Stefan Samuelsson

Individual Prediction of Dyslexia by Single Versus Multiple Deficit Models

The overall goals of this study were to test single versus multiple cognitive deficit models of dyslexia (reading disability) at the level of individual cases and to determine the clinical utility of these models for prediction and diagnosis of dyslexia. To accomplish these goals, we tested five cognitive models of dyslexia--two single-deficit models, two multiple-deficit models, and one hybrid model--in two large population-based samples, one cross-sectional (Colorado Learning Disability Research Center) and one longitudinal (International longitudinal Twin Study). The cognitive deficits included in these cognitive models were in phonological awareness, language skill, and processing speed and/or naming speed. To determine whether an individual case fit one of these models, we used two methods: 1) the presence or absence of the predicted cognitive deficits, and 2) whether the individuals level of reading skill best fit the regression equation with the relevant cognitive predictors (i.e., whether their reading skill was proportional to those cognitive predictors.) We found that roughly equal proportions of cases met both tests of model fit for the multiple deficit models (30-36%) and single deficit models (24-28%); hence, the hybrid model provided the best overall fit to the data. The remaining roughly 40% of cases in each sample lacked the deficit or deficits that corresponded with their best-fitting regression model. We discuss the clinical implications of these results for both diagnosis of school-age children and preschool prediction of children at risk for dyslexia.

Longitudinal Twin Study of Early Literacy Development: Preschool and Kindergarten Phases.

We conducted behavior–genetic analyses of kindergarten reading, spelling, phonological awareness, rapid naming, and spoken sentence processing in 172 pairs of monozygotic and 153 pairs of same-sex dizygotic twin kindergarten children sampled in the United States and Australia. We also modeled progress from preschool to kindergarten in literacy-related variables, with larger numbers of twins contributing to the preschool phase. Reading, phonological awareness, and rapid naming at kindergarten showed substantial effects of genes and modest effects of shared environment; spelling was influenced by genes and environment equally; and sentence processing was affected primarily by shared environment. Longitudinal analyses indicated that the same genes affect phonological awareness in preschool and kindergarten but that a new genetic factor comes into play in rapid naming as letters and digits are introduced in kindergarten. At preschool, print knowledge and phonological awareness share one source of genetic influence, which in turn affects reading and spelling in kindergarten. Phonological awareness is subject to a second genetic factor, but only the one it shares with print also influences kindergarten reading and spelling. In contrast to the genetic effects, a single source of shared environment affects preschool print knowledge and phonological awareness and kindergarten reading. The results are discussed in the context of theoretical and practical issues in literacy development.

Predicting Reading and Spelling Difficulties in Transparent and Opaque Orthographies: A Comparison between Scandinavian and US/Australian Children.

In this study, predictors of reading and spelling difficulties among children learning more transparent (Norwegian/Swedish) and less transparent (English) orthographies were examined longitudinally from preschool through Grade 2 using parallel versions of tests. A series of logistic regression analysis indicated three main findings. First, phonological awareness as a predictor of reading difficulties in the Scandinavian sample was time-limited to Grade 1, but remained as a significant predictor in the English-speaking sample. Second, phonological awareness predicted spelling difficulties similarly across orthographies. Third, preschool and kindergarten RAN was a significant predictor of reading and spelling difficulties at both Grades 1 and 2 across orthographies. The authors conclude that phonological awareness diminishes as a predictor of reading difficulties in transparent orthographies after the first years of schooling, that RAN is a better long-term predictor of reading difficulties, and that phonological awareness is associated with spelling difficulties similarly in transparent and opaque orthographies.

ADHD and Reading Disability in Male Adults Is There a Connection

The present study examined the comorbidity between attention-deficit/hyperactivity disorder (ADHD) and reading disability (RD) in male adults. Participants were 120 men, of whom 24 were classified as having ADHD. The basis for the diagnosis was two self-report scales validated by interviews and background data. An extensive battery was used to assess phonological abilities and various aspects of reading skills. No differences were obtained between adults with and without ADHD on measures of either phonological processing skills or word decoding, indicating a low comorbidity with RD. This finding was valid even when different criteria were used to diagnose RD. However, the two groups differed in reading comprehension, with individuals with ADHD performing poorly in tests of reading comprehension. These results are compatible with the view that reading comprehension involves many of the higher cognitive control functions assumed to be impaired in ADHD. However, these attentional control functions are not critical to word recognition, which is determined by a more encapsulated phonological processing module. The pattern of associations between ADHD and RD observed in the present study is explained with reference to the differential attentional demands of the two aspects of reading.

Genetic and Environmental Influences on Vocabulary and Reading Development.

Genetic and environmental relations between vocabulary and reading skills were explored longitudinally from preschool through Grades 2 and 4. At preschool there were strong shared-environment and weak genetic influences on both vocabulary and print knowledge but substantial differences in their source. Separation of etiology for vocabulary and reading continued for word recognition and decoding through Grade 4, but genetic and environmental correlations between vocabulary and reading comprehension approached unity by Grade 4, when vocabulary and word recognition accounted for all of the genetic and shared environment influences on reading comprehension.

