Genetic influences on learning disabilities: An update

Structural brain alterations associated with dyslexia predate reading onset

Functional magnetic resonance imaging studies have reported reduced activation in parietotemporal and occipitotemporal areas in adults and children with developmental dyslexia compared to controls during reading and reading related tasks. These patterns of regionally reduced activation have been linked to behavioral impairments of reading-related processes (e.g., phonological skills and rapid automatized naming). The observed functional and behavioral differences in individuals with developmental dyslexia have been complemented by reports of reduced gray matter in left parietotemporal, occipitotemporal areas, fusiform and lingual gyrus and the cerebellum. An important question for education is whether these neural differences are present before reading is taught. Developmental dyslexia can only be diagnosed after formal reading education starts. However, here we investigate whether the previously detected gray matter alterations in adults and children with developmental dyslexia can already be observed in a small group of pre-reading children with a family-history of developmental dyslexia compared to age and IQ-matched children without a family-history (N = 20/mean age: 5:9 years; age range 5:1-6:5 years). Voxel-based morphometry revealed significantly reduced gray matter volume indices for pre-reading children with, compared to children without, a family-history of developmental dyslexia in left occipitotemporal, bilateral parietotemporal regions, left fusiform gyrus and right lingual gyrus. Gray matter volume indices in left hemispheric occipitotemporal and parietotemporal regions of interest also correlated positively with rapid automatized naming. No differences between the two groups were observed in frontal and cerebellar regions. This discovery in a small group of children suggests that previously described functional and structural alterations in developmental dyslexia may not be due to experience-dependent brain changes but may be present at birth or develop in early childhood prior to reading onset. Further studies using larger sample sizes and longitudinal analyses are needed in order to determine whether the identified structural alterations may be utilized as structural markers for the early identification of children at risk, which may prevent the negative clinical, social and psychological outcome of developmental dyslexia.

Neurobiologische Grundlagen der Lese-Rechtschreib-Schwäche

Zusammenfassung: Diese Überblicksarbeit stellt die wichtigsten neurobiologischen Theorien zu Ursachen der entwicklungsbedingten Lese-Rechtschreib-Schwäche und daraus abgeleitete Therapieansätze vor. Es wird zunächst ein kurzer Überblick über die Symptomatik und die vermuteten genetischen Grundlagen der Störung gegeben. Die phonologische Defizithypothese als eine allgemein akzeptierte, auf der kognitiven Ebene angesiedelte Theorie wird vorgestellt. Neurobiologische Modelle versuchen zu erklären, auf welchen neuroanatomischen/physiologischen Ursachen das phonologische Defizit beruht. Die magnozelluläre Defizithypothese, die Hypothese eines grundlegenden Verarbeitungsdefizits für sich zeitlich schnell verändernde Reize, die cerebelläre Defizithypothese sowie neuroanatomische Unterschiede zwischen normalen und legasthenen Lesern werden vorgestellt. Neurobiologisch fundierte Therapieansätze der Lese-Rechtschreib-Schwäche versuchen, in Ergänzung zu traditionellen, eher linguistisch orientierten Therapien die betroffenen neuronalen Systeme in ihrer Funktionsfähigkeit zu trainieren und dadurch die Voraussetzungen für die betroffenen Kinder zu schaffen, vom normalen Lese- und Schreibunterricht zu profitieren. Einige dieser Ansätze werden dargestellt. Abschließend wird darauf hingewiesen, dass eine differenzielle Zuweisung von Patienten zu bestimmten Therapieprogrammen auf der Basis einer fundierten (Ursachen-)Diagnostik in der Praxis stärker als bislang üblich durchgeführt werden sollte.

Specific language impairment in families: evidence for co-occurrence with reading impairments

Two family aggregation studies report the occurrence and co-occurrence of oral language impairments (LIs) and reading impairments (RIs). Study 1 examined the occurrence (rate) of LI and RI in children with specific language impairment (SLI probands), a matched control group, and all nuclear family members. Study 2 included a larger sample of SLI probands, as well as their nuclear and extended family members. Probands and their family members who met specific criteria were classified as language and/or reading impaired based on current testing. In Study 1, the rates of LI and RI for nuclear family members (excluding probands) were significantly higher than those for control family members. In the SLI families, affected family members were more likely to have both LI and RI than either impairment alone. In Study 2, 68% of the SLI probands also met the diagnostic classification for RI. The language and RI rates for the other family members, excluding probands, were 25% and 23% respectively, with a high degree of co-occurrence of LI and RI (46%) in affected individuals. Significant sex ratio differences were found across generations in the families of SLI probands. There were more male than female offspring in these families, and more males than females were found to have both LIs and RIs. Results demonstrate that when LIs occur within families of SLI probands, these impairments generally co-occur with RIs. Our data are also consistent with prior findings that males show impairments more often than females.

