Corpus callosum morphology, as measured with MRI, in dyslexic men

Preventing the Development of Dyslexia: A Premature Writing Hypothesis

It has been argued that dyslexia may develop in strongly left eye dominant children through learning to write using ipsilateral, right hemisphere motor pathways. New light on this theory has been cast by recent findings of atypical enhanced corpus callosum white matter in children with dyslexia, reflecting right to left hemisphere communication that is resistant to intensive remedial reading intervention. Enhanced corpus callosum white matter is consistent with uninhibited right to left hemisphere ipsilateral mirror-motor innervation, manifested as frequent mirror-letter writing errors in children with dyslexia. Delaying writing instruction until 7-8 years of age may prevent these errors and as well as the development of dyslexia. During the 7-8 year age period, visual-proprioceptive integration enables a child to mentally map whole word visual images onto kinaesthetic/proprioceptive letter engrams (memory representations). Hypothetically, this process is facilitated by anterior commissure activity involving inter-hemispheric transfer of ipsilateral mirror-to-non mirror motor movement. This postulate, involving delayed writing instruction pending further maturation, also receives indirect support from the remarkable proficiency leap among second graders reading Hebrew as Hebrew involves a leftward orthography in which ipsilateral right to left hemisphere innervation is uninhibited. Additionally, and more directly, normal reading comprehension for learning English among children with agenesis of the corpus callosum suggests that letter-sound decoding is not the sole route to proficient reading comprehension. In this paper, I make recommendations for obtaining empirical evidence of premature writing as a cause of dyslexia.

Applying microstructural models to understand the role of white matter in cognitive development

Diffusion MRI (dMRI) holds great promise for illuminating the biological changes that underpin cognitive development. The diffusion of water molecules probes the cellular structure of brain tissue, and biophysical modeling of the diffusion signal can be used to make inferences about specific tissue properties that vary over development or predict cognitive performance. However, applying these models to study development requires that the parameters can be reliably estimated given the constraints of data collection with children. Here we collect repeated scans using a typical multi-shell diffusion MRI protocol in a group of children (ages 7-12) and use two popular modeling techniques to examine individual differences in white matter structure. We first assess scan-rescan reliability of model parameters and show that axon water faction can be reliably estimated from a relatively fast acquisition, without applying spatial smoothing or de-noising. We then investigate developmental changes in the white matter, and individual differences that correlate with reading skill. Specifically, we test the hypothesis that previously reported correlations between reading skill and diffusion anisotropy in the corpus callosum reflect increased axon water fraction in poor readers. Both models support this interpretation, highlighting the utility of these approaches for testing specific hypotheses about cognitive development.

Cortical auditory evoked potentials and hemispheric specialization of speech in individuals with learning disability and healthy controls: A preliminary study

Background: Dichotic listening (DL) technique is a behavioral non-invasive tool which is used in studying hemispheric lateralization. Previous studies using behavioral DL have hypothesized that individuals with learning disabilities (LD) exhibit a lack of cortical specialization for processing speech stimulus. However, there is no event related potential (ERP) evidence, hence the main objective of the study is to explore hemispheric asymmetry using cortical auditory evoked potential (CAEPs) in normal hearing adults and also to compare the same in children with LD and healthy controls.

Methods: CAEPs were recorded in 16 normal hearing young adults, eight right-handed children with LD and their age matched controls. Two stop constants (/Pa/ – voiceless, bilabial, stop: /Ta/ - voiceless, alveolar, stop) were chosen for this experiment and presented in each ear and dichotically in two different orders (/pa-ta/, /ta-pa/). ERPs were processed using a standard pipeline, and electrodes readings over the left and right hemispheres were averaged to create left and right regions of interest (ROI). The CAEPs were analyzed for mean amplitude and peak latency of P1-N1-P2 components.

Results: The current study results suggest no statistically significant difference between the two stimulus in monaural condition and absence of order effect in dichotic condition. In healthy controls the CAEP latencies were shorter over the left hemisphere in both monaural and dichotic conditions in adults and control children. However, it was very evident that such a difference was lacking in children with LD.

Conclusions: Hemispheric asymmetry can be detected using CAEPs for speech stimulus. The measures are consistent and void of stimulus or order effect. Taken together, the findings of current study, both monaural and dichotic condition illustrates the hemispheric differences in processing speech stimuli in normal hearers. Absence of latency differences between hemispheres in children with LD indicate a lack of hemispheric asymmetry.

Rapid and widespread white matter plasticity during an intensive reading intervention

White matter tissue properties are known to correlate with performance across domains ranging from reading to math, to executive function. Here, we use a longitudinal intervention design to examine experience-dependent growth in reading skills and white matter in grade school-aged, struggling readers. Diffusion MRI data were collected at regular intervals during an 8-week, intensive reading intervention. These measurements reveal large-scale changes throughout a collection of white matter tracts, in concert with growth in reading skill. Additionally, we identify tracts whose properties predict reading skill but remain fixed throughout the intervention, suggesting that some anatomical properties stably predict the ease with which a child learns to read, while others dynamically reflect the effects of experience. These results underscore the importance of considering recent experience when interpreting cross-sectional anatomy–behavior correlations. Widespread changes throughout the white matter may be a hallmark of rapid plasticity associated with an intensive learning experience.

White matter properties correlate with cognitive performance in a number of domains. Here the authors show that altering a child’s educational environment though a targeted intervention program induces rapid, large-scale changes in the white matter, and that these changes track the learning process.

Regional brain volume reduction and cognitive outcomes in preterm children at low risk at 9 years of age

OBJECTIVE

More information is needed on "low-risk" preterm infants' neurological outcome so that they can be included in follow-up programs. A prospective study was performed to examine the regional brain volume changes compared to term children and to assess the relationship between the regional brain volumes to cognitive outcome of the low-risk preterm children at 9 years of age.