Why Do Children Differ in Their Development of Reading and Related Skills

Modern behavior-genetic studies of twins in the United States, Australia, Scandinavia, and the United Kingdom show that genes account for most of the variance in childrens reading ability by the end of the 1st year of formal reading instruction. Strong genetic influence continues across the grades, though the relevant genes vary for reading words and comprehending text, and some of the genetic influence comes through a gene–environment correlation. Strong genetic influences do not diminish the importance of the environment for reading development in the population and for helping struggling readers, but they question setting the same minimal performance criterion for all children.

Genetic and Environmental Influences on Inattention, Hyperactivity-Impulsivity, and Reading: Kindergarten to Grade 2.

Twin children from Australia, Scandinavia, and the United States were assessed for inattention, hyperactivity-impulsivity, and reading across the first 3 school years. Univariate behavior-genetic analyses indicated substantial heritability for all three variables in all years. Longitudinal analyses showed one genetic source operating across the time span and a second entering in the second school year for each variable, though possibly not reliable for inattention. Other analyses confirmed previous findings of pleiotropy (shared genes) between inattention and reading and showed that this genetic overlap is in place from kindergarten onwards and is restricted to one of the genetic sources that affect reading and inattention. The results extend previous conclusions about the developmental trajectories of inattention, hyperactivity-impulsivity, and reading and their relationships. Limitations of this study are discussed, as are educational implications.

Very-low-birth-weight children at school age: academic achievement, behavior and self-esteem and relation to risk factors

Objective: To investigate school performance, behavior and self-esteem of children with very low birth weight (VLBW). Methods: All children with birth weight below 1501 g (VLBW) and normal birth weight controls, born in the south-east region of Sweden during a 15-month period in 1987-88, were enrolled in a prospective follow-up study. At the age of 9 years, 81% and 82%, respectively, were re-examined regarding growth, neurofunctional classification, academic achievement tests, need for special education and behavioral problems. At 12 years, 89% and 76%, respectively, were re-examined regarding growth, neurofunctional classification, visual acuity and self-esteem. Results: VLBW children were shorter and lighter, and differed from the controls with regard to neurological functional classification. They produced poorer results in most academic achievement tests. When the comparison was restricted to children with normal intelligence, almost all the differences in other academic achievements disappeared. VLBW children had more reading difficulties but were less often than expected defined as dyslexics compared to control children. We did not find any major disparity in visual acuity and self-esteem between the groups. Low Apgar scores, intracranial hemorrhage and the need for mechanical ventilation neonatally were associated with poorer results in most outcome measures. Neurofunctional assessments in early childhood were associated with most outcome measures. The mothers education was related to delayed reading skills and need for special education. Conclusions: Although VLBW children performed less well in most academic achievement tests and on some behavioral subscales, those who had a normal intellectual capacity did not differ in any important aspects from the controls.

A cross-lagged model of the development of ADHD inattention symptoms and rapid naming speed

Although previous research has identified contemporaneous associations between cognitive deficits and symptom phenotypes in Attention Deficit/Hyperactivity Disorder, no studies have as yet attempted to identify direction of effect. The present study used cross-lagged path modeling to examine competing hypotheses about longitudinal associations between rapid naming speed and symptoms of inattention in children. 1,506 school-age twins from Australia and the U.S. were tested for inattention, hyperactivity/impulsivity, and rapid naming speed at three and four time points, respectively. Symptom severity of inattention from Kindergarten to fourth grade is consistently predicted by previous rapid naming, over and above auto-regressive and correlational associations in the model. Likewise, inattention symptoms have a small but significant predictive effect on subsequent rapid naming. The findings support a reciprocal relationship between naming speed and ADHD inattentive symptoms.

Reading and Writing Difficulties Among Prison Inmates: A Matter of Experiential Factors Rather Than Dyslexic Problems

One common feature in research addressing the prevalence of dyslexic problems among prison inmates has been to ignore the impact of sociocultural and educational factors that might influence reading and writing skills among adults. As a consequence, prison inmates or juvenile delinquents have been consistently compared with norms obtained for either an adult population or same-age comparison groups with more favorable opportunities to develop reading and writing skills. This study was primarily designed to minimize these shortcomings. Data from a sample of 82 prison inmates derived from three different prisons were collected. They were compared to a group of adults equated on educational level, reading habits, and socioeconomic status. A reading-level matched group of students between the ages of 13 and 15 years was also included for comparisons. The results showed that prison inmates and adult controls performed almost identically on all tests used to measure reading and writing skills and that the prevalence of dyslexic problems among the inmates varied between 6.1% and 14.6%, depending on the definition employed. It was also observed that prison inmates performed better than or at the same level as the reading-level matched group on the same measures, including phonological-processing tasks. Less than 10% of the inmates exhibited phonological deficits assumed to be the core deficits in dyslexia. Our conclusion was that prison inmates in Sweden possess reading and writing skills that are comparable to those found in an adult population and that the occurrence of dyslexic problems is very close to population incidences.