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Cerebellar Deficits Distinguish between IQ Discrepant and Non-discrepant Poor Readers: Evidence for a Discrepancy Definition in Dyslexia

Rebecca L Brookes

Department of Psychology, University of Sheffield r.l.brookes@shef.ac.uk

Abstract

Dyslexia is typically defined as poor reading and spelling in individuals with above average IQ, there has however been some debate over the value of including an IQ discrepancy in these criteria (Stanovich, 1996). Evidence for this approach comes from research indicating that IQ discrepant and non-discrepant poor readers show similar magnitudes of deficit in phonological processing and digit span. However, little research has been conducted to examine differences between the two groups on behavioural patterns outside of the phonological sphere. We report a study examining the behavioural profiles of controls, non[IQ]-discrepant poor readers and IQ discrepant poor readers on phonological skills, static and dynamic cerebellar tasks, and short-term memory. Preliminary analyses indicate that dyslexics show behavioural deficits when compared with controls on all static cerebellar, phonological and memory sub-tests; and that dyslexics and non-discrepant poor readers showed quantitatively different profiles on static cerebellar tasks. The results are discussed in terms of the cerebellar deficit hypothesis as an explanatory framework for distinguishing between dyslexics and non-dyslexics and for the use of a discrepancy definition in dyslexia.

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