Developmental dyslexia is a language-based learning disability primarily characterized by difficulties decoding and recognizing printed words accurately and/or fluently. There is a general consensus that deficits in phonological processing represent the core area of difficulty in most cases of developmental dyslexia, with particular emphasis placed on the role of phonological awareness. However, a deficit at an early stage of phonetic processing, when the complex spectrotemporal patterns in the speech signal are analyzed and assigned to phonemic categories, could cause a deficit in phonological awareness. Our principal hypothesis is that there is a subtype of dyslexia in which the primary impairment is a specific difficulty with the categorization of complex sounds like speech, and this deficit is not the result of a more general auditory perceptual impairment. Furthermore, we hypothesize that this categorical perception deficit is associated with abnormal patterns of activation in left superior and middle temporal regions involved in pre-lexical phonetic perception. Neuropsychological measures, behavioral identification and discrimination measures of categorical perception (CP), and fMRI and DTI techniques will be used to test this hypothesis in forty 8- to 17-year-old children with dyslexia and forty typically developing children. In the first experiment, an fMRI study using an AX discrimination task will be performed to examine activation patterns associated with categorical perception in both groups. In the second experiment, both an auditory presentation and a print version of a phonological segmentation task will be used, which will allow us to examine activation patterns associated with phonological awareness both separate from and in combination with other processes comprising reading such as grapheme-phoneme mapping. Activation patterns during these tasks as well as behavioral profiles and structural connectivity will be compared between children with dyslexia and a CP deficit and those without this deficit. The heterogeneous nature of dyslexia has limited advances in our understanding of this common developmental disorder. Combining across several behavioral and neuroimaging measures is an innovative approach to the differential diagnosis of subtypes in dyslexia. The integration of multiple sources of information in addition to the use of tasks that allow examination of multiple processing levels such as auditory perceptual processing, phonemic perception, phonological awareness, and grapheme-phoneme mapping permits a greater depth of investigation than seen in previous studies and will likely yield novel information regarding not only dyslexia but also typical reading development.
This project involves the integration of behavioral profiles, brain activation patterns, and structural connectivity data in order to provide a comprehensive investigation of the underlying deficit in developmental dyslexia and in order to identify and characterize specific subtypes of this disorder. Particular emphasis is placed on one hypothesized subtype in which the core deficit involves an impairment in the ability to categorize complex sounds like speech. The identification of more homogeneous diagnostic subtypes in dyslexia will enable the development, evaluation, and use of targeted intervention strategies and is an essential stepping stone for future investigations of the genetic bases of dyslexia, which may ultimately lead to earlier identification of children at risk for this disorder. ? ? ?