PATIENTS

Subjects comprised 22 preterm children who were determined to be at low risk for neurodevelopmental deficits with a gestational age between 28 and 33 weeks without a major neonatal morbidity in the neonatal period and 24 age-matched term control children term and matched for age, sex, and parental educational and occupational status.

METHODS

Regional volumetric analysis was performed for cerebellum, hippocampus, and corpus callosum area. Cognitive outcomes of both preterm and control subjects were assessed by Weschler Intelligence Scale for Children Revised (Turkish version), and attention and executive functions were assessed by Wisconsin Card Sorting Test and Stroop Test TBAG version.

RESULTS

Low-risk preterm children showed regional brain volume reduction in cerebellum, hippocampus, and corpus callosum area and achieved statistical significance when compared with term control. When the groups were compared for all WISC-R subscale scores, preterm children at low risk had significantly lower scores on information, vocabulary, similarities, arithmetics, picture completion, block design, object assembly, and coding compared to children born at term. Preterm and term groups were compared on the Stroop Test for mistakes and corrections made on each card, the time spent for completing each card, and total mistakes and corrections. In the preterm group, we found a positive correlation between regional volumes with IQ, attention, and executive function scores. Additionally, a significant correlation was found between cerebellar volume and attention and executive function scores in the preterm group.

CONCLUSION

Low-risk preterm children achieve lower scores in neurophysiological tests than children born at term. Preterm birth itself has a significant impact on regional brain volumes and cognitive outcome of children at 9 years of age. It is a risk factor for regional brain volume reductions in preterm children with low risk for neurodevelopmental deficits. The significant interaction between cerebellar volume reduction and executive function and attention may suggest that even in preterm children at low risk can have different trajectories in the growth and development of overall brain structure.

ROBO1 polymorphisms, callosal connectivity, and reading skills

The genetic effects on specific behavioral phenotypes are putatively mediated by specific neural functions. It remains unexplored how the axon-guidance-receptor gene ROBO1 influences reading performance through the neural system despite the identification of ROBO1 as a susceptibility gene for dyslexia. To address this issue, the present study recruited a group of children with a wide range of reading abilities. Two previously identified reading-related ROBO1 polymorphisms were genotyped, and diffusion and structural MRI were acquired to measure the fiber microstructure of the corpus callosum (CC), the major white-matter tract that connects inter-hemispheric cortical regions. The results confirmed the significant influence of the ROBO1 polymorphisms on reading scores. The fiber microstructures of the midline-CC segments around the genu and splenium were also affected by the ROBO1 polymorphisms. Moreover, a mediation analysis further revealed that the genu could significantly mediate the effects of the ROBO1 polymorphisms on word-list reading performance, which suggests a ROBO1-to-genu-to-reading pathway. The genu-linked cortical morphology, however, was not associated with either the ROBO1 polymorphisms or reading performance. These findings offer direct evidence supporting ROBO1-callosum association in humans and also provide valuable insight into the functions of ROBO1 and the gene-to-brain mechanisms that underlie human reading. Hum Brain Mapp 38:2616-2626, 2017. © 2017 Wiley Periodicals, Inc.

Network dynamics in dyslexia: Review and implications for remediation

Extant neurobiological theories of dyslexia appear fractional in focusing on isolated brain regions, mechanisms, and functional pathways. A synthesis of current research shows support for an Interactive Specialization (IS) model of dyslexia involving the dysfunctional orchestration of a widely-distributed, attentionally-controlled, hierarchical, and interhemispheric circuit of intercommunicating neuronal networks. This circuitry is comprised principally of the frontostriatal-parietal cognitive control system of networks, the posterior corpus callosum, and the left arcuate fasciculus. During development, the coalescence of these functionally specialized regions, acting together, may be essential to preventing the core phonemic and phonological processing deficits defining the dyslexic phenotype. Research demonstrating an association of each with processing phonology presents the foundational outline for a comprehensive, integrative theory of dyslexia and suggests the importance of inclusive remedial efforts aimed at promoting interactions among all three networking territories.

Brain asymmetry in the white matter making and globularity

Recent studies from the field of language genetics and evolutionary anthropology have put forward the hypothesis that the emergence of our species-specific brain is to be understood not in terms of size, but in light of developmental changes that gave rise to a more globular braincase configuration after the split from Neanderthals-Denisovans. On the grounds that (i) white matter myelination is delayed relative to other brain structures and, in humans, is protracted compared with other primates and that (ii) neural connectivity is linked genetically to our brain/skull morphology and language-ready brain, I argue that one significant evolutionary change in Homo sapiens' lineage is the interhemispheric connectivity mediated by the Corpus Callosum. The size, myelination and fiber caliber of the Corpus Callosum present an anterior-to-posterior increase, in a way that inter-hemispheric connectivity is more prominent in the sensory motor areas, whereas "high- order" areas are more intra-hemispherically connected. Building on evidence from language-processing studies that account for this asymmetry ('lateralization') in terms of brain rhythms, I present an evo-devo hypothesis according to which the myelination of the Corpus Callosum, Brain Asymmetry, and Globularity are conjectured to make up the angles of a co-evolutionary triangle that gave rise to our language-ready brain.

