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Contrast sensitivity in dyslexia

Published online by Cambridge University Press:  02 June 2009

Karen Gross-Glenn
Affiliation:
Mailman Center for Child Development, Department of Pediatrics, University of Miami, School of Medicine, Miami
Bernt C. Skottun
Affiliation:
Skottun Research, 273 Mather Street, Piedmont
William Glenn
Affiliation:
FloridaAtlantic University, Boca Raton
Alex Kushch
Affiliation:
Mailman Center for Child Development, Department of Pediatrics, University of Miami, School of Medicine, Miami
Robert Lingua
Affiliation:
Department of Ophthalmology, University of California, Irvine
Mark Dunbar
Affiliation:
Bascom Palmer Eye Institute, 900 N.W. 17th Street, Miami
Bonnie Jallad
Affiliation:
Mailman Center for Child Development, Department of Pediatrics, University of Miami, School of Medicine, Miami
Herbert A. Lubs
Affiliation:
Mailman Center for Child Development, Department of Pediatrics, University of Miami, School of Medicine, Miami
Bonnie Levin
Affiliation:
Division of Neuropsychology, Department of Neurology, University of Miami School of Medicine, Miami
Mark Rabin
Affiliation:
Mailman Center for Child Development, Department of Pediatrics, University of Miami, School of Medicine, Miami
Lesley A. Parke
Affiliation:
445 Bellevue Avenue, Suite 302, Oakland
Ranjan Duara
Affiliation:
Mailman Center for Child Development, Department of Pediatrics, University of Miami, School of Medicine, Miami

Abstract

Contrast sensitivity was determined for dyslexic and normal readers. When testing with temporally ramped (i.e. stimuli with gradual temporal onsets and offsets) gratings of 0.6, 4.0, and 12.0 cycles/deg, we found no difference in contrast sensitivity between dyslexic readers and controls. Using 12.0 cycles/deg gratings with transient (i.e. abrupt) onsets and offsets, we found that dyslexic individuals had, compared to controls, markedly inferior contrast sensitivity at the shortest stimulus durations (i.e. 17, 34, and 102 ms). This deficit may reflect more sluggish temporal summation. There was no difference in sensitivity to 0.6 cycles/deg gratings with transient onsets and offsets. Under these conditions, the two groups showed a consistent and equal increase in sensitivity relative to the ramped baseline condition at 0.6 cycles/deg at the longer stimulus durations. This demonstrates that dyslexic readers have no deficit in their ability to detect stimulus transients, a finding which appears to be inconsistent with a transient system deficit. That detection of the low-frequency stimuli was mediated by the transient system is further indicated by the fact that these stimuli were more susceptible to forward masking than were the high-frequency stimuli. The effects of masking of both high and low spatial-frequency stimuli were about equal for dyslexic readers and controls. This is not in agreement with the transient system deficit theory, according to which one would expect there to be less masking of high spatial-frequency stimuli in the case of dyslexic readers.

Type
Research Articles
Copyright
Copyright © Cambridge University Press 1995

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