Genetics of learning disabilities

Animal Models of Childhood Exposure to Lead or Manganese: Evidence for Impaired Attention, Impulse Control, and Affect Regulation and Assessment of Potential Therapies

Behavioral disorders involving attention and impulse control dysfunction, such as ADHD, are among the most prevalent disorders in children and adolescents, with significant impact on their lives. The etiology of these disorders is not well understood, but is recognized to be multifactorial, with studies reporting associations with polygenic and environmental risk factors, including toxicant exposure. Environmental epidemiological studies, while good at establishing associations with a variety of environmental and genetic risk factors, cannot establish causality. Animal models of behavioral disorders, when properly designed, can play an essential role in establishing causal relationships between environmental risk factors and a disorder, as well as provide model systems for elucidating underlying neural mechanisms and testing therapies. Here, we review how animal model studies of developmental lead or manganese exposure have been pivotal in (1) establishing a causal relationship between developmental exposure and lasting dysfunction in the domains of attention, impulse control, and affect regulation, and (2) testing the efficacy of specific therapeutic approaches for alleviating the lasting deficits. The lead and manganese case studies illustrate how animal models can advance knowledge in ways that are not possible in human studies. For example, in contrast to the Treatment of Lead Poisoned Children (TLC) human clinical trial evaluating succimer chelation efficacy to improve cognitive functioning in lead-exposed children, our developmental lead exposure animal model showed that succimer chelation can produce lasting cognitive benefits if chelation sufficiently reduces brain lead levels. In addition, this study revealed that succimer treatment in the absence of lead exposure produces lasting cognitive dysfunction, highlighting potential risks of chelation in off-label uses, such as the treatment of autistic children without a history of lead exposure. Our animal model of developmental manganese exposure has demonstrated that manganese can cause lasting attentional and sensorimotor deficits, akin to an ADHD-inattentive behavioral phenotype, thereby providing insights into the role of environmental exposures as contributors to ADHD. These studies have also shown that oral methylphenidate (Ritalin) can fully alleviate the deficits produced by early developmental Mn exposure. Future work should continue to focus on the development and use of animal models that appropriately recapitulate the complex behavioral phenotypes of behavioral disorders, in order to determine the mechanistic basis for the behavioral deficits caused by developmental exposure to environmental toxicants, and the efficacy of existing and emerging therapies.

Categorical and Dimensional Diagnoses of Dyslexia: Are They Compatible?

Dyslexia is often assessed using categorical diagnoses, and subtypes of dyslexia are also recognized in a categorical fashion. Children may meet the criteria for dyslexia, and they may more specifically meet the criteria for a subtype of it, and thus get a diagnosis. This approach to diagnosis clashes with the actual distribution of reading performance in children (which is normal and continuous), and it has received criticism. This article offers a conceptual framework for conciliating these two positions. In short, the proposal is to use a set of multicomponent continuous assessments of reading, rather than thresholds. The proposal is explained using original data obtained from a sample of 30 children (age 7 to 11), tested in the United Kingdom. Using an assessment based on categorical-thresholds, only five children in our sample qualify for extra assistance, and only one may get a diagnosis of dyslexia, while with the mixed system proposed, a few additional children in the gray area would receive attention. This approach would not discard previous categorical approaches such as those distinguishing between surface and phonological dyslexia, but it would rather see these subtypes of dyslexia as the instance of a lower score on the continuum obtained on a single component of the multicomponent assessment.

Reading as a Predictor of Complex Syntax. The Case of Relative Clauses

Background

The current study aims at better characterizing the role of reading skills as a predictor of comprehension of relative clauses. Well-established cross-linguistic evidence shows that children are more accurate in the comprehension of subject-extracted relative clauses in comparison to the object-extracted counterpart. Children with reading difficulties are known to perform less accurately on object relatives at the group level compared to typically developing children. Given that children’s performance on reading tasks is shown to shape as a continuum, in the current study we attempted to use reading skills as a continuous variable to predict performance on relative clauses.

Methods

We examined the comprehension of relative clauses in a group of 30 English children (7–11 years) with varying levels of reading skills. Reading skills varied on a large spectrum, from poor readers to very skilled readers, as assessed by the YARC standardized test. The experimental task consisted of a picture-matching task. Children were presented with subject and object relative clauses and they were asked to choose one picture - out of four - that would best represent the sentence they heard. At the same time, we manipulated whether the subject and object nouns were either matching (both singular or both plural) or mismatching (one singular, the other plural) in number.

Results

Our analysis of accuracy shows that subject relatives were comprehended more accurately overall than object relatives, that responses to sentences with noun phrases mismatching in number were more accurate overall than the ones with matching noun phrases and that performance improved as a function of reading skills. Within the match subset, while the difference in accuracy between subject and object relatives is large in poor readers, the difference is reduced with better reading skills, almost disappearing in very skilled readers.

