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Zhong-Lin Lu, Anne J. Sperling, Franklin R. Manis, Mark S. Seidenberg; Deficits in forming perceptual templates may underlie the etiology of developmental dyslexia. Journal of Vision 2005;5(8):808. doi: 10.1167/5.8.808.
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Formation of optimal phonological and orthographical templates is critical for speech perception and reading. A general deficit in forming perceptual templates distorts speech perception in infancy, retarding development of phonological categories. It can also affect letter recognition and encoding of letter patterns and sequential redundancies. The behavioral signature for non-optimal perceptual templates is reduced ability in processing information embedded in high external noise ("TV snow”), compared to normal behavior in processing clean signals without added external noise (1). In this study, we compared contrast sensitivity of dyslexic and non-dyslexic children using sine wave gratings designed to activate either magnocellular (M) or parvocellular (P) processing. The gratings were either displayed without noise, or embedded in a noise patch. Dyslexics had higher contrast thresholds than non-dyslexics when the gratings were displayed in high noise, in both the M and P versions. Dyslexics performed as well as non-dyslexics, however, when the gratings were displayed without noise, again in both M and P versions. In addition, contrast thresholds in high external noise conditions correlated with language measures, as well as word reading and orthographic measures. Dyslexics with language impairments tended to have the highest thresholds. The same pattern of results was obtained in several related studies using different tasks. Our results suggest that dyslexic children may have a general deficit in forming perceptual templates instead of having just magnocellular deficits. The inability to form optimal perceptual templates primarily impairs language development, which may in itself contribute to deficits in phonological processing.
1. Z. -L. Lu, B. Dosher. (1998) Vision Research. 38, 1183-1198.
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