The epigenesis of planum temporale asymmetry in twins

Intergenerational transmission of the structure of the auditory cortex and reading skills

High-level cognitive skill development relies on genetic and environmental factors, tied to brain structure and function. Inter-individual variability in language and music skills has been repeatedly associated with the structure of the auditory cortex: the shape, size and asymmetry of the transverse temporal gyrus (TTG) or gyri (TTGs). TTG is highly variable in shape and size, some individuals having one single gyrus (also referred to as Heschl's gyrus, HG) while others presenting duplications (with a common stem or fully separated) or higher-order multiplications of TTG. Both genetic and environmental influences on children's cognition, behavior, and brain can to some to degree be traced back to familial and parental factors. In the current study, using a unique MRI dataset of parents and children (135 individuals from 37 families), we ask whether the anatomy of the auditory cortex is related to reading skills, and whether there are intergenerational effects on TTG(s) anatomy. For this, we performed detailed, automatic segmentations of HG and of additional TTG(s), when present, extracting volume, surface area, thickness and shape of the gyri. We tested for relationships between these and reading skill, and assessed their degree of familial similarity and intergenerational transmission effects. We found that volume and area of all identified left TTG(s) combined was positively related to reading scores, both in children and adults. With respect to intergenerational similarities in the structure of the auditory cortex, we identified structural brain similarities for parent-child pairs of the 1st TTG (Heschl's gyrus, HG) (in terms of volume, area and thickness for the right HG, and shape for the left HG) and of the lateralization of all TTG(s) surface area for father-child pairs. Both the HG and TTG-lateralization findings were significantly more likely for parent-child dyads than for unrelated adult-child pairs. Furthermore, we established characteristics of parents' TTG that are related to better reading abilities in children: fathers' small left HG, and a small ratio of HG to planum temporale. Our results suggest intergenerational transmission of specific structural features of the auditory cortex; these may arise from genetics and/or from shared environment.

Duplicated Heschl's gyrus associations with phonological decoding

The reason(s) for why a complete duplication of the left hemisphere Heschl's gyrus (HG) has been observed in people with reading disability are unclear. This study was designed to replicate and advance understanding of the HG and phonological decoding association, as well as test competing hypotheses that this HG duplication association is specifically localized to the HG or could be due to co-occurring atypical development of other brain regions that support reading and language development. Participants were selected on the basis of having a duplicated left hemisphere HG (N = 96) or a single HG (N = 96) and matched according to age, sex, and research site in this multi-site study. Duplicated and single HG morphology specific templates were created to determine the extent to which HG sizes were related to phonological decoding within each HG morphology group. The duplicated HG group had significantly lower phonological decoding (F = 4.48, p = 0.04) but not verbal IQ (F = 1.39, p = 0.41) compared to the single HG group. In addition, larger HG were significantly associated with lower phonological decoding in the duplicated HG group, with effects driven by the size of the lateral HG after controlling for age, sex, research site, and handedness (ps < 0.05). Brain regions that exhibited structural covariance with HG did not clearly explain the HG and phonological decoding associations. Together, the results suggest that presence of a duplicated HG indicates some risk for lower phonological decoding ability compared to verbal IQ, but the reason(s) for this association remain unclear.

Handedness in twins: meta-analyses

BACKGROUND

In the general population, 10.6% of people favor their left hand over the right for motor tasks. Previous research suggests higher prevalence of atypical (left-, mixed-, or non-right-) handedness in (i) twins compared to singletons, and in (ii) monozygotic compared to dizygotic twins. Moreover, (iii) studies have shown a higher rate of handedness concordance in monozygotic compared to dizygotic twins, in line with genetic factors playing a role for handedness.

METHODS

By means of a systematic review, we identified 59 studies from previous literature and performed three sets of random effects meta-analyses on (i) twin-to-singleton Odds Ratios (21 studies, n = 189,422 individuals) and (ii) monozygotic-to-dizygotic twin Odds Ratios (48 studies, n = 63,295 individuals), both times for prevalence of left-, mixed-, and non-right-handedness. For monozygotic and dizygotic twin pairs we compared (iii) handedness concordance Odds Ratios (44 studies, n = 36,217 twin pairs). We also tested for potential effects of moderating variables, such as sex, age, the method used to assess handedness, and the twins' zygosity.

