Numerous studies have documented that children with reading disabilities have problems in language development. Most investigations have involved concurrent examination of language problems in children with existing reading disabilities (Vogel, 1974; Bradley and Bryant, 1983; McArthur et al., 2000), while a few have studied the early language abilities of children who later became reading disabled (Scarborough, 1990; Catts et al., 1999). The latter approach is critical to determining the direction of causality.
Children who become poor readers often have language problems, or at least a history of these problems.
Poor readers may have difficulties in vocabulary, grammar, or text-level processing (Vogel, 1974; Catts et al., 1999; McArthur et al., 2000). In at least some cases, these deficits are severe enough for children to have been identified as language impaired (Catts et al., 1999; McArthur et al., 2000).
In addition to these language deficits, children with reading disabilities have difficulties in other areas of language processing, specifically phonological processing (Bradley and Bryant, 1983; Fletcher et al., 1994; Catts et al., 1999). The most noteworthy of these deficits are problems in phonological awareness. Phonological awareness is the explicit awareness of, or sensitivity, to the sounds of speech. It is one's ability to attend to, reflect on, or manipulate phonemes. Children with reading disabilities are consistently more impaired in phonological awareness than in any other single ability (Torgesen, 1996). Poor readers often have difficulties making judgments about the sounds in words or in their ability to segment or blend phonemes. Such problems make it difficult for children to learn how the alphabet represents speech and how this knowledge can be used to decode printed words.
Children with reading disabilities have also been reported to have other deficits in phonological processing (Wagner and Torgesen, 1987). Poor readers have problems in phonological retrieval (i.e., rapid naming), phonological memory, and phonological production (reviewed in Catts and Kamhi, 1999). Although these difficulties in phonological processing often co-occur with those in phonological awareness, there are notable exceptions. For example, one current theory proposes that phonological awareness and rapid naming are somewhat independent, so that poor readers may have deficits in either area alone or in combination (Wolf and Bowers, 1999). However, it is proposed that children with deficits in both areas, or what is termed a double deficit, are at greatest risk for reading difficulties.
Although most children with reading disabilities have a history of language problems, the overlap between language and reading disabilities is not complete. In each group of poor readers who have been studied, at least some participants do not appear to have a history of language problems. When language problems are defined on the basis of difficulties in vocabulary, grammar, or text-level processing, about half of poor readers show no evidence of a language impairment (Mattis, 1978; Catts et al., 1999; McArthur et al., 2000). However, when phonological processing deficits are also included, the percentage of unaffected poor readers is about 25%-30% (Catts et al., 1999). These results are due, at least in part, to the fact that other nonlinguistic factors likely contribute to reading disabilities. Current theories include visual deficits and speed of processing problems as alternative or additional causes of reading problems (Eden et al., 1995; Nicholson and Fawcett, 1999). However, the lack of an apparent association between reading disabilities and language impairments may also be the result of discontinuities in the growth of various aspects of language and reading abilities. These discontinuities may obscure the relationship between language and reading disabilities at certain points in time and highlight the association at others (Scarborough, 2001).
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