accurately and fluently. Dyslexic individuals often have difficulty "breaking the code" of sound-letter association (the alphabetic principle), and they may also reverse or transpose letters when writing or confuse letters such as b, d, p, q, especially in childhood. However, dyslexia is not a visual problem that involves reading letters or words backwards or upside down, nor are such reversals a defining characteristic of dyslexia.
Many individuals with dyslexic symptoms involving reading, writing, and spelling also exhibit symptoms in other domains such as poor short-term memory skills, poor personal organizational skills and problems processing spoken language. Murphy, Martin F (2004). Dyslexia, An Explanation. Flyleaf Press. ISBN. These symptoms may coexist with or overlap with characteristics among others of attention-deficit hyperactivity disorder, auditory processing disorder, Deborah Moncrieff (2004-4-12). "Temporal Processing Deficits in Children with Dyslexia". Audiology Online. developmental dyspraxia, dyscalculia, and/or dysgraphia. However, Dyslexia and attention deficit/hyperactivity disorder are not correlated developmental problems.Dalby, JT (1985). "Taxonomic separation of attention deficit disorders and developmental reading disorders.". Contemporary Educational Psychology 10: 228-234.
Evidence that Dyslexia is of neurological origin is substantial. Research also suggests an association with biochemical and genetic markers.Cope, N; Harold D, Hill G, Moskvina V, Stevenson J, Holmans P, Owen MJ, O\'Donovan MC, Williams J (April 2005). "Strong evidence that KIAA0319 on chromosome 6p is a susceptibility gene for developmental Dyslexia". American Journal of Human Genetics 76 (4): 581-91. PMID. Meng, H; Smith SD, Hager K, Held M, Liu J, Olson RK, Pennington BF, DeFries JC, Gelernter J, O\'Reilly-Pol T, Somlo S, Skudlarski P, Shaywitz SE, Shaywitz BA, Marchione K, Wang Y, Paramasivam M, LoTurco JJ, Page GP, Gruen JR (November 22 2005). "DCDC2 is associated with reading disability and modulates neuronal development in the brain". Proceedings of the National Academy of Sciences 102 (47): 17053-8. PMID. Schumacher, J; Anthoni H, Dahdouh F, Konig IR, Hillmer AM, Kluck N, Manthey M, Plume E, Warnke A, Remschmidt H, Hulsmann J, Cichon S, Lindgren CM, Propping P, Zucchelli M, Ziegler A, Peyrard-Janvid M, Schulte-Korne G, Nothen MM, Kere J (Jan 2006). "Strong genetic evidence of DCDC2 as a susceptibility gene for Dyslexia". American Journal of Human Genetics 78 (1): 52-62. PMID. However, experts disagree over the precise definition and criteria for diagnosis, and some advocate that the term Dyslexia be dropped altogether and replaced with the term reading disorder or reading disability (RD). Because reading skills occur on a continuum with no clear distinction between typical readers and dyslexic readers, some experts assert that the term Dyslexia should be reserved for the two to five percent with the most severe reading deficits. Lyon, G.Reid; Fletcher, J.M.; Shaywitz, S.E.; Shaywitz, B.A.; Torgesen, J.K.; Wood, F.B.; Schulte, A.; Olson, R. (2001). "Rethinking learning disabilities". Rethinking special education for a new century: 259-287. Retrieved on 2007-06-11.
Dyslexia is a lifelong condition for which there is no cure, but appropriate remedial instruction and compensatory strategies can help the dyslexic individual mitigate or overcome their difficulties with written language. Clark, Diana B, et.al. (1995). Dyslexia: Theory and Practice of Remedial Instruction. York Press. A large body of evidence shows which types of instruction dyslexics need to be successful. National Institute of Child Health and Human Development, 2000; Snow et al., 1998; Torgesen, 2000). Retrieved from http://www.ldonline.org/article/14907 October 16, 2007. Many dyslexics overcome early problems with literacy and go on to earn advanced degrees and pursue successful careers; a high level of motivation coupled with strong encouragement and mentorship have been identified as factors leading to their success. Fink, Rosalie (2006). Why Jane and John Couldn\'t Read--And How They Learned: A New Look at Striving Readers. International Reading Association. ISBN 978-0872075924.
Identified for the first time by Berkhan in 1881, BERKHAN O. Neur. Zent 28 1917 the term \'dyslexia\' was coined in 1887 by Rudolf Berlin, an ophthalmologist practicing in Stuttgart, Germany."Uber Dyslexie". Archiv fur Psychiatrie 15: 276-278. He used the term to refer to a case of a young boy who had a severe impairment in learning to read and write in spite of showing typical intellectual and physical abilities in all other respects.
