Neurology of developmental dyslexia

The multiple-level framework of developmental dyslexia: the long trace from a neurodevelopmental deficit to an impaired cultural technique

Developmental dyslexia is a neurodevelopmental disorder characterized by an unexpected impairment in literacy acquisition leading to specific poor academic achievement and possible secondary symptoms. The multi-level framework of developmental dyslexia considers five levels of a causal pathway on which a given genotype is expressed and hierarchically transmitted from one level to the next under the increasing influence of individual learning-relevant traits and environmental factors moderated by cultural conditions. These levels are the neurobiological, the information processing and the skill level (prerequisites and acquisition of literacy skills), the academic achievement level and the level of secondary effects. Various risk factors are present at each level within the assumed causal pathway and can increase the likelihood of exhibiting developmental dyslexia. Transition from one level to the next is neither unidirectional nor inevitable. This fact has direct implications for prevention and intervention which can mitigate transitions from one level to the next. In this paper, various evidence-based theories and findings regarding deficits at different levels are placed in the proposed framework. In addition, the moderating effect of cultural impact at and between information processing and skill levels are further elaborated based on a review of findings regarding influences of different writing systems and orthographies. These differences impose culture-specific demands for literacy-specific cognitive procedures, influencing both literacy acquisition and the manifestation of developmental dyslexia.

Quantitative EEG in Childhood Attention Deficit Hyperactivity Disorder and Learning Disabilities

The clinical use of the quantitative EEG (QEEG) from the pioneering work of John has received a new impetus thanks to new neuroimaging techniques and the possibility of using a number of normative databases both of normal subjects and of subjects with definite pathologies. In this direction, the term personalized medicine is becoming more and more common, a medical procedure that separates patients into different groups based on their predicted response to the quantitative EEG. This has allowed the study of single subjects and to customize health care, with decisions and treatments tailored to each individual patient, as well as improvement of knowledge of the pathophysiological mechanisms of specific diseases. This review article will present the most recent evidence in the field of developmental neuropsychiatric disorders obtained from the application of quantitative EEG both in clinical group studies (attention deficit hyperactivity disorder, developmental dyslexia, oppositional defiant disorder) and in individual case studies not yet published.

The neuronal migration hypothesis of dyslexia: A critical evaluation 30 years on

The capacity for language is one of the key features underlying the complexity of human cognition and its evolution. However, little is known about the neurobiological mechanisms that mediate normal or impaired linguistic ability. For developmental dyslexia, early postmortem studies conducted in the 1980s linked the disorder to subtle defects in the migration of neurons in the developing neocortex. These early studies were reinforced by human genetic analyses that identified dyslexia susceptibility genes and subsequent evidence of their involvement in neuronal migration. In this review, we examine recent experimental evidence that does not support the link between dyslexia and neuronal migration. We critically evaluate gene function studies conducted in rodent models and draw attention to the lack of robust evidence from histopathological and imaging studies in humans. Our review suggests that the neuronal migration hypothesis of dyslexia should be reconsidered, and the neurobiological basis of dyslexia should be approached with a fresh start.

Clinical and Electrophysiological Differences between Subjects with Dysphonetic Dyslexia and Non-Specific Reading Delay