Discussion

Beside replicating the well-established findings on the subject-object asymmetry, number facilitation in the comprehension of relative clauses, and a better overall performance by skilled readers, these results indicate that strong reading skills may determine a reduction of the processing difficulty associated with the hardest object relative clause condition (i.e., match), causing a reduction of the subject-object asymmetry.

A connectionist perspective on the development of reading skills in children

The development of decoding skills has traditionally been viewed as a stage-like process during which children's reading strategies change as a consequence of the acquisition of phonological awareness. More explicit accounts of the mechanisms involved in learning to read are provided by recent connectionist models in which children learn mappings initially between orthography and phonology, and later between orthography, phonology and semantics. Evidence from studies of reading development suggests that learning to read is determined primarily by the status of a child's phonological representations and is therefore compromised in dyslexic children who have phonological deficits. Children who have language impairments encompassing deficits in semantic representations have qualitatively different reading problems centring on difficulties with reading comprehension and in learning to read exception words.

Neuroanatomical structures and segregated circuits

Segregated neural circuits that effect particular domain-specific behaviors can be differentiated from neuroanatomical structures implicated in many different aspects of behavior. The basal ganglionic components of circuits regulating nonlinguistic motor behavior, speech, and syntax all function in a similar manner. Hence, it is unlikely that special properties and evolutionary mechanisms are associated with the neural bases of human language.

How to grow a human

I enlarge on the theme that the brain mechanisms required for languageand other aspects of the human mind evolved through selective changes in the regulatory genes governing growth. Extension of the period of postnatal growth increases the role of the environment in structuring the brain, and spatiotemporal programming (heterochrony) ofgrowth might explain hierarchical representation, hemispheric specialization, and perhaps sex differences.

Genes, specificity, and the lexical/functional distinction in language acquisition

Contrary to Müller's claims, and in support of modular theories, genetic factors play a substantial and significant role in language. The finding that some children with specific language impairment (SLI) have nonlinguistic impairments may reflect improper diagnosis of SLI or impairments that are secondary to linguistic impairments. Thus, such findings do not argue against the modularity thesis. The lexical/functional distinction appears to be innate and specifically linguistic and could be instantiated in either symbolic or connectionist systems.

Autonomy of syntactic processing and the role of Broca's area

Both autonomy and local specificity are compatible with observed interconnectivity at the cell level when considering two different levels: cell assemblies and brain systems. Early syntactic structuring processes in particular are likely to representan autonomous module in the language/brain system.

Neurobiological approaches to language: Falsehoods and fallacies

The conclusion that language is not really innate or modular is based on several fallacies. I show that the target article confuses communicative skills with linguistic abilities, and that its discussion of brain/language relations is replete with factual errors. I also criticize its attempt to contrast biological and linguistic principles. Finally, I argue that no case is made for the “alternative” approach proposed here.

Is human language just another neurobiological specialization?

One can disagree with Müller that it is neurobiologically questionable to suppose that human language is innate, specialized, and species-specific, yet agree that the precise brain mechanisms controlling language in any individual will be influenced by epigenesis and genetic variability, and that the interplay between inherited and acquired aspects of linguistic capacity deserves to be investigated.

Evolutionary principles and the emergence of syntax

The belief that syntax is an innate, autonomous, species-specific module is highly questionable. Syntax demonstrates the mosaic nature of evolutionary change, in that it made use of (and led to the enhancement of) numerous preexisting neurocognitive features. It is best understood as an emergent characteristic of the explosion of semantic complexity that occurred during hominid evolution.

Neurobiology and linguistics are not yet unifiable

Neurobiological models of language need a level of analysis that can account for the typical range of language phenomena. Because linguistically motivated models have been successful in explaining numerous language properties, it is premature to dismiss them as biologically irrelevant. Models attempting to unify neurobiology and linguistics need to be sensitive to both sources of evidence.

An innate language faculty needs neither modularity nor localization

Müller misconstrues autonomy to mean strict locality of brain function, something quite different from the functional autonomy that linguists claim. Similarly, he misperceives the interaction of learned and innate components hypothesized in current generative models. Evidence from sign languages, Creole languages, and neurological studies of rare forms of aphasia also argues against his conclusions.

Sign language and the brain: Apes, apraxia, and aphasia

The study of signed languages has inspired scientific' speculation regarding foundations of human language. Relationships between the acquisition of sign language in apes and man are discounted on logical grounds. Evidence from the differential hreakdown of sign language and manual pantomime places limits on the degree of overlap between language and nonlanguage motor systems. Evidence from functional magnetic resonance imaging reveals neural areas of convergence and divergence underlying signed and spoken languages.

