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Anne Sperling, Zhong-Lin Lu, Franklin Manis, Mark Seidenberg; Deficits in external noise exclusion underlie the Etiology of Dyslexia. Journal of Vision 2007;7(9):773. doi: 10.1167/7.9.773.
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Although dyslexia is usually diagnosed by poor reading and phonological impairment, the underlying neurobiological cause remains unclear. Individuals with dyslexia frequently exhibit impairments in other, non-linguistic areas, such as visual processing. The variety of observed impairments suggests that reading problems may be only one of several outcomes of neurological deficits. To identify the source of processing inefficiencies in dyslexia, we employed the external noise method and the observer model approach that was originally developed to study mechanisms of attention (Lu & Dosher, 1998). In the first study, signal-noise discrimination was evaluated in children with and without dyslexia, using magnocellular and parvocellular visual stimuli presented either with or without high noise. We found that dyslexic children had elevated contrast thresholds when stimuli of either type were presented in high noise, but performed as well as non-dyslexic children when either type was displayed without noise. In the second study, we compared motion-direction discrimination thresholds of adults and children with good or poor reading performance, using coherent motion displays embedded in external noise. We found that both adult and child poor readers had higher thresholds in the presence of high external noise, but did not differ from their respective peers in low external noise or when the signal was clearly demarcated. We conclude that deficits in noise exclusion, not magnocellular or temporal processing, contribute to the etiology of dyslexia. Our results may provide a critical step toward understanding the neural basis for phonological and other proximal causes of reading difficulties in dyslexia.
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