In 1896, W. Pringle Morgan, a British physician, from Seaford, East Sussex, England published a description of a reading-specific learning disorder in a report to the British Medical Journal titled "Congenital Word Blindness". This described the case of a 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.Snowling, Margaret J. (1996-11-02). "Dyslexia: a hundred years on". BMJ 313 (7065): 1096. Retrieved on 2007-06-08.
During the 1890s and early 1900s, James Hinshelwood, a British ophthalmologist, published a series of articles in medical journals describing similar cases of congenital word blindness, which he defined as "a congenital defect occurring in children with otherwise normal and undamaged brains characterised by a difficulty in learning to read." In his 1917 book Congenital Word Blindness, Hinshelwood asserted that the primary disability was in visual memory for words and letters, and described symptoms including letter reversals, and difficulties with spelling and reading comprehension.Hinshelwood, J. (1917). Congenital Word-blindness. HK Lewis \& Co., ltd..
A key early researcher in dyslexia was Samuel T. Orton, a neurologist who worked primarily with stroke victims. In 1925 Orton met a boy who could not read and who exhibited symptoms similar to stroke victims who had lost the ability to read. Orton began studying reading difficulties and determined that there was a syndrome unrelated to brain damage that made learning to read difficult. Orton called the condition strephosymbolia (meaning \'twisted signs\') to describe his theory that individuals with dyslexia had difficulty associating the visual forms of words with their spoken forms.Orton, ST (2519). "\'Word-blindness\' in school children.". Archives of Neurology and Psychiatry 14: 285–516. Orton observed that reading deficits in dyslexia did not seem to stem from strictly visual deficits.Henry, MK (1998). "Structured, sequential, multisensory teaching: The Perlow legacy". Annals of Dyslexia. He believed the condition was caused by the failure to establish hemispheric dominance in the brain.Orton, S.T. (1928). "Specific reading disability—strephosymbolia". Journal of the American Medical Association 90 (14): 1095-1099. He also observed that the children he worked with were disproportionately left- or mixed-handed, although this finding has been difficult to replicate.Geschwind, N (1982). "Biological associations of left-handedness". Annals of Dyslexia 33: 29-40. Orton\'s hypothesis concerning hemispheric specialization was borne out by postmortem studies in the 1980s and 1990s establishing that the left planum temporale, a brain area associated with language processing, is physically larger than the corresponding right area in the brains of non-dyslexic subjects, but that these brain areas are symmetrical or slightly larger on the right for dyslexic subjects.Galaburda, A.M.; Menard, M.T.; Rosen, G.D. (1994-08-16). "Evidence for Aberrant Auditory Anatomy in Developmental Dyslexia". Proceedings of the National Academy of Sciences 91 (17): 8010-8013. doi:10.1073/pnas.91.17.8013. Retrieved on 2007-06-17. FMRI imaging studies of children and young adults reported in 2003 provide further support, demonstrating that increases in age and reading level are associated with a suppression of right hemispheric activity.Was Orton Right? New Study Examines How The Brain Works In Reading; Offers Key To Better Understanding Dyslexia. Science Daily (May 19, 2003). Retrieved on 2007-06-17. Turkeltaub, P.E.; Gareau, L.; Flowers, D.L.; Zeffiro, T.A.; Eden, G.F. (2003). "Development of neural mechanisms for reading". Nature Neuroscience 6 (7): 767-773. Retrieved on 2007-06-17.
Influenced by the kinesthetic work of Helen Keller and Grace Fernald, and looking for a way to teach reading using both left and right brain functions,Orton, Samuel. "Word Blindness in School Children". Archives or Neurology and Psychiatry 14:285-516. Orton later worked with psychologist and educator Anna Gillingham to develop an educational intervention that pioneered the use of simultaneous multisensory instruction. The Orton-Gillingham approach to remedial reading instruction is still widely used and forms the basis of many reading intervention programs. Goeke, Jennifer (2006). "Orton-Gillingham and Orton-Gillingham-based reading instruction: a review of the literature". Journal of Special Education.
In contrast, Dearborn, Gates, Bennet and Blau considered a faulty guidance of the seeing mechanism to be the cause. The data collected in 1931 by Tinker and Goodenough (The J.Educ. Psych.) seemed to support this thesis. They wanted to know 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. To this end the authors asked four adults to read a text reflected in a mirror for ten minutes a day for a period of five months. It was confirmed that in all the subjects, the words were not perceived in their globality but required a meticulous analysis of the letters and syllables.