Individual differences in crossmodal brain activity predict arcuate fasciculus connectivity in developing readers

Crossmodal integration of auditory and visual information, such as phonemes and graphemes, is a critical skill for fluent reading. Previous work has demonstrated that white matter connectivity along the arcuate fasciculus (AF) is predicted by reading skill and that crossmodal processing particularly activates the posterior STS (pSTS). However, the relationship between this crossmodal activation and white matter integrity has not been previously reported. We investigated the interrelationship of crossmodal integration, both in terms of behavioral performance and pSTS activity, with AF tract coherence using a rhyme judgment task in a group of 47 children with a range of reading abilities. We demonstrate that both response accuracy and pSTS activity for crossmodal (auditory-visual) rhyme judgments was predictive of fractional anisotropy along the left AF. Unimodal (auditory-only or visual-only) pSTS activity was not significantly related to AF connectivity. Furthermore, activity in other reading-related ROIs did not show the same AV-only AF coherence relationship, and AV pSTS activity was not related to connectivity along other language-related tracts. This study is the first to directly show that crossmodal brain activity is specifically related to connectivity in the AF, supporting its role in phoneme-grapheme integration ability. More generally, this study helps to define an interdependent neural network for reading-related integration.

White matter lateralization and interhemispheric coherence to auditory modulations in normal reading and dyslexic adults

Neural activation of slow acoustic variations that are important for syllable identification is more lateralized to the right hemisphere than activation of fast acoustic changes that are important for phoneme identification. It has been suggested that this complementary function at different hemispheres is rooted in a different degree of white matter myelination in the left versus right hemisphere. The present study will investigate this structure-function relationship with Diffusion Tensor Imaging (DTI) and Auditory Steady-State Responses (ASSR), respectively. With DTI we examined white matter lateralization in the cortical auditory and language regions (i.e. posterior region of the superior temporal gyrus and the arcuate fasciculus) and white matter integrity in the splenium of the corpus callosum. With ASSR we examined interhemispheric coherence to slow, syllabic-rate (i.e. 4 Hz) and fast, phonemic-rate (i.e. 20 Hz) modulations. These structural and functional techniques were applied in a group of normal reading adults and a group of dyslexic adults for whom previously reduced functional interhemispheric connectivity at 20 Hz has been reported (Poelmans et al. (2012). Ear and Hearing, 33, 134-143). This sample was chosen since it is hypothesized that in dyslexic readers insufficient hemispheric asymmetry in myelination might relate to their auditory and phonological problems. Results demonstrate reduced white matter lateralization in the posterior superior temporal gyrus and the arcuate fasciculus in the dyslexic readers. Additionally, white matter lateralization in the posterior superior temporal gyrus and white matter integrity in the splenium of the corpus callosum related to interhemispheric coherence to phonemic-rate modulations (i.e. 20 Hz). Interestingly, this correlation pattern was opposite in normal versus dyslexic readers. These results might imply that less pronounced left white matter dominance in dyslexic adults might relate to their problems to process phonemic-rate acoustic information and to integrate them into the phonological system.

The effects of Kiaa0319 knockdown on cortical and subcortical anatomy in male rats

Developmental dyslexia is a disorder characterized by a specific deficit in reading despite adequate overall intelligence and educational resources. The neurological substrate underlying these significant behavioral impairments is not known. Studies of post mortem brain tissue from male and female dyslexic individuals revealed focal disruptions of neuronal migration concentrated in the left hemisphere, along with aberrant symmetry of the right and left the planum temporale, and changes in cell size distribution within the medial geniculate nucleus of the thalamus (Galaburda et al., 1985; Humphreys et al., 1990). More recent neuroimaging studies have identified several changes in the brains of dyslexic individuals, including regional changes in gray matter, changes in white matter, and changes in patterns of functional activation. In a further effort to elucidate the etiology of dyslexia, epidemiological and genetic studies have identified several candidate dyslexia susceptibility genes. Some recent work has investigated associations between some of these genetic variants and structural changes in the brain. Variants of one candidate dyslexia susceptibility gene, KIAA0319, have been linked to morphological changes in the cerebellum and functional activational changes in the superior temporal sulcus (Jamadar et al., 2011; Pinel et al., 2012). Animal models have been used to create a knockdown of Kiaa0319 (the rodent homolog of the human gene) via in utero RNA interference in order to study the gene's effects on brain development and behavior. Studies using this animal model have demonstrated that knocking down the gene leads to focal disruptions of neuronal migration in the form of ectopias and heterotopias, similar to those observed in the brains of human dyslexics. However, further changes to the structure of the brain have not been studied following this genetic disruption. The current study sought to determine the effects of embryonic Kiaa0319 knockdown on volume of the cortex and hippocampus, as well as midsagittal area of the corpus callosum in male rats. Results demonstrate that Kiaa0319 knockdown did not change the volume of the cortex or hippocampus, but did result in a significant reduction in the midsagittal area of the corpus callosum. Taken in the context of previous reports of behavioral deficits following Kiaa0319 knockdown (Szalkowski et al., 2012), and reports that reductions of corpus callosum size are related to processing deficits in humans (Paul, 2011), these results suggest that Kiaa0319 has a specific involvement in neural systems important for temporal processing.

Spherical harmonic analysis of cortical complexity in autism and dyslexia

Alterations in gyral form and complexity have been consistently noted in both autism and dyslexia. In this present study, we apply spherical harmonics, an established technique which we have exapted to estimate surface complexity of the brain, in order to identify abnormalities in gyrification between autistics, dyslexics, and controls. On the order of absolute surface complexity, autism exhibits the most extreme phenotype, controls occupy the intermediate ranges, and dyslexics exhibit lesser surface complexity. Here, we synthesize our findings which demarcate these three groups and review how factors controlling neocortical proliferation and neuronal migration may lead to these distinctive phenotypes.