RESULTS

We found (i) evidence for higher prevalence of left- (Odds Ratio = 1.40, 95% Confidence Interval = [1.26, 1.57]) and non-right- (Odds Ratio = 1.36, 95% Confidence Interval = [1.22, 1.52]), but not mixed-handedness (Odds Ratio = 1.08, 95% Confidence Interval = [0.52, 2.27]) among twins compared to singletons. We further showed a decrease in Odds Ratios in more recent studies (post-1975: Odds Ratio = 1.30, 95% Confidence Interval = [1.17, 1.45]) compared to earlier studies (pre-1975: Odds Ratio = 1.90, 95% Confidence Interval = [1.59-2.27]). While there was (ii) no difference between monozygotic and dizygotic twins regarding prevalence of left- (Odds Ratio = 0.98, 95% Confidence Interval = [0.89, 1.07]), mixed- (Odds Ratio = 0.96, 95% Confidence Interval = [0.46, 1.99]), or non-right-handedness (Odds Ratio = 1.01, 95% Confidence Interval = [0.91, 1.12]), we found that (iii) handedness concordance was elevated among monozygotic compared to dizygotic twin pairs (Odds Ratio = 1.11, 95% Confidence Interval = [1.06, 1.18]). By means of moderator analyses, we did not find evidence for effects of potentially confounding variables.

CONCLUSION

We provide the largest and most comprehensive meta-analysis on handedness in twins. Although a raw, unadjusted analysis found a higher prevalence of left- and non-right-, but not mixed-handedness among twins compared to singletons, left-handedness was substantially more prevalent in earlier than in more recent studies. The single large, recent study which included birth weight, Apgar score and gestational age as covariates found no twin-singleton difference in handedness rate, but these covariates could not be included in the present meta-analysis. Together, the secular shift and the influence of covariates probably make it unsafe to conclude that twinning has a genuine relationship to handedness.

It Is Not Just in the Genes

Asymmetries in the functional and structural organization of the nervous system are widespread in the animal kingdom and especially characterize the human brain. Although there is little doubt that asymmetries arise through genetic and nongenetic factors, an overarching model to explain the development of functional lateralization patterns is still lacking. Current genetic psychology collects data on genes relevant to brain lateralizations, while animal research provides information on the cellular mechanisms mediating the effects of not only genetic but also environmental factors. This review combines data from human and animal research (especially on birds) and outlines a multi-level model for asymmetry formation. The relative impact of genetic and nongenetic factors varies between different developmental phases and neuronal structures. The basic lateralized organization of a brain is already established through genetically controlled embryonic events. During ongoing development, hemispheric specialization increases for specific functions and subsystems interact to shape the final functional organization of a brain. In particular, these developmental steps are influenced by environmental experiences, which regulate the fine-tuning of neural networks via processes that are referred to as ontogenetic plasticity. The plastic potential of the nervous system could be decisive for the evolutionary success of lateralized brains.

What contributes to individual differences in brain structure?

Individual differences in adult human brain structure have been found to reveal a great deal of information about variability in behaviors, cognitive abilities and mental and physical health. Driven by such evidence, what contributes to individual variation in brain structure has gained accelerated attention as a research question. Findings thus far appear to support the notion that an individual’s brain architecture is determined largely by genetic and environmental influences. This review aims to evaluate the empirical literature on whether and how genes and the environment contribute to individual differences in brain structure. It first considers how genetic and environmental effects may separately contribute to brain morphology, by examining evidence from twin, genome-wide association, cross-sectional and longitudinal studies. Next, evidence for the influence of the complex interplay between genetic and environmental factors, characterized as gene-environment interactions and correlations, is reviewed. In evaluating the extant literature, this review will conclude that both genetic and environmental factors play critical roles in contributing to individual variability in brain structure.

Functional and structural comparison of visual lateralization in birds - similar but still different

