Toward an integrated understanding of dyslexia: genetic, neurological, and cognitive mechanisms

A Scoping Review on Movement, Neurobiology and Functional Deficits in Dyslexia: Suggestions for a Three-Fold Integrated Perspective

Developmental dyslexia is a common complex neurodevelopmental disorder. Many theories and models tried to explain its symptomatology and find ways to improve poor reading abilities. The aim of this scoping review is to summarize current findings and several approaches and theories, focusing on the interconnectedness between motion, emotion and cognition and their connection to dyslexia. Consequently, we present first a brief overview of the main theories and models regarding dyslexia and its proposed neural correlates, with a particular focus on cerebellar regions and their involvement in this disorder. After examining different types of intervention programs and remedial training, we highlight the effects of a specific structured sensorimotor intervention named Quadrato Motor Training (QMT). QMT utilizes several cognitive and motor functions known to be relevant in developmental dyslexia. We introduce its potential beneficial effects on reading skills, including working memory, coordination and attention. We sum its effects ranging from behavioral to functional, structural and neuroplastic, especially in relation to dyslexia. We report several recent studies that employed this training technique with dyslexic participants, discussing the specific features that distinguish it from other training within the specific framework of the Sphere Model of Consciousness. Finally, we advocate for a new perspective on developmental dyslexia integrating motion, emotion and cognition to fully encompass this complex disorder.

Construction of an Arabic computerized battery for cognitive rehabilitation of children with specific learning disabilities

PURPOSE

The aim of this study was to design an Arabic computerized battery of cognitive skills for training children with specific learning disabilities (SLD).

SUBJECTS AND METHODS

Nineteen students from fourth grade primary schools in Assiut, Egypt, who were previously diagnosed with SLD, agreed to participate in the rehabilitation program. The study passed through four stages: first stage, detailed analysis of the cognitive profile of students with SLD (n=19), using a previously constructed diagnostic cognitive skill battery, to identify deficits in their cognitive skills; second stage, construction of an Arabic computerized battery for cognitive training of students with SLD; third stage, implementation of the constructed training program for the students, each tailored according to his/her previously diagnosed cognitive skill deficit/deficits; and fourth stage, post-training re-evaluation of academic achievement and cognitive skills' performance.

RESULTS

Students with SLD have a wide range of cognitive skill deficits, which are more frequent in auditory cognitive skills (68.4%) than in visual cognitive skills (64.1%), particularly in phonological awareness and auditory sequential memory (78.9%). Following implementation of the training program, there was a statistically significant increase (P<0.001) in the mean scores of total auditory and visual cognitive skills, as well as in academic achievement (P<0.001) of the study group.

CONCLUSION

Remediation-oriented diagnosis of cognitive skills, when tailored according to previously diagnosed cognitive deficits, leads to the improvement in learning abilities and academic achievement of students with SLD.

A review of the neurobiological basis of dyslexia in the adult population

INTRODUCTION

Adult dyslexia affects about 4% of the population. However, studies on the neurobiological basis of dyslexia in adulthood are scarce compared to paediatric studies.

AIM

This review investigates the neurobiological basis of dyslexia in adulthood.

DEVELOPMENT

Using PsycINFO, a database of psychology abstracts, we identified 11 studies on genetics, 9 neurostructural studies, 13 neurofunctional studies and 24 neurophysiological studies. Results from the review show that dyslexia is highly heritable and displays polygenic transmission. Likewise, adult neuroimaging studies found structural, functional, and physiological changes in the parieto-occipital and occipito-temporal regions, and in the inferior frontal gyrus, in adults with dyslexia.

CONCLUSION

According to different studies, aetiology in cases of adult dyslexia is complex. We stress the need for neurobiological studies of dyslexia in languages with transparent spelling systems.

