How brain asymmetry relates to performance - a large-scale dichotic listening study

Neuroimaging in thyroid eye disease: A systematic review

Thyroid eye disease (TED) is an organ-specific autoimmune disease secondary largely to hyperthyroid Graves' disease, which profoundly affects patients' visual function, appearance, and physical and mental well-being. Emerging neuroimaging studies have reported alterations in the brains of patients with TED, suggesting that the impact of this autoimmune disease may extend beyond the orbit. This systematic review aims to consolidate the neuroimaging evidence that describes the brain alterations of TED. We analyzed information from thirty-one related studies involving 1349 TED patients and 710 healthy controls, employing multimodal neuroimaging techniques such as structural magnetic resonance imaging (MRI), functional MRI, diffusion MRI, and metabolic MRI. These studies define the brain alterations in regions associated with vision, cognition, and emotion regulation, such as gray matter volume changes, altered functional connectivity and activity, and microstructural modifications, revealing the neurological impact of TED beyond the orbit. Notably, there was convergence across these studies indicating predominant abnormalities within the occipital and parietal lobes. This review underscores the critical role of advanced neuroimaging techniques in unraveling the complex neuropathological mechanism of TED, laying a foundation for future research and potential therapeutic targets.

Analysis of distributions reveals real differences on dichotic listening scores between left- and right-handers

About 95% of right-handers and 70% of left-handers have a left-hemispheric specialization for language. Dichotic listening is often used as an indirect measure of this language asymmetry. However, while it reliably produces a right-ear advantage (REA), corresponding to the left-hemispheric specialization of language, it paradoxically often fails to obtain statistical evidence of mean differences between left- and right-handers. We hypothesized that non-normality of the underlying distributions might be in part responsible for the similarities in means. Here, we compare the mean ear advantage scores, and also contrast the distributions at multiple quantiles, in two large independent samples (Ns = 1,358 and 1,042) of right-handers and left-handers. Right-handers had an increased mean REA, and a larger proportion had an REA than in the left-handers. We also found that more left-handers are represented in the left-eared end of the distribution. These data suggest that subtle shifts in the distributions of DL scores for right- and left-handers may be at least partially responsible for the unreliability of significantly reduced mean REA in left-handers.

Sex/Gender Differences in Brain Lateralisation and Connectivity

There is now a significant body of literature concerning sex/gender differences in the human brain. This chapter will critically review and synthesise key findings from several studies that have investigated sex/gender differences in structural and functional lateralisation and connectivity. We argue that while small, relative sex/gender differences reliably exist in lateralisation and connectivity, there is considerable overlap between the sexes. Some inconsistencies exist, however, and this is likely due to considerable variability in the methodologies, tasks, measures, and sample compositions between studies. Moreover, research to date is limited in its consideration of sex/gender-related factors, such as sex hormones and gender roles, that can explain inter-and inter-individual differences in brain and behaviour better than sex/gender alone. We conclude that conceptualising the brain as 'sexually dimorphic' is incorrect, and the terms 'male brain' and 'female brain' should be avoided in the neuroscientific literature. However, this does not necessarily mean that sex/gender differences in the brain are trivial. Future research involving sex/gender should adopt a biopsychosocial approach whenever possible, to ensure that non-binary psychological, biological, and environmental/social factors related to sex/gender, and their interactions, are routinely accounted for.

Language lateralization in very preterm children: associating dichotic listening to interhemispheric connectivity and language performance

BACKGROUND

Language difficulties of very preterm (VPT) children might be related to weaker cerebral hemispheric lateralization of language. Language lateralization refers to the development of an expert region for language processing in the left hemisphere during the first years of life. Children born VPT might not develop such a dominant left hemisphere for language processing. A dichotic listening task may be a functional task to show the dominance of the left hemisphere during language processing. During this task, different acoustic events are simultaneously presented to both ears. Due to crossing fibers in the brain, right ear stimuli are transferred directly to the left hemisphere, and left ear stimuli are transferred first to the right hemisphere and then, through the corpus callosum (CC), to the left hemisphere. Dichotic listening typically shows a right ear advantage, assuming to reflect left hemispherical language dominance. The CC, in particular the splenium, is associated with auditory processing and is considered important for language lateralization. The objective of this work was to explore whether dichotic listening performance in school-aged VPT children are associated with language performance and interhemispheric connectivity.

METHODS

This is a cross-sectional study of 58 VPT children and 30 full term controls at age 10 years. Language performance and dichotic digit test (DDT) were assessed. In 44 VPT children, additionally diffusion weighted imaging (DWI) was performed using a 3 T MRI scanner. Fractional anisotropy (FA) and mean diffusivity (MD) values of the splenium of the CC were extracted.

RESULTS

Poorer right ear DDT scores were associated with poorer language performance in VPT children only (p = 0.015). Association between right ear DDT scores and MD of the splenium approached the level of significance (p = 0.051).

CONCLUSIONS

These results support the hypothesis that poor language performance in VPT children may be a consequence of weaker lateralized language organization, due to a poorly developed splenium of the CC. Dichotic listening may reflect the level of language lateralization in VPT children.