Vertebrate brains display physiological and anatomical left-right differences, which are related to hemispheric dominances for specific functions. Functional lateralizations likely rely on structural left-right differences in intra- and interhemispheric connectivity patterns that develop in tight gene-environment interactions. The visual systems of chickens and pigeons show that asymmetrical light stimulation during ontogeny induces a dominance of the left hemisphere for visuomotor control that is paralleled by projection asymmetries within the ascending visual pathways. But structural asymmetries vary essentially between both species concerning the affected pathway (thalamo- vs. tectofugal system), constancy of effects (transient vs. permanent), and the hemisphere receiving stronger bilateral input (right vs. left). These discrepancies suggest that at least two aspects of visual processes are influenced by asymmetric light stimulation: (1) visuomotor dominance develops within the ontogenetically stronger stimulated hemisphere but not necessarily in the one receiving stronger bottom-up input. As a secondary consequence of asymmetrical light experience, lateralized top-down mechanisms play a critical role in the emergence of hemispheric dominance. (2) Ontogenetic light experiences may affect the dominant use of left- and right-hemispheric strategies. Evidences from social and spatial cognition tasks indicate that chickens rely more on a right-hemispheric global strategy whereas pigeons display a dominance of the left hemisphere. Thus, behavioral asymmetries are linked to a stronger bilateral input to the right hemisphere in chickens but to the left one in pigeons. The degree of bilateral visual input may determine the dominant visual processing strategy when redundant encoding is possible. This analysis supports that environmental stimulation affects the balance between hemispheric-specific processing by lateralized interactions of bottom-up and top-down systems.

Genetics of brain structure: contributions from the Vietnam Era Twin Study of Aging

Understanding the genetics of neuropsychiatric disorders requires an understanding of the genetics of brain structure and function. The Vietnam Era Twin Study of Aging (VETSA) is a longitudinal behavioral genetic study focused on cognitive and brain aging. Here, we describe basic science work carried out within the VETSA MRI study that provides meaningful contributions toward the study of neuropsychiatric disorders. VETSA produced the first comprehensive assessment of the heritability of cortical and subcortical brain structure sizes, all within the same individuals. We showed that neocortical thickness and surface area are largely genetically distinct. With continuous neocortical thickness maps, we demonstrated regional specificity of genetic influences, and that genetic factors did not conform to traditional regions of interest (ROIs). However, there was some evidence for different genetic factors accounting for different types of cortex, and for genetic relationships across cortical regions corresponding to anatomical and functional connectivity and brain maturation patterns. With continuous neocortical surface area maps, we confirmed the anterior-posterior gradient of genetic influences on cortical area patterning demonstrated in animal models. Finally, we used twin methods to create the first map of cortical ROIs based entirely on genetically informative data. We conclude that these genetically based cortical phenotypes may be more appropriate for genetic studies than traditional ROIs based on structure or function. Our results also suggest that cortical volume-the product of thickness and surface area-is a problematic phenotype for genetic studies because two independent sets of genes may be obscured. Examples supporting the validity of these conclusions are provided.

The Corpus Callosum in Monozygotic Twins Concordant and Discordant for Handedness and Language Dominance

We used diffusion tensor imaging to assess callosal morphology in 35 pairs of monozygotic twins, of which 17 pairs were concordant for handedness and 18 pairs were discordant for handedness. Functional hemispheric language dominance was established for each twin member using fMRI, resulting in 26 twin pairs concordant and 9 twin pairs discordant for language dominance. On the basis of genetic models of handedness and language dominance, which assume one “right shift” (RS) gene with two alleles, an RS+ allele biasing toward right-handedness and left cerebral language dominance and an RS− allele leaving both asymmetries to chance, all twins were classified according to their putative genotypes, and the possible effects of the gene on callosal morphology was assessed. Whereas callosal size was under a high genetic control that was independent of handedness and language dominance, twin pairs with a high probability of carrying the putative RS+ allele showed a connectivity pattern characterized by a genetically controlled, low anisotropic diffusion over the whole corpus callosum. In contrast, the high connectivity pattern exhibited by twin pairs more likely to lack the RS+ allele was under significantly less genetic influence. The data suggest that handedness and hemispheric dominance for speech production might be at least partly dependent on genetically controlled processes of axonal pruning in the corpus callosum.

Genetic and environmental influences on neuroimaging phenotypes: a meta-analytical perspective on twin imaging studies