Visual motion sensitivity and reading

Reading is more difficult than speaking because an arbitrary set of visual symbols must be rapidly identified, ordered and translated into the sounds they represent. Many poor readers have particular problems with the rapid visual processing required for these tasks because they have a mild impairment of the visual magnocellular system. This deficit has been demonstrated using neuropathological, evoked potential, functional magnetic resonance imaging and psychophysical techniques. The sensitivity of the M-system in both good and bad readers correlates with their orthographic abilities, suggesting that the M-system plays an important part in their development. This role is probably to mediate steady direction of visual attention and eye fixations on words. Thus many children with reading difficulties have unsteady eye control and this causes the letters they are trying to read to appear to move around, so that they cannot tell what order they are meant to be in. Therefore, boosting M-performance using yellow filters, or training eye fixation, can improve reading performance very significantly. Several genetic linkage studies have associated reading difficulties with the MHC control region on the short arm of chromosome 6. This system has recently been shown to help regulate the differentiation of M-cells. This association could also explain the high incidence of autoimmune conditions in poor readers. Other chromosomal sites are associated with the metabolism of polyunsaturated fatty acids (PUFAs) as found in fish oils, and this could explain why PUFA supplements can improve reading.

The sensory basis of reading problems

Learning to read is much more difficult than learning to speak. Most children teach themselves to speak with little or no difficulty. Yet a few years later when they come to learn to read they have to be taught how to do it; they do not pick up reading by themselves. This is because we speak in words and syllables, but we write in phonemes. Syllables do not naturally break down into the sounds of letters and letter units (i.e., phonemes) because these do not correspond to physiologically distinct articulatory gestures (Liberman, Shankweiler, & Studdert-Kennedy, 1967). Alphabetic writing was only invented when people realized that syllables could be artificially divided into smaller acoustically distinguishable phonemes that could be represented by a small number of letters. But these distinctions are arbitrary cultural artifacts, and their mastery was originally confined to a select social class. And until about 100 years ago it did not matter much if the majority of people could not read; the acquisition of reading probably had no serious disadvantages. Reading requires the integration of at least two kinds of analysis (Castles & Coltheart, 1993; Ellis, 1984; Manis, Seidenberg, Doi, McBride-Chang, & Petersen, 1996; Morton, 1969; Seidenburg, 1993). First, the visual form of words, the shape of letters, their order in words, and common spelling patterns, which is termed their orthography, has to be processed visually. Their orthography yields the meaning of familiar words very rapidly without needing to sound them out. But for unfamiliar words, and all words are fairly unfamiliar to the beginning reader, the letters have to be translated into the speech sounds (i.e., phonemes) that they stand for, and then those sounds have to be melded together in inner speech to yield the word and its meaning. Reading exclusively by the phonological route is more time consuming than if words can be accessed directly without requiring phonological mediation.

Emerging issues in the genetics of dyslexia: a methodological preview

A review of the classic and recent evidence on the genetics of reading disability (RD) shows encouraging progress, and accumulating evidence of genetic risk factors that operate within families and are separately localizable to more than one chromosomal region. The accelerating pace of these findings, however, suggests the need to consider some methodological issues about the design and interpretation of current and future studies. A major issue is the shape of the distribution of reading ability in the population, and we offer three tests of increasing rigor for determining whether those distributions are categorical, and hence not suitable for analyses that depend on the assumption of a continuous normal distribution. These tests are as follows: a nonnormal preponderance of cases with RD (i.e., the hump in the lower end of the distribution); a difference in the within-group variance-covariance matrices for typical readers compared to those with RD; and a correlation between a neurogenetically relevant criterion and a categorical reading variable that is larger than the correlation between the same criterion and a continuous version of the same reading variable. We emphasize also the importance of interactive relationships between multiple genetic loci, the variations in genotypic range as well as type of affectedness, the need to account for remediation variance, and the importance of lifespan changes in the phenotypes.