IMPACT

Poor language performance in VPT children may be a consequence of weaker lateralized language organization, due to a poorly developed splenium of the CC. Dichotic listening performance may reflect the level of language lateralization in VPT children and right ear scores of a dichotic listening task are associated with both the splenium of the corpus callosum and language performance. If our results could be validated in future research, it suggests that poor CC development may indicate VPT children at risk for long-term language problems.

Cortical asymmetries at different spatial hierarchies relate to phonological processing ability

The ability to map speech sounds to corresponding letters is critical for establishing proficient reading. People vary in this phonological processing ability, which has been hypothesized to result from variation in hemispheric asymmetries within brain regions that support language. A cerebral lateralization hypothesis predicts that more asymmetric brain structures facilitate the development of foundational reading skills like phonological processing. That is, structural asymmetries are predicted to linearly increase with ability. In contrast, a canalization hypothesis predicts that asymmetries constrain behavioral performance within a normal range. That is, structural asymmetries are predicted to quadratically relate to phonological processing, with average phonological processing occurring in people with the most asymmetric structures. These predictions were examined in relatively large samples of children (N = 424) and adults (N = 300), using a topological asymmetry analysis of T1-weighted brain images and a decoding measure of phonological processing. There was limited evidence of structural asymmetry and phonological decoding associations in classic language-related brain regions. However, and in modest support of the cerebral lateralization hypothesis, small to medium effect sizes were observed where phonological decoding accuracy increased with the magnitude of the largest structural asymmetry across left hemisphere cortical regions, but not right hemisphere cortical regions, for both the adult and pediatric samples. In support of the canalization hypothesis, small to medium effect sizes were observed where phonological decoding in the normal range was associated with increased asymmetries in specific cortical regions for both the adult and pediatric samples, which included performance monitoring and motor planning brain regions that contribute to oral and written language functions. Thus, the relevance of each hypothesis to phonological decoding may depend on the scale of brain organization.

EEG Power Band Asymmetries in Children with and without Classical Ensemble Music Training

Much evidence shows that music training influences the development of functional brain organization and cerebral asymmetry in an auditory-motor integrative neural system also associated with language and speech. Such overlap suggests that music training could be used for interventions in disadvantaged populations. Accordingly, we investigated neurofunctional changes associated with the influence of socially based classical ensemble music (CEM) training on executive auditory functions of children from low socioeconomic status (LSES), as compared to untrained counterparts. We conducted a novel ROI-focused reanalysis of stimulus-locked event-related electroencephalographic (EEG) band power data previously recorded from fifteen LSES children (9–10 years), with and without CEM, while performing a series of auditory Go/No-Go trials (involving 1100 Hz or 2000 Hz tones). An analysis of collapsed Alpha2, Beta1, Beta2, Delta, and Theta EEG bands showed significant differences in increased and decreased left asymmetry between the CEM and the Comparison group in key frontal and central electrodes typically associated with learning music. Overall, in Go trials, the CEM group responded more quickly and accurately. Linear regression analyses revealed both positive and negative correlations between left hemispheric asymmetry and behavioral measures of PPVT score, auditory sensitivity, Go accuracy, and reaction times. The pattern of results suggests that tone frequency and EEG asymmetries may be attributable to a shift to left lateralization as a byproduct of CEM. Our findings suggest that left hemispheric laterality associated with ensemble music training may improve the efficiency of productive language processing and, accordingly, may be considered as a supportive intervention for LSES children and youth.

Handedness and depression: A meta-analysis across 87 studies

Alterations in functional brain lateralization, often indicated by an increased prevalence of left- and/or mixed-handedness, have been demonstrated in several psychiatric and neurodevelopmental disorders like schizophrenia or autism spectrum disorder. For depression, however, this relationship is largely unclear. While a few studies found evidence that handedness and depression are associated, both the effect size and the direction of this association remain elusive. Here, we collected data from 87 studies totaling 35,501 individuals to provide a precise estimate of differences in left-, mixed- and non-right-handedness between depressed and healthy samples and computed odds ratios (ORs) between these groups. Here, an OR > 1 signifies higher rates of atypical handedness in depressed compared to healthy samples. We found no differences in left- (OR = 1.04, 95% CI = [0.95, 1.15], p = .384), mixed- (OR = 1.64, 95% CI = [0.98, 2.74], p = .060) or non-right-handedness (OR = 1.05, 95% CI = [0.96, 1.15], p = .309) between the two groups. We could thus find no link between handedness and depression on the meta-analytical level.

Executive Function and Sensory Processing in Dichotic Listening of Young Adults with Listening Difficulties

Previous studies have suggested that varying attention demands in dichotic listening (DL) tasks might be a clinically feasible method to distinguish ‘bottom-up’ from ‘top-down’ deficits in listening. This study aims to investigate DL processing in adults with listening difficulties (LD). We assessed the performance of a listening difficulties group (LDG) (n = 24, mean age = 24, backward digit span = 4.0) and a control group (CG) (n = 25, mean age = 29.2, backward digit span = 6.4) in DL tests involving non-forced and both right and left-forced attention. The results indicated an overall significantly worse performance of LDG compared to the CG, which was greater for forced-left condition. This same result was observed when controlling for working memory (WM) variance. Both groups presented an overall right ear advantage with no difference in terms of the magnitude of advantage. These results indicate that LD presented by the LDG might be due to a combination of sensory and cognitive deficits, with emphasis on the cognitive component. However, the WM, although impaired in the LDG group, was not the main factor in segregating both groups. The role of the additional cognitive processes such as inhibitory control in LD is discussed.

