Neurobiological basis for learning disorders with a special emphasis on reading disorders

Chung, Hee Jung

Abstract

Learning disorders are diagnosed when the individual's achievement on standardized tests in reading, mathematics, or written expression is substantially below that expected for age, schooling, and level of intelligence. Subtypes of learning disorders may be classified into two groups, language-based type learning disorders including reading and writing disorder, and nonverbal type learning disorder (NLD) such as those relating to mathematics & visuospatial skills, and those in the autism spectrum. Converging evidence indicates that reading disorder represents a disorder within the language system and more specifically within a particular subcomponent of that system, phonological processing. Recent advances in neuroimaging technology, particularly the development of fMRI, provide evidences of a neurobiological basis for reading disorder, specifically a disruption of two left hemisphere posterior brain systems, one parieto-temporal, the other occipito-temporal. The former is the reading system for beginner reading, the latter for skilled reading. Compensatory engagement of anterior systems around the inferior frontal gyrus(Broca's area) and a posterior(right occipito-temporal) system is noted in persistent poor readers in long-term follow up study. The theoretical model proposed to explain NLD's source is not right hemisphere damage, but rather the white matter model. The working hypothesis of the white matter model is that the underdevelopment of, damage to, or dysfunction of cerebral white matter(long myelinated fibers) is the source of this disorder. The role of an evidence-based effective intervention in the remediation of children with learning disorder is discussed.