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Despite the popular acceptance of dyslexia as a type of learning disability, particularly as a persistent and significant form of reading difficulty, the debate surrounding its existence, definition, and diagnosis persists (Doyle 2003). For many years now, school systems around the world have developed and implemented strategies in order to address dyslexia because it affects a considerable percentage in the population. In the United Kingdom, one out of 10 people are considered dyslexic, 375,000 children (Hayes 2005) and in other studies, 5% of the children's population (Ramus 2003). Dyslexia is widely thought to be a deficit that can be overcome with proper learning interventions; some very prominent personalities have been said to succeed over dyslexia: Hans Christian Andersen and Nelson Rockefeller, for instance (Hoien and Lundberg 2003).
Scientific and academic discourse on dyslexia, which is derived from the Greek words dys meaning difficult and lexia meaning reading, is plagued by a definitional crisis (Snowling 2000; Ingram, Pianu and Welsh 2007). Until now, there is still no consensus on the definition of dyslexia. Moreover, issues on categorisation also face the dyslexia community. This is perhaps because poor reading is influenced by a myriad of factors: socio-economic disadvantage, poor diet, poor living conditions, and others. This paper centralises on the controversy on whether or not a) it is possible to differentiate 'dyslexics' from 'garden-variety poor readers' and b) it is useful to make such a differentiation. The issue of differentiation has significant implications and until now, psychologists and academic writers are still debating over it. Treating dyslexia as a distinct entity seems logically sound. In every class, there are students who do very poorly in reading but exhibit higher ability in other competencies. There are also those who do poorly in reading and in generally all other competencies. In terms of diagnosis, some claim that differentiating dyslexics from poor readers will enable educators to development the most suitable interventions. Nonetheless, the definitional crisis over dyslexia makes it a dilemma to make that diagnosis. Labelling someone as 'dyslexic' actually makes the assumption that existing literature and empirical data have generate more precise insight into what dyslexia is. The fact of the matter is experts have only agreed to disagree on the criteria by which dyslexia is diagnosed, including the types of intervention necessary to help children overcome their reading difficulties (Rich and Brooks 2004). The peril of differentiating 'dyslexics' and 'poor readers' without any clear and conclusive parameters for diagnosis is that in the end, students with temporary reading difficulties may be branded dyslexics needlessly and students who are in genuine danger may not be getting the appropriate learning interventions they need.
Due to the unstable and inconsistent definitions of dyslexia, some academic writers have questioned whether the disorder really exists. Some people view dyslexia as a 'social and emotional construct' (Elliott 2005, p. 485) and politicians claim the impairment as a 'fictional malady' (Hayes 2005, p. 1) to suggest that dyslexia is an artificial disorder concocted by the school system to promote their interests or to hide weaknesses in language instruction. Some have even claimed that dyslexia has a class bias: that it is the middle-class way of covering up intellectual weakness. Despite these allegations, the dyslexia movement worldwide has gained wide acceptance and scientific endeavours, although inconclusive have provided greater insight into how this impairment can be overcome by children.
This paper explored recent literature on dyslexia, its definitions, causes, and critically discussed the issue of differentiation between dyslexics and garden-variety poor readers, how such a difference could be diagnosed, and its implications. The paper concludes that while there are proposed methods of evaluation, there remains no conclusive means of differentiating 'dyslexics' and 'garden-variety poor readers.'
1 Dyslexia: definitions and causes
1.1 The reading process
In order to critically examine dyslexia, models of the reading process need to be reviewed. There have been two predominant theoretical models to answer the questions: 'how do children read?' and 'how do they learn how to read?' (Harrison 2000, p. 35).
Kenneth Goodman (1970) explained that reading was a selective process which requires children to use minimal language cues but maximum contextual information. In what he calls the 'psycholinguistic guessing game,' Goodman argued that the objective of reading is not word or letter recognition but to make sense of it - the construction of meaning.
Current evidence however shows that in order to become fluent readers, visual processing of text is both fairly complete and very fast, and that, most of the time, engaging in hypothesis-testing behavious seems to play a minimal role in word recognition. The currently accepted view of reading among psychologists is that an interactive model of reading best fits the available data, with the reading process largely following a bottom-up model, but with input from top-down processes being used when necessary (Rayner and Pollatsek, 1989). The current view, is that for fluent readers in normal reading, rapid, automatic, context-free word recognition is what occurs most of the time, with fixation duration largely related to the relative word frequency of different words.
