Recent work by Nicolson and Fawcett suggests the primary source of dysfunction in dyslexia is the cerebellum. In order to examine the cerebellar deficit hypothesis of dyslexia, 20 children with dyslexia (11 had co-morbid ADHD) and 20 without dyslexia (11 had ADHD, 9 were typically developing controls) were assessed with neuropsychological testing and quantitative MRI. Results demonstrated that volumes of both hemispheres and the vermis were not significantly different between groups (ps > .10). However, children without dyslexia demonstrated greater rightward cerebellar hemisphere asymmetry compared to children with dyslexia, F(1,33) = 4.09, p < .05. In addition, children with ADHD only were more comparable to controls in cerebellar asymmetry (none had reversed asymmetry); whereas those with co-morbid dyslexia and ADHD were more comparable to those with dyslexia. The relationship between cerebellar morphology and phonological processing also was assessed. For those without dyslexia, bilateral hemisphere volume moderately correlated with phonological awareness and phonological short-term memory (ps < .05); hemisphere asymmetry moderately correlated with rapid naming errors (p.10), anterior vermis volume was moderately correlated with inattention, hyperactivity and impulsivity (ps < .05) and right hemisphere volume was moderately correlated with inattention and hyperactivity (ps < .05). Overall, our findings provide mixed support for the cerebellar deficit hypothesis of dyslexia. Although cerebellum morphology is atypical in some individuals with dyslexia, cerebellar morphology does not appear to be related to cognitive or motor dysfunction consistently. In our sample, cerebellum morphology may be related to about a third of our cases of dyslexia. Hence, dyslexia may be best accounted for by a combination of cortical and cerebellar contributions.
Kibby, Michelle Y., Fancher, Jill B., Markanen, Rochelle and Hynd, George W. "A Quantitative Magnetic Resonance Imaging Analysis of the Cerebellar Deficit Hypothesis of Dyslexia." Journal of Child Neurology 23, No. 4 (Apr 2008): 368-80. doi:10.1177/0883073807309235.