Dyslexia is the most common learning difficulty and most recognized reading disorder. There are other reading difficulties that are unrelated to dyslexia.
Some see dyslexia as distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or poor or inadequate reading instruction. There are three proposed cognitive subtypes of dyslexia (auditory, visual and attentional), although individual cases of dyslexia are better explained by specific underlying neuropsychological deficits and co-occurring learning difficulties(e.g. an auditory processing disorder, an attention deficit hyperactivity disorder, a visual processing disorder) and co-occurring learning difficulties (e.g. dyscalculia and dysgraphia). Although it is considered to be a receptive language-based learning disability in the research literature, dyslexia also affects one's expressive language skills. Researchers at MIT found that people with dyslexia exhibited impaired voice-recognition abilities. Interestingly, a study published online (and in the July issue of the American Journal of Human Genetics), reported a genetic origin to the disorder, and other learning disabilities, that could help lead to earlier diagnoses and more successful interventions. 
Internationally, dyslexia has no single definition; more than 70 names are used to describe its manifestations, characterizations or causes. 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". The National Institute of Neurological Disorders and Stroke definition also adds, "difficulty with spelling, phonological processing (the manipulation of sounds), and/or rapid visual-verbal responding." Many published definitions from researchers and organizations around the world are purely descriptive or embody causal theories. These definitions for the disorder, defined as dyslexia, encompass a number of reading skills, deficits and difficulties with a number of causes rather than a single condition.
In early childhood, early symptoms that correlate with a later diagnosis of dyslexia include delays in speech, letter reversal or mirror writing,difficulty knowing left from right and directions, and being easily distracted by background noise. This pattern of early distractibility is partially explained by the co-occurrence of dyslexia and attention-deficit/hyperactivity disorder. Although each disorder occurs in approximately 5% of children, 25-40% of children with either dyslexia or ADHD meet criteria for the other disorder.
Dyslexic children of school age can have various symptoms; including difficulty identifying or generating rhyming words, or counting syllables in words (phonological awareness), a difficulty segmenting words into individual sounds, or blending sounds to make words, a difficulty with word retrieval or naming problems (see anomic aphasia), commonly very poor spelling, which has been called dysorthographia or dysgraphia (orthographic coding), whole-word guesses, and tendencies to omit or add letters or words when writing and reading are considered classic signs.
Signs persist into adolescence and adulthood with trouble with summarizing a story, memorizing, reading aloud, and learning a foreign language. Adult dyslexics can read with good comprehension, although they tend to read more slowly than non-dyslexics and perform more poorly at spelling and nonsense word reading, a measure of phonological awareness.
A common misconception about dyslexia is that dyslexic readers write words backwards or move letters around when reading – this only occurs in a very small population of dyslexic readers. Individuals with dyslexia are better identified by reading accuracy, fluency, and writing skills that do not seem to match their level of intelligence from prior observations.
The complexity of a language's orthography (i.e., its conventional spelling system, see orthographic depth) has a direct impact upon how difficult it is to learn to read that language. English has a comparatively deep orthography within the Latin alphabetwriting system, with a complex orthographic structure that employs spelling patterns at several levels: principally, letter-sound correspondences, syllables, and morphemes. Other languages, such as Spanish, have mostly alphabetic orthographies that employ letter-sound correspondences, so-called shallow orthographies. It is relatively easy to learn to read languages like Spanish; it is much more difficult to learn to read languages with more complex orthographies such as English.Logographic writing systems, notably Japanese and Chinese characters, have graphemes that are not linked directly to their pronunciation, which pose a different type[specify] of dyslexic difficulty.
From a neurological perspective, different types of writing systems (e.g., 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 one in a language with another orthography. The neurological skills required to perform the tasks of reading, writing, and spelling can vary between writing systems. As a result, different neurological deficits can cause dyslexic problems in relation to different orthographies.
Several learning disabilities often occur with dyslexia, but it is unclear whether these learning disabilities share underlying neurological causes with dyslexia. These disabilities include:
Dysgraphia – a disorder which expresses itself primarily through writing or typing, although in some cases it may also affect eye–hand coordination, direction- or sequence-oriented processes such as tying knots or carrying out a repetitive task. In dyslexia, dysgraphia is often multifactorial, due to impaired letter writing automaticity, finger motor sequencing challenges, organizational and elaborative difficulties, and impaired visual word form which makes it more difficult to retrieve the visual picture of words required for spelling. Dysgraphia is distinct from developmental coordination disorder in that developmental coordination disorder is simply related to motor sequence impairment.