Because brain structure and function are affected in neurological and psychiatric disorders, it is important to disentangle the sources of variation in these phenotypes. Over the past 15 years, twin studies have found evidence for both genetic and environmental influences on neuroimaging phenotypes, but considerable variation across studies makes it difficult to draw clear conclusions about the relative magnitude of these influences. Here we performed the first meta-analysis of structural MRI data from 48 studies on >1,250 twin pairs, and diffusion tensor imaging data from 10 studies on 444 twin pairs. The proportion of total variance accounted for by genes (A), shared environment (C), and unshared environment (E), was calculated by averaging A, C, and E estimates across studies from independent twin cohorts and weighting by sample size. The results indicated that additive genetic estimates were significantly different from zero for all meta-analyzed phenotypes, with the exception of fractional anisotropy (FA) of the callosal splenium, and cortical thickness (CT) of the uncus, left parahippocampal gyrus, and insula. For many phenotypes there was also a significant influence of C. We now have good estimates of heritability for many regional and lobar CT measures, in addition to the global volumes. Confidence intervals are wide and number of individuals small for many of the other phenotypes. In conclusion, while our meta-analysis shows that imaging measures are strongly influenced by genes, and that novel phenotypes such as CT measures, FA measures, and brain activation measures look especially promising, replication across independent samples and demographic groups is necessary.

Right, left, and center: how does cerebral asymmetry mix with callosal connectivity?

BACKGROUND

Prior research has shown that cerebral asymmetry is associated with differences in corpus callosum connectivity. Such associations were detected in histological and anatomical studies investigating callosal fiber size and density, in neuroimaging investigations based on structural and diffusion tensor imaging, as well as in neuropsychological experiments. However, little is known about typical associations between these factors, and even less about the relative influences of magnitude and direction of cerebral asymmetries. Here, we investigated relationships between callosal connectivity and cerebral asymmetry using precise measures of callosal thickness and selected cerebral structures. We considered both the direction and magnitude of the asymmetries.

METHODS

Associations between cerebral asymmetry and callosal thickness were investigated in 348 cognitively healthy older individuals.

RESULTS

The magnitude and direction of cerebral lateralization were significant independent predictors of callosal thickness. However, associations were small. Leftward asymmetry and increased magnitude of asymmetry were generally associated with increased callosal thickness, mostly in the callosal midbody and isthmus.

CONCLUSIONS

When a large sample of normal individuals is considered, cerebral asymmetries are only subtly associated with callosal thickness.

Cortical surface characteristics among offspring of schizophrenia subjects

BACKGROUND

A systematic study of cortical surface parameters in adolescent offspring of schizophrenia subjects before clinical manifestation could clarify neurodevelopmental antecedents of increased genetic risk. We examined these measures obtained on structural magnetic resonance imaging (MRI) scans at baseline and one year on a series of offspring of schizophrenia parents and healthy subjects.

METHODS

We measured cortical surface area, curvature and thickness using BRAINS2 on structural MRI scans acquired using 1.5 T GE whole body scanner on all subjects. We examined the differences between study groups at baseline using mixed-effects models, and longitudinal trajectory of these measures using linear mixed-effects models.

RESULTS

At baseline, offspring of schizophrenia parents showed reduced gyral surface area in the fronto-parietal lobes along with increased sulcal curvature and parietal gyral cortical thinning compared to healthy subjects. Prospective follow up of these subjects for one year showed shrinking of the total surface area, especially in the bilateral frontal and occipital regions along with preservation of cortical thickness among offspring of schizophrenia parents whereas healthy subjects showed preserved or increased surface area and cortical thinning. Correlation of these measures with lobar volumes was not observed at baseline cross-sectional comparisons but was observed in longitudinal examinations.

DISCUSSION

Our observations suggest that adolescents with genetically elevated risk for schizophrenia show altered cortical surface measures affecting cortical surface area and thickness differentially suggesting a divergent trajectory of neurodevelopment. Cortical surface measures appear to be more sensitive to genetic liability to schizophrenia compared to volumetric measures.

Lateralized genetic and environmental influences on human brain morphology of 8-year-old twins

It has been increasing rapidly interest in understanding genetic effects on brain structure and function in recent years. In this study, we examined the genetic and environmental influences on the variation in cortical thickness and specific tissue volumes in a large cohort of 8-year-old healthy twins. The present study can provide a better estimation of the genetic and environmental effects by virtue of the homogeneously aged pediatric twin pairs with a similar growing environment. We found that common environmental factors contributed significantly to the variations of the right lateral ventricle (36%) and corpus callosum (36%) volumes while genetic factors accounted for most of the phenotypic variance in other brain tissue volumes. In the case of cortical thickness, several regions in the left hemisphere showed statistically significant additive genetic factors, including the middle and inferior frontal gyri, lateral fronto-orbital and occipitotemporal gyri, pars opercularis, planum temporale, precentral and parahippocampal gyri and the medial region of the primary somatosensory cortex. Relatively high common environmental influence (>50%) was observed in the right anterior cingulate cortex and insula. Our findings indicate that the genetic and common environmental influences on individual human brain structural differences are lateralized, with the language-dominant left cerebral cortex under stronger genetic control than the right.

