The neurobiology of dyslexia

Within the last two decades overwhelming evidence from many laboratories has converged to indicate the cognitive basis for dyslexia: dyslexia represents a disorder within the language system and more specifically within a particular subcomponent of that system, phonological processing, Recent advances in imaging technology and the development of tasks which sharply isolate the subcomponent processes of reading now allow the localization of phonological processing in brain, and, as a result, provide for the first time, the potential for elucidating a biological signature for reading and reading disability. Converging evidence from a number of laboratories using functional brain imaging indicates a pattern of brain organization in dyslexia different from that seen in nonimpaired readers; specifically these studies show a disruption of left hemisphere posterior brain systems in adult dyslexic readers while performing reading tasks with an additional suggestion for an associated increased reliance on frontal lobe circuits. The discovery of neural systems serving reading has significant implications. At the most fundamental level. it is now possible to investigate specific hypotheses regarding the neural substrate of dyslexia, and to verify, reject or modify suggested cognitive models, From a more clinical perspective. the identification of neural systems for reading offers the promise for more precise identification and diagnosis of dislexia in children, adolescents and adults. (C) 2001 Association for Research in Nervous and Mental Disease. Published by Elsevier Science B.V. All rights reserved.