Innateness, autonomy, universality? Neurobiological approaches to language

The concepts of the innateness, universality, species-specificity, and autonomy of the human language capacity have had an extreme impact on the psycholinguistic debate for over thirty years. These concepts are evaluated from several neurobiological perspectives, with an emphasis on the emergence of language and its decay due to brain lesion and progressive brain disease.

Evidence of perceptuomotor homologies and preadaptations for human language in nonhuman primates suggests a gradual emergence of language during hominid evolution. Regarding ontogeny, the innate component of language capacity is likely to be polygenic and shared with other developmental domains. Dissociations between verbal and nonverbal development are probably rooted in the perceptuomotor specializations of neural substrates rather than the autonomy of a grammar module. Aphasiologicaldata often assumed to suggest modular linguistic subsystems can be accounted for in terms of a neurofunctional model incorporating perceptuomotor-based regional specializationsand distributivity of representations. Thus, dissociations between grammatical functors and content words are due to different conditions of acquisition and resulting differences in neural representation. Human brains are characterized by multifactorial interindividual variability, and strict universality of functional organization is biologically unrealistic.

A theoretical alternative is proposed according to which (1) linguistic specialization of brain areas is due to epigenetic and probabilistic maturational events, not to genetic ”hard-wiring,” and (2) linguistic knowledge is neurally represented in distributed cell assemblies whose topography reflects the perceptuomotor modalities involved in the acquisition and use of a given item of knowledge.

Reflections on research on reading disability with special attention to gender issues

This commentary reviews some of the issues involved in the definition of reading disability and demonstrates how definitions can influence the conclusions reached by a review. In particular, the discrepancy definition of reading disability is shown to be logically flawed. Data from a large unbiased sample show that there are no significant differences between boys and girls in the incidence of reading difficulties.

Male vulnerability to reading disability is not likely to be a myth: a call for new data

Whether boys are more vulnerable than girls to reading disabilities (RD) is controversial. We review studies that were designed to minimize ascertainment bias in the selection of individuals with RD. These include population-based studies that identified children with RD by objective, unbiased methods and studies that examined the gender ratios among the affected relatives of those diagnosed with RD. We conclude that even when ascertainment biases are minimized, there is still a significant preponderance of boys with RD, although the gender ratio of the affected relatives of those with RD manifests the weakest male bias. Furthermore, we demonstrate that potentially confounding factors such as attentional or neurological problems, race, IQ, and severity of RD cannot account for the observed gender bias. We end with a clarion call to future researchers to (a) consider analyzing gender differences by means of more than one definition of RD, (b) compare gender ratios when boys and girls are ranked against the performance of their own gender as opposed to an average across genders, and (c) report group differences in variability and effect sizes of obtained gender ratios.

A family-based association study does not support DYX1C1 on 15q21.3 as a candidate gene in developmental dyslexia

We applied a family-based association approach to investigate the role of the DYX1C1 gene on chromosome 15q as a candidate gene for developmental dyslexia (DD) to 158 families containing at least one dyslexic child. We directly sequenced exons 2 and 10 of the DYX1C1 gene and found eight single nucleotide polymorphism (SNPs), three of which (-3G>A, 1249 G>T, 1259 C>G) were suitable for the genetic analyses. We performed single- and multimarker association analyses with DD as a categorical trait by FBAT version 1.4 and TRANSMIT version 2.5.4 programs. Our sample had a power of at least 80% to detect an association between the selected phenotypes and the informative polymorphisms at a significance level of 5%. The results of the categorical analyses did not support the involvement of the DYX1C1 gene variants in this sample of dyslexics and their relatives. Quantitative and multimarker analyses, which provide greater power to detect loci with a minor effect, consistently yielded nonsignificant results. While D1X1C1 is a good candidate gene for DD, we were unable to replicate the original findings between DYX1C1 gene and DD, perhaps due to genetic heterogeneity.

Brain Event-Related Potentials (ERPs) Measured at Birth Predict Later Language Development in Children with and Without Familial Risk for Dyslexia

We report associations between brain event-related potentials (ERPs) measured from newborns with and without familial risk for dyslexia and these same children's later language and verbal memory skills at 2.5, 3.5, and 5 years of age. ERPs to synthetic consonant-vowel syllables (/ba/, /da/, /ga/; presented equiprobably with 3,910-7,285 msec interstimulus intervals) were recorded from 26 newborns at risk for familial dyslexia and 23 control infants participating in the Jyväskylä Longitudinal Study of Dyslexia. The correlation and regression analyses showed that the at-risk type of response pattern at birth (a slower shift in polarity from positivity to negativity in responses to /ga/ at 540-630 msec) in the right hemisphere was related to significantly poorer receptive language skills across both groups at the age of 2.5 years. The similar ERP pattern in the left hemisphere was associated with poorer verbal memory skills at the age of 5 years. These results demonstrate that ERPs of newborns may be valid predictors of later language and neurocognitive outcomes.