Endophenotypes in normal brain morphology and Alzheimer's disease: a review

Late-onset Alzheimer's disease is a common complex disorder of old age. Though these types of disorders can be highly heritable, they differ from single-gene (Mendelian) diseases in that their causes are often multifactorial with both genetic and environmental components. Genetic risk factors that have been firmly implicated in the cause are mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) genes, which are found in large multi-generational families with an autosomal dominant pattern of disease inheritance, the apolipoprotein E (APOE)epsilon4 allele and the sortilin-related receptor (SORL1) gene. Environmental factors that have been associated with late-onset Alzheimer's disease include depressive illness, various vascular risk factors, level of education, head trauma and estrogen replacement therapy. This complexity may help explain their high prevalence from an evolutionary perspective, but the etiologic complexity makes identification of disease-related genes much more difficult. The "endophenotype" approach is an alternative method for measuring phenotypic variation that may facilitate the identification of susceptibility genes for complexly inherited traits. The usefulness of endophenotypes in genetic analyses of normal brain morphology and, in particular for Alzheimer's disease will be reviewed as will the implications of these findings for models of disease causation. Given that the pathways from genotypes to end-stage phenotypes are circuitous at best, identifying endophenotypes more proximal to the effects of genetic variation may expedite the attempts to link genetic variants to disorders.

Rightward hemispheric asymmetries in auditory language cortex in children with autistic disorder: an MRI investigation

PURPOSE

determine if language disorder in children with autistic disorder (AD) corresponds to abnormalities in hemispheric asymmetries in auditory language cortex.

METHODS

MRI morphometric study in children with AD (n = 50) to assess hemispheric asymmetries in auditory language cortex. A key region of interest was the planum temporale (PT), which is larger in the left hemisphere in most healthy individuals.

RESULTS

(i) Heschl's gyrus and planum polare showed typical hemisphere asymmetry patterns; (ii) posterior Superior Temporal Gyrus (pSTG) showed significant rightward asymmetry; and (iii) PT showed a trend for rightward asymmetry that was significant when constrained to right-handed boys (n = 30). For right-handed boys, symmetry indices for pSTG were significantly positively correlated with those for PT. PT asymmetry was age dependent, with greater rightward asymmetry with age.

CONCLUSIONS

results provide evidence for rightward asymmetry in auditory association areas (pSTG and PT) known to subserve language processing. Cumulatively, our data provide evidence for a differing maturational path for PT for lower functioning children with AD, with both pre- and post-natal experience likely playing a role in PT asymmetry.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11689-009-9010-2) contains supplementary material, which is available to authorized users.

Paracingulate asymmetry in anterior and midcingulate cortex: sex differences and the effect of measurement technique

Many structural brain asymmetries accompany left hemisphere language dominance. For example, the cingulate sulcus is larger in the medial cortex of the right hemisphere, while the more dorsal paracingulate sulcus is larger on the left. The functional significance of these asymmetries is unknown because fMRI studies rarely attempt to localize activation to specific sulci, possibly due to difficulties in consistent sulcal identification. In medial cortex, for example, there are many regions of partial sulcal overlap where MRI images do not provide sufficient information to unambiguously distinguish a paracingulate sulcus from a displaced anterior cingulate segment. As large samples of postmortem material are rarely available for cytoarchitectural studies of sulcal variation, we have investigated the effect of variation in boundary and sulcal definition on paracingulate asymmetry in the MRI scans of 200 healthy adults (100 men, 100 women). Although women displayed a reliable asymmetry in the size of the paracingulate sulcus, regardless of boundary definition or technique, asymmetry was greatest when (1) the measurement was limited to the midcingulate region between the genu and the anterior commissure; and (2) the more dorsal of two overlapping sulci was always classified as a paracingulate sulcus (rather than as a displaced cingulate segment). The fact that paracingulate asymmetry is maximal in the midcingulate region suggests that this region may play a particular role in hemispheric specialization for language. Future work should investigate the structural and functional correlates of sulcal variation in this region.

