Brain Asymmetry Research Articles

Functional brain asymmetry for emotions: psychological stress-induced reversed hemispheric asymmetry in emotional face perception

Empirical evidence hasdemonstrated functional (mostly right-biased)brain asymmetry for emotion perception, whereas recent studies indicate that acute stress may modulate left and/or right hemisphere activation. However, it is still unknown whether emotion perception can be influenced by stress-induced hemispheric activation since behavioral studies report inconsistent or even contradictory results. We sought to reevaluate this gap. Eighty-eight healthy Caucasian university students participated in the study. In half of the randomly selected participants, acute psychological stress was induced by displaying a brief stressful movie clip (the stress condition), whereas the other half were shown a neutral movie clip (the non-stress condition). Prior to (the baseline) and following the movie clip display an emotion perception task was applied by presenting an emotional (happy, surprised, fearful, sad, angry, or disgusted) or neutral face to the left or right visual field. We found a more accurate perception of emotional and neutral faces presented to the LVF (the right hemisphere) in the baseline. However, we revealed that after watching a neutral movie clip, behavioral performance in emotional and neutral face perception accuracy became relatively equalized for both visual fields, whereas after watching a stressful movie clip, the RVF (the left hemisphere) even became dominant in emotional face perception. We propose a novel hemispheric functional-equivalence model: the brain is initially right-biased in emotional and neutral face perception by default; however, psychophysiological activation of a distributed brain-network due to watching neutral movie clips redistributes hemispheric performance toward relative equivalence. Moreover, even reversed hemispheric asymmetry may occur.

Asymmetric Behavior in Ptyodactylus guttatus: Can a Digit Ratio Reflect Brain Laterality?

The digit ratio, an indicator of brain laterality, is the ratio of the second and fourth digits on the left (L24) or right foot (R24). Much of the research on the digit ratio and brain laterality focuses on primates, rather than other species such as reptiles. We tested whether the digit ratio in the gecko Ptyodactylus guttatus was associated with behaviors attributed to brain laterality. We examined risk-taking behavior (time spent under cover), foot preference (which foot was the first to start moving) and the side from which geckos bypassed an obstacle, in relation to the digit ratio. Geckos with longer fourth digits on their left hind foot (higher digit ratio) spent more time under cover. Geckos starting to move with their left leg were much more likely to bypass obstacles from the right side, and vice versa. This is the first evidence of laterality being associated with the digit ratio in reptiles. Comparisons among vertebrates are needed in order to decipher the evolutionary origin of the commonalities and peculiarities of brain asymmetry and disentangle the patterns and drivers of our evolutionary tree.

Reproducibility of leftward planum temporale asymmetries in two genetically isolated populations of chimpanzees (Pan troglodytes).

Once considered a hallmark of human uniqueness, brain asymmetry has emerged as a feature shared with several other species, including chimpanzees, one of our closest living relatives. Most notable has been the discovery of asymmetries in homologues of cortical language areas in apes, particularly in the planum temporale (PT), considered a central node of the human language network. Several lines of evidence indicate a role for genetic mechanisms in the emergence of PT asymmetry; however, the genetic determinants of cerebral asymmetries have remained elusive. Studies in humans suggest that there is heritability of brain asymmetries of the PT, but this has not been explored to any extent in chimpanzees. Furthermore, the potential influence of non-genetic factors has raised questions about the reproducibility of earlier observations of PT asymmetry reported in chimpanzees. As such, the present study was aimed at examining both the heritability of phenotypic asymmetries in PT morphology, as well as their reproducibility. Using magnetic resonance imaging, we evaluated morphological asymmetries of PT surface area (mm2) and mean depth (mm) in captive chimpanzees (n = 291) derived from two genetically isolated populations. Our results confirm that chimpanzees exhibit a significant population-level leftward asymmetry for PT surface area, as well as significant heritability in the surface area and mean depth of the PT. These results conclusively demonstrate the existence of a leftward bias in PT asymmetry in chimpanzees and suggest that genetic mechanisms play a key role in the emergence of anatomical asymmetry in this region.

Mapping the principal gradient onto the corpus callosum

Gradients capture some of the variance of the resting-state functional magnetic resonance imaging (rsfMRI) signal. Amongst these, the principal gradient depicts a functional processing hierarchy that spans from sensory-motor cortices to regions of the default-mode network. While the cortex has been well characterised in terms of gradients little is known about its underlying white matter. For instance, comprehensive mapping of the principal gradient on the largest white matter tract, the corpus callosum, is still missing. Here, we mapped the principal gradient onto the midsection of the corpus callosum using the 7T human connectome project dataset. We further explored how quantitative measures and variability in callosal midsection connectivity relate to the principal gradient values. In so doing, we demonstrated that the extreme values of the principal gradient are located within the callosal genu and the posterior body, have lower connectivity variability but a larger spatial extent along the midsection of the corpus callosum than mid-range values. Our results shed light on the relationship between the brain's functional hierarchy and the corpus callosum. We further speculate about how these results may bridge the gap between functional hierarchy, brain asymmetries, and evolution.

