A study into the causes of Dyslexia
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A study into the causes of Dyslexia Any discussion of the causes of dyslexia must, as we shall see, be a complex and controversial undertaking; even the title may raise controversy: what is 'dyslexia'? Is it the same as 'specific learning difficulties'? Can it be defined as a single problem? Is there a continuum of dyslexic-type difficulties? Should we be using a label at all? The questions are many, and with very few satisfactory answers. For this reason, it is necessary to begin with a working definition of dyslexia, before discussing what may cause it. Once this has been established, it will be seen that the cited causes may be usefully grouped into several categories ( genetic, perceptual, neurological and linguistic). Each of these areas will be examined in some depth, and the evidence evaluated. It will be found that, as yet, no single cause can be definitively established, but that studies into information and language processing in the brain seem to offer the most promising possibilities for the future. Due to the complex nature of the problem, and the many different ways in which it manifests itself, it is impossible to find a definition upon which everyone may agree. Nevertheless, several definitions exist which may serve as a starting point. An early definition by Critchley is that dyslexia is "a disorder of children who, despite conventional classroom experience, fail to attain the language skills of reading, writing and spelling commensurate with their intellectual abilities." This raises two main points: what is 'conventional classroom experience', and how can we prove what level of language skills are 'commensurate with intellectual ability'? This definition was improved upon in 1978 by Critchley and Critchley: "children with specific learning difficulties are those who in the absence of sensory defect or overt organic damage, have an intractable learning problem in one or more of reading, spelling or maths, and who do not respond to normal teaching," (see also, Critchley and Critchley in Augur, 1981).
It seems that the way in which the brain is organised is relevant to this field; the human brain is divided into two cerebral hemispheres, each of which has its own highly specialised functions to perform, and which are connected by a bundle of fibres called the corpus callosum. Broadly speaking, the right hemisphere is specialised for non-verbal reasoning, practical skills, music, intuition etc., while the left hemisphere, which is usually larger in size, is specialised for language. Within the left hemisphere, there is yet greater specialisation, for example, Broca's area (expressive language), Wernicke's area (receptive language) and the angular gyrus (which associates a written symbol with a sound), (see Hornsby, 1992). There is also a very inefficient language area in the right hemisphere. Whilst this accounts for the majority of the population, a small minority show the reversed pattern, with language being based in the right hemisphere (Jorm, 1983) - this is more often the case with people who are left handed. Zangwill (1967) found that 98% of right handers experienced severe speech difficulties after damage to the left hemisphere, but speech was affected in less than 2% of those whose right hemisphere was damaged There are two possible descriptions of the brains of dyslexics: either they do not have a dominant hemisphere for language, or the language centre in the left hemisphere is smaller. Miles (1974) conducted an experiment whose results suggested that in dyslexic brains, language is shared more equally between the two hemispheres, and that this means that more messages have to be passed from one hemisphere to the other, resulting in a confusing 'traffic jam' of nerve signals in the corpus callosum connecting the two language areas. To support this, Galaburda and Kemper(1981) examined the brain of a young dyslexic man who had been killed, and found unusual arrangements of cells which suggested that the language areas were distributed more evenly.
Grogan (1995) recommends games to train verbal STM skills, such as those already used by the British Dyslexia Institute are worthwhile, as deficiencies in this area are related to reading problems. Perhaps the most frequently stressed factor for teaching is phonic training - Snowling (1995) says that if a child does not have phonemic awareness at entry to school, then they are not ready to benefit from reading instruction, i.e. teachers should ensure that their pupils have a good level of PA before trying to teach reading, and if they do not, then they should try and train this area. All of the suggestions are saying the same thing: Teaching for children with dyslexia should be multisensory, i.e. all the senses should be stimulated and trained in order to give the child the maximum opportunity to improve in reading ability. It seems apparent from the above discussion that the answer to the cause of dyslexia is still unresolved, although with modern techniques and knowledge, progress is being made all the time. I believe it is limiting to think in terms of a single cause; just as there are many variations of dyslexia, so the causes must be varied. This point is supported by Miles (1970), who suggests a combination of adverse factors, and Thomson and Watkins (1991), who state that there is a point where perception, coding and memory overlap with each other. It mat be that there are different causes for different subtypes (for example, visual, auditory and sequencing; see Watson and Willows, 1995), and that a weakness in one area can adversely affect related areas. Finally, I would like to suggest a model which begins with genetic factors, which affect brain development, which affects perception, which in turn affects language, and thereby, reading and writing. Obviously, this is a simplified pathway and the reality is far more complex, and I will be very interested to see whether future research supports this view. Approx 4,500 words.
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