Subtypes Based on Nature of Word Recognition Deficits (page 2)
Another body of research suggests that individual differences specific to word recognition abilities may be a useful way to classify poor readers. There are two routes for word recognition. One is the visual route in which words are recognized directly on the basis of their spelling or orthographic patterns. The other is the phonological route in which words are recognized indirectly by using sound-letter correspondence rules to decode the word. Much attention has been devoted to individual differences in children's abilities to use these word recognition routes.
A popular view in "folk psychology" and education is that children can be divided into two distinct subgroups based on whether they learn to read more easily by the visual route or phonological route (Carbo, 1987, 1992; Dunn, 1990). Carbo (1992), for example, divides children into global learners and analytic learners. Global learners or readers are argued to learn to recognize words best through a sight-word approach that makes use of the visual route. Analytic readers, on the other hand, learn to read best by a phonics method that takes advantage of the phonological route. Many in early education also refer to these groups as visual and auditory learners and believe that teachers should identify a child's learning style and teach to that style.
Despite the widespread appeal of reading/learning styles, the evidence is not very compelling that children can be divided into homogeneous subgroups on the basis of their reading strengths (or preferences), or that teaching to these strengths is an effective strategy for improving reading ability (Kavale, Forness, & Bender, 1987; Stahl, 1988; Stahl & Kuhn, 1995; Turner & Dawson, 1978). Relatively few studies have actually addressed this issue. Those studies that have offered some support for reading style subgroups and instruction (Holt & O'Tuel, 1990; Thomasson, 1990) have typically been reported outside the peer-review process. As a result, this work has not had the level of scrutiny and evaluation that is needed in order to effectively guide educational practice. This view is also contrary to most current theories of reading development. Most research suggests the importance of both the visual and the phonological routes in learning to read (Share, 1995; Share & Stanovich, 1995). Children need to have good phonological decoding skills to break the alphabetic code (i.e., self-teach) as well as good orthographic skills to develop accurate and automatic word recognition.
Dysphonetic, Dyseidetic, and Alexic Subgroups
Although evidence supporting the existence of word recognition subgroups in the population as a whole is not strong, there is converging research that indicates that such a classification system may be of value for subgrouping poor readers. There is a long history of poor readers being classified on the basis of individual differences in reading by the phonological versus the visual route (Boder, 1971, 1973; Ingram, 1964). Ingram (1964), for example, grouped poor readers into audio-phonetic dyslexics and visuo-spatial dyslexics. The audio-phonetic dyslexics were argued to have problems in sound discrimination and blending and to be poor in phonological decoding. The visuo-spatial dyslexics, on the other hand, were proposed to have difficulties in visual discrimination and spatial skills and problems reading by the sight-word route.
Elena Boder (1971, 1973) developed a classification system that recognized three subgroups of poor readers based on misreadings and/or misspellings: the dysphonetic, dyseidetic, and alexic. The dysphonetic subgroup has a primary deficit in auditory analytic skills. Children in this subgroup have great difficulty learning and using the phonological route. These children display misreadings and misspellings that are phonetically inaccurate. For example, the dysphonetic reader might pronounce block as book or spell scramble as sleber. Dyseidetic readers, on the other hand, have a deficit in the visual route. Consequently, they have particular problems with exception words (e.g., have, colonel). These words are misspelled or misread as phonetic renditions: for example, reading talc for talk or spelling laugh as laf. Finally, the alexic subgroup have a deficit in both phonetic and visual reading/spelling skills. This subgroup is the most handicapped of the three groups.
