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There is a PowerPoint file available to accompany this presentation. The BDA Conference 2001 CD-ROM contains 61 PowerPoint files. For details of how to obtain the CD-ROM, please contact the BDA.
Thursday stream K Session 17.45 - 18.30 Length 25 minutes
John Stein, Joel Talcott and Caroline Witton
University of Oxford john.stein@physiol.ox.ac.uk
Abstract
Dyslexics often have problems with separating the sounds in words, so that they have difficulty accessing their phonemic structure to match with their spellings. Development of these phonological skills seems to depend on the ability to track changes in sound frequency and amplitude, since these transients distinguish the different phonemes. Individuals' sensitivity to these transients can be measured psychophysically by recording their thresholds for perceiving sound frequency (FM) and amplitude modulations (AM). Hence their FM and AM sensitivity correlates highly with their phonological skill measured by nonword reading ability. Also many dyslexics have difficulty determining the order of letters in a word, because the letters often seem to blur and move around when they are trying to read. Anatomical, electrophysiological, psychophysical and brain imaging studies all suggest that these visual confusions may result from impaired development of the magnocellular component of the visual system which is specialized for processing fast temporal information. The m-stream culminates in the posterior parietal cortex and cerebellum, which play important roles in guiding visual attention and eye and limb movements. Thus temporal processing in auditory, visual and motor systems all seem to be impaired in dyslexics, perhaps as a result of genetic dysregulation of the development of magnocells throughout the brain.
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