Reading is essentially a two-channel function, requiring the integration of intact visual and auditory processes both peripheral and central. It is essential for normal reading that these component processes go forward automatically. Based on this model, Boder described three main subtypes of dyslexia: dysphonetic dyslexia (DD), dyseidetic, mixed and besides a fourth group defined non-specific reading delay (NSRD). The subtypes are identified by an algorithm that considers the reading quotient and the % of errors in the spelling test. Chiarenza and Bindelli have developed the Direct Test of Reading and Spelling (DTRS), a computerized, modified and validated version to the Italian language of the Boder test. The sample consisted of 169 subjects with DD and 36 children with NSRD. The diagnosis of dyslexia was made according to the DSM-V criteria. The DTRS was used to identify the dyslexia subtypes and the NSRD group. 2⁻5 min of artefact-free EEG (electroencephalogram), recorded at rest with eyes closed, according to 10⁻20 system were analyzed. Stability based Biomarkers identification methodology was applied to the DTRS and the quantitative EEG (QEEG). The reading quotients and the errors of the reading and spelling test were significantly different in the two groups. The DD group had significantly higher activity in delta and theta bands compared to NSRD group in the frontal, central and parietal areas bilaterally. The classification equation for the QEEG, both at the scalp and the sources levels, obtained an area under the robust Receiver Operating Curve (ROC) of 0.73. However, we obtained a discrimination equation for the DTRS items which did not participate in the Boder classification algorithm, with a specificity and sensitivity of 0.94 to discriminate DD from NSRD. These results demonstrate for the first time the existence of different neuropsychological and neurophysiological patterns between children with DD and children with NSRD. They may also provide clinicians and therapists warning signals deriving from the anamnesis and the results of the DTRS that should lead to an earlier diagnosis of reading delay, which is usually very late diagnosed and therefore, untreated until the secondary school level.

Development of Tract-Specific White Matter Pathways During Early Reading Development in At-Risk Children and Typical Controls

Developmental dyslexia is a neurodevelopmental disorder with a strong genetic basis. Previous studies observed white matter alterations in the left posterior brain regions in adults and school-age children with dyslexia. However, no study yet has examined the development of tract-specific white matter pathways from the pre-reading to the fluent reading stage in children at familial risk for dyslexia (FHD+) versus controls (FHD-). This study examined white matter integrity at pre-reading, beginning, and fluent reading stages cross-sectionally ( n = 78) and longitudinally (n = 45) using an automated fiber-tract quantification method. Our findings depict white matter alterations and atypical lateralization of the arcuate fasciculus at the pre-reading stage in FHD+ versus FHD- children. Moreover, we demonstrate faster white matter development in subsequent good versus poor readers and a positive association between white matter maturation and reading development using a longitudinal design. Additionally, the combination of white matter maturation, familial risk, and psychometric measures best predicted later reading abilities. Furthermore, within FHD+ children, subsequent good readers exhibited faster white matter development in the right superior longitudinal fasciculus compared with subsequent poor readers, suggesting a compensatory mechanism. Overall, our findings highlight the importance of white matter pathway maturation in the development of typical and atypical reading skills.

Brain mechanisms and reading remediation: more questions than answers

Dyslexia is generally diagnosed in childhood and is characterised by poor literacy skills with associated phonological and perceptual problems. Compensated dyslexic readers are adult readers who have a documented history of childhood dyslexia but as adults can read and comprehend written text well. Uncompensated dyslexic readers are adults who similarly have a documented history of reading impairment but remain functionally reading-impaired all their lives. There is little understanding of the neurophysiological basis for how or why some children become compensated, while others do not, and there is little knowledge about neurophysiological changes that occur with remedial programs for reading disability. This paper will review research looking at reading remediation, particularly in the context of the underlying neurophysiology.

Neural intersections of the phonological, visual magnocellular and motor/cerebellar systems in normal readers: implications for imaging studies on dyslexia

We used fMRI to explore the extent of the anatomical overlap of three neural systems that the literature on developmental dyslexia associates with reading: the auditory phonological, the visual magnocellular, and the motor/cerebellar systems. Twenty-eight normal subjects performed four tasks during fMRI scans: word and pseudoword reading, auditory rhyming for letter names, visual motion perception, and a motor sequence learning task. We found that the left occipitotemporal cortex (OTC), which previous studies reported to be dysfunctional in dyslexia, can be fractionated into different functional areas: an anterior and lateral area that was activated by both reading and auditory rhyming tasks; a posterior area that was commonly activated by both the reading and the motion perception task and a medial/intermediate area, including the so-called Visual Word Form Area, which was specifically activated by the reading task. These results show that the left OTC is an area of segregated convergence of different functional systems. We compared our results with the hypoactivation pattern reported for reading in a previous cross-cultural PET study on 36 dyslexic subjects from three countries. The region of decreased activation in dyslexia overlapped with regions that are specific for reading and those activated during both the auditory rhyming task and the single word and pseudoword reading task described in the present fMRI study. No overlap was found with the activation patterns for the visual motion perception task or for the motor sequence learning task. These observations challenge current theories of dyslexia.

