Cerebellar symmetry in dyslexics

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Developmental dyslexia is a relatively common neurocognitive disorder characterized by a difficulty in acquiring reading skills, despite an otherwise normal IQ. In addition to their reading problem, individuals with dyslexia often demonstrate a variety of subtle sensory and motor deficits. The condition is associated with several visual and auditory processing deficits and with anomalous cerebral asymmetry. Dyslexia has also been associated with abnormal balance, motor control and muscle tone, which, with recent evidence of biochemical and functional abnormalities in the right cerebellum of individuals with dyslexia, suggests cerebellar involvement in the disorder.The findings of a recent study support this suggestion. Rae et al. used magnetic resonance imaging to document anomalies in dyslexics’ cerebellar morphology [1xCerebellar morphology in developmental dyslexia. Rae, C et al. Neuropsychologia. 2002; 40: 1285–1292Crossref | PubMed | Scopus (115)See all References][1]. They examined the proportion of grey and white matter in the cerebellar hemispheres of adult male dyslexics and controls who were comparable on measures of non-verbal intelligence. The subjects’ phonological skills were measured and handedness was assessed both by reported hand preference and tests of hand skill. A significant asymmetry in cerebellar grey matter was found to be present in the control subjects. The volume of the left hemispheres was smaller than that of the right, a pattern consistent with the assumption of left cerebral-hemisphere dominance in the predominantly right-handed participants. This cerebellar asymmetry was absent in the dyslexic group. Those with dyslexia showed considerable symmetry (the ratio of left cerebellar grey matter to right grey matter was greater than in controls). There were no differences in white matter symmetry between the groups.The grey matter symmetry in the cerebella of dyslexics could be related to the severity of their dyslexia. Nonsense-word reading time, a measure of phonological skill that is sensitive to differences between dyslexics and normal readers, correlated significantly with the grey matter symmetry ratio in dyslexics, but not in controls. The phonological measures also correlated with right-hand speed on the hand-skill test, and control subjects were significantly faster using their right hands than the dyslexic subjects. The degree of cerebellar asymmetry might therefore be related to the degree of shift towards right-hand dominance. Taking these findings together, the authors suggest that anomalies in the neurological organization of the cerebellum might relate to phonological decoding skills, as well as to motor skills and handedness, although further work will be needed to confirm this hypothesis.