Visuomotor processing and hand force coordination in dyslexic children during a visually guided manipulation task

Developmental Dyslexia negatively affects children's reading and writing ability and, in most cases, performance in sensorimotor tasks. These deficits have been associated with structural and functional alterations in the cerebellum and the posterior parietal cortex (PPC). Both neural structures are active during visually guided force control and in the coordination of load force (LF) and grip force (GF) during manipulation tasks. Surprisingly, both phenomena have not been investigated in dyslexic children. Therefore, the aim of this study was to compare dyslexic and non-dyslexic children regarding their visuomotor processing ability and GF-LF coordination during a static manipulation task. Thirteen dyslexic (8-14 YO) and 13 age- and sex-matched non-dyslexic (control) children participated in the study. They were asked to grasp a fixed instrumented handle using the tip of all digits and pull the handle upward exerting isometric force to match a ramp-and-hold force profile displayed in a computer monitor. Task performance (i.e., visuomotor coordination) was assessed by RMSE calculated in both ramp and hold phases. GF-LF coordination was assessed by the ratio between GF and LF (GF/LF) calculated at both phases and the maximum value of a cross-correlation function (rmax) and its respective time lag calculated at ramp phase. The results revealed that the RMSE at both phases was larger in dyslexic than in control children. However, we found that GF/LF, rmax, and time lags were similar between groups. Those findings indicate that dyslexic children have a mild deficit in visuomotor processing but preserved GF-LF coordination. Altogether, these findings suggested that dyslexic children could present mild structural and functional alterations in specific PPC or cerebellum areas that are directly related to visuomotor processing.

An interaction-dominant perspective on reading fluency and dyslexia

The background noise of response times is often overlooked in scientific inquiries of cognitive performances. However, it is becoming widely acknowledged in psychology, medicine, physiology, physics, and beyond that temporal patterns of variability constitute a rich source of information. Here, we introduce two complexity measures (1/f scaling and recurrence quantification analysis) that employ background noise as metrics of reading fluency. These measures gauge the extent of interdependence across, rather than within, cognitive components. In this study, we investigated dyslexic and non-dyslexic word-naming performance in beginning readers and observed that these complexity metrics differentiate reliably between dyslexic and average response times and correlate strongly with the severity of the reading impairment. The direction of change in the introduced metrics suggests that developmental dyslexia resides from dynamical instabilities in the coordination among the many components necessary to read, which could explain why dyslexic readers score below average on so many distinct tasks and modalities.

Predicting dyslexia at age 11 from a risk index questionnaire at age 5

This study focused on predicting dyslexia in children ahead of formal literacy training. Because dyslexia is a constitutional impairment, risk factors should be seen in preschool. It was hypothesized that data gathered at age 5 using questions targeting the dyslexia endophenotype should be reliable and valid predictors of dyslexia at age 11. A questionnaire was given to caretakers of 120 5-year-old children, and a risk index score was calculated based on questions regarding health, laterality, motor skills, language, special needs education and heredity. An at-risk group (n = 25) and matched controls (n = 24) were followed until age 11, when a similar questionnaire and literacy tests were administered to the children who participated in the follow-up study (22 at risk and 20 control). Half of the at-risk children and two of the control children at age 5 were identified as having dyslexia at age 11 (8 girls and 5 boys). It is concluded that it is possible to identify children at the age of 5 who will have dyslexia at the age of 11 through a questionnaire approach.

Perspectives on dyslexia

Dyslexia, or a reading disability, occurs when an individual has significant difficulty with speed and accuracy of word decoding. Comprehension of text and spelling are also affected. The diagnosis of dyslexia involves the use of reading tests, but the continuum of reading performance means that any cutoff point is arbitrary. The IQ score does not play a role in the diagnosis of dyslexia. The cognitive difficulties of dyslexics include problems with speech perception, recognizing and manipulating the basic sounds in a language, language memory, and learning the sounds of letters. Dyslexia is a neurological condition with a genetic basis. There are abnormalities in the brains of dyslexic individuals. There are also differences in the electrophysiological and structural characteristics of the brains of dyslexics. Physicians play a particularly important role in recognizing children who are at risk for dyslexia and helping their parents obtain the proper assessment.

