Visual half-field contrast sensitivity in children with dyslexia.

We address the question whether left-hemispheric and/or right-hemispheric contrast thresholds differ between children with dyslexia and controls, and whether there are interhemispheric differences. In order to answer these questions we examined [1] thresholds for the detection of Contrast-Defined (CD) forms to left and right half-field stimulation, and [2] half-field pattern onset Evoked Potentials (EPs) as a function of stimulus contrast in 21 children with dyslexia of 8-15 years of age and in 17 age-matched healthy controls. It was found that (A) children with dyslexia were less sensitive to CD forms than controls irrespective of half field of stimulation. In children with dyslexia as well as controls the forms were more easily detected in the right visual half-field than in the left one: (B) Peak latencies of the pattern onset EPs to contrast levels above 20% were up to 10 ms longer in children with dyslexia than in controls irrespective of half field of stimulation, and (C) children with dyslexia had smaller mean amplitudes for all contrast levels used. (D) Neither children with dyslexia nor controls showed hemispheric differences in the pattern onset EPs.