Brain Lateralization and Cognitive Capacity

One way to increase cognitive capacity is to avoid duplication of functions on the left and right sides of the brain. There is a convincing body of evidence showing that such asymmetry, or lateralization, occurs in a wide range of both vertebrate and invertebrate species. Each hemisphere of the brain can attend to different types of stimuli or to different aspects of the same stimulus and each hemisphere analyses information using different neural processes. A brain can engage in more than one task at the same time, as in monitoring for predators (right hemisphere) while searching for food (left hemisphere). Increased cognitive capacity is achieved if individuals are lateralized in one direction or the other. The advantages and disadvantages of individual lateralization are discussed. This paper argues that directional, or population-level, lateralization, which occurs when most individuals in a species have the same direction of lateralization, provides no additional increase in cognitive capacity compared to individual lateralization although directional lateralization is advantageous in social interactions. Strength of lateralization is considered, including the disadvantage of being very strongly lateralized. The role of brain commissures is also discussed with consideration of cognitive capacity.

Testing the Darwinian function of lateralization. Does separation of workload between brain hemispheres increase cognitive performance?

Brain lateralization is a fundamental aspect of the organization of brain and behavior in the animal kingdom, begging the question about its Darwinian function. We tested the possibility that lateralization enhances cognitive performance in single- and dual-tasks. Previous studies reported mixed results on this topic and only a handful of studies have measured functional brain lateralization and performance independently and simultaneously. We therefore examined a possible positive effect of the strength and direction of lateralization on two demanding cognitive tasks: A visuospatial task (mental rotation MR), and a language task (word generation WG), executed either as a singletask or as dual-task. Participants (n = 72) performed these tasks while their single-task brain lateralization was assessed with functional Transcranial Doppler for both tasks. From these measurements we determined strength and direction of lateralization for both tasks and the individual pattern of lateralization (contralateral or ipsilateral) was derived. These factors, along with sex, were used in a GLM analysis to determine if they predicted the respective performance measure of the tasks. We found that for MR there was a significant medium effect of direction of lateralization on performance with better performance in left-lateralized (atypical) participants (partial eta squared 0.061; p = .039). After correction for outliers, there was a significant effect for strength (p = .049). For the dual-task, there was a significant positive medium effect of strength of lateralization on performance (partial eta squared 0.062; p = .038, respectively) No other association between direction or strength in either tests were found. We conclude that there is no evidence for hemispheric crowding, and that strength of lateralization may be a factor that contributes to the evolutionary selection of functional brain lateralization. Pattern of lateralization does not, explaining the large inter-individual variation in these traits.

Structural and functional brain asymmetries in the early phases of life: a scoping review

Asymmetry characterizes the brain in both structure and function. Anatomical asymmetries explain only a fraction of functional variability in lateralization, with structural and functional asymmetries developing at different periods of life and in different ways. In this work, we perform a scoping review of the cerebral asymmetries in the first brain development phases. We included all English-written studies providing direct evidence of hemispheric asymmetries in full-term neonates, foetuses, and premature infants, both at term post-conception and before. The final analysis included 57 studies. The reviewed literature shows large variability in the used techniques and methodological procedures. Most structural studies investigated the temporal lobe, showing a temporal planum more pronounced on the left than on the right (although not all data agree), a morphological asymmetry already present from the 29th week of gestation. Other brain structures have been poorly investigated, and the results are even more discordant. Unlike data on structural asymmetries, functional data agree with each other, identifying a leftward dominance for speech stimuli and an overall dominance of the right hemisphere in all other functional conditions. This generalized dominance of the right hemisphere for all conditions (except linguistic stimuli) is in line with theories stating that the right hemisphere develops earlier and that its development is less subject to external influences because it sustains functions necessary to survive.

Hemispheric Asymmetries in Electroencephalogram Oscillations for Long-Term Memory Retrieval in Healthy Individuals

The hemispherical encoding retrieval asymmetry (HERA) model, established in 1991, suggests that the involvement of the right prefrontal cortex (PFC) in the encoding process is less than that of the left PFC. The HERA model was previously validated for episodic memory in subjects with brain traumas or injuries. In this study, a revised HERA model is used to investigate long-term memory retrieval from newly learned video-based content for healthy individuals using electroencephalography. The model was tested for long-term memory retrieval in two retrieval sessions: (1) recent long-term memory (recorded 30 min after learning) and (2) remote long-term memory (recorded two months after learning). The results show that long-term memory retrieval in healthy individuals for the frontal region (theta and delta band) satisfies the revised HERA asymmetry model.

Can cerebral lateralisation explain heterogeneity in language and increased non-right handedness in autism? A literature review

BACKGROUND

Autism is characterised by phenotypic variability, particularly in the domains of language and handedness. However, the source of this heterogeneity is currently unclear.

