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Jason Yeatman, Alex White, Douglas Strodtman, Patrick Donnelly, Sung Jun Joo; Visual deficits and individual differences in developmental dyslexia. Journal of Vision 2017;17(10):643. doi: 10.1167/17.10.643.
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What is the cause of dyslexia? This question has been the topic of intense debate with competing theories arguing that dyslexia is the result of impaired magnocellular visual sensitivity, crowding, phonological awareness, or various other perceptual and language functions. Here we demonstrate that this is a fundamentally flawed question: the complex and heterogenous phenotype that has been labeled dyslexia can not be reduced to a single core deficit. 59 subjects (25 with dyslexia) participated in multiple sessions including reading and language assessments as well as psychophysical experiments measuring thresholds in tasks targeting the main classes of impairments that have been proposed to cause reading difficulties: (a) motion discrimination using random dot motion stimuli; (b) critical spacing between a target and flankers to correctly identify the target; (c) covert spatial attention using a cued visual search paradigm. We found that multiple psychophysical measures were correlated with reading skills, but no one measure accurately discriminated the dyslexic subjects from the control subjects. For example, some, but not all, of the dyslexics had abnormal motion discrimination thresholds. A subset of dyslexics had heightened visual crowding (elevated critical spacing thresholds), and these dyslexics read significantly faster when text was rendered with increased letter and line spacing. We did not observe a relationship between reading skills and performance on the cued visual search task. Moreover, a subset of the dyslexic subjects had normal thresholds on all psychophysical measures but still performed poorly across reading measures. Analyzing all the measures together, we found that some people with dyslexia cluster into distinct subgroups with specific perceptual impairments. In conclusion, by measuring an array of perceptual, language and reading skills, we demonstrated that there are multiple dimensions to reading difficulties in dyslexia; determining the optimal treatment for an individual will depend on the nature of their specific impairments.
Meeting abstract presented at VSS 2017
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