Trends in Neurosciences
Volume 24, Issue 9, 1 September 2001, Pages 508-511
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Developmental dyslexia: the cerebellar deficit hypothesis

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Abstract

Surprisingly, the problems faced by many dyslexic children are by no means confined to reading and spelling. There appears to be a general impairment in the ability to perform skills automatically, an ability thought to be dependent upon the cerebellum. Specific behavioural and neuroimaging tests reviewed here indicate that dyslexia is indeed associated with cerebellar impairment in about 80% of cases. We propose that disorders of cerebellar development can in fact cause the impairments in reading and writing characteristic of dyslexia, a view consistent with the recently appreciated role of the cerebellum in language-related skills. This proposal has implications for early remedial treatment.

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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