Reproducibility in the absence of selective reporting: Anillustration from large-scale brain asymmetry research.

The problem of poor reproducibility of scientific findings has received much attention over recent years, in a variety of fields including psychology and neuroscience. The problem has been partly attributed to publication bias and unwanted practices such as p‐hacking. Low statistical power in individual studies is also understood to be an important factor. In a recent multisite collaborative study, we mapped brain anatomical left–right asymmetries for regional measures of surface area and cortical thickness, in 99 MRI datasets from around the world, for a total of over 17,000 participants. In the present study, we revisited these hemispheric effects from the perspective of reproducibility. Within each dataset, we considered that an effect had been reproduced when it matched the meta‐analytic effect from the 98 other datasets, in terms of effect direction and significance threshold. In this sense, the results within each dataset were viewed as coming from separate studies in an “ideal publishing environment,” that is, free from selective reporting and p hacking. We found an average reproducibility rate of 63.2% (SD = 22.9%, min = 22.2%, max = 97.0%). As expected, reproducibility was higher for larger effects and in larger datasets. Reproducibility was not obviously related to the age of participants, scanner field strength, FreeSurfer software version, cortical regional measurement reliability, or regional size. These findings constitute an empirical illustration of reproducibility in the absence of publication bias or p hacking, when assessing realistic biological effects in heterogeneous neuroscience data, and given typically‐used sample sizes.

Atypical Brain Asymmetry in Autism—A Candidate for Clinically Meaningful Stratification

BackgroundAutism spectrum disorder (“autism”) is a highly heterogeneous neurodevelopmental condition with few effective treatments for core and associated features. To make progress we need to both identify and validate neural markers that help to parse heterogeneity to tailor therapies to specific neurobiological profiles. Atypical hemispheric lateralization is a stable feature across studies in autism, but its potential as a neural stratification marker has not been widely examined. MethodsIn order to dissect heterogeneity in lateralization in autism, we used the large EU-AIMS (European Autism Interventions—A Multicentre Study for Developing New Medications) Longitudinal European Autism Project dataset comprising 352 individuals with autism and 233 neurotypical control subjects as well as a replication dataset from ABIDE (Autism Brain Imaging Data Exchange) (513 individuals with autism, 691 neurotypical subjects) using a promising approach that moves beyond mean group comparisons. We derived gray matter voxelwise laterality values for each subject and modeled individual deviations from the normative pattern of brain laterality across age using normative modeling. ResultsIndividuals with autism had highly individualized patterns of both extreme right- and leftward deviations, particularly in language, motor, and visuospatial regions, associated with symptom severity. Language delay explained most variance in extreme rightward patterns, whereas core autism symptom severity explained most variance in extreme leftward patterns. Follow-up analyses showed that a stepwise pattern emerged, with individuals with autism with language delay showing more pronounced rightward deviations than individuals with autism without language delay. ConclusionsOur analyses corroborate the need for novel (dimensional) approaches to delineate the heterogeneous neuroanatomy in autism and indicate that atypical lateralization may constitute a neurophenotype for clinically meaningful stratification in autism.

Investigating the impact of supervoxel segmentation for unsupervised abnormal brain asymmetry detection

Several brain disorders are associated with abnormal brain asymmetries (asymmetric anomalies). Several computer-based methods aim to detect such anomalies automatically. Recent advances in this area use automatic unsupervised techniques that extract pairs of symmetric supervoxels in the hemispheres, model normal brain asymmetries for each pair from healthy subjects, and treat outliers as anomalies. Yet, there is no deep understanding of the impact of the supervoxel segmentation quality for abnormal asymmetry detection, especially for small anomalies, nor of the added value of using a specialized model for each supervoxel pair instead of a single global appearance model. We aim to answer these questions by a detailed evaluation of different scenarios for supervoxel segmentation and classification for detecting abnormal brain asymmetries. Experimental results on 3D MR-T1 brain images of stroke patients confirm the importance of high-quality supervoxels fit anomalies and the use of a specific classifier for each supervoxel. Next, we present a refinement of the detection method that reduces the number of false-positive supervoxels, thereby making the detection method easier to use for visual inspection and analysis of the found anomalies.

