Reading Disability Subtypes and the Test of Memory and Learning

Archives of Clinical Neuropsychology, Vol. 14, No. 3, pp. 317–339, 1999 Copyright © 1999 National Academy of Neuropsychology Printed in the USA. All rights reserved 0887-6177/99 $–see front matter PII S0887-6177(98)00025-0 Reading Disability Subtypes and the Test of Memory and Learning N. L. Howes, E. D. Bigler, J. S. Lawson, and G. M. Burlingame Brigham Young University For Study 1, 24 readers with dysphonetic dyslexia and 21 with dyseidetic dyslexia, classified by Boder criteria, were compared to 90 control group participants (45 matched for age and 45 for reading level) on the Composite Memory Index (CMI) score from the Test of Memory and Learning (TOMAL). CMI scores were significantly lower for children with dyslexia (p , .0001). Plotting average subtest score profiles for all reader groups revealed auditory sequential memory impairments for both types of readers with dyslexia, and multiple strengths for good readers. Dysphonetic and dyseidetic dyslexia profiles were nearly identical. For Study 2, average linkage cluster analysis was performed using principal components derived from subtests of the TOMAL. Homogeneous clusters of normal readers and children with reading disabilities emerged. Results indicated that qualitatively distinct subtypes of readers with dyslexia exist. © 1999 National Academy of Neuropsychology. Published by Elsevier Science Ltd Reading disabilities, often referred to as developmental dyslexia, are characterized by cognitive processing deficits that have a selective effect on word decoding and recognition abilities. Individuals with reading disabilities experience reading failure despite normal intelligence, articulate verbal expression, and adequate listening comprehension (Aaron, Kuchta, & Grapenthin, 1988; Apthorp, 1995). Because of the specific and limited nature of the impaired processes underlying reading failure, researchers have faced significant challenges in detecting and measuring cognitive deficits associated with dyslexia. Neuroanatomical and neuroimaging studies have provided substantial evidence of structural brain abnormalities in individuals with dyslexia (Bigler, 1992; Galaburda, 1993). In addition to multiple structural and cytoarchitectonic anomalies in brain areas associated with language, irregular brain morphology has also been detected in regions associated with visual processing and executive functions, and in subcortical areas (Galaburda, 1993; Hynd et al., 1995; Riccio & Hynd, 1996). Rather than arising from a single, focal abnormality, dyslexia appears to involve a widespread neurocognitive system of integrated brain functions and regions. Parts of this research are based on the dissertation of the first author. The assistance of David G. Weight, PhD and Gawain M. Wells is gratefully acknowledged. Address correspondence to Erin D. Bigler, PhD, Department of Psychology, Brigham Young University, Provo, UT 84602. 317 318 N. L. Howes et al. The heterogeneous nature of the performance of individuals with dyslexia on reading tasks, along with neuroanatomical, neurophysiological, and brain injury evidence of the involvement of multiple bilateral brain regions in reading failure, has prompted investigators to theorize the existence of distinct dyslexia subtypes (Masutto, Bravar, & Fabbro, 1994; Riccio & Hynd, 1996; Van Strien, Bakker, Bouma, & Koops, 1990). In general, subtypes are identified based upon whether their most prominent deficits are related to auditory-phonemic processing failures (inability to sound out words, blend sounds, or spell phonetically), or visual-orthographic processing failures (inability to recognize irregularly spelled words, poor sight word vocabulary, letter and word reversals, but good phonetic skills), or failures in both processing domains. Readers with auditoryphonemic impairments are often categorized as having “dysphonetic” dyslexia, those with visual-orthographic impairments as having “dyseidetic” dyslexia, and those with both types of processing failures as having “mixed” or “global” dyslexia (Boder, 1973; Flynn, Deering, Goldstein, & Rahbar, 1992) . An important implication of these hypothesized categories of deficits is that subtypes of readers with dyslexia would be expected to exhibit differences in cerebral and cognitive functioning that correspond to the type of processing deficit associated with their reading difficulties. Neurophysiological investigations of subtypes of readers with dyslexia have provided considerable support for the existence of differential abnormalities of brain function. Electrophysiological and regional cerebral blood flow studies (Flynn et al., 1992; Hynd, Hynd, Sullivan, & Kingsbury, 1987; McPherson, Ackerman, Oglesby, & Dykman, 1996; Seri & Cerquiglini, 1993) indicate that subtype-specific “compensation syndromes” may operate characterized by overreliance on effortful and inefficient brain systems. Both subtypes have demonstrated electroencephalogram (EEG) spectrum patterns characteristic of generalized immature, inefficient brain processing (Ackerman, Dykman, Oglesby, & Newton, 1995; Flynn et al., 1992). Consistent with neurophysiological evidence, behavioral investigations employing statistical clustering techniques to categorize the performance of individuals with dyslexia on batteries of neuropsychological or psychoeducational measures have yielded fairly homogeneous clusters corresponding to the proposed dysphonetic and dyseidetic subtypes (McIntosh & Gridley, 1993; Swanson, Cochran, & Ewers, 1990; Watson & Willows, 1995). Cognitive theorists have characterized learning as a complex behavior in which all information processing activities reach a final common pathway of encoding, retrieval, and response (Gagne, 1985; Levine, 1989; Wickens, 1984). Successful word reading is thought to depend upon the fluid interaction of systems that encode, store, and retrieve grapheme-phoneme correspondences, configural information about orthographic word units (cr-, th-, -ist, -ate), and positional/relational information about letters and word units (Bowey & Hansen, 1994; Bowey & Underwood, 1996; Foorman, Francis, Fletcher, & Lynn, 1996; Thompson, Cottrell, & Fletcher-Flinn, 1996). According to theories of reading development, children with verbal-phonological processing deficits do not encode correct grapheme-phoneme correspondences or accurately blend a series of phonemes to form a word, thus depending upon visual recognition of the entire word configuration in order to read. Alternatively, those with visual-orthographic deficits do not encode accurate representations of positions and relationships of letters or recognize letter combinations as orthographic units. Consequently, they may (a) attempt to decode all words by serial grapheme-to-phoneme conversion, exceeding working memory capacity on longer words (b) fail to recognize common, irregularly pronounced orthographic units (ch-, -ight), or (c) demonstrate errors of letter position or orientati (...truncated)