dyslexia

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

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

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

Classification

The World Federation of Neurology defines dyslexia as "a disorder manifested by difficulty in learning to read despite conventional instruction, adequate intelligence and sociocultural opportunity".^[18]

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

The National Institute of Neurological Disorders and Stroke gives the following definition for dyslexia:

"Dyslexia is a brain-based type of learning disability that specifically impairs a person's ability to read. These individuals typically read at levels significantly lower than expected despite having normal intelligence. Although the disorder varies from person to person, common characteristics among people with dyslexia are difficulty with spelling, phonological processing (the manipulation of sounds), and/or rapid visual-verbal responding. In adults, dyslexia usually occurs after a brain injury or in the context of dementia. It can also be inherited in some families and so on, and recent studies have identified a number of genes that may predispose an individual to developing dyslexia".^[1]

Other published definitions are purely descriptive or embody causal theories. Varying definitions are used for dyslexia from researchers and organizations around the world; it appears that this disorder encompasses a number of reading skills, deficits and difficulties with a number of causes rather than a single condition.^[20][21]

Castles and Coltheart describe phonological and surface types of developmental dyslexia by analogy to classical subtypes of alexia (acquired dyslexia) which are classified according to the rate of errors in reading non-words.^[22][23] However, the distinction between surface and phonological dyslexia has not replaced the old empirical terminology of dysphonetic versus dyseidetic types of dyslexia.^[21][23][24] The surface/phonological distinction is only descriptive, and devoid of any etiological assumption as to the underlying brain mechanisms.^[25] Studies have, however, alluded to potential differential underlying brain mechanisms in these populations given performance differences.^[26][27][28] The dysphonetic/dyseidetic distinction refers to two different mechanisms; one that relates to a speech discrimination deficit, and another that relates to a visual perception impairment.

Signs and symptoms

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

Cause

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

Effect of language orthography

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

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

Cross-cultural prevalence

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

Exacerbating conditions

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

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

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

Management

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

For dyslexia intervention with alphabet writing systems the fundamental aim is to increase a child's awareness of correspondences between graphemes and phonemes, and to relate these to reading and spelling. It has been found that training focused towards visual language and orthographic issues yields longer-lasting gains than mere oral phonological training.^[71]

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

History

Dyslexia was identified by Oswald Berkhan in 1881,^[75] but the term dyslexia was later coined in 1887 by Rudolf Berlin,^[76] an ophthalmologist practicing in Stuttgart, Germany.^[77] He used the term to refer to a case of a young boy who had a severe impairment in learning to read and write in spite of showing typical intellectual and physical abilities in all other respects.

In 1896, W. Pringle Morgan, a British physician, from Seaford, East Sussex published a description of a reading-specific learning disorder in a report to the British Medical Journal titled "Congenital Word Blindness". This described the case of a 14-year-old boy who had not yet learned to read, yet showed normal intelligence and was generally adept at other activities typical of children of that age.^[78]

Research

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

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

In the area of neurological research into dyslexia, modern neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) have produced clear evidence of structural differences in the brains of children with reading difficulties. It has been found that people with dyslexia have a deficit in parts of the left hemisphere of the brain involved in reading, which includes the inferior frontal gyrus, inferior parietal lobule, and middle and ventral temporal cortex.^[86]

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

Gene–environment interaction

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

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

See also

• Child development
• History of the alphabet
• Learning theory (education)
• Linguistics
• List of artistic depictions of dyslexia
• List of people diagnosed with dyslexia
• Neurolinguistics
• Philosophy of language
• Writing system
• Neurodiversity
• Dyslexie

References

External links

Organizations

• International Dyslexia Association
• Dyslexia Action (UK)
• British Dyslexia Association
• Dyslexia Research Trust
• Learning Ally, formerly Recording for the Blind & Dyslexic
• The Brain Foundation.Singapore