Sylvian fissure morphology in Prader-Willi syndrome and early-onset morbid obesity

PURPOSE

Prader-Willi syndrome is a well-defined genetic cause of childhood-onset obesity that can serve as a model for investigating early-onset childhood obesity. Individuals with Prader-Willi syndrome have speech and language impairments, suggesting possible involvement of the perisylvian region of the brain. Clinical observations suggest that many individuals with early-onset morbid obesity have similar speech/language deficits, indicating possible perisylvian involvement in these children as well. We hypothesized that similar perisylvian abnormalities may exist in both disorders.

METHODS

Participants included individuals with Prader-Willi syndrome (n = 27), their siblings (n = 16), individuals with early-onset morbid obesity (n = 13), and their siblings (n = 10). Quantitative and qualitative assessments of sylvian fissure conformation, insula closure, and planum temporale length were performed blind to hemisphere and diagnosis.

RESULTS

Quantitative measurements verified incomplete closure of the insula in individuals with Prader-Willi syndrome. Planar asymmetry showed its normal bias toward leftward asymmetry in all groups except those with Prader-Willi syndrome maternal uniparental disomy. Individuals with Prader-Willi syndrome and siblings had a normal distribution of sylvian fissure types in both hemispheres, while individuals with early-onset morbid obesity and their siblings had a high proportion of rare sylvian fissures in the right hemisphere.

CONCLUSIONS

The contrast between the anatomic findings in individuals with Prader-Willi syndrome and early-onset morbid obesity suggests that the language problems displayed by children with these two conditions may be associated with different neurodevelopmental processes.

MEG auditory evoked fields suggest altered structural/functional asymmetry in primary but not secondary auditory cortex in bipolar disorder

OBJECTIVES

Objective physiological indices independently characterizing affective and schizophreniform psychoses would contribute to our understanding of the nature of their relationships. Magnetoencephalography (MEG)-based metrics of altered structural/functional asymmetry in the superior temporal gyrus have previously been found to characterize schizophrenia at the level of both the primary auditory (AI) and the secondary auditory (AII) cortex. This study examines these markers in patients with bipolar disorder, with the goal of improved understanding of the patterns of brain asymmetry that may independently characterize affective and schizophreniform psychosis.

METHODS

We studied 17 euthymic bipolar subjects and 17 matched controls. Auditory evoked fields were generated by both 40 Hz auditory stimuli eliciting steady state gamma band (SSR), activating the AI cortex, and discrete 1 kHz tone pips, activating the AII cortex. MEG was recorded from the hemisphere contralateral to the ear stimulated using a 37-channel MEG system. Source location estimates were calculated in both left and right hemispheres. Neuroanatomical location estimates for medial Heschl's gyri were determined from magnetic resonance images for correlation with MEG source locations.

RESULTS

Bipolar subjects failed to demonstrate normal laterality of SSR AI responses, indicating altered patterns of asymmetry at the level of AI cortex, but demonstrated normal asymmetry of AII responses (right anterior to left). Medial Heschl's gyri centroids were similarly lateralized in both groups, however (right anterior to left), dissociating function from structure in the AI cortex in the bipolar group.

CONCLUSIONS

The findings are compatible with altered functional/structural relationships, including diminished left-right hemisphere asymmetry of the AI, but not the AII cortex in bipolar disorder. In schizophrenia, both the AI and AII cortices exhibit such derangements; thus, the findings support both shared and nonshared features of auditory cortical disruption between the two disorders. This functional disorganization may help explain previously reported decreases in amplitude and phase synchrony of SSR gamma band responses in bipolar subjects, suggesting impaired neocortical synchrony in AI, possibly at a cortico-thalamic level, but perhaps not extending to heteromodal association cortex, and may relate to the cognitive impairments found in bipolar disorder.

Review of twin and family studies on neuroanatomic phenotypes and typical neurodevelopment

This article reviews the extant twin studies employing magnetic resonance imaging data (MRI), with an emphasis on studies of population-based samples. There have been approximately 75 twin reports using MRI, with somewhat under half focusing on typical brain structure. Of these, most are samples of adults. For large brain regions such as lobar volumes, the heritabilities of large brain volumes are consistently high, with genetic factors accounting for at least half of the phenotypic variance. The role of genetics in generating individual differences in the volumes of small brain regions is less clear, mostly due to a dearth of information, but rarely because of disagreement between studies. Multivariate analyses show strong genetic relationships between brain regions. Cortical regions involved in language, executive function, and emotional regulation appear to be more heritable than other areas. Studies of brain shape also show significant, albeit lower, genetic effects on population variance. Finally, there is evidence of significant genetically mediated relationships between intelligence and brain structure. At present, the majority of twin imaging studies are limited by sample sizes small by the standards of behavioral genetics; nevertheless the literature at present represents a pioneering effort in the pursuit of answers to many challenging neurobiological questions.