Moreover they also demonstrated either total or partial inversions even sometimes affecting the order of the words in a sentence. What is more, they revealed a curious impression of not just horizontal but also vertical inversions.These are errors that exist amongst dyslexics and they suffer from the aggravating circumstance inherent in all learning. What remained to be demonstrated was that there exists a preference amongst dyslexics, without sensory deficiency, or mental retardation, or any backwardness in speech or language, towards scanning with the eyes from right to left. Proof of this was provided in a work conducted under Prof. Clement Launay in 1949 (thesis G. Mahec Paris 1951). In adult subjects the reading of a series of 66 tiny lowercase letters, 5mm high, spaced 5mm apart, first from left to right and then from right to left was more easily and quickly done in the left to right direction. For former dyslexic children, a substantial number read a series of 42 letters with equal speed in both directions and some (10%) read better from right to left than from left to right. 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. This reading test can also be used to diagnose serious cases of dyslexia.
In the 1970s, a new hypothesis, based in part on Orton\'s theories, emerged that dyslexia stems from a deficit in phonological processing or difficulty in recognizing that spoken words are formed by discrete phonemes (for example, that the word CAT comes from the sounds [k], [æ], and [t]). As a result, affected individuals have difficulty associating these sounds with the visual letters that make up written words. Key studies of the phonological deficit hypothesis include the finding that the strongest predictor of reading success in school age children is phonological awareness,Bradley, L; Bryant, PE (1983). "Categorizing sounds and learning to read: A Causal connection.". Nature 30 (2): 419-421. and that phonological awareness instruction can improve decoding skills for children with reading difficulties.Alexander, A; Anderson, H, Heilman, P, Voeller, K, Torgesen, J (1991). "Phonological awareness training and the remediation of analytic decoding deficits in a group of severe dyslexics". Annals of Dyslexia 41: 193-206.
The advent of neuroimaging techniques to study brain structure and function enhanced the research in the 1980s and 1990s. Since then, interest in the neurologically based causes has persisted. Current models of the relation between the brain and dyslexia generally focus on some form of defective or delayed brain maturation. More recently, genetic research has provided increasing evidence supporting a genetic origin of dyslexia Collins, David and Rourke, Byron (October 2003). "[taylorandfrancis.metapress.com/index/H47FWM65HHJAP0K6.pdf Learning-disabled Brains: A Review of the Literature]". Journal of Clinical and Experimental Neuropsychology 25 (7): 1011-1034. Retrieved on 2007-07-11. .
Researchers are currently searching for a link between the neurological and genetic findings, and the reading disorder. There are many previous and current theories of dyslexia, but the one that has the most support from research is that, whatever the biological cause, dyslexia is a matter of reduced phonogical awareness, the ability to analyze and link the units of spoken and written languages. Lyytinen, Heikki, Erskine, Jane, Aro, Mikko, Richardson, Ulla (2007), "Reading and reading disorders", in Hoff, Erika, Blackwell Handbook of Language Development, Blackwell, pp. 454-474, ISBN 9781405132534.
Castles and Coltheart prove through their study that developmental dyslexia includes at least two prevalent and distinct varieties or subtypes of dyslexia. Subtypes include surface dyslexia and phonological dyslexia. Understanding these subtypes is useful in diagnosing learning patterns and developing approaches for overcoming impairments that may be visual perception impairments or speech discrimination deficits. These subtypes are based on differing patterns of underlying symptoms, as supported by a finding using large-scale data from comparative studies of reading patterns in dyslexic and normal readers Castles, A., & Coltheart, M. (1993). Varieties of developmental dyslexia. Cognition, 47(2), 149-180 . In the study by Castles and Coltheart, 56 dyslexic boys and 56 non-dyslexic boys as a control group were tested. During the test, the boys read aloud words and non words that were presented to them. The researchers found that surface dyslexics (subjects who have poor lexical skills, or can’t make out irregular words well) had a mean difference of 14.4 words between reading regular words versus irregular words, however, the mean difference in subjects with phonological dyslexia (subjects who can’t use sub lexical skills, or can’t make out non words) was only 7.75 words which was comparable to the control group .The majority of their subjects showed signs of phonological dyslexia. Twenty-nine subjects showed that their non word reading skills were poorer than their irregular word reading skills. However, sixteen subjects showed the opposite where their irregular word reading skills were poorer than their non word reading skills and were called surface dyslexics .
Surface Dyslexia
Surface dyslexia is characterized by subjects who can read non words but who have trouble reading words that are irregular Castles, A., & Coltheart, M. (1993). Varieties of developmental dyslexia. Cognition, 47(2), 149-180 .Surface dyslexia is the outcome of an individual who cannot function using the lexical procedure for reading out loud. The lexical procedure includes sounding out a word though the use of a past word already known . In Castles and Coltheart’s study, the researchers matched 30 regular words with 30 irregular words and asked their surface dyslexic subjects to read the word out loud. As hypothesized in their research, the difference between the surface dyslexic subjects and control subjects was about 4.6 words which was 2.55 words higher than the difference between the phonological dyslexics to the control group . In Castles and Coltheart’s study, both control and dyslexic subjects were shown a card with a word that is irregular or that isn’t pronounced as it looks. Fifteen of the 51 dyslexics were below the confidence limit set by the control subjects on ability to read irregular words. These subjects were then called surface dyslexics Manis, F., Seidenberg, M., Doi, L., McBride-Chang, C., Petersen, A., (1996). On the basis of two subtypes of developmental dyslexia. Cognition, 59, 157-195 .