Emerging phonological awareness differentiates children with and without familial risk for dyslexia after controlling for general language skills

Emerging phonological awareness was compared in two groups of 3.5-year-old children belonging to the Jyväskylä Longitudinal Study of Dyslexia (JLD): children with familial risk of dyslexia (at-risk group n = 98) and children without such risk (control group n = 91). Four computer animated tasks were used: Word-level and Syllable-level Segment Identification, Synthesis, and Continuation of Phonological Units. The control group children manifested higher mastery than children in the at-risk group in phonological awareness, and the proportion of children with a low phonological awareness mean score was 2.5 times higher in the at-risk group than in the control group. In both groups, phonological awareness at 3.5 years was predicted by early language skills assessed between 14 and 26 months of age, and it was also associated with concurrent language. The difference between the at-risk and control group at 3.5-year in phonological awareness remained significant, even when the effect of other language skills such as productive and receptive vocabulary, and mastery of inflections, measured both at earlier ages and concurrently were controlled for. Our findings indicate that familial risk for dyslexia is reliably reflected in emerging phonological awareness already at this early age and it can be assessed independently of other language skills.

Speech perception of infants with high familial risk for dyslexia differ at the age of 6 months

As yet relatively little is known of the earliest signs of dyslexia. We present evidence showing that the speech perception of 6-month-old infants from dyslexic families differs significantly from that of infants from control families with normal reading parents; the former group needed a significantly longer duration to categorize speech sounds as long. The same difference appeared in their dyslexic parents. This study shows that differences in categorizing speech sounds according to duration, which is crucial to intelligibility in Finnish, are a factor associated with familial risk for dyslexia already at infancy, which persists until adulthood in many of those suffering from dyslexia.

Mismatch negativity: clinical research and possible applications

This article provides a selective review on the perspectives of the clinical research and application of the mismatch negativity (MMN), a component of the auditory event-related potential generated by the brain's automatic response to any discriminable change in auditory stimulation. The MMN (and its magnetic equivalent MMNm) currently provide the only objective measure of auditory discrimination and sensory memory. It can be registered in the absence of attention and with no task requirements, which makes it particularly suitable for studying different clinical populations and infants.

The role of motion direction selective extrastriate regions in reading: a transcranial magnetic stimulation study

Why reading ability is correlated with motion processing ability is perplexing. Activity in motion direction processing regions (Area V5/MT+) was perturbed by means of repetitive transcranial magnetic stimulation (rTMS) to examine its effect on reading. A functional probe (significant shortening of the motion aftereffect) was used to identify Area V5/MT+. Right-handed participants (8 m, 8 f) received three 7.5 min blocks of rTMS, after which two phonological and one orthographic reading tasks were administered. Application of rTMS to Area V5/MT+ (as compared to a non-rTMS baseline) significantly decreased performance only during non-word naming. The pattern of naming errors and the absence of deficits on the second phonological task were not consistent with a role for Area V5/MT+ in phonological decoding. Instead, its role in reading may be limited to image stabilization and/or letter localization.

Brain responses to changes in speech sound durations differ between infants with and without familial risk for dyslexia

A specific learning disability, developmental dyslexia, is a language-based disorder that is shown to be strongly familial. Therefore, infants born to families with a history of the disorder are at an elevated risk for the disorder. However, little is known of the potential early markers of dyslexia. Here we report differences between 6-month-old infants with and without high risk of familial dyslexia in brain electrical activation generated by changes in the temporal structure of speech sounds, a critical cueing feature in speech. We measured event-related brain responses to consonant duration changes embedded in ata pseudowords applying an oddball paradigm, in which pseudoword tokens with varying /t/ duration were presented as frequent standard (80%) or as rare deviant stimuli (each 10%) with an interval of 610 msec between the stimuli. The infants at risk differ from control infants in both their initial responsiveness to sounds per se and in their change-detection responses dependent on the stimulus context. These results show that infants at risk due to a familial background of reading problems process auditory temporal cues of speech sounds differently from infants without such a risk even before they learn to speak.

