Reading and spelling disorders: clinical features and causes.

Developmental dyslexia (specific reading and specific spelling disorder) is thought to stem from specific features in cognitive processing strongly related to biological maturation of the central nervous system which interact with non-biological learning conditions. The specific learning disorder should not be accounted for by mental age, gross neurological deficits, emotional disturbances or inadequate schooling. As a clinical guideline, the child's level in reading and spelling must be significantly below that expected for the population of children of the same mental age. The persistence rate is high and dyslexia is often associated with psychiatric problems. The etiology is not known. From the biological point of view, dyslexia is supposed to have a neurological basis. Neuroanatomical, neurophysiological, and neuropsychological correlates have been studied by means of autopsy, brain imaging, neurophysiological and neuropsychological methods. There is good evidence that dyslexia is determined by heritable cognitive components of reading and spelling processing. Experimental research focuses on characteristics of brain structure and cognitive skills related to the central nervous systems of auditory-phonological and visual information processing.