Dyslexia is one of several specific learning difficulties and has been defined by the 1993 Education Act as a Special Educational Need which interferes with the acquisition of language (Orton Dyslexia Society, 1994). Although some maintain that dyslexia may be caused by low level impairments of the visual system (Eden et al, 1996, Lovegrove et al, 1993) There is a greater body of evidence to suggest that the aetiology of dyslexic difficulties is that it is a verbal deficit (Vellutino, 1979) and is therefore classified as a language disorders not a visual disorder (Snowling, 1996).
However, to understand what causes this language is important in understanding dyslexia. It is believed that the acquisition of language is largely done through the brains working memory (sometimes referred to as short term memory). Working memory affects many aspects of comprehension and recall. Among children, the ability to repeat back unfamiliar words affects both language comprehension and the learning of new words. Gathercole et al, (1994) found that children with dyslexia have difficulty in this task and conclude that the ability of a child to repeat back unfamiliar words is constrained by the capacity of their working memory. This affects their ability to learn new words, caused by the lack of ability to comprehend what they hear or read. Therefore, inefficient working memory has been implicated as an underlying factor in dyslexia. The discussion below will look into reasons why this conclusion has arrived - through the examination of the symptoms of dyslexia - i.e. what language difficulties dyslexic people may encounter and how this affects their ability to recall information, to read or write. In addition to looking at the symptoms of this disorder, the discussion will examine reasons that have been put forward for its underlying cause and what implications this has for the dyslexic learner.
Dyslexia can manifest itself in a variety of guises, for example, the individual may have difficulty with the identification of letter sounds and the visual represents of letters when reading, spelling and writing. Dyslexia can also affect language - for example, the speed and fluency of language may be affected - leading to an individual experiencing difficulty with word recognition, word retrieval or pronunciation. Furthermore, individuals with dyslexia may have difficulty processing information. The individual may have poor short-term and/or working auditory or visual sequential memory, which means that difficulty may be experienced when attempting to fulfill specific tasks such as holding a list of instructions in memory or remembering particular sequences. There is a body of evidence to argue that phonological processes underpin the development of a phonological decoding strategy in reading - i.e. decoding the sound of a word. Working memory is believed to play a significant role in this strategy. This enables sounds and/or phonological codes to be held in short-term store until these can be recognised as a word and its meaning accessed in long-term memory. Dyslexics, who tend to have weaknesses in phonological processing and short-term memory, tend to have difficulty in learning to read. Working memory is a resource of the brain that saves information and ideas that we can draw from - once the information is received and understood the dyslexic has no special difficulties working things out and remembering them, it just takes longer for someone with dyslexia to perform reading tasks-due to the delayed ability to retrieve the appropriate information.
Working memory is used to hold new information in the mind before it is discarded or transferred into long term memory. Memory handles information in a variety of different formats. These formats being - audio memory (includes phonology), visual memory (includes the recognition of words), movement or procedural memory (sometimes called habit memory, for example driving a car) and semantic memory (meaningfulness). An individual with dyslexia can experience a weakness in any of these pathways and this will put pressure on the others (Mortimore, 2003). For example, in the classroom situation this might mean that the dyslexic child cannot concentrate on both the mechanics (spelling, grammar, punctuation) and the content of written work. The dyslexic individuals are likely to experience difficulties in listening to the teacher and trying to understand alongside making notes. Mortimore (2003) suggests that this pattern of events displays how the dyslexic learners may have difficulties in keeping temporary hold of material whilst they perform another task - e.g. learning to ride a bike, whilst learning to balance. The lack of ability to retain previous knowledge in the short term memory capacity may mean that the dyslexic individual may need to re-focus and de-code letter-sound links. This inevitably reduces the attention available for the process of understanding what has been read or heard.
Brain imaging techniques reveal a range of functional and structural cerebral anomalies of individuals with dyslexia. Brain imaging studies have located the components of working memory in the brain - the central executive in the frontal lobes, the visual-spatial sketchpad ('inner eye') in the right hemisphere and the phonological loop ('inner ear') in the left hemisphere (Carter, 1998). This may help to explain why some dyslexics have poor grapheme-phoneme conversion skills - in that they have difficulty converting visual words into sounds. This may lead to the poorer reading of non-words, or words that are difficult to pronounce. It has been cited that the dyslexic learners may make less efficient use of phonological codes in their working memory. Pickering (2000) suggests that dyslexic individuals have problems translating visual information into phonological form; this then affects their ability to learn new words when reading. This links to research into Broca's/Wernicke's areas. In that damage to the Wernicke's area-which is responsible for the storage of memories of the sequence of sounds that constitute into words -often produces poor comprehension and speech production. Due to this left hemispheric impairment, the problems in retaining letter-sound associations might be explained by the individual reading via the right hemisphere (Broca's area which is predominantly responsible for comprehension) and not speech production.
It has been assumed and negatively stereotyped that dyslexic people are not as intellectual as non-dyslexic individuals. This may be a consequence of difficulties in school exams. For example, failure to organise learned facts effectively in exams may have a profound affect on their grade. Also, if work is written in a disjointed fashion with omissions of words and phrases - simply because the individual has lost track of what s/he is trying to express - can also have a negative affect on the outcome of the exam. However, it may be the case that non-dyslexic learners adapt a different method of revision that aids them to perform better in exams. For example, revision techniques often applied by students such as making lists, rehearsing material, or verbally repeating information into a tape recorder, would be a difficult and stressful task to a dyslexia learner who has difficulty retaining phonological information using rehearsal or repetition and/or problems with converting phonological material into written format. Furthermore due to difficulties with semantic information and memory, dyslexic individuals find attaching verbal labels to pictures ineffective as a learning technique.
Therefore, it may not be that they do not know the information required of them in an exam situation, it is more to do with their lack of expertise in retrieving and expressing their knowledge that is the obstacle. Therefore it is important in the educational setting for dyslexia to be screened for from an early age - so children with dyslexia can learn different strategies of learning information and be brought up to their normal classroom level.
In light of the evidence above, it would seem that the aetiology of dyslexia has its roots in the ability of the human brain to access its working memory. For example, referring back to some of the symptoms displayed in dyslexic individuals - for example, difficulties with lists and recalling information - it seems very plausible that aspects of working memory or short term retrieval of memory are not working as effectively as they should be. Biological evidence also leads to the conclusion that damage to the left hemisphere (the part of the brain responsible for comprehension) is common in the dyslexic sufferer. Furthermore, the evidence suggested above implies a neurological dysfunction of the human brain and the way it accesses and utilises the memory compartment. Therefore can it be assumed that there may be a genetic link to dyslexia (see Byrne et al (2002) for support)? However, the support for the genetic assumption of dyslexia is mixed, and like a lot of twin studies, confounding results may be explained by environmental stimulus as well, or more than, genetic factors. Therefore, not only is it important to recognise and diagnose dyslexia from an early age, it is also important for the individual to be encouraged in the family and educational setting to enjoy and develop upon their weak language skills. The awareness that aspects of memory are a large contributor to the learning abilities of dyslexic suffers will help families and schooling to adapt to more friendly ways of teaching - such as multi-sensory teaching methods - so that the child can access other stimulus to trigger their working memory.
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