The play and language behavior of mothers with and without dyslexia and its association to their toddlers' language development

The play and language behavior of mothers with (n = 49) and without (n = 49) specific reading disabilities (RD) was investigated during play with their 14-month-old children. The contribution of maternal behavior to the language development of their children was examined. The children's receptive and expressive language skills were assessed longitudinally at 14, 18, and 30 months, using the MacArthur Communicative Development Inventories and the Reynell Developmental Language Scales. Children with and without familial risk for RD did not differ from each other in any play or language measures at these ages. No group differences were found for mothers' manifestations of nonsymbolic play and language. However, the typically reading (TR) mothers produced significantly more symbolic play and language in play interactions with their child than did the mothers with RD. The correspondence between mother-child symbolic play and maternal play-related language was also higher for the TR group than it was for the RD group. The mothers' symbolic play did not show any relation to their children's language development, but their expressions of symbolic language did show a relationship. The mothers' child-directed symbolic language contributed significantly toward the comprehension skills of 14- and 18-month-olds in the RD group. This association, however, was lower at 30 months, at which point it was reliably present for the first time in the TR group.

Atypical brain development: a conceptual framework for understanding developmental learning disabilities

This article presents ideas that are, in part, a response to the ambiguity in the neurological research on learning disorders, the growing awareness that developmental disabilities are typically nonspecific and heterogeneous, and the growing scientific literature showing that comorbidity of symptoms and syndromes is the rule rather than the exception. This article proposes the term atypical brain development (ABD) as a unifying concept to assist researchers and educators trying to come to terms with these dilemmas. ABD is meant to serve as an integrative concept of etiology, the expression of which is variable within and across individuals. ABD does not itself represent a specific disorder or disease. It is a term that can be used to address the full range of developmental disorders that are found to be overlapping much of the time in any sample of children. Although similar in spirit to the older term of minimal brain dysfunction (MBD), in that it closely links neurology with behavioral difficulties, ABD as proposed here differs in several ways. In support of the ABD conceptual framework, first, we consider the ABD concept in terms of its superiority to the older notion of MBD. Second, we provide a brief review of the burgeoning literature on the overlap of the various developmental disabilities. Third, we review some of the scientific literature that supports the ABD concept. Our sole purpose in proposing this concept is to initiate dialogue and debate on several critical issues across a wide variety disciplines. Hence, this article is not intended to be a definitive statement of a rigid perspective. It reflects neither a nonmalleable philosophical position, nor any type of condemnation of other perspectives. It does, however, reflect a data-based and philosophical trend visible in the field of learning disabilities, as well as the broader area of childhood developmental disorders.

Dyslexia and familial high blood pressure: an observational pilot study

BACKGROUND

Developmental dyslexia is a neurodevelopmental learning disability characterised by unexpectedly poor reading and unknown aetiology. One hypothesis proposes excessive platelet activating factor, a potent vasodilator, as a contributor, implying that there should be a negative association between dyslexia and high blood pressure (HBP). Since both conditions have a partial genetic basis, this association may be apparent at the familial level.

AIMS

To test this prediction in dyslexic and non-dyslexic children.

METHODS

Individuals and families with (HBP+) and without (HBP-) a family history of HBP were compared.

RESULTS

Proportionately fewer dyslexics (49/112) than controls (11/12) were HBP+. Families with multiple, all dyslexic children were less likely to be HBP+ (7/16) than those with a non-dyslexic child (11/11). Within families, mean child scores on reading were higher in the HBP+ group (mean 44.3, SE 0.95) than in the HBP- group (mean 40.3, SE 0.87).

CONCLUSION

HBP+ family history is associated with better performance on reading. The prediction of a negative association between dyslexic status and familial high blood pressure is therefore confirmed.

Event-related potentials and consonant differentiation in newborns with familial risk for dyslexia

We measured event-related potentials (ERPs) to synthetic consonant-vowel syllables (/ba/, /da/, /ga/) from 26 newborns with familial risk for dyslexia and 23 control infants participating in the Jyväskylä Longitudinal Study of Dyslexia. The syllables were presented with equal probability and with interstimulus intervals ranging from 3,010 to 7,285 ms. Analyses of averaged ERPs from the latencies identified on the basis of principal component analysis (PCA) revealed significant group differences in stop-consonant processing in several latency ranges. At the latencies of 50-170 ms and 540-630 ms, the responses to /ga/ were larger and more positive than those to /ba/ and /da/ in the right hemisphere in the at-risk group. Between 740 and 940 ms, the responses to /ba/ and /da/ differed between anterior and posterior electrode sites in the control group. These results indicate that the cortical electric activation evoked by speech elements differs between children with and without risk for dyslexia even immediately after birth. Group-related effects at early and late latency ranges of ERPs suggest differences both in the early sound processing based on activation of afferent elements and in later phases of syllable encoding.

Incidence of reading disability in a population-based birth cohort, 1976-1982, Rochester, Minn

OBJECTIVE

To report the incidence of reading disability among school-aged children.