Differential expression of microRNAs in the human fetal left and right cerebral cortex.

Human brain is anatomically and functionally asymmetric. How brain asymmetry is initiated and established during fetal development is poorly understood. Accumulating evidence has shown that microRNAs (miRNAs) play crucial roles in brain development and function. In this study, we investigate miRNA expression profiles in left and right hemispheres of human fetal brains at 12weeks post conception (PC), and identify 42 miRNAs showing differential expression between two hemispheres using Affymetrix microarray analyses. Target genes for left- and right-biased miRNAs are largely involved in developmental and functional regulations in the cortex such as axon guidance, GABAergic synapse and dopaminergic synapse pathways. Moreover, we find that predicted targets associated with canonical and non-canonical WNT signaling pathway show variations and differential expression between two hemispheres in response to left- and right-biased miRNAs. Our results highlight a potential role of miRNAs in regulating asymmetric development of human fetal brains.

From Smart Apes to Human Brain Boxes. A Uniquely Derived Brain Shape in Late Hominins Clade

Modern humans have larger and more globular brains when compared to other primates. Such anatomical features are further reflected in the possession of a moderately asymmetrical brain with the two hemispheres apparently rotated counterclockwise and slid anteroposteriorly on one another, in what is traditionally described as the Yakovlevian Torque. Developmental disturbance in human brain asymmetry, or lack thereof, has been linked to several cognitive disorders including schizophrenia and depression. More importantly, the presence of the Yakovlevian Torque is often advocated as the exterior manifestation of our unparalleled cognitive abilities. Consequently, studies of brain size and asymmetry in our own lineage indirectly address the question of what, and when, made us humans, trying to trace the emergence of brain asymmetry and expansion of cortical areas back in our Homo antecedents. Here we tackle this same issue studying the evolution of human brain size, shape and asymmetry on a phylogenetic tree including nineteen apes and Homo species, inclusive of our fellow ancestors. We found that a significant positive shift in the rate of brain shape evolution pertains to the clade including modern humans, Neanderthals and Homo heidelbergensis. Although the Yakovlevian Torque is well evident in these species and levels of brain asymmetry are correlated to changes in brain shape, further early Homo species possess the torque. Even though, a strong allometric component is present in hominoids brain shape variability, this component seems unrelated to asymmetry and to the rate shift we recorded. These results suggest that changes in brain size and asymmetry were not the sole factors behind the fast evolution of brain shape in the most recent Homo species. The emergence of handedness and early manifestations of cultural modernity in the archaeological record nicely coincide with the same three species sharing the largest and most rapidly evolving brains among all hominoids.

Brain Lateralization: A Comparative Perspective.

Comparative studies on brain asymmetry date back to the 19th century but then largely disappeared due to the assumption that lateralization is uniquely human. Since the reemergence of this field in the 1970s, we learned that left-right differences of brain and behavior exist throughout the animal kingdom and pay off in terms of sensory, cognitive, and motor efficiency. Ontogenetically, lateralization starts in many species with asymmetrical expression patterns of genes within the Nodal cascade that set up the scene for later complex interactions of genetic, environmental, and epigenetic factors. These take effect during different time points of ontogeny and create asymmetries of neural networks in diverse species. As a result, depending on task demands, left- or right-hemispheric loops of feedforward or feedback projections are then activated and can temporarily dominate a neural process. In addition, asymmetries of commissural transfer can shape lateralized processes in each hemisphere. It is still unclear if interhemispheric interactions depend on an inhibition/excitation dichotomy or instead adjust the contralateral temporal neural structure to delay the other hemisphere or synchronize with it during joint action. As outlined in our review, novel animal models and approaches could be established in the last decades, and they already produced a substantial increase of knowledge. Since there is practically no realm of human perception, cognition, emotion, or action that is not affected by our lateralized neural organization, insights from these comparative studies are crucial to understand the functions and pathologies of our asymmetric brain.

Impact of the Use of the Beanie on the Neurodevelopmental Outcomes of Preterm Infants With Plagiocephaly: A Pilot Study.

