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In order to develop a causal model of RD (reading disability) that can inform intervention, it is necessary to develop a theory at the cognitive level of explanation.… Although some causes of RD have a genetic origin, and environmental factors play an important role, cognition mediates brain-behaviour relationships and at the present time, the cognitive level offers a necessary and sufficient level of explanation for the development of principled interventions. In short, we need to understand the cognitive difficulties that underpin reading problems, regardless of whether their origin is constitutional or environmental.
This chapter examines the evidence concerning the nature and role of a number of cognitive processes that have been proposed as influential in dyslexia/reading disability (i.e., phonological awareness, rapid naming, short-term/working memory, low-level sensory auditory and visual processing, scotopic sensitivity, attentional factors, and motor processing). In each case the implications of the available research evidence for clinical and educational intervention will be considered. As this chapter demonstrates, this highly complex field is rendered additionally problematic by often contrasting and inconsistent research findings that have resulted in much debate and little consensus, beyond wide-spread agreement that reading disability appears to be explained by multiple deficits.
The phonological deficit hypothesis
For most of the past four decades, the phonological deficit hypothesis (Stanovich, 1988; Stanovich & Siegel, 1994) has been the dominant cognitive explanation (Vellutino et al., 2004). Studies of cognitive functioning in children with reading problems have consistently found three processes – phonological processing, short-term/working memory, and processing speed – that appear particularly significant when comparisons are made with typically achieving readers (Johnson et al., 2010). For each of these processes there appears to be a phonological component that is particularly important for reading. According to some theorists, the phonological deficit incorporates elements that map onto these three major dimensions: phonological awareness, verbal short-term memory, and slow retrieval of phonological information stored in long-term memory, as exemplified in rapid automatic naming tasks (Wagner & Torgesen, 1987). However, this grouping is controversial, and others argue that it is not helpful to conceive of verbal short-term/working memory and naming speed as core elements of phonology (Nicolson & Fawcett, 2008).