Developmental malformation of the corpus callosum: a review of typical callosal development and examples of developmental disorders with callosal involvement

This review provides an overview of the involvement of the corpus callosum (CC) in a variety of developmental disorders that are currently defined exclusively by genetics, developmental insult, and/or behavior. I begin with a general review of CC development, connectivity, and function, followed by discussion of the research methods typically utilized to study the callosum. The bulk of the review concentrates on specific developmental disorders, beginning with agenesis of the corpus callosum (AgCC)-the only condition diagnosed exclusively by callosal anatomy. This is followed by a review of several genetic disorders that commonly result in social impairments and/or psychopathology similar to AgCC (neurofibromatosis-1, Turner syndrome, 22q11.2 deletion syndrome, Williams yndrome, and fragile X) and two forms of prenatal injury (premature birth, fetal alcohol syndrome) known to impact callosal development. Finally, I examine callosal involvement in several common developmental disorders defined exclusively by behavioral patterns (developmental language delay, dyslexia, attention-deficit hyperactive disorder, autism spectrum disorders, and Tourette syndrome).

Abnormalities of the corpus callosum in non-psychotic high-risk offspring of schizophrenia patients

Alterations in the structure of the corpus callosum (CC) have been observed in schizophrenia. Offspring of schizophrenia parents have 10-15 times higher risk for developing schizophrenia. We examined CC volume in offspring at genetic high-risk (HR) subjects. Since the sub-regions of the CC are topographically mapped to cortical brain regions, we hypothesized that HR subjects may show a decrement in total volume and differential volume decreases in sub-regions of the CC. The offspring of schizophrenia parents (HR; n=70; 36 males) and healthy volunteers with no family or personal history of psychotic disorders (healthy controls (HC); n=73; 37 males) matched for age, gender and education were selected for the study. Magnetic resonance images were collected using a GE 1.5 T scanner and processed using FreeSurfer image analysis software. The CC was divided into five neuroanatomically based partitions. The volume of total CC and the five sub-regions were measured blind to clinical information. With covariation for intracranial volume, HR subjects had significantly reduced total CC, more prominently observed in the anterior splenium. An age-related increase in CC volume was found in the anterior and posterior splenium of healthy controls but not in HR subjects. The volume reduction was greater in male than female HR subjects. The volume reduction in the CC may reflect a reduction in axonal fibers crossing the hemispheres and/or myelination between the left and right temporo-parietal cortices. The absence of an age-related volume increase suggests an abnormal developmental trajectory that may underlie susceptibility to schizophrenia.

Corpus callosum abnormalities and potential age effect in men with schizophrenia: an MRI comparative study

The goal of this investigation was to evaluate corpus callosum (CC) morphometry in schizophrenia. In consideration of possible confounders such as age, gender and handedness, our study sample was restricted to right-handed male subjects, aged 18-55 years. In addition, we controlled for age at onset, illness duration and exposure to antipsychotic medication. Midsagittal CC linear and area Magnetic Resonance Imaging (MRI) measurements were performed on 50 subjects with schizophrenia and 50 healthy controls. After controlling for midsagittal cortical brain area and age, Analysis of Covariance (ANCOVA) revealed an overall effect of diagnosis on CC splenium width and CC anterior midbody area and a diagnosis by age interaction. Independent Student t tests revealed a smaller CC splenium width in the 36- to 45-year-old age group among the patients with schizophrenia and a smaller CC anterior midbody area in the 18- to 25-year-old age group among the patients with schizophrenia compared with controls. Age, age at onset, illness duration and psychopathology ratings did not show any significant correlations with the whole CC MRI measurements. A negative correlation was found between CC rostrum area and the estimated lifetime neuroleptic consumption. The results are discussed in terms of the possibility that CC structural changes may underlie the functional impairments, frequently reported in schizophrenia, of the associated cortical regions.

Corpus Callosum Shape Analysis with Application to Dyslexia

Morphometric studies of the corpus callosum suggest its involvement in a number of psychiatric conditions. In the present study we introduce a novel pattern recognition technique that offers a point-by-point shape descriptor of the corpus callosum. The method uses arc lengths of electric field lines in order to avoid discontinuities caused by folding anatomical contours. We tested this technique by comparing the shape of the corpus callosum in a series of dyslexic men (n = 16) and age-matched controls (n = 14). The results indicate a generalized increase in size of the corpus callosum in dyslexia with a concomitant diminution at its rostral and caudal poles. The reported shape analysis and 2D-reconstruction provide information of anatomical importance that would otherwise passed unnoticed when analyzing size information alone.

Microsurgical anatomy of the ventral callosal radiations: new destination, correlations with diffusion tensor imaging fiber-tracking, and clinical relevance

OBJECT

In the current literature, there is a lack of a detailed map of the origin, course, and connections of the ventral callosal radiations of the human brain.

METHODS

The authors used an older dissection technique based on a freezing process as well as diffusion tensor imaging to investigate this area of the human brain.

RESULTS

The authors demonstrated interconnections between areas 11, 12, and 25 for the callosal radiations of the trunk and rostrum of the corpus callosum; between areas 9, 10, and 32 for the genu; and between areas 6, 8, and 9 for the ventral third of the body. The authors identified new ventral callosal connections crossing the rostrum between both temporal poles and coursing within the temporal stem, and they named these connections the "callosal radiations of Peltier." They found that the breadth of the callosal radiations slightly increases along their course from the rostrum to the first third of the body of the corpus callosum.

CONCLUSIONS

The fiber dissection and diffusion tensor imaging techniques are complementary not only in their application to the study of the commissural system in the human brain, but also in their practical use for diagnosis and surgical planning. Further investigations, neurocognitive tests, and other contributions will permit elucidation of the functional relevance of the newly identified callosal radiations in patients with disease involving the ventral corpus callosum.