SUBJECTS AND METHODS

In this population-based, retrospective birth cohort study, subjects included all 5718 children born between 1976 and 1982 who remained in Rochester, Minn, after the age of 5 years. Based on records from all public and nonpublic schools, medical facilities, and private tutorial services and on results of all individually administered IQ and achievement tests, extensive medical, educational, and socioeconomic information were abstracted. Reading disability was established with use of research criteria based on 4 formulas (2 regression-based discrepancy, 1 non-regression-based discrepancy, and 1 low achievement).

RESULTS

Cumulative incidence rates of reading disability varied from 5.3% to 11.8% depending on the formula used. Boys were 2 to 3 times more likely to be affected than girls, regardless of the identification methods applied.

CONCLUSIONS

In this population-based birth cohort, reading disability was common among school-aged children and significantly more frequent among boys than girls, regardless of definition.

Language development and symbolic play in children with and without familial risk for dyslexia

The purposes of this study were to investigate (a) whether children in families with a positive history of dyslexia were more likely to show delays in language development than children without family risk and (b) whether a delayed onset of expressive language (late talking) predicted later language development. We analyzed the language development of 200 children longitudinally at 14, 24, 30, and 42 months and assessed their symbolic play at 14 months. Half of the children (N = 106) were from families with a history of dyslexia (the Dyslexia Risk [DR] group), and other children served as age-matched controls. Parental reports and structured tests were used to assess children's receptive and expressive language and symbolic play. No differences emerged between the two groups in receptive language, symbolic play, or on the Bayley MDI. The groups, however, diverged in expressive language measures. The maximum sentence length at 2 years and object naming and inflectional morphology skills at 3.5 years were higher for the control group than for the DR group. Reynell receptive score at 2.5 years provided the greatest unique contribution to the prediction of the children's receptive and expressive language. Children's risk status did not contribute to receptive language, but provided a significant contribution to their expressive language at 3.5 years, even after the variance associated with parental education and children's previous language skills was controlled. Late talkers in the DR group differed from the other members of the DR group in both receptive and expressive language at 3.5 years, although in the control group children with a late-talking history performed at age-level expectations. The findings suggest that children with a familial risk for dyslexia and with a history of late talking are at higher risk for delays in language acquisition than children without the familial risk for dyslexia.

The mismatch negativity in evaluating central auditory dysfunction in dyslexia

The mismatch negativity (MMN), a brain response elicited by a discriminable change in any repetitive aspect of auditory stimulation even in the absence of attention, has been widely used in both basic and clinical research during recent years. The fact that the MMN reflects the accuracy of auditory discrimination and that it can be obtained even from unattentive subjects makes it an especially attractive tool for studying various central auditory-system dysfunctions both in adults and children. In this review, we will discuss the applicability of the MMN to studies in dyslexia, which is currently thought, in the majority of the cases, to primarily result either from a dysfunction of the phonological system or a more general auditory deficit. Recent evidence indicates that the MMN enables one to determine which aspects of auditory information are deficiently processed in dyslexia. The MMN might also be helpful in the early definition of the dyslexia type, which would make it possible to start correctly-targeted training programmes before any major learning delays occur. Furthermore, the MMN holds promise of showing plastic changes in the brain of dyslexic individuals underlying the alleviation or remediation of dyslexia in the course of a successful training programme.

Atypical organisation of the auditory cortex in dyslexia as revealed by MEG

Neuroanatomical and -radiological studies have converged to suggest an atypical organisation in the temporal bank of the left-hemispheric Sylvian fissure for dyslexia. Against the background of this finding, we applied high temporal resolution magnetoencephalography (MEG) to investigate functional aspects of the left-hemispheric auditory cortex in 11 right-handed dyslexic children (aged 8-13 years) and nine matched normal subjects (aged 8-14 years). Event-related field components during a passive oddball paradigm with pure tones and consonant-vowel syllables were evaluated. The first major peak of the auditory evoked response, the M80, showed identical topographical distributions in both groups. In contrast, the generating brain structures of the later M210 component were located more anterior to the earlier response in children with dyslexia only. Control children exhibited the expected activation of more posterior source locations of the component that appeared later in the processing stream. Since the group difference in the relative location of the M210 source seemed to be independent of stimulus category, it is concluded that dyslexics and normally literate children differ as to the organisation of their left-hemispheric auditory cortex.