AIMS

To synthesise findings regarding the relationship between language, handedness, and cerebral lateralisation in autistic people and consider how future research should be conducted in order to progress our understanding of phenotypic variability.

METHODS AND PROCEDURES

Following a literature search and selection process, 19 papers were included in this literature review. Studies using behavioural, structural, and functional measures of lateralisation are reviewed.

OUTCOMES AND RESULTS

The studies reviewed provided consistent evidence of differential cerebral lateralisation in autistic people, and this appears to be related to between-group differences in language. Evidence relating this to handedness was less consistent. Many of the studies did not include heterogeneous samples, and/or did not specify the language process they investigated.

CONCLUSIONS AND IMPLICATIONS

This review suggests that further research is needed to fully understand the relationship between cerebral lateralisation and phenotypic variability within autism. It is crucial that future studies in this area include heterogeneous samples, specify the language process they are investigating, and consider taking developmental trajectories into account.

A multimodal study of the effects of tDCS on dorsolateral prefrontal and temporo-parietal areas during dichotic listening

The underlying neural mechanisms of transcranial direct current stimulation (tDCS), especially beyond the primary motor cortex, remain unclear. Several studies examined tDCS effects on either functional activity, neurotransmitters or behavior but few investigated those aspects together to reveal how the brain responds to tDCS. The objective is to elucidate the underlying mechanisms of tDCS using a multimodal approach that extends from behavioral to neurotransmitter levels of explanation. Thirty-two healthy participants performed an auditory dichotic listening task at two visits, one session with sham and one session with real tDCS (2 mA) while simultaneously undergoing functional magnetic resonance imaging (fMRI). The anode and cathode were placed over the left temporo-parietal cortex (TPC) and dorsolateral prefrontal cortex, respectively. Before and after simultaneous dichotic listening/fMRI/tDCS, combined glutamate and glutamine (Glx) and myo-inositol levels were assessed in the stimulated areas. While fMRI and dichotic listening showed expected functional activity and behavioral effects, neither method demonstrated differences between real and sham stimulation. Glx only showed a statistical trend towards higher levels after real tDCS in both stimulated brain areas. There were no significant correlations between behavior and Glx. Despite a reasonable sample size, electrical field strength, and replication of behavioral and functional activity results, tDCS had little to no effect on dichotic listening, Glx, and functional activity. The study emphasizes that findings about the underlying neural mechanisms of the primary motor cortex cannot simply be generalized to other brain areas. Particularly, the TPC might be less sensitive to tDCS. Moreover, the study demonstrates the general feasibility of multimodal approaches.

Left Ear Hearing Predicts Functional Activity in the Brains of Patients with Alzheimer's Disease Dementia

OBJECTIVES

To determine whether central speech processing ability, as measured by hearing in noise, differs between right and left ears in adults with Alzheimer's disease related dementia (AD) as well as whether differences in central speech processing ability correlate with an fMRI-based measurement of global functional brain connectivity.

METHODS

This prospective study was carried out at a tertiary referral center. Patients with an AD diagnosis and pure tone averages 40 dB HL or better were included. They were examined using resting-state fMRI and underwent central audiometric testing using the Dichotic Sentence Identification Test (DSI), the Dichotic Digits Test (DD), and the Synthetic Sentence Identification Test (SS), which test hearing in noise. DSI scores were correlated with resting-state fMRI connectivity between 361 distinct gray matter brain regions of interest (ROIs). Average global connectivity was calculated as mean functional connectivity between an ROI and the other 360 regions, a quantitative marker representing overall functional connectivity in the brain.

RESULTS

Sixteen subjects had adequate fMRI and hearing data. The average age was 71.5 years old (±6.0). The average DSI score for the left ear was 40% (±34%) compared to 90% (±10%) in the right ear (P < .001). No difference between ears was noted on the DD. SS does not differentiate between ears, but worsening scores were noted with increasing background noise. Of the fMRI ROIs, 269 of the 361 had multiple comparison corrected significant correlations between global connectivity and DSI of the left ear (P = .004, r = .673), and all 269 showed higher functional connectivity for individuals with higher left DSI score. No correlations between DSI of the right ear and functional connectivity were found.

CONCLUSIONS

Correlation was noted between left sided DSI and functional connectivity in patients with AD. Auditory input from the left ear was more susceptible to impairment, suggesting that side-specific auditory input may influence central auditory processing.

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.

A large-scale estimate on the relationship between language and motor lateralization

Human language is dominantly processed in the left cerebral hemisphere in most of the population. While several studies have suggested that there are higher rates of atypical right-hemispheric language lateralization in left-/mixed-handers, an accurate estimate of this association from a large sample is still missing. In this study, we comprised data from 1,554 individuals sampled in three previous studies in which language lateralization measured via dichotic listening, handedness and footedness were assessed. Overall, we found a right ear advantage indicating typical left-hemispheric language lateralization in 82.1% of the participants. While we found significantly more left-handed individuals with atypical language lateralization on the categorical level, we only detected a very weak positive correlation between dichotic listening lateralization quotients (LQs) and handedness LQs using continuous measures. Here, only 0.4% of the variance in language lateralization were explained by handedness. We complemented these analyses with Bayesian statistics and found no evidence in favor of the hypothesis that language lateralization and handedness are related. Footedness LQs were not correlated with dichotic listening LQs, but individuals with atypical language lateralization also exhibited higher rates of atypical footedness on the categorical level. We also found differences in the extent of language lateralization between males and females with males exhibiting higher dichotic listening LQs indicating more left-hemispheric language processing. Overall, these findings indicate that the direct associations between language lateralization and motor asymmetries are much weaker than previously assumed with Bayesian correlation analyses even suggesting that they do not exist at all. Furthermore, sex differences seem to be present in language lateralization when the power of the study is adequate suggesting that endocrinological processes might influence this phenotype.

