Neurobiological Approaches on Brains of Children with Dyslexia

Learning difficulties commonly comprise a heterogeneous group of disorders manifested by unexpected problems in some children’s experiences in the academic performance arena. These problems especially comprise of a variety of disorders, which one of the most well-recognized learning difficulties is reading disability or dyslexia. The aim of this review is to explain the postmortem, structural or functional neuroimaging, and electrophysiological studies of human brains in children. The findings about these neuropathological and neurofunctional characteristics of developmental dyslexia, prospective studies beginning early in the life span and studies targeting remedial intervention will help to set the research agendas for future studies to follow.

Affecting up to 4–10% of the population, reading disability (RD) or developmental dyslexia (DD) is a highly prevalent, childhood onset developmental disorder adversely influencing multiple domains of adaptive functioning throughout the lifespan. DD is characterized by an unexpected difficulty in reading in children and adults who otherwise possess the intelligence and motivation considered necessary for accurate and fluent reading. In the International Classification of Diseases 10 manual, dyslexia is coded as a Specific Reading Disorder that emphasizes problems with poor learning to read, spell, and write despite adequate intellectual capacity, educational resources, and social background by the inclusion of sensory acuity deficits, neurological, and psychiatric diseases. According to the Diagnostic and Statistical Manual of Mental Disorders IV, dyslexia is coded as Reading Disorder, is characterized by difficulties with accurate or fluent word recognition, and by poor spelling and decoding abilities (omissions [eg, “ply” instead of “play”], substitutions [eg, “arm” for “hand”], inversions [eg, “aks” instead of “ask”], or additions [eg, “of” and “to”] of words or fragments of words).

Unfortunately, affected children are not diagnosed until they have tried and failed to learn to read, although they have these symptoms in preschool age. In primary school, they have difficulties in learning the alphabet, naming letters, producing rhymes, and categorizing speech sounds. Children with dyslexia frequently decode slowly and make errors in comprehension while reading passages. For this purpose, I have focused on DD from a converging methodological perspective of diagnose.

In the present paper, I will overview important arguments and experimental data obtained to date in a favor of neurological basis of dyslexia. The major part of this presentation will be devoted to the morphological, functional, and electrophysiological in vivo investigations of the dyslexic brain. Finally, I will propose a conclusion of the studies in this domain.

First, I will provide a brief historical survey of the postmortem studies of developmental dyslexia.

Postmortem studies

The notion that impaired reading and writing conditions in dyslexia may have a neurological origin and due to defective development of brain was initially thought with the Scottish ophthalmologist Hinshelwood’s report . He proposed that the young patients with dyslexia had a defective development in the left occipital and parietal lobes that was injured in adult alexic patients. After 1 year later, Morgan described a case which of an intelligent 14-year-old boy who could not learn how to read and is considered as one of the first reports about “congenital word blindness.” He assumed that the disorder was caused by a defective development of the left angular gyrus in this report and is recognized as the father of DD.

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Structural neuroimaging studies

