Trends in Neurosciences
OpinionDevelopmental dyslexia: the cerebellar deficit hypothesis
Section snippets
Behavioural symptoms of dyslexia
All major theories make a reasonable attempt at explaining the major behavioural symptoms – reading, writing and spelling. Consequently, crucial tests often derive from domains outside literacy. In the studies mentioned below, the dyslexic subjects are defined in terms of: significant reading delay (at least 18 months); IQ of at least 90; without attention deficit hyperactive disorder (ADHD) or serious emotional problems. Control subjects were matched with the dyslexic subjects for age and IQ,
Toward a causal explanation
The above analyses indicate a correlation between dyslexia and abnormal cerebellar function in ∼80% of the dyslexic children tested. A key question that arises is whether cerebellar impairment can provide a causal explanation of the development of the specific cognitive difficulties of dyslexic children.
Fig. 2 (adapted from Ref. 15) outlines one hypothetical ontogenetic causal chain, linking cerebellar problems, phonological difficulties and eventual reading problems. Note that the three
Summary and conclusions
In summary, we have argued the following points.
- 1.
A high percentage of diagnosed dyslexic children show behavioural evidence of abnormal cerebellar function – in skill automatisation, in time estimation, balance and the classic cerebellar signs of dystonia.
- 2.
In the dyslexic adults tested, the behavioural evidence of cerebellar abnormality was accompanied by direct evidence of abnormal cerebellar function, both for executing an ‘automatic’ sequence of button presses and for learning a new sequence
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