Brain imaging findings in dyslexia

Dyslexia is a brain-based disorder that has been intensively studied in the Western world for more than a century because of its social burden. However, affected individuals in Chinese communities are neither recognized nor formally diagnosed. Previous studies have concentrated on the disadvantages of reading deficits, and few have addressed non-linguistic skills, which are included in the symptoms. In addition, certain dyslexics possess visual spatial talents that have usually been ignored. In this review, we discuss the available information regarding brain imaging studies of dyslexia based on studies in Caucasian subjects. Gray matter deficits have been demonstrated in dyslexics using structural magnetic resonance imaging. Reduced neural activities in the left temporal and left parietal cortices, and diffuse widespread activation patterns in the cerebellum could be detected using functional magnetic resonance imaging. Changes in lactate levels, N-acetylaspartate/choline-containing compounds and N-acetylaspartate/creatine ratios, and phosphomonoester peak area were detected in magnetic resonance spectroscopy studies. Lower fractional anisotropy values in bilateral white matter tracts have been demonstrated by diffusion tensor imaging. Abnormal Broca's area activation was found using positron emission tomography imaging. Increased activities in the right frontal and temporal brain regions were detected using electroencephalography. Reduced hemispheric asymmetry and increased left inferior frontal activation were reported following magnetoencephalography. Although these imaging modalities are not currently diagnostic or prognostic, they are able to provide information on the causes of dyslexia beyond what was previously provided by behavioral or cognition studies.

An anatomical signature for literacy

Language is a uniquely human ability that evolved at some point in the roughly 6,000,000 years since human and chimpanzee lines diverged. Even in the most linguistically impoverished environments, children naturally develop sophisticated language systems. In contrast, reading is a learnt skill that does not develop without intensive tuition and practice. Learning to read is likely to involve ontogenic structural brain changes, but these are nearly impossible to isolate in children owing to concurrent biological, environmental and social maturational changes. In Colombia, guerrillas are re-integrating into mainstream society and learning to read for the first time as adults. This presents a unique opportunity to investigate how literacy changes the brain, without the maturational complications present in children. Here we compare structural brain scans from those who learnt to read as adults (late-literates) with those from a carefully matched set of illiterates. Late-literates had more white matter in the splenium of the corpus callosum and more grey matter in bilateral angular, dorsal occipital, middle temporal, left supramarginal and superior temporal gyri. The importance of these brain regions for skilled reading was investigated in early literates, who learnt to read as children. We found anatomical connections linking the left and right angular and dorsal occipital gyri through the area of the corpus callosum where white matter was higher in late-literates than in illiterates; that reading, relative to object naming, increased the interhemispheric functional connectivity between the left and right angular gyri; and that activation in the left angular gyrus exerts top-down modulation on information flow from the left dorsal occipital gyrus to the left supramarginal gyrus. These findings demonstrate how the regions identified in late-literates interact during reading, relative to object naming, in early literates.

Regional corpus callosum morphometry: effect of field strength and pulse sequence

PURPOSE

To investigate whether scanning at different field strengths and pulse sequences would influence reproducibility of corpus callosum (CC) morphometric measurements as variations in scanning parameters may result in differences in contrast properties between resulting images that are independent of the underlying tissue but rather reflect the physics of the imaging process.

MATERIALS AND METHODS

Ten subjects were scanned twice at 3T using magnetization-prepared rapid gradient-echo imaging (MPRAGE) and modified driven equilibrium Fourier transform (MDEFT) sequences and once at 1.5T using MPRAGE. Cross-sectional area measurements of four callosal regions were performed on midsagittal magnetic resonance imaging (MRI) sections.

RESULTS

Repeated measures analysis of variance with four regions as dependent variables and three scanning protocols (1.5T MPRAGE, 3T MPRAGE and 3T MDEFT) as independent variables revealed no significant region by protocol interaction: F(6,54) = 0.69, P = 0.52. Reliability measures for (3T MPRAGE/3T MDEFT) and (1.5T MPRAGE/3T MPRAGE; 1.5T MPRAGE/3T MDEFT) comparisons were high, ranging between 0.90 and 0.97.

CONCLUSION

Based on our results, combining and comparing CC morphometric data obtained at different field strengths and/or with different pulse sequences appears possible.

Neurobiological approaches on brains of children with dyslexia: review

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.

Splenium microstructure is related to two dimensions of reading skill

Inconsistent differences in the corpus callosum (CC) structure between dyslexic readers (DRs) and typical readers (TRs) have been reported. We examine differences in CC splenium microstructure and the association of splenium microstructure with reading-related skills. Nine DRs and 18 TRs completed a reading skills battery and diffusion tensor imaging. DRs had higher splenium fractional anisotropy (FA) and axial diffusivity (LA) as compared with TRs. Retrieval of orthographic information from the language lexicon was negatively associated with FA and LA within both reading groups. Phonological awareness was positively associated with splenium FA and LA in TRs but not DRs. This study suggests two white matter pathways that may be differentially associated with reading skills in the CC splenium.

M, Jain S. Corpus callosal area differences and gender dimorphism in neuroleptic-naïve, recent-onset schizophrenia and healthy control subjects

The study of corpus callosal morphometry is important to unravel the underlying connectivity disturbance in schizophrenia. We studied the corpus callosal area in schizophrenia subjects compared to healthy subjects, while controlling for several confounders that could affect morphometric measures of the corpus callosum (CC). Areas of the whole CC and its sub-regions obtained by two geometric partitioning schemes were studied in 23 right-handed neuroleptic-naïve, recent-onset, schizophrenia patients and compared with 23 right-handed age-, sex- and education-matched healthy subjects. The patients did not differ from controls in whole CC area. On tripartite division of the CC, the area of the anterior sub-region was significantly higher in patients compared to controls. On radial division into 5 sub-regions, the anterior truncus area was significantly higher in patients compared to controls. There was a significant effect of gender (F>M) on the area measures; however there was no significant diagnosis()gender effect. Age, age of onset, duration of illness and psychopathology ratings did not show any significant correlations with whole CC area and area of CC sub-regions. The finding of increased area of the anterior truncus that possibly comprises white fibres connecting the temporal association cortices could be indicative of an "abnormal functional hyperconnection" involving these regions in positive symptom schizophrenia. Additionally, the finding of females having larger areas of the whole CC and of the anterior and middle sub-regions could reflect a "normal hyperconnection" underlying increased ambilaterality in females.