Mismatch negativity (MMN) as a tool for investigating auditory discrimination and sensory memory in infants and children

For decades behavioral methods, such as the head-turning or sucking paradigms, have been the primary methods to investigate auditory discrimination, learning and the function of sensory memory in infancy and early childhood. During recent years, however, a new method for investigating these issues in children has emerged. This method makes use of the mismatch negativity (MMN), the brain's automatic change-detection response, which has been used intensively in both basic and clinical studies in adults for twenty years. This review demonstrates that, unlike many other components of event-related potentials, the MMN is developmentally quite stable and can be obtained even from pre-term infants. Further, MMN amplitude is only slightly smaller in infants than is usually reported in school-age children and it does not seem to differ much from that obtained in adults. MMN latency has been reported to be slightly longer in infants than in adults but reaches adult values by the early school-age years. Child MMN does not seem to be analogous to adult MMN, however. For example, contrary to the results of adult studies, a prominent MMN can be obtained from in all waking- and sleep states in infants. Moreover, MMN scalp distribution seems to be broader and more central in children than in adults.

Brain morphometry in reading-disabled twins

OBJECTIVE

To test for brain structure differences in reading disability (RD) by means of MRI-based morphometry.

BACKGROUND

Consensus is lacking on the brain structural correlates of RD. The current study reports on a wider set of structures in the largest sample yet studied, controlling for age, gender, IQ, and attention deficit hyperactivity disorder (ADHD).

METHODS

A case-control study was performed that was comprised of 75 individuals with RD (mean age, 17.43+/-4.29 years) and 22 control subjects without RD (mean age, 18.69+/-3.75 years), each a single member of a twin pair. The two groups were similar in age, gender, and handedness, but differed in full-scale IQ (FSIQ), with the RD group having a lower mean FSIQ (101.8+/-9.9 versus 118.3+/-10.3). Using three group-by-structure analyses of covariance, groups were compared in terms of volume (in cubic centimeters) of major neocortical subdivisions, subcortical structures, and midsagittal areas (in square millimeters) of three subdivisions of the corpus callosum.

RESULTS

Controlling for age, gender, and IQ, the authors found a significant group-by-structure interaction for the major neocortical subdivisions (p = 0.002), reflecting a different developmental pattern in the RD group, with the insula and anterior superior neocortex being smaller and the retrocallosal cortex being larger in the RD group. In contrast, they found no group main or interaction effects for the subcortical or callosal structures. The pattern of results was essentially the same in subjects without ADHD.

CONCLUSIONS

Most brain structures do not differ in size in RD, but cortical development is altered subtly. This study replicates in a larger sample previous findings of insular differences in RD and demonstrates further that those differences are not attributable to comorbid ADHD.

Cerebrocortical microdysgenesis is enhanced in c57BL/6J mice exposed in utero to acetazolamide

A small percentage of C57BL/6 mice spontaneously develop focal collections of neurons in the molecular layer of the cerebral neocortex. Usually only one "ectopia" is present in each affected brain. Studies in other mouse strains have shown that these ectopias occur before birth, probably because of a breach in the superficial glial membrane during neuronal migration. The ectopias are heritable and are caused by multiple genes. C57BL/6J mice exposed prenatally to acetazolamide, a carbonic anhydrase-specific inhibitor and teratogen, develop an increased frequency of limb malformations, especially in the right forelimb. In the present study, we hypothesized that the prevalence and severity of ectopias would be increased in acetazolamide-exposed mice because carbonic anhydrase plays a key role in brain development. Further, we wanted to determine whether there was a correlation between the side of limb deformity and the hemisphere containing an ectopia. Thus, we injected C57BL/6J time-mated mice intraperitoneally on embryonic day 9 with either sodium acetazolamide (750 mg/kg) or water. Histological analysis of the brains from 105 acetazolamide-exposed offspring and 89 control offspring revealed no difference in the overall prevalence of cerebrocortical ectopias between the acetazolamide and control groups: 34% of the acetazolamide-exposed and 28% of the control mice had ectopias. There was, however, a striking difference in the shape and size of ectopias: 67% of the ectopias were large in the acetazolamide-exposed group in comparison to 32% in controls. The acetazolamide-exposed offspring also were more likely to have multiple ectopias. Thus, there may be a genetic predisposition for developing ectopias in some mouse strains, but epigenetic factors such as prenatal exposure to acetazolamide can influence their severity.

Cortical responses of infants with and without a genetic risk for dyslexia: I. Age effects

We studied auditory event-related potentials (ERP) in newborns and 6-month-old infants, about half of whom had a familial risk for dyslexia. Syllables varying in vowel duration were presented in an oddball paradigm, in which ERPs to deviating stimuli are assumed to reflect automatic change detection in the brain. The ERPs of newborns had slow positive deflections typical of their age, but significant stimulus and group effects were found only by the age of 6 months. In both groups, the responses to the deviant /ka/ were more positive than those to the standard /kaa/ stimuli, contrary to the findings of adult ERPs to duration changes. The results also suggested differences in brain activation pattern between the groups.