Listening Difficulties in Children: Behavior and Brain Activation Produced by Dichotic Listening of CV Syllables

Listening difficulties (LiD) are common in children with and without hearing loss. Impaired interactions between the two ears have been proposed as an important component of LiD when there is no hearing loss, also known as auditory processing disorder (APD). We examined the ability of 6-13 year old (y.o.) children with normal audiometric thresholds to identify and selectively attend to dichotically presented CV syllables using the Bergen Dichotic Listening Test (BDLT; www.dichoticlistening.com). Children were recruited as typically developing (TD; n = 39) or having LiD (n = 35) based primarily on composite score of the ECLiPS caregiver report. Different single syllables (ba, da, ga, pa, ta, ka) were presented simultaneously to each ear (6 × 36 trials). Children reported the syllable heard most clearly (non-forced, NF) or the syllable presented to the right [forced right (FR)] or left [forced left (FL)] ear. Interaural level differences (ILDs) manipulated bottom-up perceptual salience. Dichotic listening (DL) data [correct responses, laterality index (LI)] were analyzed initially by group (LiD, TD), age, report method (NF, FR, FL), and ILD (0, ± 15 dB) and compared with speech-in-noise thresholds (LiSN-S) and cognitive performance (NIH Toolbox). fMRI measured brain activation produced by a receptive speech task that segregated speech, phonetic, and intelligibility components. Some activated areas [planum temporale (PT), inferior frontal gyrus (IFG), and orbitofrontal cortex (OFC)] were correlated with dichotic results in TD children only. Neither group, age, nor report method affected the LI of right/left recall. However, a significant interaction was found between ear, group, and ILD. Laterality indices were small and tended to increase with age, as previously reported. Children with LiD had significantly larger mean LIs than TD children for stimuli with ILDs, especially those favoring the left ear. Neural activity associated with Speech, Phonetic, and Intelligibility sentence cues did not differ significantly between groups. Significant correlations between brain activity level and BDLT were found in several frontal and temporal locations for the TD but not for the LiD group. Overall, the children with LiD had only subtle differences from TD children in the BDLT, and correspondingly minor changes in brain activation.

Is laterality adaptive? Pitfalls in disentangling the laterality-performance relationship

Unlike non-human animal studies that have progressively demonstrated the advantages of being asymmetrical at an individual, group and population level, human studies show a quite inconsistent picture. Specifically, it is hardly clear if and how the strength of lateralization that an individual is equipped with relates to their cognitive performance. While some of these inconsistencies can be attributed to procedural and conceptual differences, the issue is aggravated by the fact that the intrinsic mathematical interdependence of the measures of laterality and performance produces spurious correlations that can be mistaken for evidence of an adaptive advantage of asymmetry. Leask and Crow [Leask, S. J., & Crow, T. J. (1997), How far does the brain lateralize?: an unbiased method for determining the optimum degree of hemispheric specialization. Neuropsychologia, 35(10), 1381-1387] devised a method of overcoming this problem that has been subsequently used in several large-sample studies investigating the asymmetry-performance relationship. In our paper we show that the original Leask and Crow method and its later variants fall victim to inherent nonlinear dependencies and produce artifacts. By applying the Leask and Crow method to random data and with mathematical analysis, we demonstrate that what has been believed to describe the true asymmetry-performance relation in fact only reflects the idiosyncrasies of the method itself. We think that the approach taken by Leask in his later paper [Leask, S. (2003), Principal curve analysis avoids assumptions of dependence between measures of hand skill. Laterality, 8(4), 307-316. doi:10.1080/13576500342000004] might be preferable.

A function for the bicameral mind

Why do the left and right sides of the brain have different functions? Having a lateralized brain, in which each hemisphere processes sensory inputs differently and carries out different functions, is common in vertebrates, and it has now been reported for invertebrates too. Experiments with several animal species have shown that having a lateralized brain can enhance the capacity to perform two tasks at the same time. Thus, the different specializations of the left and right sides of the brain seem to increase brain efficiency. Other advantages may involve control of action that, in Bilateria, may be confounded by separate and independent sensory processing and motor outputs on the left and right sides. Also, the opportunity for increased perceptual training associated with preferential use of only one sensory or motoric organ may result in a time advantage for the dominant side. Although brain efficiency of individuals can be achieved without the need for alignment of lateralization in the population, lateral biases (such as preferences in the use of a laterally-placed eye) usually occur at the population level, with most individuals showing a similar direction of bias. Why is this the case? Not only humans, but also most non-human animals, show a similar pattern of population bias (i.e., directional asymmetry). For instance, in several vertebrate species (from fish to mammals) most individuals react faster when a predator approaches from their left side, although some individuals (a minority usually ranging from 10 to 35%) escape faster from predators arriving from their right side. Invoking individual efficiency (lateralization may increase fitness), evolutionary chance or simply genetic inheritance cannot explain this widespread pattern. Using mathematical theory of games, it has been argued that the population structure of lateralization (with either antisymmetry or directional asymmetry) may result from the type of interactions asymmetric organisms face with each other.