The primary evidence for the validity of these subgroups comes from a study of 107 dyslexic children (Boder, 1973). Using an in-depth analysis of reading and spelling abilities, 100 of these children were divided into one of the three subgroups. Boder reported that 67 of the dyslexic children were dysphonetic, 10 were dyseidetic, and 23 were alexic. Boder and a colleague (Boder & Jarrico, 1982) later developed a diagnostic screening test for subtyping dyslexia. Researchers, utilizing this test, have provided some evidence of behavioral and electrophysiological differences between subtypes of dyslexics (Dalby & Gibson, 1981; Flynn & Deering, 1989). Flynn and Deering (1989), for example, found that dyseidetic children demonstrated greater EEG activity in the left temporal-parietal region during reading than did dysphonetic children. They suggested that this was evidence of different processing capabilities between these subgroups. Others, however, have failed to uncover reading-related differences between these subgroups of poor readers (Godfrey, Lasky, Millag, & Knox, 1981; van den Bos, 1982). Godfrey and colleagues (1981), for example, failed to find an advantage in speech perception abilities among dyseidetic dyslexics as compared to dysphonetic dyslexics. Such a difference would be expected if dysphonetic dyslexics had a phonological processing problem.
Deep, Phonological, and Surface Dyslexia
Cognitive neuropsychologists have also considered subgroups similar to those proposed by Boder (Coltheart, Patterson, & Marshall, 1980; Marshall & Newcombe, 1973). This work, however, has used terminology and procedures borrowed from the study of acquired dyslexia. Acquired dyslexia is a reading disability in previously literate individuals following neurological damage. Three syndromes are often identified: deep, phonological, and surface dyslexia. Individuals with deep and phonological dyslexia have considerable difficulty in phonological decoding. They are identified primarily on the basis of their problems pronouncing nonwords such as zun or vope. Such words cannot be recognized by the visual route and must be sounded out using sound-letter correspondence rules. Individuals with deep dyslexia, unlike those with phonological dyslexia, also make semantic errors in reading. For example, when asked to read a word like tulip they might say "crocus" or they might read "sun" for moon. Other symptoms include visual errors (confusing words like wife and life), morphological errors (misreading prefixes or suffixes), and greater facility recognizing content words as opposed to function words (Thomson, 1984). Finally, individuals with surface dyslexia have problems with the visual route. They are identified on the basis of their misreading of exception words. Whereas the terms phonological and surface dyslexia roughly correspond to dysphonetic and dyseidetic readers, the former terms have become more popular in recent years.
Using primarily case studies, cognitive neuropsychologists have subtyped individuals with developmental reading disabilities as phonological or surface dyslexics (Coltheart, Materson, Byng, Prior, & Riddoch, 1983; Holmes, 1978; Marshall, 1984; Temple & Marshall, 1983; see Rayner & Pollatsek, 1989, for review). For example, Temple and Marshall (1983) described a case of developmental phonological dyslexia. This student, a 17 old girl, had considerable difficulty reading nonwords compared to real words. Her responses to nonwords were typically real words that were visually similar to the target words. Marshall (1984) noted that this developmental case was very similar to the case of acquired phonological dyslexia reported by Patterson (1982). Coltheart and colleagues (1983) and Holmes (1978), on the other hand, identified a number of cases of developmental surface dyslexia. Holmes reported on four boys, between 9 and 13 years of age, who had great difficulty reading exception words. They often made phonetic errors, regularizing words like bread as "breed." Coltheart and colleagues (1983) identified a 15-year-old dyslexic girl who had many problems with homophones. For example, she was noted to read "pane" correctly, but to define it as "something that hurts."
Heterogeneity without Clusters
The classification system proposed by cognitive neuropsychologists may lead to the impression that poor readers can be divided into distinct and homogeneous subgroups based on word recognition deficits. Ellis (1985), however, has argued that while there may be heterogeneity among poor readers in terms of word recognition strengths and weaknesses, poor readers do not form distinct subgroups. He proposed that word recognition abilities can be viewed according to two dimensions: one dimension corresponding to reading by the visual route and the other dimension representing reading by the phonological route. He maintained that readers' abilities are distributed continuously along each of these dimensions. Readers may show similar abilities in these dimensions or have abilities in one dimension that are significantly better than those in the other.