Learning disabilities, dyslexia, and vision

Learning disabilities constitute a diverse group of disorders in which children who generally possess at least average intelligence have problems processing information or generating output. Their etiologies are multifactorial and reflect genetic influences and dysfunction of brain systems. Reading disability, or dyslexia, is the most common learning disability. It is a receptive language-based learning disability that is characterized by difficulties with decoding, fluent word recognition, rapid automatic naming, and/or reading-comprehension skills. These difficulties typically result from a deficit in the phonologic component of language that makes it difficult to use the alphabetic code to decode the written word. Early recognition and referral to qualified professionals for evidence-based evaluations and treatments are necessary to achieve the best possible outcome. Because dyslexia is a language-based disorder, treatment should be directed at this etiology. Remedial programs should include specific instruction in decoding, fluency training, vocabulary, and comprehension. Most programs include daily intensive individualized instruction that explicitly teaches phonemic awareness and the application of phonics. Vision problems can interfere with the process of reading, but children with dyslexia or related learning disabilities have the same visual function and ocular health as children without such conditions. Currently, there is inadequate scientific evidence to support the view that subtle eye or visual problems cause or increase the severity of learning disabilities. Because they are difficult for the public to understand and for educators to treat, learning disabilities have spawned a wide variety of scientifically unsupported vision-based diagnostic and treatment procedures. Scientific evidence does not support the claims that visual training, muscle exercises, ocular pursuit-and-tracking exercises, behavioral/perceptual vision therapy, "training" glasses, prisms, and colored lenses and filters are effective direct or indirect treatments for learning disabilities. There is no valid evidence that children who participate in vision therapy are more responsive to educational instruction than children who do not participate.

Effects of bottom-up and top-down intervention principles in emergent literacy in children at risk of developmental dyslexia: a longitudinal study

This longitudinal study focused on the effects of two different principles of intervention in children at risk of developing dyslexia from 5 to 8 years old. The children were selected on the basis of a background questionnaire given to parents and preschool teachers, with cognitive and functional magnetic resonance imaging results substantiating group differences in neuropsychological processes associated with phonology, orthography, and phoneme-grapheme correspondence (i.e., alphabetic principle). The two principles of intervention were bottom-up (BU), "from sound to meaning", and top-down (TD), "from meaning to sound." Thus, four subgroups were established: risk/BU, risk/TD, control/BU, and control/TD. Computer-based training took place for 2 months every spring, and cognitive assessments were performed each fall of the project period. Measures of preliteracy skills for reading and spelling were phonological awareness, working memory, verbal learning, and letter knowledge. Literacy skills were assessed by word reading and spelling. At project end the control group scored significantly above age norm, whereas the risk group scored within the norm. In the at-risk group, training based on the BU principle had the strongest effects on phonological awareness and working memory scores, whereas training based on the TD principle had the strongest effects on verbal learning, letter knowledge, and literacy scores. It was concluded that appropriate, specific, data-based intervention starting in preschool can mitigate literacy impairment and that interventions should contain BU training for preliteracy skills and TD training for literacy training.

Brain asymmetry for language in dyslexic children

Studies of brain asymmetry in dyslexia have yielded mixed results both with respect to patterns of asymmetry and their cognitive correlates. This paper assessed language lateralisation inferred from dichotic listening with forced attention in a clinical group of young dyslexic subjects. As a whole the dyslexic group showed a deviant asymmetry pattern compared to a control group. Subdivision into two subgroups by receptive language abilities yielded differential patterns within the dyslexia group. Both subgroups exhibited a weaker response pattern to right ear stimuli than the control group. In addition, the subgroup with no language impairment showed bilateral language representation, whereas the subgroup with impaired receptive language abilities showed reversed dominance patterns.

A thirteen-year follow-up study of young Norwegian adults with dyslexia in childhood: reading development and educational levels

As part of a larger follow-up study, 75 students were diagnosed with dyslexia at 10 years of age. At the end of secondary school, grades for the whole cohort of 16-year olds were examined. At age 23 the whole study group responded to a questionnaire and the subgroup with dyslexia who responded to this questionnaire were invited to come for testing and interviews. The persistence of dyslexia into young adulthood and educational levels were examined. The results showed that almost all students still suffered dyslexia and showed lower decoding ability. However, they only showed slightly lower educational attainment levels compared with a representative sample of half the cohort (n=530) they were part of.