Phonological Dyslexia
Phonological dyslexia is characterized by subjects who can read aloud both regular and irregular words but have difficulties with non words and with connecting sounds to symbols, or with sounding out words . Phonological processing tasks predict reading accuracy and comprehension. This subtype is the most predominant form of dyslexia Castles, A., & Coltheart, M. (1993). Varieties of developmental dyslexia. Cognition, 47(2), 149-180 .In Castles and Coltheart’s study, they had 56 dyslexic boys and 56 non dyslexic boys read words and non words given to them. The majority of the boys, 55%, showed a phonological dyslexic pattern .In Castles and Coltheart’s study, dyslexic subjects and control subjects were asked to read non words listed on a card, 17 out of 51 cases of dyslexics were below the confidence limit in non word reading, which was derived by the control group of subjects their own age. These phonological dyslexics have a lower non word reading level than expected by reviewing their irregular word reading level Manis, F., Seidenberg, M., Doi, L., McBride-Chang, C., Petersen, A., (1996). On the basis of two subtypes of developmental dyslexia. Cognition, 59, 157-195 . Phonological dyslexia is the outcome of a subject who cannot function using the sub lexical (pronunciations are constructed from smaller orthographic components) procedure for reading out loud .In Castles and Coltheart’s study, dyslexic and control subjects read words off a note card; the researchers found that while reading irregular words, the dyslexic subjects scored comparable to the control subjects because sub lexical skills weren’t involved in this test .
Double Deficit Dyslexia
Other researchers have identified a deficit related to “naming speed”, which relates to the ability of students to rapidly verbalize the names of symbols such as letters and numbers when tested Wolf, M., Bowers, P. G., (1999). The double-deficit hypothesis for the developmental dyslexias. Journal of educational psychology, 91, 415-438 . In their study, Wolf and Bowers tested out naming speed by having their subjects name a symbol as quickly as possible when shown on a flash card. Difficulties in naming speed exist in conjunction with a phonological deficit, is characterized as double deficit dyslexia Wolf, M., Bowers, P. G., (1999). The double-deficit hypothesis for the developmental dyslexias. Journal of educational psychology, 91, 415-438 .
Dyslexia is a learning disability. It has many underlying causes that are believed to be a brain-based condition that influences the ability to read written language. It is identified in individuals who fail to learn to read in the absence of a verbal or nonverbal intellectual impairment, sensory deficit (e.g., a visual deficit or hearing loss), pervasive developmental deficit or a frank neurological impairment.
The following conditions may also be contributory or overlapping factors, or underlying cause of the dyslexic symptoms as they can lead to difficulty reading:
The following theories should not be viewed as competing, but viewed as theories trying to explain the underlying causes of a similar set of symptoms from a variety of research perspectives and backgrounds.:
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. Dalby, J.T. (1986). "An ultimate view of reading ability". International Journal of Neuroscience 30 (3): 227-230. Gordon and Breach, Science Publishers, Inc.. PMID 3759349.
The phonological hypothesis postulates that dyslexics have a specific impairment in the representation, storage and/or retrieval of speech sounds. It explains dyslexics\' reading impairment on the basis that learning to read an alphabetic system requires learning the grapheme/phoneme correspondence, i.e. the correspondence between letters and constituent sounds of speech. If these sounds are poorly represented, stored or retrieved, the learning of grapheme/phoneme correspondences, the foundation of reading by phonic methods for alphabetic systems, will be affected accordingly.Ramus, Franck; Stuart Rosen, Steven C. Dakin, Brian L. Day, Juan M. Castellote, Sarah White and Uta Frith (April 2003). "Theories of developmental dyslexia: insights from a multiple case study of dyslexic adults". Brain 126 (4): 841-865. Oxford University Press. doi:10.1093/brain/awg076. Retrieved on 5/27/07.
