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The perceptual profile of adult dyslexics

Sygal Amitay ,, Gal Ben-Yehudah , Karen Banai , Merav Ahissar and Israel Nelken ,

(1) Center for Neural Computation (2) Dept of Neurobiology (3) Dept of Psychology, the Hebrew University of Jerusalem, Israel (4) Dept of Physiology, Hebrew University - Hadassah Medical School, Jerusalem, Israel (5) MRC Institute of Hearing Research, Nottingham, UK sygal@ihr.mrc.ac.uk

Abstract

Most dyslexics suffer some degree of perceptual difficulty. In this study we set out to test the prevalence and extent of these difficulties, by directly testing the predictions of the magnocellular hypothesis. This influential theory predicts a selective deficit for processing brief and transient sensory stimuli. We designed a psychophysical test battery that consisted of tasks that should elicit poor performance in a dyslexic population, and those that should present no such difficulty if a specific magnocellular deficit is hypothesised. About a third of the dyslexics tested on the full battery of auditory tasks displayed consistent difficulties. About 20% of the dyslexics tested on the full battery of visual tasks showed substantial deficits on tasks specifically designed to probe magnocellular processing. All individuals with deficits on tasks probing magnocellular function also showed remarkable deficits in processing stimuli that were neither brief nor transient. The extent of the perceptual processing deficits was unrelated to the degree of reading difficulty, but was significantly correlated with several cognitive test scores. Thus, not a single reading-disabled individual in our sample can be described as having a specific magnocellular deficit.

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