Developmental dyslexia is a common disorder of reading whose etiology is poorly understood. The behavioral manifestations of dyslexia are complex, involving abnormalities in both language and visual perception. It is well known that reading ability is correlated with measures of phonological awareness. Because dyslexics exhibit abnormalities in various forms of phonological processing, this has been suggested as an explanation for their reading disability. While less clinically evident, specific visual processing problems have also been identified in dyslexia and it has been suggested that they contribute to the observed reading disorder. Surprisingly, few studies have attempted to reconcile these two apparently disparate views concerning the etiology of this disorder. Recent advances in functional neuroimaging techniques now provide a way to identify the regional specialization and spatial congruence of the cortical areas engaged in visual sensory and language processing. These new techniques make possible novel investigations of the pathophysiology of the reading disorder developmental dyslexia. We propose a series of integrated studies combining behavioral measures with functional magnetic resonance imaging (fMRI) to investigate the relationship between the disorders in phonological processing and visual motion processing in dyslexia. In the proposed research, functional maps obtained from control subjects during reading tasks will be compared to those obtained in individuals with dyslexia. Our central hypothesis posits the existence of a disorder in temporal information processing in dyslexia that exhibits itself in deficits in both sensory and language processing. In our previous work we have demonstrated a sensory deficit specific to the visual motion processing system. Using similar techniques we now propose to extend these investigations and relate them to dysfunction of phonological processing mechanisms. We hypothesize that we will identify spatial overlap in the cortical areas altered in dyslexia during both phonological and visual processing in the temporoparietal areas, as this area has shown to directly subserve both sensory visual motion and cognitive language processes. Specific questions addressed include: (1) What are the neural systems involved in phonological awareness (segmentation, rhyme generation and coding in verbal memory) and visual motion processing in normal subjects? (2) Do the neural systems involved in phonological awareness show sensitivity to performance rate, modality of presentation (oral versus visual) or response (covert versus overt)? (3) Is there spatial overlap in the neuronal systems involved in the performance of phonological awareness tasks and visual motion processing? (4) Since performance on these tasks reliably differentiates dyslexics from controls, are the neural systems involved differently activated in dyslexics and controls during the performance of these tasks?
| Maisog, Jose M; Einbinder, Erin R; Flowers, D Lynn et al. (2008) A meta-analysis of functional neuroimaging studies of dyslexia. Ann N Y Acad Sci 1145:237-59 |