Asymmetry of language activation relates to regional callosal morphology following early cerebral injury

The anatomical factors underlying reorganization of language representation are yet to be elucidated, although correlations between asymmetric structures and language lateralization have been identified. Previous research has implicated the corpus callosum in the development of language lateralization. This study examined the relationship between callosal morphology and language asymmetry, using letter fluency functional magnetic resonance imaging, in 13 patients with focal epilepsy and 8 healthy controls. Regional callosal thickness was determined without relying on a priori delineation of callosal segments. We predicted that language asymmetry measured by fMRI activation laterality scores would be correlated with regional callosal thickness in both groups. However, only the degree of language activation asymmetry was significantly correlated with callosal thickness in the isthmus and the midbody of patients, and there was a significant interaction between the groups with respect to callosal thickness and language activation asymmetry. These data suggest that callosal pathways may be important for language reorganization in the context of early cerebral injury.

Interhemispheric cooperation and non-cooperation during word recognition: evidence for callosal transfer dysfunction in dyslexic adults

Participants report briefly-presented words more accurately when two copies are presented, one in the left visual field (LVF) and another in the right visual field (RVF), than when only a single copy is presented. This effect is known as the 'redundant bilateral advantage' and has been interpreted as evidence for interhemispheric cooperation. We investigated the redundant bilateral advantage in dyslexic adults and matched controls as a means of assessing communication between the hemispheres in dyslexia. Consistent with previous research, normal adult readers in Experiment 1 showed significantly higher accuracy on a word report task when identical word stimuli were presented bilaterally, compared to unilateral RVF or LVF presentation. Dyslexics, however, did not show the bilateral advantage. In Experiment 2, words were presented above fixation, below fixation or in both positions. In this experiment both dyslexics and controls benefited from the redundant presentation. Experiment 3 combined whole words in one visual field with word fragments in the other visual field (the initial and final letters separated by spaces). Controls showed a bilateral advantage but dyslexics did not. In Experiments 1 and 3, the dyslexics showed significantly lower accuracy for LVF trials than controls, but the groups did not differ for RVF trials. The findings suggest that dyslexics have a problem of interhemispheric integration and not a general problem of processing two lexical inputs simultaneously.

Temporal-callosal pathway diffusivity predicts phonological skills in children

The development of skilled reading requires efficient communication between distributed brain regions. By using diffusion tensor imaging, we assessed the interhemispheric connections in a group of children with a wide range of reading abilities. We segmented the callosal fibers into regions based on their likely cortical projection zones, and we measured diffusion properties in these segmented regions. Phonological awareness (a key factor in reading acquisition) was positively correlated with diffusivity perpendicular to the main axis of the callosal fibers that connect the temporal lobes. These results could be explained by several physiological properties. For example, good readers may have fewer but larger axons connecting left and right temporal lobes, or their axon membranes in these regions may be more permeable than the membranes of poor readers. These measurements are consistent with previous work suggesting that good readers have reduced interhemispheric connectivity and are better at processing rapidly changing visual and auditory stimuli.

White matter pathways in reading

Skilled reading requires mapping of visual text to sound and meaning. Because reading relies on neural systems spread across the brain, a full understanding of this cognitive ability involves the identification of pathways that communicate information between these processing regions. In the past few years, diffusion tensor imaging has been used to identify correlations between white matter properties and reading skills in adults and children. White matter differences have been found in left temporo-parietal areas and in posterior callosal tracts. We review these findings and relate them to possible pathways that are important for various aspects of reading. We describe how the results from diffusion tensor imaging can be integrated with functional results in good and poor readers.

Role of the corpus callosum in speech comprehension: interfacing syntax and prosody

The role of the corpus callosum (CC) in the interhemispheric interaction of prosodic and syntactic information during speech comprehension was investigated in patients with lesions in the CC, and in healthy controls. The event-related brain potential experiment examined the effect of prosodic phrase structure on the processing of a verb whose argument structure matched or did not match the prior prosody-induced syntactic structure. While controls showed an N400-like effect for prosodically mismatching verb argument structures, thus indicating a stable interplay between prosody and syntax, patients with lesions in the posterior third of the CC did not show this effect. Because these patients displayed a prosody-independent semantic N400 effect, the present data indicate that the posterior third of the CC is the crucial neuroanatomical structure for the interhemispheric interplay of suprasegmental prosodic information and syntactic information.

Interhemispheric communication involving multiple tasks: A study of children with dyslexia

The study investigated whether inefficient interhemispheric communication is involved in developmental dyslexia using multiple tasks. A finger localization task, rhyming judgment task, primed lexical decision task, and a visual half-field presentation paradigm were used. Nineteen dyslexic children (mean age = 13.1 years) were compared with 26 chronological age-matched normal children. Although the dyslexic group demonstrated significantly slower and less accurate performance in all three tasks, there was no significant group difference in term of interhemispheric communication. However, priming effects demonstrated by the dyslexic group (p < .05) further indicate that their reading problems may stem from the word retrieval process from the long term memory.