Topography of syllable change-detection electrophysiological indices in children and adults with reading disabilities

INTRODUCTION

Developmental dyslexia (DD) is a frequent language-based learning disorder. The predominant etiological view postulates that reading problems originate from a phonological impairment.

METHOD

We studied mismatch negativity (MMN) and Late Discriminative Negativity (LDN) to syllables change in both children (n=12; 8-12 years) and young adults (n=15; 14-23 years) with DD compared with controls.

RESULTS/DISCUSSION

The present study confirmed abnormal automatic discrimination of syllable changes in both children and adults with developmental dyslexia. MMN topographic, amplitude and latency group differences were evidenced, suggesting different brain mechanisms involved in elementary auditory stimulus change-detection in DD, especially in the left hemisphere. The LDN results demonstrated that the auditory disorder of temporal processing in DD children becomes more serious at late stages of information processing and that the apparent cerebral hypo reactivity to speech changes in DD actually may correspond to additional processes. The age-related differences observed in both MMN and LDN topographies, amplitudes and latency between subjects with DD and controls could indicate different developmental courses in the neural representation of basic speech sounds in good and poor readers, with a tendency to normalization with increasing age.

CONCLUSION

Our results showing atypical electrophysiological concomitants of speech auditory perception in DD strongly support the hypothesis of deviant cortical organization in DD.

Some indicators of (un)successful reading

In the paper we set the aim to determine whether phonological awareness and short-term verbal memory are indicators of a subsequent reading acquisition. The sample consisted of 194 first graders from two primary schools in Belgrade. The results of our research showed that the most significant indicator of the subsequent (un)successful reading was phonological awareness. The short-term verbal memory is, to a lesser extent, an indicator of the subsequent reading acquisition defined by the time needed for reading the text and by the text comprehension. Our findings offer basis for proposing that the phonological awareness assessment should be implemented into the regular procedure of assessing child's readiness for school. In this way children in need for preventive stimulation within the inclusive programme in primary schools could be identified.

The genetic bases of speech sound disorders: evidence from spoken and written language

The purpose of this article is to review recent findings suggesting a genetic susceptibility for speech sound disorders (SSD), the most prevalent communication disorder in early childhood. The importance of genetic studies of SSD and the hypothetical underpinnings of these genetic findings are reviewed, as well as genetic associations of SSD with other language and reading disabilities. The authors propose that many genes contribute to SSD. They further hypothesize that some genes contribute to SSD disorders alone, whereas other genes influence both SSD and other written and spoken language disorders. The authors postulate that underlying common cognitive traits, or endophenotypes, are responsible for shared genetic influences of spoken and written language. They review findings from their genetic linkage study and from the literature to illustrate recent developments in this area. Finally, they discuss challenges for identifying genetic influence on SSD and propose a conceptual framework for study of the genetic basis of SSD.

“Scientific roots” of dualism in neuroscience

Although the dualistic concept is unpopular among neuroscientists involved in experimental studies of the brain, neurophysiological literature is full of covert dualistic statements on the possibility of understanding neural mechanisms of human consciousness. Particularly, the covert dualistic attitude is exhibited in the unwillingness to discuss neural mechanisms of consciousness, leaving the problem of consciousness to psychologists and philosophers. This covert dualism seems to be rooted in the main paradigm of neuroscience that suggests that cognitive functions, such as language production and comprehension, face recognition, declarative memory, emotions, etc., are performed by neural networks consisting of simple elements. I argue that neural networks of any complexity consisting of neurons whose function is limited to the generation of electrical potentials and the transmission of signals to other neurons are hardly capable of producing human mental activity, including consciousness. Based on results obtained in physiological, morphological, clinical, and genetic studies of cognitive functions (mainly linguistic ones), I advocate the hypothesis that the performance of cognitive functions is based on complex cooperative activity of "complex" neurons that are carriers of "elementary cognition." The uniqueness of human cognitive functions, which has a genetic basis, is determined by the specificity of genes expressed by these "complex" neurons. The main goal of the review is to show that the identification of the genes implicated in cognitive functions and the understanding of a functional role of their products is a possible way to overcome covert dualism in neuroscience.