The rapid auditory processing theory is an alternative to the phonological deficit theory, which specifies that the primary deficit lies in the perception of short or rapidly varying sounds. Support for this theory arises from evidence that dyslexics show poor performance on a number of auditory tasks, including frequency discrimination and temporal order judgment. Backward masking tasks, in particular, demonstrate a 100-fold (40 dB) difference in sensitivity between normals and dyslexics. Wright, Beverlyqwefojq wefi o this is a stupid site.; Linda J. Lombardino, Wayne M. King, Cynthia S. Puranik, Christiana M. Leonard and Michael M. Merzenich (1997). "Deficits in auditory temporal and spectral resolution in language-impaired children". Nature 387: 176-8. doi:10.1038/387176a0. Abnormal neurophysiological responses to various auditory stimuli have also been demonstrated. The failure to correctly represent short sounds and fast transitions would cause further difficulties in particular when such acoustic events are the cues to phonemic contrasts, as in /ba/ versus /da/. There is also evidence that dyslexics may have poorer categorical perception of certain contrasts.
The visual theory (Lovegrove et al., 1980; Livingstone et al., 1991; Stein and Walsh, 1997) reflects another longstanding tradition in the study of dyslexia, that of considering it as a visual impairment giving rise to difficulties with the processing of letters and words on a page of text. This may take the form of unstable binocular fixations, poor vergence, or increased visual crowding. The visual theory does not exclude a phonological deficit, but emphasizes a visual contribution to reading problems, at least in some dyslexic individuals. At the biological level, the proposed aetiology of the visual dysfunction is based on the division of the visual system into two distinct pathways that have different roles and properties: the magnocellular and parvocellular pathways. The theory postulates that the magnocellular pathway is selectively disrupted in certain dyslexic individuals, leading to deficiencies in visual processing, and, via the posterior parietal cortex, to abnormal binocular control and visuospatial attention. Evidence for magnocellular dysfunction comes from anatomical studies showing abnormalities of the magnocellular layers of the lateral geniculate nucleus (Livingstone et al., 1991), psychophysical studies showing decreased sensitivity in the magnocellular range, i.e. low spatial frequencies and high temporal frequencies in dyslexics, and brain imaging studies.
Yet another view is represented by the automaticity/ cerebellar theory of dyslexia. Here the biological claim is that the dyslexic\'s cerebellum is mildly dysfunctional and that a number of cognitive difficulties ensue. First, the cerebellum plays a role in motor control and therefore in speech articulation. It is postulated that retarded or dysfunctional articulation would lead to deficient phonological representations. Secondly, the cerebellum plays a role in the automatization of overlearned tasks, such as driving, typing and reading. A weak capacity to automatize would affect, among other things, the learning of grapheme±phoneme correspondences. Support for the cerebellar theory comes from evidence of poor performance of dyslexics in a large number of motor tasks, in dual tasks demonstrating impaired automatization of balance, and in time estimation, a non-motor cerebellar task. Brain imaging studies have also shown anatomical, metabolic and activation differences in the cerebellum of dyslexics.
There is a unifying theory that attempts to integrate all the findings mentioned above. A generalization of the visual theory, the magnocellular theory postulates that the magnocellular dysfunction is not restricted to the visual pathways but is generalized to all modalities (visual and auditory as well as tactile). Furthermore, as the cerebellum receives massive input from various magnocellular systems in the brain, it is also predicted to be affected by the general magnocellular defect (Stein et al., 2001). Through a single biological cause, this theory therefore manages to account for all known manifestations of dyslexia: visual, auditory, tactile, motor and, consequently, phonological. Beyond the evidence pertaining to each of the theories described previously, evidence specifically relevant to the magnocellular theory includes magnocellular abnormalities in the medial as well as the lateral geniculate nucleus of dyslexics\' brains, poor performance of dyslexics in the tactile domain, and the co-occurrence of visual and auditory problems in certain dyslexics.
The concept of a perceptual noise exclusion (Visual-Noise) deficit is an emerging hypothesis, supported by research showing that dyslexic subjects experience difficulty in performing visual tasks such as motion detection in the presence of perceptual distractions, but do not show the same impairment when the distracting factors are removed in an experimental setting.Sperling, Anne J.; Zhong-Lin Lu, Franklin R. Manis, Mark S. Seidenberg (2006). "Motion-Perception Deficits and Reading Impairment: It\'s the Noise, Not the Motion". Psychological Science 17 (12): 1047-1053. Association for Psychological Science. 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.Sperling, Anne J.; Lu, Z.L.; Manis, F.R.; Seidenberg, M.S. (2005). "Deficits in perceptual noise exclusion in developmental dyslexia". Nature Neuroscience 8: 862-863. doi:10.1038/nn1474. ISSN 1097-6256. Retrieved on 2007-06-12.
