In an increasingly literate world, the inability to read becomes a significant disability that may affect academic success, employment, and self-concept. Because widespread literacy is a recent historical development, it should not be surprising that it was only about 100 years ago that the first case of alexia or "word blindness" was described. By definition, the term alexia describes a condition where reading ability is lost as the result of some neurological insult, such as head injury or stroke. With alexia, it is assumed that the individual had adequate reading achievement prior to the neurological insult.
The clinical and experimental literature on alexia has greatly increased understanding of dyslexia. The term dyslexia refers to an inborn or congenital inability to learn to read. The term rose out of the medical literature and is not completely accepted by many educators who work with children with reading difficulties. Many educators and psychologists prefer the terms developmental reading disorder or reading disability, because they avoid the implication that the etiology of the reading disorder is due to neurological deficits. Consequently, the term dyslexia is most appropriate when it describes a severe reading disability presumed to be neurologic in origin. Although prevalence estimates vary, the incidence of dyslexia has been estimated at 3% to 8% of the school-aged population. .Although defini tions of dyslexia vary across contexts, most definitions center on a significant discrepancy between reading achievement, particularly word recognition ability, and a measure of general cognitive ability or intellectual functioning.
The biological bases of dyslexia have gained significant research attention since the early 1990s. Many researchers have targeted the left hemisphere's perisylvian region, or the area surrounding the Sylvian fissure on the lateral surface of the cortex, in research examining morphology and planum temporale symmetry. Morphology refers to the pattern of gyri and sulci on the surface of the brain. Subtle deviations in perisylvian morphology, although not at a level of gross neurological significance, have been associated with dyslexia. Research on the biological basis of dyslexia has also focused on the planum temporale. Symmetry or rightward asymmetry in the length or size of the planum temporale have been associated with dyslexia. Polymicro-gyri, additional small folds in the cortical surface, and het-erotopias, neuronal migration errors, in this region have also been associated with dyslexia.
Although the perisylvian region has received significant research attention, it is not entirely clear why brains with symmetry or atypical morphology are more likely to be found in individuals with dyslexia. The symmetry noted in these brains may result from neuronal migration errors or poor pruning of redundant pathways. Because the individual has fewer functionally connected cells, those that are functional may be less able to compensate for other cortical atypicalities. Additionally, the individual may have fewer pathways available to compensate for the atypicalities.
Reading is a complex cognitive task that combines numerous skills, including attention, memory, phonological processing, rapid naming, and so forth. Consequently, any findings related to individual chromosomal involvement in the development of dyslexia might actually pertain to language tasks in general. Despite this caveat, research has implicated chromosomes 6 and 15 in dyslexia, resulting from the action of an autosomal dominant gene with variable expression and incomplete penetrance. Although there is significant evidence to support genetic involvement in dyslexia, environmental factors also play a role. Behavioral genetics research has suggested that slightly more than 50% of the variance in reading performance is the result of differences in genetic heritability.
Although research supports the involvement of genetics and the differences in the brain as being risk factors for dyslexia, these areas do not answer questions about the underlying cognitive processes that are involved in reading and that are aberrant in dyslexia. Research on dyslexia suggests that phonological processing and orthographic coding are two cognitive processes that play significant roles in reading ability and dyslexia.
Phonological processing allows an individual to hear and manipulate individual sounds in spoken language. Although there are only 26 letters in the English language, there are 44 phonemes. Phonological processing is part of the larger skill of auditory perception and discrimination, but it is involved only with sounds used in speech. Rather than being one unitary skill demonstrable in a single behavior, phonological processing skills are actually a group of skills, including letter-sound association, rhyming, blending, segmenting, and sound replacement. Phonological processing skills are developing in children before they enter school, and these early skills appear to predict future reading achievement. Furthermore, these skills continue to develop as the child learns to read, such that the relationship between phonological processing and reading is symbiotic. Research with dyslexic children suggests that phonological deficits may be the core deficits impeding their reading acquisition. Furthermore, there is evidence to suggest that these skills are influenced by genetics and the underlying brain structures in the perisylvian region.
Orthographic processing is a second area of research investigation. It involves the interpretation of abstract representations, specifically, series of letters that form words during the reading process. Orthographic processing is most closely related to sight word reading, in which the individual does not use decoding strategies to read words but, rather, recognizes the entire word on sight. It appears to be influenced mostly by environmental factors, such as home reading environment, rather than genetic heritability. Functional magnetic resonance imaging (fMRI) has been employed to chart possible deficiencies in phonological and orthographic processes in the brain.
Intervention for dyslexia typically consists of remediation and compensation strategies. When children are diagnosed with dyslexia during the early school years, the emphasis is on teaching them phonetic skills to improve their decoding ability and teaching them to recognize sight words to increase reading speed. For many children, their reading improves and they are able to experience normal developmental outcomes, such as employment and, in some cases, higher-level education. For those individuals who do not develop adequate reading ability by adolescence and adulthood, the emphasis changes to include compensatory strategies. As adults, dyslexic individuals typically have access to books on tape and other compensatory approaches.
Carlin J. Miller
City University of New York, Queens College
George W. Hynd
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This is a comprehensive guide covering the basics of dyslexia to a wide range of diagnostic procedures and tips to help you manage with your symptoms. These tips and tricks have been used on people with dyslexia of every varying degree and with great success. People just like yourself that suffer with adult dyslexia now feel more comfortable and relaxed in social and work situations.