Genetic influences on human brain structure: a review of brain imaging studies in twins

Twin studies suggest that variation in human brain volume is genetically influenced. The genes involved in human brain volume variation are still largely unknown, but several candidate genes have been suggested. An overview of structural Magnetic Resonance (brain) Imaging studies in twins is presented, which focuses on the influence of genetic factors on variation in healthy human brain volume. Twin studies have shown that genetic effects varied regionally within the brain, with high heritabilities of frontal lobe volumes (90-95%), moderate estimates in the hippocampus (40-69%), and environmental factors influencing several medial brain areas. High heritability estimates of brain structures were revealed for regional amounts of gray matter (density) in medial frontal cortex, Heschl's gyrus, and postcentral gyrus. In addition, moderate to high heritabilities for densities of Broca's area, anterior cingulate, hippocampus, amygdala, gray matter of the parahippocampal gyrus, and white matter of the superior occipitofrontal fasciculus were reported. The high heritability for (global) brain volumes, including the intracranium, total brain, cerebral gray, and white matter, seems to be present throughout life. Estimates of genetic and environmental influences on age-related changes in brain structure in children and adults await further longitudinal twin-studies. For prefrontal cortex volume, white matter, and hippocampus volumes, a number of candidate genes have been identified, whereas for other brain areas, only a few or even a single candidate gene has been found so far. New techniques such as genome-wide scans may become helpful in the search for genes that are involved in the regulation of human brain volume throughout life.

Neuroimaging endophenotypes: strategies for finding genes influencing brain structure and function

It is vitally important to identify the genetic determinants of complex brain-related disorders such as autism, dementia, mood disorders, and schizophrenia. However, the search for genes predisposing individuals to these illnesses has been hampered by their genetic and phenotypic complexity and by reliance upon phenomenologically based qualitative diagnostic systems. Neuroimaging endophenotypes are quantitative indicators of brain structure or function that index genetic liability for an illness. These indices will significantly improve gene discovery and help us to understand the functional consequences of specific genes at the level of systems neuroscience. Here, we review the feasibility of using neuroanatomic and neuropsychological measures as endophenotypes for brain-related disorders. Specifically, we examine specific indices of brain structure or function that are genetically influenced and associated with neurological and psychiatric illness. In addition, we review genetic approaches that capitalize on the use of quantitative traits, including those derived from brain images.

Hand preference and performance in 20 pairs of monozygotic twins with discordant handedness

The differences between right (RH) and left (LH) handers reported in the literature on fine motor tasks, has traditionally been interpreted relative to purported functioning of the cerebral hemispheres. However, conclusive evidence for performance differences which are intrinsic to handedness per se is difficult to obtain unless left and right handers are compared who are similar in their genetic and environmental background. The present study therefore, employed a monozygotic (MZ) twin design which minimizes differences in genetic variation between the two groups. Forty female monozygotic twins (20 pairs) were selected on the basis of discordance of writing hand. Their laterality preferences were assessed and they were tested for differences on hand performance tasks (dot filling, finger tapping, and peg moving). The results revealed that on the hand and foot preference inventories, the right-handers were more strongly lateralized than their left-handed sisters, and that the left-handers had greater variation in their laterality scores. There were substantial correlations between preference and performance scores. The analyses not only revealed the obvious strong main effects of writing hand on performance tasks, but interaction effects of handedness on the peg-moving task. The dot filling task differentiated the writing versus non-writing hand considerably better than either of the other two performance tasks. However, no evidence was found to indicate that twins who wrote with their left hands showed poorer performance than their right-handed twin sisters.