Studies have linked several forms of dyslexia to genetic markers.Grigorenko, EL (Jan 2001). "Developmental dyslexia: an update on genes, brains, and environments". Journal of child psychology and psychiatry, and allied disciplines 42 (1): 91-125. PMID. Grigorenko, EL; Wood FB, Meyer MS, Hart LA, Speed WC, Shuster A, Pauls DL (Jan 1997). "Susceptibility loci for distinct components of developmental dyslexia on chromosomes 6 and 15". American journal of human genetics 60 (1): 27-39. PMID. Grigorenko, EL; Wood FB, Meyer MS, Pauls DL (Feb 2000). "Chromosome 6p influences on different dyslexia-related cognitive processes: further confirmation". American journal of human genetics 66 (2): 715-23. PMID. One major genetic study identified a region on chromosome 6, DCDC2, as possibly linked to dyslexia.
As of 2007, genetic research in families with dyslexia have identified nine chromosome regions that may be associated with susceptibility to dyslexia. However, several of the major studies have not been replicated.Schumacher, Johannes; Per Hoffmann, Christine Schmäl, Gerd Schulte-Körne and Markus M Nöthen (16 February 2007). "Genetics of dyslexia: the evolving landscape". Journal of Medical Genetics 44: 289-297. BMJ Publishing Group. doi:10.1136/jmg.2006.046516. Retrieved on 5/27/07.
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."Deficient orthographic and phonological representations in children with dyslexia revealed by brain activation patterns". Journal of Child Psychology and Psychiatry (November 2006) 47: 1041-1050. doi:10.1111/j.1469-7610.2006.01684.x. Retrieved on 2007-10-06. "The role of functional magnetic resonance imaging in understanding reading and dyslexia.". PubMed (2006). Retrieved on 2007-10-06.
Scientific studies of brains donated to medical research have revealed that there are anatomical differences in two parts of the dyslexic brain: the cerebral cortex and the thalamus. In 1979 Albert Galaburda of Harvard Medical School noticed anatomical differences in the language center in a dyslexic brain, showing microscopic differences known as ectopias and microgyria. Both affect the typical six-layer structure of the cortex. An ectopia is a collection of neurons that have pushed up from the lower layers of the cortex into the outermost one. A microgyrus is an area of cortex that includes only four layers instead of six. These differences affect connectivity and functionality of the cortex in critical areas related to auditory processing and visual processing, which seems consistent with the hypothesis that dyslexia stems from a phonological awareness deficit.
Studies of both autopsied brains and living brains using neuroimaging techniques have shown that the brains of dyslexic children are symmetrical, unlike the asymmetrical brains of non-dyslexic readers who had larger left hemispheres.Haslam, R, Dalby, J.T., Johns, RD and Rademaker,A.W. (1981). "Cerebral asymmetry in developmental dyslexia". Archives of Neurology 27: 23-25. PMID 7305694.
Scientists do not claim that all people with dyslexia have these structural brain differences. However, the studies are evidence that some children\'s reading problems are brain based. The challenge for researchers is to determine how these structural differences affect reading acquisition. Sherman, Gordon. Can Neuroscience Help to Demystify Dyslexia? Schwab Learning. Retrieved from http://www.schwablearning.org/articles.aspx?r=430 October 8, 2007.
Some studies have concluded that speakers of languages whose orthography has a highly consistent correspondence between letter and sound (e.g., Italian) suffer less from the effects of dyslexia than speakers of languages where the letter-sound correspondence is less consistent (e.g. English and French)."Scientists Say Severity of Dyslexia Depends on Language", The Tech News Briefs, Los Angeles Times, March 16, 2001. Retrieved on 2006-06-06.
In one of these studies, reported in Seymour et al.,Seymour, P. H. K., Aro, M., & Erskine, J. M. (2003). Foundation literacy acquisition in European orthographies. British Journal of Psychology, 94, 143 – 174. the word-reading accuracy of first-grade children of different European languages was measured. English children had an accuracy of just 40%, whereas among children of most other European languages accuracy was about 95%, with French and Danish children somewhere in the middle at about 75%; Danish and French are known to have an irregular pronunciation.
However, this does not mean that dyslexia is caused by orthography: instead, Ziegler et al.Johannes C. Ziegler, Conrad Perry, Anna Ma-Wyatt, Diana Ladner, and Gerd Schulte-Körne, Developmental dyslexia in different languages: Language-specific or universal? Journal of Experimental Child Psychology) 169 – 193 claim that the dyslexia suffered by German or Italian dyslexics is of the same kind as the one suffered by the English ones, supporting the theory that the origin of dyslexia is biological. In a study by Paulescu et al. (Science, 2001) English, French, and Italian dyslexics were found to have the same brain function signature when studied with functional magnetic resonance imaging (fMRI), a signature that differed from non-dsylexic readers. However, dyslexia has more pronounced effects on orthographically difficult languages, e.g., dyslexics have more difficulty in English than Italian. The next theory of some forms of dyslexia uses orthography to test a hypothesis of psychological causation.