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MRI Studies

The asymmetry of planum temporale

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Table 1

Structural Neuroimaging Investigations in Children with RD

Study Materials and Methods Regions of Brain Results Larsen et al 19 dyslexics and 17 control; MRI Plana temporale Planum symmetry among the 70% of dyslexics Hynd et al 10 dyslexic, 10 ADHD, and 10 control children; MRI Plana temporale, frontal lobe, and insula Smaller right anterior-width in dyslexic or ADHD children Bilaterally smaller insular region and significantly smaller left planum temporale in the dyslexics Duara et al 21 dyslexics and 29 control; MRI Right and left hemispheres Right side being larger in the frontal half of the horizontal brain section Left side being larger in the posterior to only the occipital polar segment Kushch et al 21 dyslexics and 29 control; MRI Superior surface of the temporal lobe Leftward asymmetry in both of anterior and posterior halves of the SSTL area in controls, but symmetry in dyslexics Heiervang et al 20 right-handed boys with and without RD; sagittal MRI Planum temporale and planum parietale Smaller left planum temporale and rightward planum parietale asymmetry in dyslexics Best and Demb Dyslexics with magnocellular deficit and controls; sagittal MRI Planum temporale No differences in asymmetry Duara et al 21 dyslexics and 29 control; MRI Corpus callosum Larger splenium in dyslexic subjects or dyslexic female subjects than non-dyslexic or dyslexic male subjects Larsen et al Dyslexics and controls; MRI Total corpus callosum or the splenium No differences in asymmetry Hynd et al 16 children with DD and matched control; MRI Corpus callosum The anterior region carpus callosum smaller in the dyslexic children von Plessen et al 20 right-handed boys with DD and controls; shape analysis with MRI Corpus callosum Shorter corpus callosum shape in the dyslexic group Leonard et al 13 subjects (9 phonological dyslexic) with RD and 15 controls; volumetric MRI scanning Cerebellum Marked rightward and leftward asymmetry of the anterior lobe of the cerebellum in phonological dyslexics Eckert et al 14 males, 4 females dyslexics and 19 males, 13 females controls; MRI Posterior temporal lobe, inferior frontal gyrus, cerebellum and whole brain Significantly smaller right anterior lobes of the cerebellum, pars triangularis bilaterally, and brain volume in dyslexics Casanova et al 16 dyslexics and 14 controls; MRI Corpus callosum and cerebellum Significantly smaller total cerebral volume and reduced gyrification index in dyslexic patients Eckert et al 13 dyslexic and 13 normal children; VBM 1. Right cerebellar anterior lobe and right and left pars triangularis Gray and white matter differences in first analyzed arena and gray matter volume differences in second analyzed arena 2. Left and right lingual gyrus, left inferior parietal lobule and cerebellum Vinckenbosch et al 10 males with dyslexia and 14 matched controls; VBM Gray and white matter lobar volumes Significantly reduced gray matter volumes in both temporal lobes; reduced gray matter density in the middle and inferior temporal gyri and increased gray matter density in the precentral gyri bilaterally in dyslexics Hoeft et al 19 dyslexic adolescents and 19 controls; VBM Gray and white matter lobar volumes Reduced gray matter volume only in the left parietal region of the dyslexic group relative to both control groups Jancke et al 21 children with developmental language disorder and 21 controls; VBM Gray and white matter lobar volumes Decreased white matter volumes in a left-hemispheric network comprising the motor cortex in dyslexics Deutsch et al 14 children with a wide range of reading performance levels; DTI White matter structure Correlation between white matter structure and behavioral measurements of reading, spelling, and rapid naming performance Niogi and McCandliss Children; DTI White matter structure Correlation between FA values in a left temporoparietal white matter region and standardized reading scores of typically developing children

RD, reading disability; ADHD, attention deficit hyperactivity disorder; MRI, magnetic resonance imaging; SSTL, superior surface of the temporal lobe; VBM, voxel-based morphometry; FA, fractional anisotropy; DTI, diffusion tensor imaging.

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The volume and size of corpus callosum

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The morphology of cerebellum

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VBM Studies

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DTI

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Functional neuroimaging studies