BackgroundDeformational plagiocephaly (DP) is the abnormal flattening of the skull. Infants with DP have been found to have abnormal brain shape and asymmetry associated with worse neurodevelopmental outcomes on the Bayley Scales of Infant and Toddler Development-III (BSID-III) compared to those without DP. In 2009, the FDA approved a repositioning Beanie, the TortleTM (Tortle Products LLC, Greenwood Village, CO), for the prevention of flat head syndrome. PurposeOur goal was to assess the impact of the use of the Beanie on the neurodevelopment of preterm infants with DP admitted in the neonatal intensive care unit (NICU) using the BSID-III.MethodsSubjects were identified using a retrospective chart review of infants during January 2013-2017. Infants of less than 32 weeks of gestational age, under 1500 g birth weight, and attending the high-risk follow-up clinic were included in the study. Neurodevelopmental assessment of patients' cognition, language, motor development using the BSID-III was performed at the 12-month and 24-month follow-up visits. The BSID-III scores for patients who used the Beanie were compared to those who did not.ResultsA total of 207 patients met the inclusion criteria. The gestational age ranged from 22.5 to 31.5 weeks with a median and mean gestational age of 26.4 weeks and 26.5 weeks respectively. Of the patients, 105 were females and 102 males. The birth weight ranged between 460 g and 1460 g with a mean of 879 g and a median of 860 g. The Beanie was used in 32 patients; 31 patients were found to use the Beanie at 12 months and 16 patients at 24 months. Of note, 12-month Bayley cognition scores were found to be statistically improved in babies who used the Beanie versus those who did not (p: 0.02). The statistical significance was not appreciated at 24 months, which could be due to a decrease in the sample size.Conclusion The Beanie is an inexpensive and simple way to help prevent DP in preterm infants, which in turn could improve the aforementioned outcomes.

Clinical Implication of Brain Asymmetries in Psychiatric Disorders

Clinical Implication of Brain Asymmetries in Psychiatric Disorders

Dyslexia, Dysgraphia, Inclusion: a Literature Review in the Research Field

The history of the appearance and development of the concepts of dyslexia, dysgraphia and inclusion is considered in the article. The research methods are the method of critical interpretation, the method of conceptualization, the observational method. The terms of dyslexia and dysgraphia attracted the doctor’s attention in the Late 19th – Early 20th Century. An explanation of their causes focused on medical aspects (ophthalmological factors, brain asymmetry, etc.). Linguistic and social argumentations of these deviations have appeared in the 1970s. The number of students with difficulties in reading and writing in the world is quite large and amounts to at least 10% of the total world population. In the 1980–1990s, the question arose of the stages of the such student’s integration in the general educational process. The three pedagogical models were used in were used in education throughout the 20th century: segregation, integration and inclusion. Although the inclusion assumes the equal participation of all children, without exception, in the educational process, nevertheless there is no consensus on the widespread of this model in education among the students’ parents and pedagogical community.

Mapping brain asymmetry in health and disease through the ENIGMA consortium.

Left–right asymmetry of the human brain is one of its cardinal features, and also a complex, multivariate trait. Decades of research have suggested that brain asymmetry may be altered in psychiatric disorders. However, findings have been inconsistent and often based on small sample sizes. There are also open questions surrounding which structures are asymmetrical on average in the healthy population, and how variability in brain asymmetry relates to basic biological variables such as age and sex. Over the last 4 years, the ENIGMA‐Laterality Working Group has published six studies of gray matter morphological asymmetry based on total sample sizes from roughly 3,500 to 17,000 individuals, which were between one and two orders of magnitude larger than those published in previous decades. A population‐level mapping of average asymmetry was achieved, including an intriguing fronto‐occipital gradient of cortical thickness asymmetry in healthy brains. ENIGMA's multi‐dataset approach also supported an empirical illustration of reproducibility of hemispheric differences across datasets. Effect sizes were estimated for gray matter asymmetry based on large, international, samples in relation to age, sex, handedness, and brain volume, as well as for three psychiatric disorders: autism spectrum disorder was associated with subtly reduced asymmetry of cortical thickness at regions spread widely over the cortex; pediatric obsessive–compulsive disorder was associated with altered subcortical asymmetry; major depressive disorder was not significantly associated with changes of asymmetry. Ongoing studies are examining brain asymmetry in other disorders. Moreover, a groundwork has been laid for possibly identifying shared genetic contributions to brain asymmetry and disorders.

Mapping brain asymmetry in health and disease through the ENIGMA consortium

Mapping brain asymmetry in health and disease through the ENIGMA consortium

Unmasking the relevance of hemispheric asymmetries-Break on through (to the other side).

The pioneer works of Marc Dax and Paul Broca on the association between left hemisphere injuries and speech impairments, revealed one of the most intriguing properties of the brain - asymmetry. Since then, lateralized features have been observed in virtually all phylogenetic branches, suggesting evolutionary conservation, although its adaptive role is still not clear. In humans, the field remains greatly shaped by early observations on language, but the advent of brain imaging revealed that functional and structural laterality is not only widespread, extending to memory, decision-making and emotion, but also that it is plastic. In this review, we systematize information regarding structural and functional hemispheric asymmetries of the healthy brain and their associations with cognition and behavior. We briefly explore evolutionary theories and the pathways for asymmetry development, but mostly we focus on central nervous system asymmetries of the adult human, bridging towards the laboratory rodent for mechanistic explanations.

