We have revised the 1995 working definition on the basis of current converging evidence relevant to reading development, reading disabilities, and reading instruction. But the task is not complete. Our understanding of dyslexia is a work in progress and will continue to be just that. To be sure, over the next five years, our ability to bring to bear improved research methodologies and investigative modalities to the study of dyslexia literally guarantees the acquisition of new knowledge that will further modify the definition. The one constant is that this and future definitions reflect the best that science has to often

A much debated question is whether sex differences exist in the functional organization of the brain for language. A long-held hypothesis posits that language functions are more likely to be highly lateralized in males and to be represented in both cerebral hemispheres in females, but attempts to demonstrate this have been inconclusive. Here we use echo-planar functional magnetic resonance imaging to study 38 right-handed subjects (19 males and 19 females) during orthographic (letter recognition), phonological (rhyme) and semantic (semantic category) tasks. During phonological tasks, brain activation in males is lateralized to the left inferior frontal gyrus regions; in females the pattern of activation is very different, engaging more diffuse neural systems that involve both the left and right inferior frontal gyrus. Our data provide clear evidence for a sex difference in the functional organization of the brain for language and indicate that these variations exist at the level of phonological processing.

Learning to read requires an awareness that spoken words can be decomposed into the phonologic constituents that the alphabetic characters represent. Such phonologic awareness is characteristically lacking in dyslexic readers who, therefore, have difficulty mapping the alphabetic characters onto the spoken word. To find the location and extent of the functional disruption in neural systems that underlies this impairment, we used functional magnetic resonance imaging to compare brain activation patterns in dyslexic and nonimpaired subjects as they performed tasks that made progressively greater demands on phonologic analysis. Brain activation patterns differed significantly between the groups with dyslexic readers showing relative underactivation in posterior regions (Wernicke's area, the angular gyrus, and striate cortex) and relative overactivation in an anterior region (inferior frontal gyrus). These results support a conclusion that the impairment in dyslexia is phonologic in nature and that these brain activation patterns may provide a neural signature for this impairment.Speech enables its users to create an indefinitely large number of words by combining and permuting a small number of phonologic segments, the consonants and vowels that serve as the natural constituents of the biologic specialization for language. An alphabetic transcription brings this same ability to readers but only as they connect its arbitrary characters (letters) to the phonologic segments they represent. Making that connection requires an awareness that all words, in fact, can be decomposed into phonologic segments. Thus, it is this awareness that allows the reader to connect the letter strings (the orthography) to the corresponding units of speech (phonologic constituents) that they represent. As numerous studies have shown, however, such awareness is largely missing in dyslexic children and adults (1-4). Not surprisingly, then, perhaps the most sensitive measure of the reading problem in dyslexia is inability to read phonologically legal nonsense words (5-7). As for why dyslexic readers should have exceptional difficulty developing phonologic awareness, there is support for the notion that the difficulty resides in the phonologic component of the larger specialization for language (8)(9)(10). If that component is imperfect, its representations will be less than ideally distinct and, therefore, harder to bring to conscious awareness.Previous efforts using functional imaging methods to examine brain organization in dyslexia have been inconclusive (11-17) largely, we think, because the experimental tasks tapped the several aspects of the reading process in somewhat unsystematic ways. Our aim therefore was to develop a set of hierarchically structured tasks that control the kind of language-relevant coding required, including especially the demand on phonologic analysis, and then to compare the performance and brain activation patterns (as measured by functional MRI) of dyslexic (DYS) and nonimpaired (NI) readers. Thus, procee...

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Conflict of interest statement: No conflicts declared.Freely available online through the PNAS open access option.

The assistance of John Holahan, who bore responsibility for maintaining and updating the database, is gratefully acknowledged. Thanks are also due to Lois Dreyer, who supervised the testing of the children. In addition, we acknowledge many others at the Yale University Center for the Study of Learning and Attention Disorders and at Haskins Laboratories, whose help was essential to the completion of the project. Finally, we thank Linda Kimbrough for her work on preparation of this article.

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Individual growth curves were used to test whether the development of children with reading disabilities is best characterized by models of developmental lag or developmental deficit. Developmental changes in reading ability were modeled by using 9 yearly longitudinal assessments of a sample of 403 children classified into three groups representing (a) deficient reading achievement relative to IQ expectations (RD-D), (b) deficient reading achievement consistent with IQ expectations (LA), and (c) no reading deficiency (NRI). Using a model of quadratic growth to a plateau, the age and level at which reading scores plateaued were estimated for each child. Reading-disabled children differed on average from nondisabled children in the level but not in the age at which reading skills plateaued. The RD-D and LA groups did not differ in reading plateau or age at plateau. The subgroup of RD-D children scoring below the 25th percentile in reading differed from LA children only in reading plateau. Results suggest that the developmental course of reading skills in children with reading disability is best characterized by deficit as opposed to lag models. In addition, no support for the validity of classifications of reading disability based on IQ discrepancies was apparent.Despite 30 years of research, it remains unclear whether children who vary in reading abilities are best characterized by models involving developmental lags or deficits (Fletcher, 1981;Stanovich, Nathan, & Vala-Rossi, 1986). The deficit model assumes that children fail to read proficiently because of the absence of a skill that never develops