Brain abnormalities underlying altered activation in dyslexia: a voxel based morphometry study

Voxel-based morphometry was used to assess the consistency among functional imaging and brain morphometry data in developmental dyslexia. Subjects, from three different cultural contexts (UK, France and Italy), were the same as those described in a previous PET activation paper, which revealed a common pattern of reduced activation during reading tasks in the left temporal and occipital lobes. We provide evidence that altered activation observed within the reading system is associated with altered density of grey and white matter of specific brain regions, such as the left middle and inferior temporal gyri and the left arcuate fasciculus. This supports the view that dyslexia is associated with both local grey matter dysfunction and with altered connectivity among phonological/reading areas. The differences were replicable across samples confirming that the neurological disorder underlying dyslexia is the same across the cultures investigated in the study.

Structural brain imaging of attention-deficit/hyperactivity disorder

Many investigators have hypothesized that attention-deficit/hyperactivity disorder (ADHD) involves structural and functional brain abnormalities in frontal-striatal circuitry. Although our review suggests that there is substantial support for this hypothesis, a growing literature demonstrates widespread abnormalities affecting other cortical regions and the cerebellum. Because there is only one report studying adults with ADHD, this summary is based on children. A key limitation of the literature is that most of the studies until recently have been underpowered, using samples of fewer than 20 subjects per group. Nevertheless, these studies are largely consistent with the most comprehensive and definitive study (Castellanos et al 2002). Moreover, studies differ in the degree to which they address the influence of medications, comorbidities, or gender, and most have not addressed potentially important sources of heterogeneity such as family history of ADHD, subtype, or perinatal complications. Despite these limitations, a relatively consistent picture has emerged. The most replicated alterations in ADHD in childhood include significantly smaller volumes in the dorsolateral prefrontal cortex, caudate, pallidum, corpus callosum, and cerebellum. These results suggest that the brain is altered in a more widespread manner than has been previously hypothesized. Developmental studies are needed to address the evolution of this brain disorder into adulthood.

The effects of redundant stimuli on visuospatial processing in developmental dyslexia

The interhemispheric deficit theory of dyslexia postulates that reading difficulties can arise from abnormal communication/collaboration between the cerebral hemispheres. A currently popular way to gather information about interhemispheric processing and integration is with the redundant stimuli task, where participants respond to stimuli presented to the left visual field, right visual field, or both visual fields simultaneously. In neurologically normal individuals, response times to bilateral simple stimulus presentations are faster than response times to a single stimulus in either visual field alone (referred to as redundancy gain). In contrast, individuals with no corpus callosum exhibit greater redundancy gains than would be expected by probability summation. In the present study, 11 children with phonological dyslexia showed a similar "over violation" of the probability (race) model when responding with the left but not the right hand. This asymmetry was not found in age- and IQ-matched control children. The results are at least partially consistent with the notion of phonological dyslexia involving deficits in the transfer of information across the corpus callosum.

Brain function and structure in adults with attention-deficit/hyperactivity disorder

Cross-sectional data suggest that brain dysfunctions are a central component of attention-deficit/hyperactivity disorder (ADHD) in children, and a growing literature is suggesting the same for adults. This article reviews the current state of the literature pertaining to the structural and functional brain abnormalities that are found in adults with ADHD. Because the literature on ADHD in children is more extensive than that reported heretofore in ADHD in adults,the authors include brief summaries of the child literature to help inform that found in adults.

Anatomical risk factors that distinguish dyslexia from SLI predict reading skill in normal children

These studies investigated whether anatomical measures could separate phonologically-based reading disability (PD) from nonphonologically-based learning disabilities such as specific language impairment (SLI). In a previous study. four brain measures (cerebral asymmetry. summed planum temporale and parietale asymmetry, anterior cerebellar asymmetry, and a duplicated left Heschl's gyrus) distinguished a group of PD adults from reading disabled adults without specific phonological deficits (URD). Study 1 found that these measures did not distinguish 14 reading disabled children from 21 children with SLI. Instead, differences were found in cerebral volume, planum temporale asymmetry, and the size of a single left Heschl's gyrus. Study 2 demonstrated that including all seven measures in a discriminant analysis separated the adults and children into two groups: one with 100% of the PD adults and 75% of the reading disabled children and the other with 72% of the SLI children and 75% of the URD adults. Study 3 demonstrated that an anatomical risk factor index (ARF7) generated from the discriminant function with seven brain measures predicted reading in normal children. Children with ARF7 near 0 (normal anatomy) had superior verbal ability and phonological decoding scores that improved with age. Normal children with negative ARF7 the relatively s mall symmetrical structures that characterize SLI)had deficits in verbal ability. Children with positive ARF7 (the asymmetrical structures that characterize PD) had phonological decoding scores that decreased with age. These results suggest that PD and SLI are qualitatively different disorders associated with anatomical deviations in opposite directions from the population mean.

LEARNING OUTCOMES

As a result of this activity, the participant will be able to: (1) distinguish the neuroanatomical features that characterize PD and SLI; (2) recognize that PD is associated with large asymmetrical brain structures while SLI is associated with smaller symmetrical brain structures; (3) understand that children with moderate sized brains and whose anatomy is intermediate between symmetry and extreme asymmetry have an enhanced probability of developing good verbal ability; (4) understand that reading disabilities depend on the interaction of neurodevelopment and the environment.

A callosal transfer deficit in children with developmental language disorder

Twenty-two control children (aged 6-12 years) and forty-three children with developmental language disorder (DLD) (aged 7-12 years) received a test of callosal transfer of tactile information. Among the children with dysphasia, 30 had a diagnosis of receptive dysphasia and 13 of expressive dysphasia. Both control children and children with DLD made a significantly larger number of errors in the crossed localization condition (implying callosal transfer of tactile information) versus the uncrossed localization condition. In the crossed localization condition, children with DLD made a significantly larger number of errors than controls, while no differences were found in the two groups of children with DLD. These data suggest that the corpus callosum may be involved in the pathogenesis of DLD.