Stanovich (1980) attempted to incorporate what is known about skilled and unskilled reading into the interactive-compensatory model. A key concept is that "a process at any level can compensate for deficiencies at any other level" (p. 36). Just as the bottom-up models have problems, so do the top-down models. One of the problems for the top-down model is that for many texts, the reader has litlle knowledge of the topic and cannot generate predictions. While the top-down models may explain beginning reading, with slow rates of word recognition, they do not accurately describe skilled reading behavior. Stanovich (1980) argued that good readers recognised words rapidly because their recognition was automatic. It was poor readers, by contrast, who needed to make the greatest use of context in order to facilitate word recognition, and they did so at the expense of needing to devote extra time to this part of the processes. The obviously implication of this model is that accurate, rapid word recognition is really important in fluent reading.
1.2 Defining Dyslexia
Dyslexia is difficult to define. Although there have been several definitions introduced since its inception, experts have relied on identifying exclusionary and inclusionary characteristics to define who has dyslexia (Rice and Brooks 2004). Fletcher and Lyon (2008) offered three primary reasons why dyslexia is hard to define. First, dyslexia is 'unobservable construct' meaning that attempts to measure it are imperfect and people suffering from the disorder cannot objectively report it. Second, dyslexia is 'dimensional' meaning that there are varying degrees to which individuals may experience difficulty, from minor, severe, and even in-between. Third, what characteristics to include and what to exclude have been a site for great disagreement among practitioners and psychologists. Nevertheless, common points in several definitions can be seen are elaborated in this paper.
A specific learning disability
Defining dyslexia as one disorder among a whole umbrella of learning disabilities (LD) has been criticised (Shaywitz, Morris and Shaywitz 2003; Snowling 2000). Studies have shown that among all learning disabilities, reading disability is the most prevalent, affecting 80% of people diagnosed with learning disabilities. In the past, LD encompassed a broad selection of difficulties experienced by children and adults in several competencies: reading, listening, writing, mathematics, and speaking. Dyslexia is now defined as a 'specific learning disability' (Shaywitz, Morris, and Shaywitz 2003, p. 2).
Some definitions on dyslexia describe the different areas impaired. The British Association of Dyslexia (2010) defines dyslexia as 'a specific learning difficulty which mainly affects the development of literacy and language related skills' wherein difficulties may experienced in rapid naming, working memory, phonological processing, processing speed, and other skills. The definition also elaborates that dyslexia may occur among people who are of normal or average intelligence. Shaywitz, Morris, and Shaywitz (2003) explained that among the indicators of dyslexia may be poor spelling, inaccurate word recognition, and poor decoding abilities. The National Centre for Learning Disabilities treats dyslexia as a 'language processing disorder' which may impair reading, writing, spelling, and speaking.
Dyslexia Scotland (2010) defined dyslexia as a specific leaning difficulty affecting almost 10% of the population. Dyslexics are said to experience difficulty in reading, spelling, writing, and mathematics. It explained that dyslexia occurs as a consequence of poor short-term memory; hence, dyslexics do poorly in learning abilities because they are unable to maintain the sequencing of numbers, words, or simple instructions.
The Miles Dyslexia Centre (2010) defined dyslexia 'as a combination of abilities and difficulties which affect the learning process in one or more of reading, spelling, writing and sometimes numeracy/language' (n.p.). Dyslexics have weaknesses in the following areas: memory, processing speed, short-term memory, visual perception, auditory perception, spoken language, and motor skills. Nevertheless, their definition of dyslexia also reiterates that in place of such weaknesses, dyslexics are gifted with creative or oral skills and that the impairment occurs regardless of socioeconomic or linguistic background.
In addition, the National Strategy for Improving Adult Literacy and Numeracy Skills point out that dyslexia is the result of poor phonological processing and poor short-term memory. Hence, dyslexics often struggle with following instructions, copying notes from the board, and difficulty in reading (Department for Education and Skills 2004).
The Dyslexia Institute (2002) also outlined some of the shortcomings of dyslexic individuals, such as: 1) difficulty in processing real-time information due to poor short-term memory or working memory, 2) problems in the phonological process which make them struggle in associating letters to sounds, and 3) problems in visual processing.
A neurobiological dysfunction
Many definitions focus on the neurobiological origins of dyslexia (Shaywitz, Morris, and Shaywitz 2003). Dyslexia is seen not as a consequence of impaired vision or poor intelligence, but as a neurological disorder which has caused brains of individuals to interpret and process information in a different manner (National Centre for Learning Disabilities 2007). The National Institute of Health (2008) also viewed dyslexia as 'a problem with the brain's ability to recognise and process symbols' which may affect children's capacity to separate sounds in spoken words or in rhyming - skills which are fundamental in the reading process.
