THE PHONOLOGICAL DEFICIT HYPOTHESIS
Broadly, there are two forms of reading disorder: difficulties with decoding (dyslexia) and difficulties with comprehension (Cain, 2010; Hulme & Snowling, 2009). In this view, Snowling (1991) had proposed a single deficit causal definition of dyslexia until recently, which she maintained until Snowling (2000). Snowling (2013) admitted that 'that book needed to be rewritten' because she and her colleagues had carried out family studies and a meta-analysis study that confirmed that dyslexia was an outcome based on biological-cognitive, as well as, behavioural-environmental factors.
A growing body of literature following the development of children who are ‘at risk’ of dyslexia by virtue of having one affected parent, speaks directly to this issue and underlines the view that there is a broad spectrum of reading disorders. Family studies of dyslexia are important because, by investigating differences between children from dyslexic and non-dyslexic families before they enter formal schooling, they can elucidate the developmental precursors of learning disorders. Furthermore, the comparison of affected and unaffected family members allows the identification of probabilistic risk factors that may predispose a child to the disorder (but not lead to it when additional risks are absent). Within this view, children with single deficits are less likely to succumb to failure than children with multiple risk factors (Snowling and Hulme, 2012: 606).
People are not born dyslexic, but rather inherit the 'at risk' factors and develop dyslexia in conditions of whether or not they have the opportunity of 'protective' factors in their environment, that is, in forms of teaching and explicit intervention (Snowling, 2013).
'The idea that there is more than one cognitive ‘endophenotype’ associated with dyslexia suggests that the phonological deficit view that has dominated the field for many years is inadequate. More generally, single deficit accounts of disorders have fallen from favour, and are being replaced by theories that posit multiple deficits underpinning categorical disorders' (Bishop, 2006; Pennington, 2006), (Snowling & Hulme, 2012: 600).
The consensus view for many years has been that dyslexia has its origins in a phonological deficit (Vellutino, Fletcher, Snowling & Scanlon, 2004). Snowling's (1991) model had a lot of impact on intervention models and their implementation. It has been influential as a framework for thinking about the distinction between difficulties that affect decoding and those, which affect reading comprehension in the Simple View of Reading intervention method.
Even though Snowling (2013) has changed her perspective of the causal mechanisms of 'becoming' dyslexic, aspects of her basic 'core deficit' model (Snowling, 2000) "strict version" still remain operational in her less strict version. For example, her basic phonological structures, by which she believes reading progresses (or not) are still the same, and captured in her model of 'decoding'. The only difference is in the question of whether these structural weaknesses manifest in dyslexia are 'caused' by a core deficit in a single working memory mechanism 'The Phonological Loop', or whether they are 'caused' by the environment, for example, the way reading and spelling are taught, with her answer being a resounding: they are caused by interactions between both in that they are a primary contributory factor in 'causing' dyslexia (Snowling, 2013).
MS at Dyslexia Tutor/Researcher
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