Dyslexia From diagnoses to theory

Dyslexia is generally considered to be a disorder of accurate and/or fluent word recognition and spelling and decoding abilities. However, theories about what causes dyslexia differ to a large extent which prevents international agreement about how dyslexia should be diagnosed.
In this thesis, various aspects of diagnostic methods were investigated, and new methods were proposed. It was found that existing methods can be improved by using item scores instead of sum scores, by applying multiple classifications, and by carefully evaluating criteria of dyslexia. Furthermore, it was found that self-report statements provide more reliable diagnoses than test results, mainly because self-report statements do not depend on general intelligence and level of schooling. Additionally, a classification accuracy of 80% was found using anatomical brain imaging techniques.
Some findings of this thesis are relevant for the interpretation of theoretical perspectives about dyslexia. First, a severity score of dyslexia showed two separate normal distributions for people with and without dyslexia. Second, it was found that dyslexia is characterised by at least six cognitive variables. Third, some of these variables showed significant correlations with various areas in the brain. Fourth, support was found for the idea that anatomical brain differences are mainly the result of individual differences in training.
Based on the findings in this thesis, it was proposed that dyslexia may not be a disorder, but a perceptual variation, originating in the subcortex and with widespread effects on various areas in the cortex. Especially processes of inhibition may be impaired in people with dyslexia.