Prenatal disturbance alters the size of the corpus callosum in young monkeys

Many factors during fetal life and early infancy have been found to affect the development of the brain. The following study investigated whether maternal stress during pregnancy would influence the size and shape of one sensitive brain region, the corpus callosum, in infant monkeys. For 30% of the gestation period, from Days 90 to 140 postconception, gravid females were disturbed using an acoustical startle protocol for 10 min per day. Magnetic resonance imaging was then employed to obtain sagittal and coronal scans of their infants' brains. Morphometric measures of the corpus callosum were compared in 16 monkeys (5 controls and 11 from disturbed pregnancies). Prenatal conditions altered the corpus callosum, but in a differential manner for male and female monkeys. Based on the midsagittal and parasagittal scans, prenatally disturbed male offspring showed a decrease in overall size of the corpus callosum whereas the prenatal disturbance resulted in an increased area in females. An evaluation of callosal height from the coronal images suggested that the volumetric change was associated with a shift in anterior-to-posterior shape from the genu back toward the splenium. These findings concur with observations in other animals and humans, which have indicated that prenatal and postnatal factors can influence the development of the corpus callosum, possibly affecting communication between the hemispheres.

Cognitive brain potentials to novel acoustic stimuli in adult dyslexic readers

Event-related brain potentials were recorded in two three-stimulus oddball tasks in 13 adult dyslexic and 13 age- and IQ-matched normal readers. The stimuli consisted of a random series of frequent (80%) and non-frequent tones (10%) as well as occasionally inserted novel sounds (10%). The experiment comprised an active (response to the rare target tone) and a passive listening condition. No performance differences were found for dyslexic and normal readers in the active task. In both conditions, novel sounds evoked a centrally distributed P3a-component followed by a P3b-component most prominent at parietal electrodes for target and novel sounds. Additionally, a slow negativity emerged after presentation of novel sounds at frontal electrodes. In the active condition only, peak amplitude of the P3a and the frontal slow negativity to novel stimuli were slightly enlarged for dyslexic readers. These findings indicate a larger distractability of dyslexic readers (enhancement of P3a to novel tones). Furthermore, we propose that dyslexics need to employ more cognitive resources to refocus on the task at hand (as indicated by the enlarged slow frontal negativity).

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.

Individual differences in neurobehavioral measures of laterality and interhemispheric function as measured by dichotic listening

This article presents analysis of dichotic listening performance in 57 healthy men and women aged 20 to 72 years. The data are presented as a means to cover 2 theoretical issues relevant to the biobehavioral study of laterality and interhemispheric relations. First, the sensitivity of dichotic listening performance to factors such as sex, age, and their interactions was examined. Dichotic listening asymmetry scores were found to vary as a function of sex, age, handedness, and family history of developmental language disorders. The effects of sex and age were then explored in relation to a comparison of nonforced and unilaterally focused test conditions. The results suggest that even within a healthy, normative sample of human participants, individuals vary not only in their underlying perceptual asymmetries for auditory input, but also in the manner in which such asymmetries interact with other higher order cognitive functions.

Callosal transfer in different subtypes of developmental dyslexia

Sixteen controls (age 6-13) and 20 native Italian children with developmental dyslexia (age 7-15) received a test of callosal transfer of tactile information. Among the dyslexic children, 7 had a diagnosis of L-type, 7 of P-type and 6 of M-type dyslexia according to Bakker's classification. Both control children and children with dyslexia made a significantly larger number of errors in the crossed localization condition (implying callosal transfer of tactile information) vs. the uncrossed condition. In the same condition, children with dyslexia made a significantly larger number of errors than controls. In the crossed localization condition L-types and M-types made a significantly larger number of errors than P-types and controls, while there was no significant difference in performance between P-types and controls. These findings are discussed in terms of defective callosal transfer or deficient somatosensory representation in children with L- and M-dyslexia.

Corpus callosum size in children with developmental language disorder

Using high-resolution in-vivo magnetic resonance morphometry of the midsagittal area of the corpus callosum (CC) and four callosal subareas in 21 children with developmental language disorder (DLD) of the phonologic-syntactic type we found no significant anatomical differences in comparison to an age- and gender-matched normal control group. There was also no significant between-group difference when the approximately 7% smaller forebrain volume among children with DLD was accounted for by relating CC measures to forebrain volume. Only a tendency towards a larger anterior and middle CC in relation to forebrain volume was found in DLD children. In our DLD children we found the same relationship between CC midsagittal size and forebrain volume as recently reported for normal adults, namely, that the CC area increases to the two-third power of forebrain volume.

Reading and spelling disorders: clinical features and causes

Developmental dyslexia (specific reading and specific spelling disorder) is thought to stem from specific features in cognitive processing strongly related to biological maturation of the central nervous system which interact with non-biological learning conditions. The specific learning disorder should not be accounted for by mental age, gross neurological deficits, emotional disturbances or inadequate schooling. As a clinical guideline, the child's level in reading and spelling must be significantly below that expected for the population of children of the same mental age. The persistence rate is high and dyslexia is often associated with psychiatric problems. The etiology is not known. From the biological point of view, dyslexia is supposed to have a neurological basis. Neuroanatomical, neurophysiological, and neuropsychological correlates have been studied by means of autopsy, brain imaging, neurophysiological and neuropsychological methods. There is good evidence that dyslexia is determined by heritable cognitive components of reading and spelling processing. Experimental research focuses on characteristics of brain structure and cognitive skills related to the central nervous systems of auditory-phonological and visual information processing.