Operationally, these abilities can be displayed on a scatterplot in which performance on exception word reading represents one axis and scores on nonword reading constitutes the other. Ellis noted that when plotted like this, the distinct subtype view of cognitive neuuropsychologists assumes that there will be "galaxies" of dyslexics within the scatterplot. That is, phonological dyslexics would be expected to represent a cluster of poor readers who are separated from other readers by their distinct pattern of poor phonological decoding skills and good exception word reading skills. The surface dyslexics, on the other hand, would be predicted to cluster together in this two-dimensional space as a result of their poor exception word reading skills and good phonological decoding skills. Ellis, however, argued that a more valid conceptualization of heterogeneity is one without clusters or galaxies. He suggested that poor readers are more likely to be distributed continuously in this multidimensional space, such that "there will be a complete and unbroken gradation of intermediate dyslexics linking the extreme cases" (Ellis, 1985, p. 192). In proposing this model, Ellis does not deny individual differences, only the homogeneity of subgroups. In other words, he argues that children with RD do not fall into distinct categories in terms of their word recognition skills. While some children can be characterized as surface or phonological dyslexics, these children will differ by degree of impairment and not type of impairment.
Recently, Ellis and his colleagues (Ellis et al., 1996) tested this view of the heterogeneity of word recognition by examining a group of thirteen children with RD. These children, who were 9 to 11 years old, had normal or above normal IQs and a reading age eighteen or more months behind their chronological age. Three control groups, each consisting of thirteen children and matched for reading level to the dyslexic group, were also included. One group consisted of poor readers of the same age as the children with RD, but with lower IQ scores. Another group contained younger children who were reading at a level predicted for their age. The final group was an even younger group of precocious readers, children who were reading well above their age. All children read a list of non words and real words (half of which were exception words). A scatterplot of nonword reading abilities versus sight-word reading abilities showed considerable variability among the dyslexic children. However, there was no evidence of clustering among the dyslexic readers. Instead, the dyslexic children were distributed continuously throughout the scatterplot. Ellis and his colleagues also found similar heterogeneity in the three control groups.
Murphy and Pollatsek (1994) also examined the heterogeneity of word recognition abilities, but in a much larger sample of children with RD. Sixty-five children with RD, 10 to 13 years of age, were administered a variety of measures designed to test children's ability to read by the visual or phonological routes. These included timed and untimed reading of regular, exception, and nonwords; a lexical decision task; and a homophone definition task. Participants' phonological awareness and word retrieval abilities were also assessed.
Despite finding much hetergeneity between poor readers in word recognition abilities, they too failed to uncover distinct clusters of poor readers. Poor readers differed primarily in terms of the severity of deficits, and not in the kind of deficits. Most children with RD were poor at reading by both a visual and phonological route. In addition, a moderate correlation was found between nonword and exception word reading. If discrete subgroups had been present, such a correlation would have been negative, or at least absent. Nevertheless, there were some children with RD who did show a dissociation between phonological decoding and sight-word reading. These children, however, were still part of the same continuum and did not cluster together into discrete subgroups.
Murphy and Pollatsek (1994) further speculated on the reasons for the dissociation in some children with RD. They noted that children fitting the profile of phonological dyslexics performed less well on a phonological awareness task and better on a phonological retrieval task than did children who displayed a surface dyslexia profile. They also speculated that instructional factors may have contributed to individual differences. Several of the surface dyslexics had been enrolled in intensive phonics programs that taught them to read non words and real words, but few exception words. Such instruction could have led to the error pattern of a surface dyslexic.
The results of these studies strongly suggest that poor readers cannot be divided into homogeneous subgroups based on their word recognition abilities. Some poor readers do, however, display a dissociation in their ability to use the phonological as opposed to the visual route. This dissociation may be related to differences in cognitive processing or reading instruction/experience (Murphy & Pollatsek, 1994). The fact that poor readers do display a dissociation despite the absence of distinct and homogeneous clusters suggests that the classification of poor readers on the basis of word recognition abilities might have some clinical/educational validity.
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