To what extent do researchers agree on the causes and remediation of developmental dyslexia? How do you think any differences should be resolved?
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To what extent do researchers agree on the causes and remediation of developmental dyslexia? How do you think any differences should be resolved? Developmental dyslexia is a specific learning difficulty that primarily affects the acquisition of word reading (and spelling). During early schooling, individuals may display difficulty in acquiring adequate reading skills, reading more slowly or less accurately than expected. The following description of the subject of an early case study of dyslexia captures the challenging paradox faced by the wide variety of researchers of this developmental disorder. "He has always been a bright and intelligent boy...in no way inferior to others of his age. His great difficulty has been...his inability to learn to read...." (Morgan, 1986, cited in Brown, 2003). Why is it that these articulate, intelligent people show such a problem in what seems like one of our most routine skills? Due to this unexpectedness in reading failure and its high incidence in Western populations (between 5% and 15%), dyslexia research has instigated great passion and controversy. Early pioneers in dyslexia research, Morgan, Hinshelwood, and Orton believed that visual problems underlay an apparent 'wordblindness'. Orton (1925) introduced the term 'strephosymbolia' to specify that, although the problem was believed to be primarily visual, it was not one of blindness as such, but one of 'twisted symbols', a difficulty in distinguishing the order of letters. However, around a quarter of a century ago, there was a gradual realisation that problems of language must be, at least in part, responsible for the reading deficits (Vellutino, 1979). This general hypothesis has been refined over the years to provide what is arguably the consensus theoretical belief of most dyslexia researchers, namely that dyslexic children suffer from an early impairment in their phonological skills, and this impairment prevents them from acquiring the word decoding and blending skills that are essential for the acquisition of skilled reading. Following on from this consensus, systematic phonics instruction, as a remediation for dyslexia has become largely popular.
demonstrated that developmental dyslexics had slightly reduced contrast sensitivity at the low spatial frequencies and low luminance levels favoured by the magnocellular system, particularly during flicker. Stein and Walsh (1997), also committed proponents of the visual magnocellular hypothesis and has argued that dyslexic individuals have difficulties with binocular vision. Eden, Vanmeter, Rumsey, Maisog, Woods and Zeffiro (1996) reported a brain-imaging study in which a small sample of dyslexics showed impairments in judging the relative velocity of movement of visual stimuli. In addition they also exhibited abnormal activation in area V5/MT, part of the magnocellular system (Rayner, Foorman, Perfetti, Pesetsky & Seidenberg, 2001). It should be emphasised that these supporters of the magnocellular theory do not dispute the phonological deficit hypothesis. Rather, they challenge that phonological problems are caused by a basic deficiency in hearing sounds, and that a visual deficit might independently contribute to reading problems. Therefore, on the basis of the two alternative approaches discussed so far: the phonological deficit and magnocellular deficit, it can be seen that dyslexia researchers agree that a phonological deficit is a cause of developmental dyslexia. However differences in the exact degree of centrality of the phonological deficit in explaining the reading retardation and about the importance of apparent sensory impairments in the dyslexic population reduce the strength of the consensus and give rise to an alternative magnocellular account of dyslexia. Recently, Roderick Nicolson and his colleagues have criticised that "in spite of extensive research, these approaches have failed to account for the full range of difficulties established for dyslexic children" (Nicolson, Fawcett, Dean, 2001, p.508), specifically the more recently established impairments in balance and motor skills. Initial cognitive level arguments (Nicolson & Fawcett, 1990) emphasised an all-encompassing difficulty in skill automatisation, an inability to become completely fluent in cognitive and motor skills to the extent that they no longer need conscious control. The same researchers have now implicated abnormal function of the cerebellum, a system long known to be responsible for motor skill execution and more recently thought to play a central role in skill automatisation and skills relating to language.
route, and the visual (whole word) route respectively (Manis, Seidenberg, Doi, McBride-Chang & Petersen, 1995). It could be then that different dyslexic research, especially concerning phonological, auditory and visual impairments, is studying different types of dyslexics. However more extensive research does need to be carried out regarding the issue of subtypes and the underlying causes of dyslexia. The occurrence of sensory and/or motor disorders more often in the dyslexic than in the non-dyslexic population gives rise to the possibility that dyslexia could be viewed as a general sensorimotor syndrome, as opposed to a specific phonological deficit on the other side of an antagonistic divide. However the prevalence is limited in comparison to that of a deficit in phonological processing. Indeed a recent large scale study (Ramus, Rosen, Dakin, Day, Castellote, White, & Frith, 2003), has found every single dyslexic in their sample to display a phonological deficit, with a third of them seeming to be spared by any concurrent sensory or motor deficit. Another caveat of the proposed sensory and motor disorders is that they seem to have limited consequences on reading skill. The relationship of visual deficits to reading retardation, for example, is hotly debated, especially as visual disorders are frequently accompanied by a phonological deficit. The visual, along with the rest of the sensory and motor hypotheses outlined above are very controversial and overall very different to the largely held phonological deficit. It is therefore unlikely that they will all be able to peacefully co-exist. But until one side of the antagonistic divide decides to give in or are proved wrong it doesn't seem like their will be any long-term resolution. In the meantime based on this current knowledge, Franck Ramus (2003) proposes a viable immediate resolution, that is the characterisation of dyslexia as a specific phonological deficit, optionally accompanied by a sensorimotor syndrome. Consequently whether or not the remediaition of the dyslexic individual will involve additional or alternative interventions to the successful phonics instruction, will depend on the absence or presence of this optional feature.
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