DYSLEXIA is caused by a faulty connection between two areas of the brain
which process language, according to British cognitive scientists. The research
may finally resolve years of debate over whether dyslexia is at root a
biological, cultural or learning problem. It may also lead to better diagnosis
and education for dyslexics.
The research team, led by Uta Frith of the Medical Research Council鈥檚
Cognitive Development Unit in London, recruited five dyslexic and five
nondyslexic adult volunteers of above-average intelligence. While the volunteers
performed linguistic tests, the researchers studied brain activity using
positron emission tomography (PET), which reveals which parts of the brain are
most active by measuring blood flow.
The people with dyslexia had problems creating spoonerisms from familiar
combinations of words: converting 鈥淛ohn Lennon鈥� to 鈥淟ohn Jennon鈥�, for example.
The PET scans revealed differences between the two groups in a part of the brain
called the left perisylvian area鈥攁 chunk of cerebral cortex just above the ear.
This contains Broca鈥檚 and Wernicke鈥檚 areas, which are involved with processing
words, plus a lump of tissue called the insula.
Advertisement
Wernicke鈥檚 area is thought to be involved in the recognition of complete
written words, says Frith, while Broca鈥檚 breaks the same words down into
segments and creates a mental image of their sound. Until now the function of
the insula was unknown, but the PET scans suggest that it forms a crucial bridge
between the two areas. In the nondyslexic volunteers, the insula and both
language areas lit up together during the linguistic tests. In the dyslexics鈥�
scans, however, the insula did not light up and each language area was activated
in isolation (Brain, vol 119, p 143).
鈥淓ach of the language areas deals with a specific aspect of word processing
and in normal people the insula synchronises this work,鈥� says Frith. So when
most people see a written word they automatically 鈥渉ear鈥� it in their head at the
same time. 鈥淚n dyslexics the areas are disconnected so instead of knowing
instinctively what a written word sounds like they have to think about each word
they see and consciously translate it from one form to another,鈥� says
Frith.
This translation may be easier in some languages than in others. Languages
that rely on picture-like symbols, such as Japanese, can be easier to read
because each symbol represents a whole word (快猫短视频, Science, 20
January, p 14). But English words often need to be broken down into segments,
and can only be processed normally if the insula is fully functioning.
Nevertheless, many dyslexics get round the problem by using other pathways in
the brain, so the condition often only shows up under stress. But being able to
cover up dyslexia may be a mixed blessing because it means that as many as half
of all sufferers remain undiagnosed.
鈥淒yslexic children have to put much more mental effort than others into work
that involves using language, and this inevitably puts them under terrific
strain,鈥� says Frith. 鈥淚f their dyslexia is not recognised they may be wrongly
diagnosed, or marked down as being of average ability when they are in fact
much, much brighter.鈥� She hopes the new study will lead to better and earlier
diagnosis, and appropriate education for dyslexics.
PET scans are never likely to be used for routine diagnosis. Because they
rely on people consuming small quantities of a radioactively labelled sugar,
radiation from each scan is equivalent to 10 X-rays. But researchers are now
developing a sensitive variant of magnetic resonance imaging, which examines the
behaviour of the brain鈥檚 constituent atoms in a strong magnetic field, and will
also measure brain activity.