The magnocellular theory of developmental dyslexia

Low literacy is termed 'developmental dyslexia' when reading is significantly behind that expected from the intelligence quotient (IQ) in the presence of other symptoms--incoordination, left-right confusions, poor sequencing--that characterize it as a neurological syndrome. 5-10% of children, particularly boys, are found to be dyslexic. Reading requires the acquisition of good orthographic skills for recognising the visual form of words which allows one to access their meaning directly. It also requires the development of good phonological skills for sounding out unfamiliar words using knowledge of letter sound conversion rules. In the dyslexic brain, temporoparietal language areas on the two sides are symmetrical without the normal left-sided advantage. Also brain 'warts' (ectopias) are found, particularly clustered round the left temporoparietal language areas. The visual magnocellular system is responsible for timing visual events when reading. It therefore signals any visual motion that occurs if unintended movements lead to images moving off the fovea ('retinal slip'). These signals are then used to bring the eyes back on target. Thus, sensitivity to visual motion seems to help determine how well orthographic skill can develop in both good and bad readers. In dyslexics, the development of the visual magnocellular system is impaired: development of the magnocellular layers of the dyslexic lateral geniculate nucleus (LGN) is abnormal; their motion sensitivity is reduced; many dyslexics show unsteady binocular fixation; hence poor visual localization, particularly on the left side (left neglect). Dyslexics' binocular instability and visual perceptual instability, therefore, can cause the letters they are trying to read to appear to move around and cross over each other. Hence, blanking one eye (monocular occlusion) can improve reading. Thus, good magnocellular function is essential for high motion sensitivity and stable binocular fixation, hence proper development of orthographic skills. Many dyslexics also have auditory/phonological problems. Distinguishing letter sounds depends on picking up the changes in sound frequency and amplitude that characterize them. Thus, high frequency (FM) and amplitude modulation (AM) sensitivity helps the development of good phonological skill, and low sensitivity impedes the acquisition of these skills. Thus dyslexics' sensitivity to FM and AM is significantly lower than that of good readers and this explains their problems with phonology. The cerebellum is the head ganglion of magnocellular systems; it contributes to binocular fixation and to inner speech for sounding out words, and it is clearly defective in dyslexics. Thus, there is evidence that most reading problems have a fundamental sensorimotor cause. But why do magnocellular systems fail to develop properly? There is a clear genetic basis for impaired development of magnocells throughout the brain. The best understood linkage is to the region of the Major Histocompatibility Complex (MHC) Class 1 on the short arm of chromosome 6 which helps to control the production of antibodies. The development of magnocells may be impaired by autoantibodies affecting the developing brain. Magnocells also need high amounts of polyunsaturated fatty acids to preserve the membrane flexibility that permits the rapid conformational changes of channel proteins which underlie their transient sensitivity. But the genes that underlie magnocellular weakness would not be so common unless there were compensating advantages to dyslexia. In developmental dyslexics there may be heightened development of parvocellular systems that underlie their holistic, artistic, 'seeing the whole picture' and entrepreneurial talents.

Psychophysiological aspects of Tourette's syndrome

Tourette's syndrome (TS), once considered a rare disorder, has been investigated extensively in the last two decades. It is inherited, usually beginning in childhood, and waxes and wanes, usually decreasing in frequency and severity in adolescence and early adulthood. Pharmacotherapy is the usual treatment approach, reducing frequency and severity of symptoms, but it is not a cure and often has side effects. Psychological help for people with TS and their families may be needed for this complex disorder.

Familial dyslexia associated with cavum vergae

Three members of one family, diagnosed as dyslexic, are described. All of them have variations of midline cavity: cavum vergae or cavum septum pellucidum, diagnosed by neuroradiological examination. In contrast, the non dyslexic members of the same family have no neuroanatomical congenital variations. We raise the possibility of a functional correlation between the dyslexia and the anatomical findings in the affected members of this family.

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

To test the hypothesis of anomalous anatomy in posterior brain regions associated with language and reading, the corpus callosum was imaged in the midsagittal plane with magnetic resonance. The areas of the anterior, middle, and posterior segments were measured in 21 dyslexic men (mean age 27 yrs, SD 6) and in 19 matched controls. As predicted, the area of the posterior third of the corpus callosum, roughly equivalent to the isthmus and splenium, was larger in dyslexic men than in controls. No differences were seen in the anterior or middle corpus callosum. The increased area of the posterior corpus callosum may reflect anatomical variation associated with deficient lateralization of function in posterior language regions of the cortex and their right-sided homologues, hypothesized to differ in patients with dyslexia.