Genetic influences on early word recognition abilities and disabilities: a study of 7-year-old twins

BACKGROUND

A fundamental issue for child psychology concerns the origins of individual differences in early reading development.

METHOD

A measure of word recognition, the Test of Word Reading Efficiency (TOWRE), was administered by telephone to a representative population sample of 3,909 same-sex and opposite-sex pairs of 7-year-old twins. Analyses allowing for sex differences in aetiology were used to estimate the extent to which genetic and environmental influences contribute to normal variation in word recognition and word recognition difficulties, defined by scores below the 5th and 10th percentiles of the unselected sample.

RESULTS

Both normal variation in word recognition and impaired word recognition abilities were substantially heritable (h2 = .65-.67; h(g)2 = .37-.72). Environmental influences were primarily shared between twins, rather than specific to each individual, and small to moderate in magnitude. There was evidence for qualitative sex differences. Quantitative sex differences were also suggested at the extremes, with genetic influences being more important as a cause of reading difficulties in boys than in girls.

CONCLUSIONS

These findings indicate that early individual differences and impairments in word recognition are principally influenced by genetic factors and may involve partly distinct genetic or environmental effects in boys and girls. Crucially, they also provide evidence that reading impairments are linked genetically to the normal distribution. Genetic risk for early impairments in word recognition is continuous rather than discrete.

A computerized treatment of dyslexia: benefits from treating lexico-phonological processing problems

Two hundred sixty-seven 10- to 14-year-old Dutch children with dyslexia were randomly assigned to one of two samples that received a treatment for reading and spelling difficulties. The treatment was computer-based and focused on learning to recognise and use the phonological and morphological structure of Dutch words. The inferential algorithmic basis of the program ensured that the instruction was highly structured. The present study examined the reliability of the effects of the treatment, and provided an evaluation of the attained levels of reading and spelling by relating them to normal levels. Both samples revealed large, generalized treatment effects on reading accuracy, reading rate, and spelling skills. Following the treatment, participants attained an average level of reading accuracy and spelling. The attained level of reading rate was comparable to the lower bound of the average range.

Lebensalterspezifische Veränderungen phonologischer Defizite bei Lese-Rechtschreibstörung

Zusammenfassung. Theoretischer Hintergrund: Die klinischen Symptome der LRS ändern sich im Laufe der Entwicklung der Patienten, wobei sich die altersabhängigen Veränderungen der Lesefähigkeit aber von normgerechter Leseentwicklung unterscheiden. Fragestellung: Der Charakter dieser Veränderungen soll anhand der Entwicklung von Lesefertigkeiten in verschiedenen Lebensaltern gezeigt werden. Methoden: 102 Personen mit LRS und 91 normallesenden Kontrollen aus drei Altersgruppen (Kinder, Jugendliche und Erwachsenen) wurden Leseaufgaben vorgegeben, die unterschiedliche phonologische Strategien verlangen: (1) lautes Lesen von Wörtern, (2) von Nonwörtern und (3) Phonemtransformation. Erfasst wurden Lesefehler und -zeiten. Ergebnisse: Eine Verbesserung der Lesefertigkeiten erfolgt kontinuierlich über die drei Altersgruppen. Erwachsene mit LRS erreichen nicht einmal das Wortleseniveau jugendlicher Kontrollen. Beim Nonwortlesen sind sie schlechter als normallesende Kinder und erreichen über die Altersstufen kaum signifikante Verbesserungen. Schlussfolgerungen: In der LRS-Gruppe bilden sich Fähigkeiten zur Graphem-Phonem-Transformation langsamer heraus und bleiben auf niedrigem Niveau. Verbesserungen der Wortlesefähigkeiten scheinen vor allem über lexikalische Strategien (Ganzwortlesen) möglich.