Exploiting human anatomical variability as a link between genome and cognome

Although talents and disabilities appear to run in families, direct links between genes and cognitive ability are difficult to establish. Investigators are currently searching for intermediate phenotypes with plausible links to both genome and cognome (the cognitive phenotype). Cortical anatomy could provide one such intermediate phenotype. Variation in cortical size, asymmetry and sulcal pattern is influenced by genetic variation in neurotrophic factors and can predict variation in verbal and mathematical talent. Anecdotal evidence suggests that individuals with a rare morphological variant of Sylvian fissure sometimes have superior visualization ability combined with verbal deficits. Documentation of such 'cognitive cortical syndromes' might prove as genetically informative as the identification of dysmorphic syndromes associated with mental retardation. A necessary prerequisite for the establishment of such syndromes is a reliable technique for the identification of cortical patterns. Recent technical advances in software for automatically labeling and measuring cortical sulci now provide the possibility of establishing standard measures for their shape, size and location. Such measures are a prerequisite for genetic studies of cortical patterns that could illuminate the neurodevelopmental pathways by which genes affect cognitive ability.

Sulcal pattern and morphology of the superior temporal sulcus

The superior temporal sulcus (STs) is the main sulcal landmark of the external temporal cortex and is very important for functional (posterior language areas on the left) mapping and surgery. The methodology we use is based on the extraction of the 3D shape of sulci and their separation into subunits called sulcal roots. Seventeen normal brains (male: 11, female: 6, age: 22-60) were systematically analyzed. Additionally, parameters generated by visual observation were recorded. Non-parametric statistics were performed to evaluate the variation of the STs and influence of side, handedness and sex. We found that the 3D architecture of the STs was consistent with our generic model in four sulcal roots and four "plis de passage" (PP) and significant differences between right and left hemispheres. These morphological differences may be related to the language-relevant cortical areas difference and are pertinent for defining the limits of morphometric variability of the STs in "normal humans".

Left and right in the amphibian world: which way to develop and where to turn?

The last decade has seen a dramatic increase in studies on the development, function and evolution of asymmetries in vertebrates, including amphibians. Here we discuss current knowledge of behavioral and anatomical asymmetries in amphibians. Behavioral laterality in the response of both adult and larval anurans to presumed predators and competitors is strong and may be related, respectively, to laterality in the telencephalon of adults and the Mauthner neurons of tadpoles. These behavior lateralities, however, do not seem to correlate with visceral asymmetries in the same animals. We briefly compare what is known about the evolution and development of asymmetry in the structure and function of amphibians with what is known about asymmetries in other chordate and non-chordate groups. Available data suggest that the majority of asymmetries in amphibians fall into two independent groups: (1) related to situs viscerum and (2) of a neurobehavioral nature. We find little evidence linking these two groups, which implies different developmental regulatory pathways and independent evolutionary histories for visceral and telencephalic lateralizations. Studies of animals other than standard model species are essential to test hypotheses about the evolution of laterality in amphibians and other chordates.

Dynamic programming generation of boundaries of local coordinatized submanifolds in the neocortex: application to the planum temporale

Dynamic programming is used to define boundaries of cortical submanifolds with focus on the planum temporale (PT) of the superior temporal gyrus (STG), which has been implicated in a variety of neuropsychiatric disorders. To this end, automated methods are used to generate the PT manifold from 10 high-resolution MRI subvolumes ROI masks encompassing the STG. A procedure to define the subvolume ROI masks from original MRI brain scans is developed. Bayesian segmentation is then used to segment the subvolumes into cerebrospinal fluid, gray matter (GM), and white matter (WM). 3D isocontouring using the intensity value at which there is equal probability of GM and WM is used to reconstruct the triangulated graph representing the STG cortical surface, enabling principal curvature at each point on the graph to be computed. Dynamic programming is used to delineate the PT manifold by tracking principal curves from the retro-insular end of the Heschl's gyrus (HG) to the STG, along the posterior STG up to the start of the ramus and back to the retro-insular end of the HG. A coordinate system is then defined on the PT manifold. The origin is defined by the retro-insular end of the HG and the y-axis passes through the point on the posterior STG where the ramus begins. Automated labeling of GM in the STG is robust with L(1) distances between Bayesian and manual segmentation in the range 0.001-0.12 (n = 20). PT reconstruction is also robust with 90% of the vertices of the reconstructed PT within about 1 voxel (n = 20) from semiautomated contours. Finally, the reliability index (based on interrater intraclass correlation) for the surface area derived from repeated reconstructions is 0.96 for the left PT and 0.94 for the right PT, thus demonstrating the robustness of dynamic programming in defining a coordinate system on the PT. It provides a method with potential significance in the study of neuropsychiatric disorders.