What is dyslexia?
Description and definitions
Scientific background
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Description and definitions
1. Our own experts have adopted the following plain language description:
Dyslexia is a neurologically-based condition, which is often hereditary.
It results in problems with:
- reading
- writing
- spelling
and is usually associated with difficulties in:
- concentration
- short term memory
- organization.
Dyslexia is not the result of stupidity.
It is not caused by:
- poor schooling
- poor home background
- poor motivation for learning
- clinically manifest poor sight, hearing or muscle control - although it may occur with these conditions.
2. In a speech at UNESCO, 5 December, 2007, Dyslexia International used the definition by the World Federation of Neurology - a non-governmental organisation in association with the World Health Organisation (Category F81 in the WHO/ICD).
WHO definition
The World Health Organisation (WHO) has the following definition of dyslexia in two of its publications:
ICD-10, The International Statistical Classification of Diseases and Related Health Problems, tenth revision
ICIDH-2, The International Classification of Impairments, Activities, and Participation
"A disorder manifested by difficulty learning to read, despite conventional instruction, adequate intelligence and sociocultural opportunity. It is dependent upon fundamental cognitive disabilities which are frequently of constitutional origin."
3. Dr. Steve Chinn, leading authority on maths and dyslexia,
uses the following description, with diagnostic features in his
interview in the BBC film Language Shock – Dyslexia across cultures.
"Dyslexia means a difficulty with language - words and letters - so that
the most obvious and persistent difficulties you will see will be with
reading and writing, and very intractable difficulties with spelling,
also with memory, especially sequences such as days of the week and
months of the year: memory will be poor; personal organisation will be
poor in almost every circumstance."
4. An academic definition is recommended by Professor Emeritus Dirk Bakker
"Dyslexia is present when the automatisation of word identification
(reading) and/or spelling does not develop or does so very incompletely or with great difficulty.
[Gersons-Wolfensberger and Ruijssenaars, 1997]
Scientific background
Researchers are pursuing a number of lines of investigation.
Genetic factors
Surveys involving many hundreds of families confirm the likelihood of
dyslexia being hereditary. Studies of identical twins have shown much
closer similarity on their scores for reading and writing than
non-identical twins of the same sex. It is generally agreed that
dyslexia is more common in boys than girls.
In the same family, if one of the members is affected by dyslexia, there
is a 50 % probability that one of his or her close relatives will be
dyslexic.
Brain factors
Practically all studies using brain imaging of the dyslexic child (8 -12
years) and of adults show under-activity in the left temporal area in
two regions: the lateral temporal cortex which deals with the management
of spoken language – phonological information - and a temporal area
lower down that is part of the lower visual pathway.
These observations support the theory of a double deficit: visual and
phonological. In brief, the functional activity of the brain of people
with dyslexia is not normal: several key areas are not sufficiently
activated at both the level of visual analysis and at the level of
phonological processing.
To observe the abnormal under-activity of this part of the brain in
people with dyslexia, researchers have examined anatomical organization.
They use a technique which images in microscopic detail layers of parts
of the cortex. It shows that in the case of people with dyslexia there
appears to be some disorganization in the left temporal region. This
disorganization arises because some neurons are not in their right
place.
Furthermore, in studies quoted by Professor Dehaene, a very new
technique which can image the long-distance connections between areas of
the brain shows there is also a degree of disorganization in these
tracts.
Other factors
Nutritional factors during the pregnancy of the mother, and in the early
childhood of the infant are implicated as well as immunological
resistance in the foetus.
Problems do not always lie solely in the cerebral cortex. The brain has to control the fine tracking of the central most sensitive part of the eye when reading and one of the pathways, the ‘magnocellular’, may also be impaired. Similarly, auditory
paths may be weak. These factors would affect the speed of the processing which is instrumental in integrating the information from our senses. There is evidence that other subcortical structures like the cerebellum (‘little brain’) are
implicated.
Excessive sensitivity to certain wavelengths of light may mean that coloured filters on spectacles can help.
If you want a good overview of these factors,
see a presentation by
Professor John Stein of Oxford University, who is Chairman of Dyslexia
International’s Scientific Advisory Committee.