Variation among developmental dyslexics: Evidence from a printed-word-learning task

A word-learning task was used to investigate variation among developmental dyslexics classified as phonological and surface dyslexics. Dyslexic children and chronological age (CA)- and reading level (RL)-matched normal readers were taught to pronounce novel nonsense words such as veep. Words were assigned either a regular (e.g., "veep") or an irregular (e.g., "vip") pronunciation. Phonological dyslexics learned both regular and exception words more slowly than the normal readers and, unlike the other groups, did not show a regular-word advantage. Surface dyslexics also learned regular and exception words more slowly than the CA group, consistent with a specific problem in mastering arbitrary item-specific pronunciations, but their performance resembled that of the RL group. The results parallel earlier findings from Manis, Seidenberg, Doi, McBride-Chang, & Petersen [Cognition 58 (1996) 157-195] indicating that surface dyslexics and phonological dyslexics have a different profile of reading deficits, with surface dyslexics resembling younger normal readers and phonological dyslexics showing a specific phonological deficit. Models of reading and reading disability need to account for the heterogeneity in reading processes among dyslexic children.

Evaluation of an exercise-based treatment for children with reading difficulties

An evaluation is reported of an exercise-based approach to remediation of dyslexia-related disorders. Pupils in three years of a Warwickshire junior school were screened for risk of literacy difficulty using the Dyslexia Screening Test (DST). The 35 children scoring 0.4 or over on the DST were divided randomly into two groups matched for age and DST score. One quarter of the participants had an existing diagnosis of dyslexia, dyspraxia or ADHD. Both groups received the same treatment at school but the intervention group used the DDAT exercise programme daily at home. Performance on the DST and specialist cerebellar/vestibular and eye movement tests were assessed initially and after six months. Cerebellar/vestibular signs were substantially alleviated following the exercise treatment whereas there were no significant changes for the control group. Even after allowing for the passage of time, there were significant improvements for the intervention group in postural stability, dexterity, phonological skill, and (one-tailed) for naming fluency and semantic fluency. Reading fluency showed a highly significant improvement for the intervention group, and nonsense passage reading was also improved significantly. Significantly greater improvements for the intervention group than the control group occurred for dexterity, reading, verbal fluency and semantic fluency. Substantial and significant improvements (compared with those in the previous year) also occurred for the exercise group on national standardized tests of reading, writing and comprehension. It is concluded that, in addition to its direct effects on balance, dexterity and eye movement control, the benefits of the DDAT exercise treatment transferred significantly to cognitive skills underlying literacy, to the reading process, and to standardized national literacy attainment tests.

Role of individual neurons and neural networks in cognitive functioning of the brain: a new insight

The prevailing concept in modern neuroscience is that neuron networks play a dominant role in the functioning of the nervous system, whereas the role of individual neurons is rather insignificant. This concept suggests that "individuality" of single neurons is primarily determined by their place in a network rather than their intrinsic properties. Here I argue that individual neurons may play an important, if not decisive, role in performing cognitive functions of the brain. This tentative viewpoint is supported by experimental and clinical insights into disorders of cognitive functions and by genetic studies of cognitive abilities and disabilities. The results obtained in these studies indicate that many specific cognitive functions are carried out by groups of highly specialized neurons whose roles in performing these functions are genetically predetermined and their activity could not be substituted by the activity of other neurons. In this context, the main role of neural networks and intercellular interactions is to form dynamic ensembles of neurons involved in performing a given cognitive function.

Specific reading disability: a multiplanar view

In the past three decades a revolution has altered the way society approaches people with disabilities. Social changes resulted in a significant increase in fundamental and applied research that seeks to improve the lives of people with disabilities by facilitating better understanding of the mechanisms, manifestations, prevention, and treatment of functional impairment. Specific Reading Disability (SRD) has benefited from this revolution. This review focuses on the evolution of SRD, new information in its neurobiology and management, and the challenges that remain. Evidence from a wide spectrum of research provides strong support for the role of phonology in Specific Reading Disability. Despite the mounting evidence, the case is far from completely established. Adults with compensated SRD read but still demonstrate disordered phonology (Felton et al. [1990] Brain Language 39:485-497). Whether poor phonology is causal or a covariant remains to be demonstrated. Of children with poor phonology, it is not known how many are poor readers. While phonology is associated with SRD, other studies have questioned the uniqueness of SRD. Challenges have been made to the method of classification, the uniqueness of phonological dysfunction as a mechanism in SRD and the response to treatment. In the final analysis all poor readers may have a common core of dysphonology, independent of whether their reading is discrepant from their IQ.