Dysfunction of the auditory thalamus in developmental dyslexia

Developmental dyslexia, a severe and persistent reading and spelling impairment, is characterized by difficulties in processing speech sounds (i.e., phonemes). Here, we test the hypothesis that these phonological difficulties are associated with a dysfunction of the auditory sensory thalamus, the medial geniculate body (MGB). By using functional MRI, we found that, in dyslexic adults, the MGB responded abnormally when the task required attending to phonemes compared with other speech features. No other structure in the auditory pathway showed distinct functional neural patterns between the two tasks for dyslexic and control participants. Furthermore, MGB activity correlated with dyslexia diagnostic scores, indicating that the task modulation of the MGB is critical for performance in dyslexics. These results suggest that deficits in dyslexia are associated with a failure of the neural mechanism that dynamically tunes MGB according to predictions from cortical areas to optimize speech processing. This view on task-related MGB dysfunction in dyslexics has the potential to reconcile influential theories of dyslexia within a predictive coding framework of brain function.

Reading text increases binocular disparity in dyslexic children

Children with developmental dyslexia show reading impairment compared to their peers, despite being matched on IQ, socio-economic background, and educational opportunities. The neurological and cognitive basis of dyslexia remains a highly debated topic. Proponents of the magnocellular theory, which postulates abnormalities in the M-stream of the visual pathway cause developmental dyslexia, claim that children with dyslexia have deficient binocular coordination, and this is the underlying cause of developmental dyslexia. We measured binocular coordination during reading and a non-linguistic scanning task in three participant groups: adults, typically developing children, and children with dyslexia. A significant increase in fixation disparity was observed for dyslexic children solely when reading. Our study casts serious doubts on the claims of the magnocellular theory. The exclusivity of increased fixation disparity in dyslexics during reading might be a result of the allocation of inadequate attentional and/or cognitive resources to the reading process, or suboptimal linguistic processing per se.

Developmental dyslexia and explicit long-term memory

The reduced verbal long-term memory capacities often reported in dyslexics are generally interpreted as a consequence of their deficit in phonological coding.The present study was aimed at evaluating whether the learning deficit exhibited by dyslexics was restricted only to the verbal component of the long-term memory abilities or also involved visual-object and visual-spatial domain. A further goal of the present study was to investigate the predictive value of non-verbal long-term memory abilities with respect to word and non-word reading in dyslexic children.In accordance with these aims, performances of 60 dyslexic children were compared with that of 65 age-matched normal readers on verbal, visual-spatial and visual-object task.Results documented a generalized impairment of episodic long-term memory capacities in dyslexic children and the results did not vary as a function of children's age.Furthermore, in addition to verbal measures, also individual differences in non-verbal long-term memory tasks turn out to be good predictors of reading difficulties in dyslexics.Our findings indicate that the long-term memory deficit in dyslexia is not limited to the dysfunction of phonological components but also involves visual-object and visual-spatial aspect, thus suggesting that dyslexia is associated to multiple cognitive deficits.

Attentional blink deficits observed in dyslexia depend on task demands

The attentional blink (AB) refers to a deficit in the ability to identify a second target following a first target when both appear randomly within a rapid sequence of distractor items. The AB of five adults with dyslexia (ADys) was compared with that of a group of normal adult readers. Two tasks were completed which differed in the conceptual category of the target items (a red digit or letter) relative to the distractor items (all black digits). In the digit condition, all ADys cases showed a longer AB compared to the control group. In the letter condition, all participants showed improvement in accuracy compared to the digit condition, but three ADys cases continued to have a longer AB. The results suggest that (a) AB performance depends on task requirements, and (b) the attentional system is compromised in dyslexia. However, examination of individual case performance suggests that prolonged attentional dwell time is not a core deficit in dyslexia. The results also illustrate the limitations of group comparisons in small sample studies.

