난독증(難讀症, dyslexia) 또는 독서 장애(讀書障礙)는 문자를 읽는 데에 어려움이 있는 증세를 말한다. 이는 읽고 말하고 철자를 구분하는 데 정확성이나 유연성에 장애가 있는 학습 장애를 가리킨다. 시각이 정상이고 다른 학업영역에서는 적절한 기술을 가지고 있음에도 불구하고 읽기를 하지 못하는 것이다. 두통이나 요통이 그렇듯이 난독증도 많은 종류가 있고 그에 따른 원인도 다양하다. 모든 사례의 난독증과 관련된 단일한 비정상적 특징은 없고, 여러 가지의 비정상성이 다양한 사례에서 관찰된다. 어떤 난독증 환자의 경우,근본적인 문제들 중 일부는 지각과 관련되어 있다. 이들은 한 번에 한 글자씩 읽을 수 있지만 여러 글자를 결합하여 인식 하지는 못한다. 어떤 한 글자를 결합하여 인식하지는 못한다. 어떤 한 글자를 의미가 없는 x로 둘러싸기만 해도 가운데에 있는 글자를 식별하기 어려워한다. 그들은 또한 구어의 음절을 잘 알아듣지 못하는데, 예를 들어 da라는 소리가 들리면 레버를 누르고 ga,pa또는 ta라는 소리가 들리면 레버를 누르지 않는 과제를 잘 수행하지 못한다.[2] 모두는 아니지만 많은 난독증 환자들의 경우,그 시각체계에서 대세포 경로가 비교적 반응성이 낮다. 많은 난독증 환자들은 단어 뿐 아니라 시각 운동 패턴을 지각하는 데도 결함을 가지고 있다. 그러나 이런 효과는 개인에 따라 많이 다르고 대세포 경로만이 이 현상을 설명할 수 있는 유일한 부위는 아니다. 난독증이 언어에 대한 불완전한 반구전문화를 반영하는 것으로 보이는 사례들도 있다. 한 광범위한 문헌 연구에 의하면, 난독증 환자들에게서는 두뇌반구의 대뇌피질이 대칭적일 가능성이 더 높은 반면에 다른 사람들의 경우에는 측두평면을 비롯한 어떤 뇌영역들이 좌반구에서 더 크다. 어떤 난독증 환자들의 경우에는 언어와 관련된 특정 뇌영역들이 사실상 우반구에서 더 크다. 이런 경과는 기능의 비정상적인 편재화를 난독증이나 기타 문제들과 연관시켜 설명하는 Geschwind와 Galaburda의 이론을 지지한다. 난독증에 관한 또 다른 설명에서는 이 현상을 주의집중 또는 방략의 차이와 관련짓는다. 한 연구에서 연구자들은 아동에게 문자 조합들(clid와 cdil과 같은)을 쌍으로 보여주고, 둘 중 어느 것이 영어 단어가 “될 수 있는 것인지”말하게 하였다. 난독증 아동들은 batmotbeam과 monglustamer 같은 가짜 단어를 제대로 발음하지는 못했지만, 정상적인 읽기 능력을 가진 아동들보다 이 과제를 사실상 더 잘 하였다. 이 결과는 난독증 아동들이 단어의 시각적 특징에 주의를 두어 단어가 될 수 있는 문자 조합과 될 수 없는 문자 조합을 인식할 수 있었지만, 생소한 단어를 말 소리로 변환시키는 능력이 부족함을 잘 보여준다.
Dyslexia
Dyslexia is characterized by difficulty with learning to read fluently and with accurate comprehension despite normal intelligence. This includes difficulty with phonological awareness, phonological decoding, processing speed, orthographic coding, auditory short-term memory, language skills/verbal comprehension, and/or rapid naming. Developmental reading disorder(DRD) is the most common learning disability. Dyslexia is the most recognized of reading disorders, however not all reading disorders are linked 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 disabilities (e.g. an auditory processing disorder, an attention-deficit/hyperactivity disorder, a visual processing disorder) and co-occurring learning disabilities (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.
Classification
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. Dyslexia can also be acquired following brain damage; commonly called alexia, it includes surface dyslexia, semantic dyslexia, phonological dyslexia, and deep dyslexia. Acquired surface dyslexia arises after brain damage in a previously literate person and results in pronunciation errors that indicate impairment of the lexical route. Numerous hypothesizes which attempt to define dyslexia by their symptoms and attempt to resolve it by proposing a neurological answer. The Dual-route hypothesis to reading aloud proposes an answer for disordered reading, including both developmental dyslexica and alexica.
Signs and symptoms
n early childhood, early symptoms that correlate with a later diagnosis of dyslexia include delays in speech, letter reversal or mirror writing, 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.
Prevalance
The prevalence of dyslexia is unknown; with estimations varying between 1% to 33% of the population.[41] 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 dyslexia, but despite the significant differences between the writing systems, Italian, German and English speaking populations suffer similarly from dyslexia.[44] Dyslexia is not limited to difficulty in converting letters into sounds, but Chinese dyslexics have difficulty in extracting shapes of Chinese characters into meanings.
Neuroanatomy
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 a correlation between functional and structural differences in the brains of children and reading difficulties. Some individuals with dyslexia show less electrical activation 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. 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.
Genetics
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.
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, 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 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.
Effect of language orthography
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 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. 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. 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.
Management
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.[74] There is some evidence that the use of specially tailored fonts may provide some measure of assistance for people who have dyslexia.[75][76] 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. Ronald Davis describes the alternative compensating skills and abilities some who have dyslexia are able to develop as a gift that can increase the capacity to think and perceive multi-dimensionally and can help further utilize the brain’s ability to alter and create perceptions. Davis acknowledges the benefits of dyslexics visual thinking and provides management solutions to overcoming the challenges faced by dyslexics.
History
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 by analogy to classical subtypes of 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 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.
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.
Exacerbating conditions
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. Attention deficit disorder – a high degree of co-morbidity has been reported between ADD/ADHD and dyslexia/reading disorders, it occurs in between 12% and 24% of those with dyslexia. 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. Developmental coordination disorder – 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. 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.
Alternative compensating skills and abilities
Ronald Davis’s book The Gift of Dyslexia presents dyslexia with a unique viewpoint and states that “dyslexia is an unintentionally self-created condition … [and] is the result of a perceptual talent… in some situations, the talent becomes a liability.” Upon looking at a list of people diagnosed with dyslexia, important historical figures have had dyslexia or succeeded because of dyslexia. Davis explains that the nonverbal picture thinking mode is superior to verbal thinking which is limited by the brain’s ability to process and think auditorially. Davis explains that the adage of a “picture is worth a thousand words” to describe Einstein’s daydream which spawned his theory of general relativity as simple because of his visual thought process.
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