Learning disabilities and central auditory dysfunction

Hearing loss, whether peripheral or central, compounds the communication and educational problems of the learning disabled student. A central auditory processing disorder uniquely interferes with both the input and integration of verbal information, further resulting in a potentially permanent cognitive dysfunction during the developmental period of acquisition of language. Illustrative cases are presented that indicate the panorama of cognitive dysfunction associated with the learning disabled status. Methods of evaluation and identification and diagnostic criteria are correlated with auditory, visual, and academic performance. Comments regarding clinical awareness, prompt recognition, and ensuing individualized remediation are submitted.

Are families of children with reading difficulties at risk for immune disorders and nonrighthandedness?

This study used questionnaire data to examine immune disorders and nonrighthandedness in the families of children enrolled in a learning disabilities school and children attending regular classrooms in public schools. Groups were organized according to their performance on a standardized test of reading comprehension to avoid overlap. In total, 468 questionnaires were returned, from which we were able to derive a final sample of carefully matched subjects: 55 subjects undergoing remediation for reading problems and 55 age- and sex-matched control subjects. The results indicated that children with reading problems and their families more frequently suffered from some immune and autoimmune disorders, particularly those involving the gastrointestinal tract and the thyroid gland. In addition, symptoms of attention deficit hyperactivity disorder were associated with Crohn's disease and migraine headache in the families. There was no evidence of an elevated prevalence of nonrighthandedness in the children with reading problems and their families.

Mother-child teaching strategies and learning disabilities

The teaching strategies used by mothers of sons with learning disabilities (LD) (n = 30) and normally achieving sons (NLD) (n = 30) were examined. The children were matched for age (8- to 11-year-olds) and for parents' socioeconomic status. The behavior of mother-child pairs was videotaped in a teaching task that was constructed to resemble a homework assignment. The results showed that the mothers of children with LD used fewer high-level strategies, and their total time used in teaching was less than that of the mothers of NLD children. The mothers of children with LD exhibited more dominance and less emotionality and cooperation than did the mothers of NLD children; however, the mothers did not differ in task motivation. The children with LD seemed to have more inactive learning strategies, evident in their weaker initiative and greater dependence on their mothers. Analyses concerning the variation of maternal strategies within the LD group revealed that the mothers' motivation, combined with their emotionality and proportion of high-level strategies, had a strong positive association with their children's success in learning.

Communication deviances and clarity among the mothers of normally achieving and learning-disabled boys

The main purpose of the study was to reexamine the association between maternal communication deviances and learning disabilities in children. In this study, we adapted and extended the procedure used by Ditton, Green, and Singer (1987). A two-part experimental task was used: one in which the child could not request any clarification of mother's instructions, and another in which the mother and child could communicate. Both communication deviances and the clarity of mothers' communication were analyzed. The subjects were 60 mother-child pairs in which half of the children had learning disabilities and the other half were normally achieving children matched for age and parents' SES. The dyads were videotaped in a laboratory setting. The mothers of learning-disabled (LD) children were found to give less exact instructions and to present more ambiguous messages to the child than the mothers of non-LD children. Despite the more inaccurate input from their mother, the LD boys did not request clarification for ambiguous statements any more than did the NLD boys.

Familial dyslexia: use of genetic linkage data to define subtypes

Specific reading disability is an example of a complex behavioral disorder which is clinically heterogeneous. It is probably also heterogeneous at the levels of etiology and process (pathogenesis), but there may not be a 1:1:1 mapping of etiology to process to clinical outcome. Thus, classification of cases by clinical features may not lead to discovery of the underlying processes or etiologies, and it may be profitable to define subgroups by etiology. There is evidence for genetic etiology in some cases, but there is genetic heterogeneity as well. Possible genetic models for specific reading disability include polygenic, oligogenic, and single gene inheritance, and there are several types of genetic analysis that can be used to determine which of these modes of inheritance may be present. Identification of individual genes is possible in single gene and oligogenic disorders. Clinical studies and molecular analysis can then be used to determine gene function.

Using genetics to understand dyslexia

This paper reviews what is currently known about the genetics of dyslexia and shows how genetic studies can help clarify which symptoms are primary and which are secondary in dyslexia. On the genetic side, current evidence supports the view that dyslexia is familial, substantially heritable, and heterogeneous in its genetic mechanisms. At least some forms of familial dyslexia appear to be autosomal dominant, with linkage studies supporting both a major locus on chromosome 15 and genetic heterogeneity. On the symptom side, current evidence supports the view that the primary symptom in dyslexia is a deficit in the phonological coding of written language. This primary symptom likewise appears to be heritable. Recent evidence suggests that the heritable precursor to this written language deficit is a spoken language deficit in the skill of phoneme segmentation and awareness.