Children with dyslexia: evidence for visual attention deficits in perception of rapid sequences of objects

The attentional blink (AB) refers to a decrease in accuracy that occurs when observers are required to identify, detect or classify the second of two rapidly-sequential targets. The AB is typically attributed to an inability to rapidly reallocate attentional resources from the first to the second target. Thus, it provides an ideal tool to investigate how visual attention is rapidly allocated to sequences of stimuli such as occurs when reading. In the present work, we compared the magnitude of the AB in children with developmental dyslexia to reading-matched and age-matched control groups. In Experiment 1, when two targets were presented in the same spatial location, the AB deficit was similar in the reading-matched and dyslexic groups, but greater in the dyslexic group than in age-matched controls. In Experiment 2, when targets were presented in different spatial locations, performance in the dyslexic group was worse than the age-matched controls and marginally worse than the reading-matched controls. Taken together, the results argue for developmental delays in the ability of children with dyslexia to allocate attention to rapidly-sequential stimuli, as well as some evidence for difficulties that are unique to this group.

The handshaking model of brain function: notes toward a theory

John Hughlings Jackson described the system-level organization of the nervous system in terms of functional-control relations between neural centers. His model emphasized hierarchical organization along a rostral-caudal dimension, with applications primarily to cases of clinical disorders such as epilepsy and motor paralysis. This paper outlines a new systems-level model of brain function, updating Jackson's original idea to account for all three orientations of body/brain organization, and additional, non-hierarchical types of relations. The approach may provide a powerful tool for addressing many aspects of human brain and behavior, ranging from normal characteristics (such as individual differences and gender differences), to cases of frank neurological injury, to other conditions, such as hyperactivity and chronic pain, conventionally considered as 'neurologically silent'.

Impaired auditory frequency discrimination in dyslexia detected with mismatch evoked potentials

Deficits in phonological skills appear to be at the heart of reading disability; however, the nature of this impairment is not yet known. The hypothesis that dyslexic subjects are impaired in auditory frequency discrimination was tested by using an attention-independent auditory brain potential, termed mismatch negativity (MMN) while subjects performed a visual distractor task. In separate blocks, MMN responses to graded changes in tone frequency or tone duration were recorded in 10 dyslexic and matched control subjects. MMN potentials to changes in tone frequency but not to changes in tone duration were abnormal in dyslexic subjects. This physiological deficit was corroborated by a similarly specific impairment in discriminating tone frequency, but not tone duration, which was assessed separately. Furthermore, the pitch discrimination and MMN deficit was correlated with the degree of impairment in phonological skills, as reflected in reading errors of regular words and nonwords. It is possible that in dyslexia a persistent sensory deficit in monitoring the frequency of incoming sound may impair the feedback control necessary for the normal development of phonological skills.

Technology in the assessment of learning disability

Recent neuroradiologic and brain imaging technologies, along with methods for displaying electrophysiologic data, have promulgated active exploration in the assessment of learning disability with attempts to improve diagnostic precision and elucidate the neurobiological substrates of learning disorders. The following article reviews these techniques and explores the research that has been conducted in this area over the past two decades. Initial investigations attempted to elucidate irregularities or abnormalities of brain morphology in individuals with learning disability utilizing computerized tomography (CT). The current standard for structural imaging of the brain is magnetic resonance (MR) imaging, which has allowed for greater specificity in identifying brain abnormalities. More recently, functional magnetic resonance imaging (fMRI) has been postulated as holding some promise in distinguishing anatomic/function differences in LD. Electrophysiological (EEG) and metabolic imaging techniques offer methods by which human brain activity can be studied during cognitive processes. Computerized EEG studies including evoked potentials (EP) or event-related potentials (ERP), spectral EEG analysis, and topographic EEG brain mapping have also identified a number of brain irregularities in individuals with learning disabilities, though no consistent exemplars have emerged. Studies with positron emission tomography (PET) and single photon emission computerized tomography (SPECT) have also demonstrated a number of abnormalities and inconsistencies in individuals with learning disabilities, but, again, no systematic research has demonstrated specific diagnostic abnormalities. Though inroads and some consistent patterns have begun to emerge in the assessment of learning disability with the preceding technologies, a number of challenges persist with neuroimaging and neurophysiological and metabolic imaging techniques. To date, no diagnostic conclusions have been drawn utilizing these methods in the assessment of the neurobiologic basis to LD.

