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Controversy over the cause of dyslexia has been stirred up following
the claim by two British scientists that the cerebellum is somehow involved.
Dyslexia causes problems with reading, writing and spelling, and affects
1 person in 25 in Britain.

Rod Nicholson and Angela Fawcett of the University of Sheffield say
there is an extremely high correlation between dyslexia in children and
impairments in the cerebellum, an area of the brain usually associated with
motor control. Whether this means that defects in the cerebellum are what
causes dyslexia remains contentious.

Dyslexia is usually diagnosed through the failure of children who have
reached the age of seven or eight to learn to read, though a child who cannot
read by this age is by no means certain to have dyslexia. The researchers
hope that testing for defects in the cerebellum will detect dyslexia in
children as young as five. At this age, remedial teaching is effective at
preventing reading difficulties in later years. Nicholson is talking to
the Psychological Corporation, a British company which publishes psychometric
tests, about the possible use of the team’s test in schools from September.

The cerebellum is a large structure at the rear of the brain above the
brain stem. It is thought to be responsible mainly for coordinating unconscious
motor activity such as posture, but in recent years there have been claims
that it is involved in language.

Since April, Nicholson and Fawcett have been measuring the performance
of children with dyslexia and children with more general learning difficulties
in the standard tests that are used to measure damage to the cerebellum.
The children were asked to hold their arms out in front of them, with their
hands dangling. The test involves measuring the angle between the child’s
hand and wrist on both the left and the right side to see if there are any
differences. ‘With dyslexics the results showed consistent differences of
about 10 degree ,’ says Fawcett. With slow learners and normal children
there were virtually no differences.

Even if a correlation is accepted, many researchers reject Nicholson
and Fawcett’s claim that the cerebellum could be involved in causing dyslexia.
Eraldo Paulesu of the Medical Research Council Cyclotron Unit in London
comments: ‘There is no evidence whatsoever in the literature that cerebellar
impairments could be the real problem.’

Graham Beaumont of the Royal Home and Hospital in southwest London is
equally cautious. ‘You can’t logically make the inference from Nicholson
and Fawcett’s studies that the cerebellum is involved in dyslexia,’ he says.
‘It’s rather like saying that a grasshopper’s hearing is in its knees, cutting
off its legs and saying ‘jump’, and when it doesn’t, claiming that your
theory is correct.’

The most common explanations offered for the cause of dyslexia involve
areas in the left temporal and frontal lobes, parts of the brain known to
play a major part in language. Other theories have suggested that people
with dyslexia have hearing and visual difficulties.

‘It could be that an impaired cerebellum might affect control of subtle
eye movements and that dyslexia is secondary to those problems,’ says Mitch
Glickstein a at University College London. He notes that there is ‘a great
deal of enthusiasm’ for the idea that higher functions such as language
are affected by the cerebellum, but says ‘there is very little evidence
indeed’ that this is actually what happens.

Nicholson and Fawcett accept that the cause of the link remains uncertain.
‘We need to do more studies into the neuroanatomy,’ says Nicholson.

Al Galaburda, at Bethisrael Hospital in Boston, Massachusetts, has offered
the Sheffield team access to his bank of dyslexic brains. ‘They have provided
some good preliminary evidence implicating the cerebellum in dyslexia,’
says Galaburda.

The correlation may be important even if its cause remains a mystery.
‘It’s extremely important we diagnose dyslexia as early as possible,’ says
Arthur Blair, an educational psychologist in Nottingham. ‘If a test is developed
which nets young dyslexic children, it is very useful in terms of organising
remedial treatment.’