The relationship between behavioral language laterality, face laterality and language performance in left-handers

Left-handers provide unique information about the relationship between cognitive functions because of their larger variability in hemispheric dominance. This study presents the laterality distribution of, correlations between and test-retest reliability of behavioral lateralized language tasks (speech production, reading and speech perception), face recognition tasks, handedness measures and language performance tests based on data from 98 left-handers. The results show that a behavioral test battery leads to percentages of (a)typical dominance that are similar to those found in neuropsychological studies even though the incidence of clear atypical lateralization (about 20%) may be overestimated at the group level. Significant correlations were found between the language tasks for both reaction time and accuracy lateralization indices. The degree of language laterality could however not be linked to face laterality, handedness or language performance. Finally, individuals were classified less consistently than expected as being typical, bilateral or atypical across all tasks. This may be due to the often good (speech production and perception tasks) but sometimes weak (reading and face tasks) test-retest reliabilities. The lack of highly reliable and valid test protocols for functions unrelated to speech remains one of the largest impediments for individual analysis and cross-task investigations in laterality research.

Weaker semantic language lateralization associated with better semantic language performance in healthy right-handed children

INTRODUCTION

The relationship between language abilities and language lateralization in the developing brain is important for our understanding of the neural architecture of language development.

METHODS

We investigated 35 right-handed children and adolescents aged 7-16 years with a functional magnetic resonance imaging language paradigm and a comprehensive language and verbal memory examination.

RESULTS

We found that less lateralized language was significantly correlated with better language performance across areas of the brain and across different language tasks. Less lateralized language in the overall brain was associated with better in-scanner task accuracy on a semantic language decision task and out-of-scanner vocabulary and verbal fluency. Specifically, less lateralized frontal lobe language dominance was associated with better in-scanner task accuracy and out-of-scanner verbal fluency. Furthermore, less lateralized parietal language was associated with better out-of-scanner verbal memory across learning, short- and long-delay trials. In contrast, we did not find any relationship between temporal lobe language laterality and verbal performance.

CONCLUSIONS

This study suggests that semantic language performance is better with some involvement of the nondominant hemisphere.

Handedness in gestural and manipulative actions in male hunter-gatherer Aka pygmies from Central African Republic

OBJECTIVES

All human populations display a right-biased handedness. Nonetheless, if studies on western populations are plenty, investigations of traditional populations living at subsistence levels are rare. Yet, understanding the geographical variation of phenotypes of handedness is crucial for testing evolutionary hypotheses. We aimed to provide a preliminary investigation of factors affecting handedness in 25 Aka pygmies from Central African Republic when spontaneously gesturing or manipulating food/tools (N_actions  = 593).

MATERIALS AND METHODS

We recorded spontaneous behaviors and characterized individuals' hand preference using GLMM with descriptive variables as target position, task complexity (unimanual/bimanual), task nature (food/tool manipulation, gesture), and task physical/cognitive constraints (precision or power for manipulative actions and informative content for gestures).

RESULTS

Individuals were lateralized to the right (93%, N = 15) when manipulating food/tools but not when gesturing. Hand preference was affected by target position but not by task complexity. While nonexplicitly informative gestures were more biased to the right compared to explicitly informative ones, no differences were found within food/tool manipulation (power or precision vs. none).

DISCUSSION

Although we do not intend to assume generalizable results due to our reduced sample, our observations provide additional information on handedness in a contemporary traditional society. Especially, the study mainly evidenced considerable cultural effects in gestures while also supporting theories considering active tool manipulation as one of the overriding factor in human handedness evolution.

Visuospatial attention in the lateralised brain of pigeons - a matter of ontogenetic light experiences

The ontogenetic mechanisms leading to complementary hemispheric specialisations of the two brain halves are poorly understood. In pigeons, asymmetrical light stimulation during development triggers the left-hemispheric dominance for visuomotor control but light effects on right-hemispheric specialisations are largely unknown. We therefore tested adult pigeons with and without embryonic light experience in a visual search task in which the birds pecked peas regularly scattered on an area in front of them. Comparing the pecking pattern of both groups indicates that the embryonic light conditions differentially influence biased visuospatial attention under mono- and binocular seeing conditions. When one eye was occluded, dark-incubated pigeons peck only within the limits of the visual hemifield of the seeing eye. Light-exposed pigeons also peck into the contralateral field indicating enlarged monocular visual fields of both hemispheres. While dark-incubated birds evinced an attentional bias to the right halfspace when seeing with both eyes, embryonic light exposure shifted this to the left. Thus, embryonic light experience modifies processes regulating biased visuospatial attention of the adult birds depending on the seeing conditions during testing. These data support the impact of light onto the emergence of functional dominances in both hemispheres and point to the critical role of interhemispheric processes.