Auditory processing disorder – A condition that affects the ability to process auditory information. Auditory processing disorder is a listening disability. It can lead to problems with auditory memory and auditory sequencing. Many people with dyslexia have auditory processing problems and may develop their own logographic cues to compensate for this type of deficit. Auditory processing disorder is recognized as one of the major causes of dyslexia.
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.
Researchers have been trying to find a biological basis of dyslexia since it was first identified by Oswald Berkhan in 1881 and the term dyslexia coined in 1887 by Rudolf Berlin. The theories of the etiology of dyslexia have and are evolving.
Brain activation studies using PET to study language have produced a breakthrough in understanding of the neural basis of language over the past decade. A neural basis for the visual lexicon and for auditory verbal short-term memory components have been proposed, with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural).
fMRI's in dyslexics have provided important data supporting the interactive role of the cerebellum and cerebral cortex as well as other brain structures.
Genetic research into dyslexia has its roots in the examination of post-autopsy brains of people with dyslexia. When they observed anatomical differences in the language center in a dyslexic brain, they showed microscopic cortical malformations known as ectopias and more rarely vascular micro-malformations, and in some instances these cortical malformations appeared as a microgyrus. These studies and those of Cohen et al. 1989 suggested abnormal cortical development which was presumed to occur before or during the sixth month of fetal brain development.
Abnormal embryonic cell formations in dyslexics found on autopsy have also been reported in non-language cerebral and subcortical brain structures. MRI data have conﬁrmed a cerebellar role in dyslexia.
Research has examined gene–environment interactions in reading disability through twin studies, which estimate the proportion of variance associated with environment and the proportion associated with heritability. Studies examining the influence of environmental factors such as parental education, and teacher quality have determined that genetics have greater influence in supportive, rather than less optimal environments. Instead, it may just allow those genetic risk factors to account for more of the variance in outcome, because environmental risk factors that affect that outcome have been minimized.
As the environment plays a large role in learning and memory, it is likely that epigenetic modifications play an important role in reading ability. Animal experiments and measures of gene expression and methylation in the human periphery are used to study epigenetic processes, both of which have limitations in extrapolating to the human brain.
Through compensation strategies and therapy, dyslexic individuals can learn to read and write with educational support. There are techniques and technical aids that can manage or even conceal symptoms of the disorder. Removing stress and anxiety alone can improve written comprehension.
For dyslexia intervention with alphabet writing systems, the fundamental aim is to increase a child's awareness of correspondences between graphemes (letters) and phonemes (sounds), and to relate these to reading and spelling by teaching him or her to blend the sounds into words. It has been found that training focused towards visual language (reading) and orthographic (spelling) yields longer-lasting gains than mere oral phonological training. Intervention early on while language areas in the brain are still developing is most successful in reducing long-term impacts of dyslexia.
There is some evidence that the use of specially tailored fonts may provide some measure of assistance for people who have dyslexia. Among these fonts are Dyslexie and OpenDyslexic, which were created with the notion that many of the letters in the latin alphabet are visually similar and confusing for people with dyslexia. Dyslexie, along with OpenDyslexic, put emphasis on making each letter more unique to assist in reading.
The prevalence of dyslexia is unknown; with estimations varying between 1% to 33% of the population. It is often estimated that the prevalence of dyslexia is around 5-10 percent of a given population although there have been no studies to indicate an accurate percentage.Internationally, there are differing definitions of dyslexia, but despite the significant differences between the writing systems, Italian, German and English speaking populations suffer similarly from dyslexia. Dyslexia is not limited to difficulty in converting letters into sounds, but Chinese dyslexics have difficulty in extracting shapes of Chinese characters into meanings.
Dyslexia was identified by Oswald Berkhan in 1881, but the term dyslexia was coined in 1887 by Rudolf Berlin, who was an ophthalmologist in Stuttgart. He used the term to refer to a case of a young boy who had a severe impairment in learning to read and write in spite of showing typical intellectual and physical abilities in all other respects.
In 1896 W. Pringle Morgan, a British physician from Seaford, East Sussex, published a description of a reading-specific learning disorder in a report to the British Medical Journal titled "Congenital Word Blindness". This described the case of Percy, 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 that age.
Castles and Coltheart describe phonological and surface types of developmental dyslexia (dysphonetic and dyseidetic, respectively) to classical subtypes of alexia which are classified according to the rate of errors in reading non-words. The surface/phonological distinction is only descriptive, and devoid of any etiological assumption as to the underlying brain mechanisms. Studies have, however, alluded to potential differential underlying brain mechanisms in these populations given performance differences. 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.
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