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fMRI Studies

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Table 2

Functional Neuroimaging Investigations in Children with RD

Study Materials and Methods Functional Activation Results Georgiewa et al 17 dyslexics and 17 normal reading children; fMRI 1) Passive viewing of letter strings Significant differences in Broca’s area and the left inferior temporal region for both, non-word reading and the phonological transformation task in dyslexics 2) Passive reading of non-words 3) Passive reading of legal words 4) Phonological transformation task Georgiewa et al 9 dyslexic and 8 control children; fMRI Non-oral reading of German words Hyperactivation in the left inferior frontal gyrus in the dyslexics Corina et al Dyslexics and controls; fMRI Phonological and lexical judgment tasks Phonological judgment in dyslexics: more activity in right than left inferior temporal gyrus and in left precentral gyrus; lexical judgment in dyslexics: less activity in bilateral middle frontal gyrus and more active than controls in left orbital frontal cortex; less active in left insula and left inferior temporal gyrus Aylward et al 10 children with RD and 11 normal readers; fMRI scanning Assigning sounds to letters task First task in dyslexics: less activation in left middle and inferior frontal gyri, right superior frontal gyrus, left middle and inferior temporal gyri, and bilateral superior parietal regions Understanding the relationship of suffixed words task Second task in dyslexics: reduced activations in left middle frontal gyrus, right superior parietal and fusiform/occipital region Shaywitz et al 70 dyslexic readers and 74 non-impaired readers; fMRI Pseudo- and real-word reading tasks for phonologic analysis Disruption in neural systems for reading involving parietotemporal and occipitotemporal area and positive correlation between reading skills and the activation in the left occipitotemporal region in dyslexics Shaywitz et al 49 subjects with RD and 28 controls; fMRI Phonologically based reading intervention Significant gains in reading fluency and demonstrated increased activation in left hemisphere regions, including the inferior frontal gyrus and the middle temporal gyrus in dyslexics Starting to activate bilateral inferior frontal gyri and left superior temporal and occipitotemporal regions of dyslexic’s brain Temple et al Dyslexics and normal readers; fMRI Rapid acoustic information processing task No differential left frontal response in dyslexic readers Temple et al Dyslexic and normal reading children; fMRI Phonological and orthographic tasks of rhyming and matching letters Letter rhyming: no activition in left temporoparietal cortex in dyslexics Letter matching: little activity in extrastriate cortex in dyslexics Temple et al 20 children with RD before and after remediation; fMRI Auditory processing and oral language training An increased activity in brain areas of children with dyslexia, including left temporoparietal cortex, left inferior frontal gyrus, right hemisphere frontal and temporal regions, and anterior cingulated gyrus A correlation between the magnitude of increased activation in left temporoparietal cortex and improvement in oral language ability in subjects with DD Hoeft et al 20 right-handed boys with DD and controls; shape analysis with MRI Rhyme judgment task Reduced activation relative to both age- and reading-matched children in the left and right parietotemporal cortex of brains of dyslexic children. Reduced activation bilaterally in the parietotemporal cortex in dyslexics Reduced left parietotemporal activation in 9 of the 10 dyslexic children Hoeft et al 19 dyslexic adolescents and 19 controls; fMRI Visual word rhyme judgment compared with visual cross-hair fixation rest Hypoactivation in left parietal and bilateral fusiform cortices and hyperactivation in left inferior and middle frontal gyri, caudate, and thalamus in dyslexics relative to age-matched controls Hypoactivation in left parietal and fusiform regions in dyslexics compared to reading-matched controls

RD, reading disability; fMRI, functional magnetic resonance imaging; DD, developmental dyslexia.

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Electrophysiological neuroimaging studies

