From ROD NICHOLSON, ANGELA FAWCETT and PAUL DEAN
Emma Young correctly noted our discovery that most dyslexic children
show symptoms characteristic of cerebellar damage (New Scientist, Science,
6 August). She then noted that correlation does not prove causation. But
the evidence is very much stronger.
Following the discovery that dyslexic children balance poorly, we tested
large panels of dyslexic and control children on 45 tasks covering the skill
spectrum. These studies have been published or accepted for publication
in the academic literature. The strongest, most consistent deficits were
in reading and spelling (as expected), and also in phonological skill,
motor skill and in blindfold balance.
Recent evidence that the cerebellum is significantly involved in each
of these latter three skills led us to formulate the ‘dyslexic cerebellar
deficit’ hypothesis. In a stringent test, funded by the Medical Research
Council, we replicated a study showing that cerebellar lesions lead to specific
deficits in time estimation. Precisely the predicted pattern of results
was obtained, with dyslexic children showing large impairments on time estimation
but none on a control loudness-estimation task. No other theory of dyslexia
predicted this pattern of results.
Furthermore, in applied work using further subject panels, we have established
that cerebellar tests are very sensitive to dyslexia, with two simple tests
correctly classifying 90 per cent of the subjects. By adding established
tests, the discrimination is further improved. This forms the basis for
our dyslexia screening tests for 5 years and upwards (available 1995).
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In short, rather than a chance correlation, the link between cerebellar
symptoms and dyslexia was predicted from years of laboratory research. The
discovery should illuminate the causes of dyslexia and the cerebellum’s
role in skill development and, crucially, should lead to better educational
prospects for dyslexic children.
Rod Nicolson, Angela Fawcett and Paul Dean University of Sheffield