Formal diagnosis of dyslexia is made by a qualified professional, such as a neurologist or an educational psychologist. Evaluation generally includes testing of reading ability together with measures of underlying skills such as tests of rapid naming, to evaluate short term memory and sequencing skills, and nonword reading to evaluate phonological coding skills. Evaluation will usually also include an IQ test to establish a profile of learning strengths and weaknesses. However, the use of a "discrepancy" between full scale IQ and reading level as a factor in diagnosis has been discredited by recent research.Fletcher first=Jack M; Francis DJ, Rourke BP, Shaywitz SE, Shaywitz BA. (November 1992). "The validity of discrepancy-based definitions of reading disabilities.". J Learn Disabil 25(9):555-61, 573. 25 (9): 555-61, 573. ISSN ISSN-0022-2194. Retrieved on 2007-06-15. It often includes interdisciplinary testing to exclude other possible causes for reading difficulties, such as a more generalized cognitive impairment or physical causes such as problems with vision or hearing.
Recent advances in neuroimaging and genetics provide evidence that could potentially help identify children with dyslexia before they learn to read in the future. However, such tests have not yet been developed and more research is needed before such testing could be considered reliable.
Speech delays may be an early warning sign of dyslexia. Many dyslexics may have problems processing and decoding auditory input prior to reproducing their own version of speech. Early stuttering or cluttering can also be warning signs of dyslexia.
Many dyslexics also can have problems with speaking clearly. They can mix up sounds in multi-syllabic words (ex: aminal for animal, bisghetti for spaghetti, hekalopter for helicopter, hangaberg for hamburger, mazageen for magazine, etc.) They also can have problems speaking in full sentences. They can have trouble correctly articulating Rs and Ls as well as Ms and Ns. They often have "immature" speech. They may still be saying "wed and gween" instead of "red and green" in second or third grade. Many dyslexics might have speech therapy in special education. They may have fast speech, cluttered speech, or hesitant speech. Stephen Wilcox - Dyslexia & Vision http://www.dyslexiaassociation.ca/english/files/universityanddyslexia.pdf
Reading requires the sounding out of words. Therefore, it makes sense that children with speech problems can end up having reading problems later. Many have problems with speech due to problems with auditory processing disorder issues.
Many dyslexics have problems with phonemic awareness. Phonemes are the smallest units in spoken language. The Auditory related underlying causes of dyslexia may be partially remediated by auditory therapy or speech therapy, which help with phonemic awareness. This may help to make sense of phonics which may help with phonological awareness, which is needed to sound out words.
Many acquire auditory processing disorder as an underlying cause of dyslexia from glue ear, Otitis Media.
Some shared symptoms of the speech/hearing deficits and dyslexia:
Because of literacy problems, an individual with dyslexia may have difficulty with handwriting. This can involve slower writing speed than average or poor handwriting characterised by irregularly formed letters. They may use inappropriate words when writing.
Some studies have also reported gross motor difficulties in dyslexia, including motor skills disorder. This difficulty is indicated by clumsiness and poor coordination. The relationship between motor skills and reading difficulties is poorly understood but could be linked to the role of the cerebellum and inner ear in the development of reading and motor abilities.Nicolson, R. and Fawcett, A. (Nov 1999). "Developmental dyslexia: the role of the cerebellum". Dyslexia: An International Journal of Research and Practice 5: 155-7.
Dyslexia should not be confused with dyscalculia, a learning disability marked by severe difficulties with mathematics. Individuals with dyslexia can be gifted in mathematics while having poor reading skills. However, in spite of this they might have difficulty with word problems (i.e., descriptive mathematics, engineering, or physics problems that rely on written text rather than numbers or formulas). Individuals with dyslexia may also have difficulty remembering mathematical facts, such as multiplication tables, learning the sequence of steps when performing calculations, such as long division, and other mathematics which involve remembering the order in which numbers appear. This may be exhibited by having a slow response in mathematical drills and difficulty with word problems.[-
There is no cure for dyslexia, but dyslexic individuals can learn to read and write with appropriate education or treatment. There is wide research evidence indicating that specialized phonics instruction can help remediate the reading deficits. The fundamental aim is to make children aware of correspondences between graphemes and phonemes, and to relate these to reading and spelling. It has been found that training, that is also focused towards visual language and orthographic issues, yields longer-lasting gains than mere oral phonological training.Lyytinen, Heikki, Erskine, Jane, Aro, Mikko, Richardson, Ulla (2007), "Reading and reading disorders", in Hoff, Erika, Blackwell Handbook of Language Development, Blackwell, pp. 454-474, ISBN 9781405132534
Effective training is given by teachers at school or kindergarten. Meta-analysis evaluating the effects of phonological awareness instruction has shown that word reading skills of all children, those with a risk for reading problems as well as those developing typically, improved their reading in systematic phonics instruction, a method that encourages a word to be recognised through the building of its constituent sounds. Basic phonemic awareness instruction did not, however, improve spelling in disabled readers.Ehri, Nunes, Willows, Schuster, Yaghoub-Zadeh, Shananan (2001). "Phonemic Awareness Instruction Helps Children Learn to Read: Evidence From the National Reading Panel\'s Meta-Analysis". Reading Research Quarterly 36 (3): 250-287. Retrieved on 2007-07-31. None of the studies included measures of reading fluency.Necoechea, D. M. (2003). "The role of reading intervention research in the identification of children with reading difficulties: A meta-analysis of the literature funded by the NICHD.". Advances in Learning and Behavioral Disabilities 16: 83-161.
