SENSITIVITY TO DYNAMIC AUDITORY AND VISUAL-STIMULI PREDICTS NONWORD READING-ABILITY IN BOTH DYSLEXIC AND NORMAL READERS

Background: Developmental dyslexia is a specific disorder of reading a nd spelling that affects 3-9% of school-age children and adults. Contr ary to the view that it results solely from deficits in processes spec ific to linguistic analysis, current research has shown that deficits in more basic auditory or visual skills may contribute to the reading difficulties of dyslexic individuals. These might also have a crucial role in the development of normal reading skills. Evidence for visual deficits in dyslexia is usually found only with dynamic and not static stimuli, implicating the magnocellular pathway or dorsal visual strea m as the cellular locus responsible. Studies of such a dissociation be tween the processing of dynamic and static auditory stimuli have not b een reported previously. Results: We show that dyslexic individuals ar e less sensitive both to particular rates of auditory frequency modula tion (2 Hz and 40 Hz but not 240 Hz) and to dynamic visual-motion stim uli. There were high correlations, for both dyslexic and normal reader s, between their sensitivity to the dynamic auditory and visual stimul i. Nonword reading, a measure of phonological awareness believed cruci al to reading development, was also found to be related to these senso ry measures. Conclusions: These results further implicate neuronal mec hanisms that are specialised for detecting stimulus timing and change as being dysfunctional in many dyslexic individuals. The dissociation observed in the performance of dyslexic individuals on different audit ory tasks suggests a sub-modality division similar to that already des cribed in the visual system. These dynamic tests may provide a non-lin guistic means of identifying children at risk of reading failure.