Electroencephalography Studies

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Table 3

Electrophysiological Neuroimaging Experiments in Children with RD

Study Materials and Methods Functional Activation Results Klimesch et al 8 dyslexics and 8 control; EEG Reading numbers, words, and pseudowords task in tonic and phasic theta band power Dyslexics showed a complete lack of pseudoword processing at tonic and phasic lower theta for occipital sites Klimesch et al Dyslexic and control children; EEG Reading numbers, words, and pseudowords task in lower and upper alpha and two beta bands In the lower alpha band: an increased phasic response to words and pseudowords at right hemispheric sites but a lack to respond to words at second grade (01) in dyslexics In the upper alpha band: general increase tonic upper alpha power in dyslexics Spironelli et al Dyslexic children with phonological deficit and controls; EEG Word reading/encoding in the verbal working memory task in theta band An altered pattern of theta activation both in the temporal dimension cortical space of the brains of dyslexia. No findings with analysis of alpha band Scheuerpflug et al 16 dyslexic and 15 control children; ERP analysis Motion-onset and random-dot-kinematogram paradigms Differences in global ERP responses between dyslexic and control children for rapid moving gratings but not for the dot coherence Maurer et al Dyslexic and control children; ERP analysis Visual activity for print advances in the same children before and after initial reading training Reduced ERPs in the inferior occipito-temporal cortex, left-lateralized in dyslexic children Gomez-Velazquez et al Fifteen 8-year-old RD children and a control group; ERP analysis Inverse serial digit detection task Higher amplitude in visual working memory in RD group Sharma et al Dyslexic and control children; MMN analysis Reading and auditory processing task Smaller /ga/-evoked MMN area in the reading disorder group A correlation between reading fluency and accuracy and non-word scores, and auditory processing disorder measures Huttunen-Scott et al Children with AD and RD; MMN analysis Auditory discrimination task The MMNs were diminished in the right hemisphere in the RD group, in all frontal and central channels in the RD + AD group, and the MMN peaks appeared earlier in frontal channels in the AD group Silva-Pereyra et al Poor readers and control children; P300 analysis Word categorization task Longer reaction times, a poorer performance, longer and larger P2 amplitudes, and smaller amplitudes and longer P300 latencies in poor readers compared to controls whereas there were no differences in the N400 component between groups Meyler and Breznitz 17 dyslexic and 16 normal college-level readers; P300 measures Phonological and orthographic word representation task P200 and P300 components of lower amplitude and later latency among dyslexics than controls for both types of word representation Ucles et al Dyslexic and control children; P300 analysis Auditory “oddball” paradigm Difference at the mean power of the wavelet-transformed 40-Hz oscillations and their in dyslexics Bonte and Blomert Poorly and normally reading children; N400 analysis Auditory lexical decision task Later N400 and deviant priming effects in earlier time windows encompassing the N1 and N2 in dyslexics Sabisch et al 16 dyslexic children and their controls; N400 and P600 analysis Auditory sentence comprehension task Different pattern by a delayed left lateralized anterior negativity, followed by a P600, whereas no difference by a N400 in dyslexic children Schulz et al 16 dyslexics and 31 controls; N400 analysis Sentence reading and semantic processing Decreased activation for sentence reading in inferior parietal and frontal regions, and for semantic processing in inferior parietal regions, and during the N400 effect in dyslexics Simos et al 11 dyslexic children and 10 controls; MSI analysis Pseudoword rhyme-matching task Reduced activity in temporoparietal areas in the left hemisphere (including the posterior part of the superior temporal, angular and supramarginal gyri) and increased activity in the right homotopic region in dyslexics group Simos et al 8 dyslexic and 8 normal children; MSI analysis Pseudoword reading task before and after 80 hours of intensive remedial instruction Before intervention: aberrant activation profiles featuring little or no activation of the posterior portion of the superior temporal gyrus, and increased activation of the corresponding right hemisphere area in dyslexics After intervention: significant improvement in reading skills, activity in the left superior temporal gyrus increased by several orders of magnitude in dyslexics Sarkari et al Dyslexic and control children; MSI analysis; Printed word mapped onto spoken language Before: greater activity in the right temporoparietal region of dyslexic children After: aberrant profile could be normalized following intensive instruction Breier et al 12 dyslexic and 11 normal children; MEG analysis Discrimination between pairs of syllables from a voice onset time series (/ga/-/ka/) Correlation between increased relative activation in right temporoparietal areas and reduced performance on phonological processing measures in dyslexics

RD, reading disability; EEG, electroencephalogram; ERP, event-related potential; MMN, mismatch negativity; AD, attention deficient; MSI, magnetic source imaging; MEG, magnetoencephalography.

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Mismatch negativity studies

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P300 studies

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N400 studies

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Magnetoencephalography studies

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Conclusion

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Neurobiological Approaches on Brains of Children with Dyslexia

Postmortem studies

Structural neuroimaging studies

MRI Studies

The asymmetry of planum temporale

The volume and size of corpus callosum

The morphology of cerebellum

VBM Studies

DTI

Functional neuroimaging studies

fMRI Studies

Electrophysiological neuroimaging studies

Electroencephalography Studies

Event-related potentials studies

Mismatch negativity studies

P300 studies

N400 studies

Magnetoencephalography studies

Conclusion

References

Further Reading

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