The National Institute of Neurological Disorders and Stroke (NINDS 2010) indicates that dyslexics experience difficulties with writing, phonological processing and rapid visual-verbal responding. It defined dyslexia as 'a brain-based type of learning disability that specifically impairs a person's ability to read. These individuals typically read at levels significantly lower than expected despite having normal intelligence' (NINDS 2010, n.p.). Although the disorder varies from person to person, common characteristics among people with dyslexia are difficulty with spelling, phonological processing (the manipulation of sounds), and/or rapid visual-verbal responding. In adults, dyslexia usually occurs after a brain injury or in the context of dementia. It can also be inherited in some families, and recent studies have identified a number of genes that may predispose an individual to developing dyslexia'.
A phonological deficit
Secondary to the biological basis of dyslexia is that it is the consequence of a phonological deficit. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge (Shaywitz, Morris, and Shaywitz 2003, p. 2).
The International Dyslexia Association (IDA 2008) describes dyslexia a language-based type of learning disorder. Dyslexics experience difficulty in developing fluent single-word decoding skills. Furthermore, Fletcher et al. (2002) supported the notion that dyslexia indicates a problem with phonological processing. Accordingly, children who suffer from dyslexia are identified when they appear a deficit in the skills of words recognition which is associated with rapid naming and phonological memory. However, dyslexia is more than just difficulties in reading, where most dyslexic children dyslexia have problems in other areas such as motor skills, attention and some aspects of the language. Moreover, they agree that dyslexic people have difficulties with reading speed and writing due to word recognition difficulties.
In addition, Snowling (2001) considered the importance of phonological processing as one of the etiology of dyslexia. Therefore, this definition focuses on the issues with word-decoding more than reading comprehension, which are controversial and critical point to distinguish between children who have specific difficulties in reading and those who have more problems and weaknesses in language. Finally, this definition is clear by clarifying that dyslexia includes some difficulties in spelling and other writing skills. Snowling (2001) attributes dyslexia to weaknesses in language that affect the development of reading and spelling with dyslexic people. The phenomenon of weakness in language do not affect the reading directly, but also affect the development of the spoken language, which is the basic foundation for learning to read. Snowling (2009) then defined dyslexia as:
'A specific form of language impairment that affects the way in which the brain encodes the phonological features of spoken words. The core deficit is in phonological processing and stems from poorly specified phonological representations. Dyslexia specifically affects the development of reading and spelling skills but its effects can be modified through development leading to a variety of behavioural manifestations.' (pp. 213-214)
This definition stresses the importance of focusing on the ability to read text not only accurately, but also rapidly and with the importance of proper expression. The acute shortage in the development of fluent reading is characteristic of dyslexia, which continues into adulthood, even if accuracy evolved. Therefore, the importance of reading fluently must be encouraged dyslexic children who read accurately but not fluently will pass unnoticed.
Fletcher et.al. (2002) agrees with this definition that most students with dyslexia frequently encounter problems in reading comprehension because of the difficulties in the accuracy and fluency of word recognition. They may also suffer from these problems even if the skills of the accuracy and fluency were improved because of weaknesses in vocabulary as well as in language abilities.
One of the most popular and widely-used definition dates back to 1928 and developed by The World Federation of Neurology: '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 constitutional in origin' (as cited in Gustafson and Samuelsson 1999, p. 127).
This definition is the basis of what is known as the discrepancy-based view of dyslexia (Fletcher et al. 2007). This definition assumes dyslexia as an 'unexpected' difficulty in reading. By 'unexpected,' dyslexia is taken to occur as a reading disorder in a child or an adult who despite having all the various factors crucial to developing fluent reading (motivation, intelligence, good instruction) still struggles with reading (Shaywitz 1998). More challenging has been the question of how to operationalise the unexpected nature of dyslexia. Thus, using differing methods and criteria, definitions have attempted to capture the "unexpected" nature of dyslexia by requiring a discrepancy of a certain degree between a child's measured IQ and his reading achievement. For example, schools have typically relied on criteria based on an absolute discrepancy, most commonly one or one-and-one-half standard deviations between standard scores on IQ and reading tests; others, including many researchers, prefer regression-based methods adjusting for the correlation of IQ and reading achievement.
The discrepancy-based definition of dyslexia provides the basis for the categorisation of 'dyslexics' with 'garden-variety poor readers'. Genuine dyslexics are those who despite having average intelligence struggle with reading while 'poor readers' are those who struggle with reading because of intellectual weakness.