Altered Hemispheric Asymmetry of White Matter Integrity in Children With Williams Syndrome

Williams syndrome (WS) is a rare condition caused by hemideletion of ∼25 genes at chromosomal location 7q11.23. Previous neuroimaging studies have demonstrated altered white matter integrity in both hemispheres in WS, but some findings, along with reported disparities in verbal versus spatial abilities, suggest that there may be underlying brain asymmetries. Because formal analyses of such putative lateralization have not been reported, here we tested for white matter asymmetry differences between children with WS and typically developing (TD) children.

Speaking of aging: Changes in gray matter asymmetry in Broca’s area in later adulthood

Several theories suggest a change in the brain's asymmetry as we get older. However, it is currently unresolved whether Broca's area, consisting of left Brodmann Areas (BA) 45 and 44, undergoes age-related changes. To address this question, we mapped associations between chronological age and gray matter asymmetry of BA45 and BA44 in a large sample (n = 485) of adults ranging between 42 and 97 years of age. Hemisphere-specific gray matter volumes and asymmetry indices were obtained by integrating cytoarchitectonic probabilities with MRI-based signal intensities. For BA44, we did not observe any significant correlation between age and gray matter asymmetry. In contrast, for BA45, the analysis revealed a significant correlation, which indicates a decreasing asymmetry from rightward to less rightward with increasing age. A subsequent characterization of hemisphere-specific volume loss revealed significant negative associations between age and gray matter volume for left and right BA45, but with weaker effects in the left hemisphere compared to the right. These findings seem to support the assumption of reduced structural asymmetries later in life, at least for BA45, which seem to be driven by a stronger tissue loss in the right hemisphere than the left hemisphere.

Network dilution and asymmetry in an efficient brain

ABSTRACT The ultimate goal of neuroscience is to ultimately understand how the brain functions. The advancement of brain imaging shows us how the brain continuously alternates complex activity patterns and experimentally reveals how these patterns are responsible for memory, association, reasoning, and countless other tasks. Two fundamental parameters, dilution (the number of connections per node), and symmetry (the number of bidirectional connections of the same weight) characterise two fundamental features underlying the networks that connect the single neurons in the brain and generate these patterns. Mammalian brains show large variations of dilution, and mostly asymmetric connectivity, unfortunately the advantages which drove evolution to these state of network dilution and asymmetry are still unknown. Here, we studied the effects of symmetry and dilution on a discrete-time recurrent neural network with McCulloch–Pitts neurons. We use an exhaustive approach, in which we probe all possible inputs for several randomly connected neuron networks with different degrees of dilution and symmetry. We find an optimum value for the synaptic dilution and symmetry, which turns out to be in striking quantitative agreement with what previous researchers have found in the brain cortex, neocortex and hippocampus. The diluted asymmetric brain shows high memory capacity and pattern recognition speed, but most of all it is the less energy-consumptive with respect to fully connected and symmetric network topologies.

Brain and Behavioral Asymmetry: A Lesson From Fish

It is widely acknowledged that the left and right hemispheres of human brains display both anatomical and functional asymmetries. For more than a century, brain and behavioral lateralization have been considered a uniquely human feature linked to language and handedness. However, over the past decades this idea has been challenged by an increasing number of studies describing structural asymmetries and lateralized behaviors in non-human species extending from primates to fish. Evidence suggesting that a similar pattern of brain lateralization occurs in all vertebrates, humans included, has allowed the emergence of different model systems to investigate the development of brain asymmetries and their impact on behavior. Among animal models, fish have contributed much to the research on lateralization as several fish species exhibit lateralized behaviors. For instance, behavioral studies have shown that the advantages of having an asymmetric brain, such as the ability of simultaneously processing different information and perform parallel tasks compensate the potential costs associated with poor integration of information between the two hemispheres thus helping to better understand the possible evolutionary significance of lateralization. However, these studies inferred how the two sides of the brains are differentially specialized by measuring the differences in the behavioral responses but did not allow to directly investigate the relation between anatomical and functional asymmetries. With respect to this issue, in recent years zebrafish has become a powerful model to address lateralization at different level of complexity, from genes to neural circuitry and behavior. The possibility of combining genetic manipulation of brain asymmetries with cutting-edge in vivo imaging technique and behavioral tests makes the zebrafish a valuable model to investigate the phylogeny and ontogeny of brain lateralization and its relevance for normal brain function and behavior.