Hostility and cognitive control: Evidence of increased cardiovascular reactivity as a function of exposure to affective stress using a dichotic listening paradigm

Indices of cognitive control were examined in men with high and low levels of trait hostility as a function of exposure to affective and cognitive stress. A dual concurrent task paradigm was used whereby participants intentionally directed focus to the left or right ear under dichotic listening conditions before and after exposure to angry infant vocalizations. Analysis of the behavioral data supports the prediction of reduced right frontal regulatory control in men with high levels of hostility as indicated by diminished capacity to suppress report of phonemes presented to the language dominant left hemisphere (right ear) in the Focus Left condition. This diminishment in the capacity to suppress report of phonemes presented to the right ear in the Focus Left condition is suggestive of reduced cognitive control. With respect to the neurophysiological data, heart rate increased for only men with high levels of hostility in the Focus Left condition, and this was especially evident in the post-affective stress condition. This increase in right hemisphere arousal provides additional evidence of reduced cognitive control and support for the capacity model of hostility by implicating poor right frontal regulatory control over right posterior cerebral regions under dual task conditions. The results are discussed in terms of integrating the construct of cognitive control into the capacity model as well as providing implications regarding reductions in the capacity to suppress predominant aggressive responses in domestic settings.

Learning Morse Code Alters Microstructural Properties in the Inferior Longitudinal Fasciculus: A DTI Study

Learning relies on neuroplasticity, which has mainly been studied in gray matter (GM). However, there is mounting evidence indicating a critical role of white matter changes involved in learning processes. One of the most important learning processes in human development is language acquisition. However, due to the length of this learning process, it has been notoriously difficult to investigate the underlying neuroplastic changes. Here, we report a novel learning paradigm to assess the role of white matter plasticity for language acquisition. By acoustically presenting Morse Code (MC) using an in house developed audio book as a model for language-type learning, we generated a well-controlled learning environment that allows for the detection of subtle white matter changes related to language type learning in a much shorter time frame than usual language acquisition. In total 12 letters of the MC alphabet were learned within six learning session, which allowed study participants to perform a word recognition MC decoding task. In this study, we found that learning MC was associated with significant microstructural changes in the left inferior longitudinal fasciculus (ILF). The fractional anisotropy (FA) of this associative fiber bundle connecting the occipital and posterior temporal cortex with the temporal pole as well as the hippocampus and amygdala was increased. Furthermore, white matter plasticity was associated with task performance of MC decoding, indicating that the structural changes were related to learning efficiency. In conclusion, our findings demonstrate an important role of white matter neuroplasticity for acquiring a new language skill.

Allusive thinking (remote associations) and auditory top-down inhibition skills differentially predict creativity and positive schizotypy

INTRODUCTION

Positive schizotypy and creativity seem to be linked. However, the question still remains why they are related, and what may make the difference? As creative ideation is hypothesised as a dual process (association and inhibition), the propensity for remote associations might be a shared mechanism. However, positive schizotypy and creative thinking might be differentially linked to inhibition. Therefore, this study investigated a potentially overlapping feature of positive schizotypy and creativity (remote associations) as well as a potential dissociative factor (auditory inhibition).

METHODS

From a large screening sample, 46 participants covering a broad range of positive schizotypy were selected. Association proneness was assessed via two association tasks, auditory inhibition skill with the forced-left condition of the Dichotic Listening Test, and creative thinking by means of two creative ideation tests.

RESULTS

Positive schizotypy and creative thinking were positively associated. Both traits were linked to lower rates of common associations. However, creative thinking was associated with higher and positive schizotypy with lower inhibitory control in the auditory domain.

CONCLUSIONS

While creativity and positive schizotypy shared some variance (related to remote associations), profound inhibition skills may be vital for creative performance and may coincide with lower levels of positive schizotypy.

Auditory top-down control and affective theory of mind in schizophrenia with and without hallucinations

Social cognitive impairments may represent a core feature of schizophrenia and above all are a strong predictor of positive psychotic symptoms. Previous studies could show that reduced inhibitory top-down control contributes to deficits in theory of mind abilities and is involved in the genesis of hallucinations. The current study aimed to investigate the relationship between auditory inhibition, affective theory of mind and the experience of hallucinations in patients with schizophrenia. In the present study, 20 in-patients with schizophrenia and 20 healthy controls completed a social cognition task (the Reading the Mind in the Eyes Test) and an inhibitory top-down Dichotic Listening Test. Schizophrenia patients with greater severity of hallucinations showed impaired affective theory of mind as well as impaired inhibitory top-down control. More dysfunctional top-down inhibition was associated with poorer affective theory of mind performance, and seemed to mediate the association between impairment to affective theory of mind and severity of hallucinations. The findings support the idea of impaired theory of mind as a trait marker of schizophrenia. In addition, dysfunctional top-down inhibition may give rise to hallucinations and may further impair affective theory of mind skills in schizophrenia.