The phonological deficit is considered as the cognitive basis to dyslexia (Snowling 2001; Vellutino et al. 2006). Nonetheless, this deficit is also viewed as secondary only to other deficits such auditory temporal processing deficits (Tallal 1980) or low-level visual deficit induced by inhibitions in the magnocellular system (Livingstone, Rosen, Drislane, and Galaburda 1991).
1.3 Visual Processing in Dyslexia
The most dominant etiological explanation for dyslexia had been that it was phonological deficit and not the result of problems in visual functioning or processing. However, in the 1980s, several researchers undertook systematic studies and found that many dyslexic readers experienced visual deficits. For instance, Getman (1985, as cited in Ramus 2003) attributed problems in visual tracking to oculomotor deficiencies as well as a supposed deficit in the transient visual system. This theory has been discredited by several controlled experiments but the hypothesised effect of the transient visual system gained empirical interest.
While proponents of theories attributing dyslexia to visual deficits do not exclude other explanations, the relationship between dyslexia and problems in visual processing hasspurred academic interest in the subject. The main theory behind such systematic reports indicates that dyslexia is caused by a deficit in the transient system or the magnocellular system in the visual pathway (Wright and Groner 1993).
The magnocellular-deficit theory explains that two parallel systems lead the visual pathway from the eyes to the visual cortex: the magno- (large)and parvo- (small) cellular systems. The larger system consisting of large cells possesses high conduction velocity as well as sensitivity to rapid movement and stimulus changes. The smaller system is sensitive to fine spatial details and color. Dyslexia occurs among people which have magnocellular systems that have reduced sensitivity (Lovegrove, Garzia, & Nicholson, 1990). The magnocellular system indirectly influences the parvocellular system by pressuring the latter in the course of the reading process. As one reads, there occurs a series of fixations followed by saccades or short rapid eye moements. The magnocellular deficit theory proposes that the larger system is stimulated by the saccades and the parvocellular system by the fixations. Dyslexia occurs when there is inhibition in the magnocellular system, consequently suppressing activity in the parvocellular system.
While a number of psychologists and writers have accepted the magnocellular deficit theory, there have also been counterproductive results. For instance, in a review of contrast sensitivity studies which support the magnocellular deficit theory, Skutton (2001) found the studies which are consistent with the theory are outnumbered by studies that disprove it. An explanation behind the conflicting results is the lack of definite parameters to diagnose dyslexia. One author has alleged that the confirmability of the magnocellular deficit theory is confined to only one subtype of dyslexics (Borsting et al., 1996).
1.4 Auditory Processing in Dyslexia
Another alternative explanation to dyslexia does not discount the deficit as phonological in nature but sees it more as one that stems from an auditory deficit in 'temporal processing.' This theory is based on Tallal (1980) who proposed that children who are poor readers have difficulty processing brief or rapidly changing acoustic events whether used in speech or not. Hence, children find it difficult to judge the temporal order of nonspeech tones presented but stop-consonant-vowel syllables as well. This deficit interferes with their overall speech perception which is crucial to normal development of language, in turn, a fundamental requirement in reading.
Tallal and colleagues arrived at this hypothesis after studying children with language impairments. When presented with high and low tones at long (400ms) and short (50 ms) insterstimulus intervals (ISIs), the children were found unable to make temporal order judgments (TOJ). Tallal (1980) found that compared to normal readers, dyslexics performed poorly when presented long ISIs. From this observation, Tallal made an inference that dyslexics have a 'non-linguistic deficit in temporal resolution of rapidly changing auditory stimuli' which impairs speech perception. Tallal also inferred that this deficit underlies the phonological problem.
Studies which replicated Tallal's work had mixed findings. Reed (1989) found that when using stop consonants and tones, dyslexics performed poorly compared to normals in making TOJs but performed equally with normals when presented with steady state vowels. Other studies lent support and contradiction to Tallal's hypothesis. For instance, Mody et al. (1997, as cited in Ramus 2003) that dyslexics suffered from speech discrimination deficits rather than TOJs.
In sum, evidence suggesting a direct link of temporal processing deficit to dyslexia has not been established. The auditory disorders that are observed in individuals with dyslexia are not particularly 'rapid' or 'temporal' in nature. These disorders are restricted to a subset of the dyslexic population, and have little influence on the development of phonology and reading. It therefore seems that the phonological deficit that is characteristic of dyslexia can arise in the absence of any auditory disorder, with the most severe auditory impairments nevertheless acting as aggravating factors (Ramus 2003).