The core deficit of dyslexia is in learning to read at the word level Clark, Diana B, et.al. (2005). Dyslexia: Theory and Practice of Remedial Instruction. York Press. , and individuals with dyslexia require more practice to master skills in their areas of deficit. In the circumstances where typically developing children need 30 to 60 hours training, the number of hours that has resulted in optimistic conclusions concerning the treatability of dyslexia is between 80 and 100 hours, or less if the intervention is started sufficiently early. Only approximately 20% of adults with early reading difficulties have acquired fluent reading skills in adulthood. Lyytinen, Heikki, Erskine, Jane, Aro, Mikko, Richardson, Ulla (2007), "Reading and reading disorders", in Hoff, Erika, Blackwell Handbook of Language Development, Blackwell, pp. 454-474, ISBN 9781405132534
Functional MRI (fMRI) studies have shown changes in the brains of dyslexic children and adults with phonics tutoring, along with improved performance on tests of phonemic awareness and text decoding.Shaywitz, B.; Shaywitz, S.; Blachman, B.; Pugh, K.R.; Fulbright, R.; Skudlarski, P.; Others, (2004). "Development of left occipitotemporal systems for skilled reading following a phonologically based intervention in children". Biological Psychiatry 55: 926-933. Eden, G.F.; Jones, K.M.; Cappell, K.; Gareau, L.; Wood, F.B.; Zeffiro, T.A.; Dietz, N.A.E.; Agnew, J.A.; Flowers, D.L. (2004). "Neural Changes following Remediation in Adult Developmental Dyslexia". Neuron 44 (3): 411-422. Retrieved on 2007-07-18. Functional MRI studies have also shown changes in the brain and spelling improvement of dyslexic children taught spelling phonetically in an orthographic manner. Dahms, Joel. (2006). "Spelling Out Dyslexia". Northwest Science & Technology.
One factor that characterises the field of dyslexia treatment is the incessant flow of alternative therapeutic approaches. These controversial treatments include sensory, dietary, and movement-based approaches as well as many others. Reid, Gavin (2005). Dyslexia A Complete Guide for Parents. Chichester: John Wiley and Sons, 213. ISBN 0 470 86312 9.
In the United States, researchers estimate the prevalence of dyslexia to range from five to nine percent of school-aged children, though some have put the figure as high as 17 percent.Shaywitz, Sally E.; Bennett A. Shaywitz (August 2001). "The Neurobiology of Reading and Dyslexia". Focus on Basics 5 (A). National Center for the Study of Adult Learning and Literacy. Learning Disabilities: Multidisciplinary Research Centers, NIH Guide, Volume 23, Number 37, October 21, 1994, Full Text HD-95-005 ("LDRC longitudinal, epidemiological studies show that RD (dyslexia) affect at least 10 million children, or approximately 1 child in 5.") Recent studies indicate that dyslexia is particularly prevalent among small business owners, with roughly 20 to 35 percent of U. S. and British entrepreneurs being affected. Researchers consider that many dyslexic entrepreneurs are successful by delegating responsibilities and excelling at verbal communication.Brent Bowers. "Tracing Business Acumen to Dyslexia", New York Times, December 6, 2007. Cites a study by by Julie Logan, professor of entrepreneurship at Cass Business School in London, among other literature.
Dyslexia\'s main manifestation is a difficulty in developing word-level reading skills in elementary school children. Those difficulties result from reduced ability to associate visual symbols with verbal sounds. While motivational factors must also be reviewed in assessing poor performance, dyslexia is considered to be developmental. Most scientific criteria for dyslexia exclude cases that can be explained as arising from environmental factors such as lack of education or total sensory deficits.
Dyslexia can be substantially compensated for with proper therapy, training, and assistive technology. Many coping strategies are developed subconsciously by the individual dyslexic.
Dyslexia has many variations dependent on the cultural choice of visual notation of speech. So the nature of the notation used in different cultures creates different types of problems for their groups of dyslexics. The differences between the English text and Chinese characters is a good example.