Music-induced changes in functional cerebral asymmetries

After decades of research, it remains unclear whether emotion lateralization occurs because one hemisphere is dominant for processing the emotional content of the stimuli, or whether emotional stimuli activate lateralised networks associated with the subjective emotional experience. By using emotion-induction procedures, we investigated the effect of listening to happy and sad music on three well-established lateralization tasks. In a prestudy, Mozart's piano sonata (K. 448) and Beethoven's Moonlight Sonata were rated as the most happy and sad excerpts, respectively. Participants listened to either one emotional excerpt, or sat in silence before completing an emotional chimeric faces task (Experiment 1), visual line bisection task (Experiment 2) and a dichotic listening task (Experiment 3 and 4). Listening to happy music resulted in a reduced right hemispheric bias in facial emotion recognition (Experiment 1) and visuospatial attention (Experiment 2) and increased left hemispheric bias in language lateralization (Experiments 3 and 4). Although Experiments 1-3 revealed an increased positive emotional state after listening to happy music, mediation analyses revealed that the effect on hemispheric asymmetries was not mediated by music-induced emotional changes. The direct effect of music listening on lateralization was investigated in Experiment 4 in which tempo of the happy excerpt was manipulated by controlling for other acoustic features. However, the results of Experiment 4 made it rather unlikely that tempo is the critical cue accounting for the effects. We conclude that listening to music can affect functional cerebral asymmetries in well-established emotional and cognitive laterality tasks, independent of music-induced changes in the emotion state.

Left-Right Axis Differentiation and Functional Lateralization: a Haplotype in the Methyltransferase Encoding Gene SETDB2 Might Mediate Handedness in Healthy Adults

Handedness is a multifactorial trait, and genes contributing to the differentiation of the left-right axis during embryogenesis have been identified as a major gene group associated with this trait. The methyltransferase SETDB2 (SET domain, bifurcated 2) has been shown to regulate structural left-right asymmetry in the vertebrate central nervous system by suppressing fgf8 expression. Here, we investigated the relation of genetic variation in SETDB2-and its paralogue SETDB1-with different handedness phenotypes in 950 healthy adult participants. We identified a haplotype on SETDB2 for which homozygous individuals showed a significantly lower lateralization quotient for handedness than the rest of the cohort after correction for multiple comparisons. Moreover, direction of handedness was significantly associated with genetic variation in this haplotype. This effect was mainly, but not exclusively, driven by the sequence variation rs4942830, as individuals homozygous for the A allele of this single nucleotide polymorphism had a significantly lower lateralization quotient than individuals with at least one T allele. These findings further confirm a role of genetic pathways relevant for structural left-right axis differentiation for functional lateralization. Moreover, as the protein encoded by SETDB2 regulates gene expression epigenetically by histone H3 methylation, our findings highlight the importance of investigating the role of epigenetic modulations of gene expression in relation to handedness.

Sex hormones affect language lateralisation but not cognitive control in normally cycling women

This article is part of a Special Issue "Estradiol and Cognition". Natural fluctuations of sex hormones during the menstrual cycle have been shown to modulate language lateralisation. Using the dichotic listening (DL) paradigm, a well-established measurement of language lateralisation, several studies revealed that the left hemispheric language dominance was stronger when levels of estradiol were high. A recent study (Hjelmervik et al., 2012) showed, however, that high levels of follicular estradiol increased lateralisation only in a condition that required participants to cognitively control (top-down) the stimulus-driven (bottom-up) response. This finding suggested that sex hormones modulate lateralisation only if cognitive control demands are high. The present study investigated language lateralisation in 73 normally cycling women under three attention conditions that differed in cognitive control demands. Saliva estradiol and progesterone levels were determined by luminescence immunoassays. Women were allocated to a high or low estradiol group. The results showed a reduced language lateralisation when estradiol and progesterone levels were high. The effect was independent of the attention condition indicating that estradiol marginally affected cognitive control. The findings might suggest that high levels of estradiol especially reduce the stimulus-driven (bottom-up) aspect of lateralisation rather than top-down cognitive control.

Hemispheric asymmetry in new neurons in adulthood is associated with vocal learning and auditory memory

Many brain regions exhibit lateral differences in structure and function, and also incorporate new neurons in adulthood, thought to function in learning and in the formation of new memories. However, the contribution of new neurons to hemispheric differences in processing is unknown. The present study combines cellular, behavioral, and physiological methods to address whether 1) new neuron incorporation differs between the brain hemispheres, and 2) the degree to which hemispheric lateralization of new neurons correlates with behavioral and physiological measures of learning and memory. The songbird provides a model system for assessing the contribution of new neurons to hemispheric specialization because songbird brain areas for vocal processing are functionally lateralized and receive a continuous influx of new neurons in adulthood. In adult male zebra finches, we quantified new neurons in the caudomedial nidopallium (NCM), a forebrain area involved in discrimination and memory for the complex vocalizations of individual conspecifics. We assessed song learning and recorded neural responses to song in NCM. We found significantly more new neurons labeled in left than in right NCM; moreover, the degree of asymmetry in new neuron numbers was correlated with the quality of song learning and strength of neuronal memory for recently heard songs. In birds with experimentally impaired song quality, the hemispheric difference in new neurons was diminished. These results suggest that new neurons may contribute to an allocation of function between the hemispheres that underlies the learning and processing of complex signals.