A case study of an English-Japanese bilingual with monolingual dyslexia

We report the case of AS, a 16 year-old English/Japanese bilingual boy, whose reading/writing difficulties are confined to English only. AS was born in Japan to a highly literate Australian father and English mother, and goes to a Japanese selective senior high school in Japan. His spoken language at home is English. AS's reading in logographic Japanese Kanji and syllabic Kana is equivalent to that of Japanese undergraduates or graduates. In contrast, his performance in various reading and writing tests in English as well as tasks involving phonological processing was very poor, even when compared to his Japanese contemporaries. Yet he has no problem with letter names or letter sounds, and his phoneme categorisation is well within the normal range of English native speakers. In order to account for our data that show a clear dissociation between AS's ability to read English and Japanese, we put forward the 'hypothesis of granularity and transparency'. It is postulated that any language where orthography-to-phonology mapping is transparent, or even opaque, or any language whose orthographic unit representing sound is coarse (i.e. at a whole character or word level) should not produce a high incidence of developmental phonological dyslexia.

Practice parameters for the assessment and treatment of children and adolescents with language and learning disorders. AACAP

These parameters describe the aims and approach to diagnosis, treatment, and monitoring of children and adolescents with language and learning disorders (LLDs). LLDs are among the most common developmental disorders the clinician is likely to encounter. About 50% of children with an LLD also have a comorbid Axis I psychiatric disorder. The diagnosis of an LLD requires a discrepancy, based on age and intelligence, between potential and achievement. The clinician collaborates with parents and school personnel to clarify the diagnosis, implement appropriate treatment and remediation, and monitor progress. The clinician is instrumental in identifying and treating comorbid conditions, including determining the appropriateness of medication. Long-term prognosis depends on the type and severity of the LLD, the availability of remediation, and the presence of a supportive family and school environment.

Double dissociation, modularity, and distributed organization

Müller argues that double dissociations do not imply underlying modularity of the cognitive system, citing neural networks as examples of fully distributed systems that can give rise to double dissociations. We challenge this claim, noting that suchdouble dissociations typically do not “scale-up,” and that even some singledissociations can be difficult to account for in a distributed system.

Familial language impairment: The evidence

Müller argues that general cognitive skills and linguistic skills are not necessarily independent. However, cross-linguistic evidence from an inherited specific language disorder affecting productive rules suggests significant degrees of modularity, innateness, and universality of language. Confident claims about the overall nature of such a complex system still await more interdisciplinary research.

Innateness, autonomy, universality, and the neurobiology of regular and irregular inflectional morphology

Müller's goal of bringing neuroscience to bear on controversies in linguistics is laudable. However, some of his specific proposals about innateness and autonomy are misguided. Recent studies on the neurobiology of regular and irregular inflectional morphology indicate that these two linguistic processes are subserved by anatomically and physiologically distinct neural subsystems, whose functional organization is likely to be under direct genetic control rather than assembled by strictly epigenetic factors.

Biology of language: Principle predictions and evidence

Müller's target article aims to summarize approaches to the question of how language elements (phonemes, morphemes, etc.) and rules are laid down in the brain. However, it suffers from being too vague about basic assumptions and empirical predictions of neurobiological models, and the empirical evidence available to test the models is not appropriately evaluated. (1) In a neuroscientific model of language, different cortical localizations of words can only be based on biological principles. These need to be made explicit. (2) Evidence for and against word class differences could be evaluated more rigorously. (3) All (and only) humans are able to learn languages with complex syntactic structures; it is, therefore, not appropriate to deny innateness and universality of syntactic principles. The real question appears to be the following: Which neurobiological principles are the linguistic principles based on?

Müller's conclusions and linguistic research

Because Müiller fails to distinguish between two senses of the term “autonomy,” there is a danger that his results will be misinterpreted by both linguists and neuroscientists. Although he may very well have been successful in refuting one sense of autonomy, he may actually have helped to provide an explanation for the correctness of autonomy in its other sense.

Speaking of language: Thoughts on associations

Müller attempts to downplay cases of dissociation between language and cognition as evidence against the modularity of language. We review cases of associations between verbal and nonverbal abilities as further evidence against the notion of language as an autonomous subsystem. We also point out a discrepancy between his proposal of homologies between nonhuman primates' communication and human language and recent proposals on the evolution of language.

It's a far cry from speech to language

We agree with Müller's epigenetic view of evolution and ontogeny and applaud his multilevel perspective. With him, we stress the importance in ontogeny of progressive specialisation rather than prewired structures. However, we argue that he slips from “speech” to “language” and that, in seeking homologies, these two levels need to be kept separate in the analysis of evolution and ontogeny.

A worthy enterprise injured by overinterpretation and misrepresentation

The synthetic position adopted by Müller is weakened by a large number of overinterpretations and misrepresentations, together with a caricatured view of innateness and modularity.