Learning disorders with a special emphasis on reading disorders: a review of the past 10 years

OBJECTIVE

To review the past 10 years of clinical and research reports on learning disorders.

METHOD

The most common and best-researched type of learning disorder is reading disability, which is the focus of this review. A selective review of the literature from Psychological Abstracts and Index Medicus from 1985 to the present was conducted. This review focused on conceptual and methodological issues, current assessment practices, epidemiology, correlates of brain function, biological factors, predictors of reading achievement, core deficits, comorbidity reading development and instructional approaches, treatment, and outcome.

RESULTS

Definitional issues, still unresolved, bedevil the field with the debate between those for and those against discrepancy definitions of reading disabilities. Nevertheless considerable progress has been made. Phonological processing problems are now considered the main core deficit responsible for reading disabilities. Correlates of brain function and possible genetic factors are noted. Comorbidity with externalizing and internalizing disorders is described, and some theories for the overlap are identified. Studies on the comorbidity with internalizing disorders are lacking. Good assessment practice and promising approaches to remediation are identified. Unless a concurrent disorder is present, the use of medication for the treatment of reading disabilities should be considered experimental. Favorable outcomes are dependent on initial severity and a supportive home and school environment.

CONCLUSIONS

Much progress has been made in our understanding of learning disabilities, especially in reading disabilities. Resolution of definitional and conceptual issues will greatly assist research into assessment, treatment, and long-term outcome of learning disabilities with and without concurrent psychiatric disorders. Further research into the nature, extent, and correlates of comorbid learning disabilities and their treatment is much needed.

Schizophrenia: a disorder of neurodevelopment?

For the first time, there is a pathogenic hypothesis of schizophrenia based upon reasonable empirical data. The hypothesis is that schizophrenia is a disorder arising from aberrant brain development. The neurodevelopmental view of schizophrenia is supported by neuropathological, epidemiological and clinical findings. Here, the evidence in favour of the model is summarized, together with a consideration of its weaknesses.

The familial incidence of symptoms of Scotopic Sensitivity/Irlen syndrome

The familial incidence of Scotopic Sensitivity/Irlen Syndrome was investigated using parents of 751 children identified with symptoms. Children were identified by methods independent of their parents' symptoms or lack of symptoms. For these children, there was an 84% chance of either one or both parents showing similar symptoms, with similar numbers of mothers identified with symptoms as fathers. The data suggest that Scotopic Sensitivity/Irlen Syndrome may be a genetically based deficit in visual processing, but the simplest genetic models do not appear to fit.

Developmental dyslexia: a motor-articulatory feedback hypothesis

Reading is mediated by parallel and widely distributed modular systems. There are, therefore, multiple loci in these systems where dysfunction may lead to developmental dyslexia. However, most normal children learn to read using the alphabetic system. Learning to use this system requires awareness that words are comprised of a system of speech sounds (phonological awareness) and the knowledge of how to convert letters (graphemes) into these speech sounds (phonemes). Most dyslexic children have deficient phonological awareness and have difficulty converting graphemes into phonemes. Studies of patients with acquired lesions who are unable to convert graph-emes into phonemes, as well as positron emission tomographic studies of normal subjects, suggest that the left inferior frontal lobe is important in phonologic reading. Phonetic gestures are represented in the brain as invariant motor commands that program the articulators. Phonologic reading may activate the left inferior frontal lobe because grapheme-to-phoneme conversion requires activation of these motor-articulatory gestures. Dyslexic children are unaware of the position of their articulators during speech. The inability to associate the position of their articulators with speech sounds may impair the development of phonological awareness and the ability to convert graphemes to phonemes. Unawareness of their articulators may be related to programming or feedback deficits.