Genetic models for handedness, brain lateralization, schizophrenia, and manic-depression

Do patients with hallucinations imagine speech right?

A direct relationship between auditory verbal hallucinations (AVHs) and decreased left-hemispheric lateralization in speech perception has been often described, although it has not been conclusively proven. The specific lateralization of AVHs has been poorly explored. However, patients with verbal hallucinations show a weak Right Ear Advantage (REA) in verbal perception compared to non AVHs listeners suggesting that left-hemispheric language area are involved in AVHs. In the present study, 29 schizophrenia patients with AVHs, 31 patients with psychotic bipolar disorder who experienced frequent AVHs, 27 patients with schizophrenia who had never experienced AVHs and 57 healthy controls were required to imagine hearing a voice in one ear alone. In line with previous evidence healthy controls confirmed the expected REA for auditory imagery, and the same REA was also found in non-hallucinator patients. However, in line with our hypothesis, patients with schizophrenia and psychotic bipolar disorder with AVHs showed no lateral bias. Results extend the relationship between abnormal asymmetry for verbal stimuli and AVHs to verbal imagery, suggesting that atypical verbal imagery may reflect a disruption of inter-hemispheric connectivity between areas implicated in the generation and monitoring of verbal imagery and may be predictive of a predisposition for AVHs. Results also indicate that the relationship between AVHs and hemispheric lateralization for auditory verbal imagery is not specific to schizophrenia but may extend to other disorders as well.

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.

MAOA variants differ in oscillatory EEG & ECG activities in response to aggression-inducing stimuli

Among the genetic variations in the monoamine oxidase A (MAOA) gene, upstream variable number tandem repeats (uVNTRs) of the promoter have been associated with individual differences in human physiology and aggressive behaviour. However, the evidence for a molecular or neural link between MAOA uVNTRs and aggression remains ambiguous. Additionally, the use of inconsistent promoter constructs in previous studies has added to the confusion. Therefore, it is necessary to demonstrate the genetic function of MAOA uVNTR and its effects on multiple aspects of aggression. Here, we identified three MAOA alleles in Koreans: the predominant 3.5R and 4.5R alleles, as well as the rare 2.5R allele. There was a minor difference in transcriptional efficiency between the 3.5R and 4.5R alleles, with the greatest value for the 2.5R allele, in contrast to existing research. Psychological indices of aggression did not differ among MAOA genotypes. However, our electroencephalogram and electrocardiogram results obtained under aggression-related stimulation revealed oscillatory changes as novel phenotypes that vary with the MAOA genotype. In particular, we observed prominent changes in frontal γ power and heart rate in 4.5R carriers of men. Our findings provide genetic insights into MAOA function and offer a neurobiological basis for various socio-emotional mechanisms in healthy individuals.

Asymmetric development of the nervous system

The human nervous system consists of seemingly symmetric left and right halves. However, closer observation of the brain reveals anatomical and functional lateralization. Defects in brain asymmetry correlate with several neurological disorders, yet our understanding of the mechanisms used to establish lateralization in the human central nervous system is extremely limited. Here, we review left-right asymmetries within the nervous system of humans and several model organisms, including rodents, Zebrafish, chickens, Xenopus, Drosophila, and the nematode Caenorhabditis elegans. Comparing and contrasting mechanisms used to develop left-right asymmetry in the nervous system can provide insight into how the human brain is lateralized. Developmental Dynamics 247:124-137, 2018. © 2017 Wiley Periodicals, Inc.

Behavioral and brain asymmetries in primates: a preliminary evaluation of two evolutionary hypotheses

Contrary to many historical views, recent evidence suggests that species-level behavioral and brain asymmetries are evident in nonhuman species. Here, we briefly present evidence of behavioral, perceptual, cognitive, functional, and neuroanatomical asymmetries in nonhuman primates. In addition, we describe two historical accounts of the evolutionary origins of hemispheric specialization and present data from nonhuman primates that address these specific theories. Specifically, we first discuss the evidence that genes play specific roles in determining left-right differences in anatomical and functional asymmetries in primates. We next consider and present data on the hypothesis that hemispheric specialization evolved as a by-product of increasing brain size relative to the surface area of the corpus callosum in different primate species. Last, we discuss some of the challenges in the study of hemispheric specialization in primates and offer some suggestions on how to advance the field.

Poor receptive joint attention skills are associated with atypical gray matter asymmetry in the posterior superior temporal gyrus of chimpanzees (Pan troglodytes)

Clinical and experimental data have implicated the posterior superior temporal gyrus as an important cortical region in the processing of socially relevant stimuli such as gaze following, eye direction, and head orientation. Gaze following and responding to different socio-communicative signals is an important and highly adaptive skill in primates, including humans. Here, we examined whether individual differences in responding to socio-communicative cues was associated with variation in either gray matter (GM) volume and asymmetry in a sample of chimpanzees. Magnetic resonance image scans and behavioral data on receptive joint attention (RJA) was obtained from a sample of 191 chimpanzees. We found that chimpanzees that performed poorly on the RJA task had less GM in the right compared to left hemisphere in the posterior but not anterior superior temporal gyrus. We further found that middle-aged and elderly chimpanzee performed more poorly on the RJA task and had significantly less GM than young-adult and sub-adult chimpanzees. The results are consistent with previous studies implicating the posterior temporal gyrus in the processing of socially relevant information.

Left brain, right brain: facts and fantasies

Handedness and brain asymmetry are widely regarded as unique to humans, and associated with complementary functions such as a left-brain specialization for language and logic and a right-brain specialization for creativity and intuition. In fact, asymmetries are widespread among animals, and support the gradual evolution of asymmetrical functions such as language and tool use. Handedness and brain asymmetry are inborn and under partial genetic control, although the gene or genes responsible are not well established. Cognitive and emotional difficulties are sometimes associated with departures from the "norm" of right-handedness and left-brain language dominance, more often with the absence of these asymmetries than their reversal.

Heritability of brain volumes in older adults: the Older Australian Twins Study

The relative contributions of genetic and environmental factors to brain structure change throughout the lifespan. Brain structures have been reported to be highly heritable in middle-aged individuals and younger; however, the influence of genes on brain structure is less studied in older adults. We performed a magnetic resonance imaging study of 236 older twins, with a mean age of 71.4 ± 5.7 years, to examine the heritability of 53 brain global and lobar volumetric measures. Total brain volume (63%) and other volumetric measures were moderately to highly heritable in late life, and these genetic influences tended to decrease with age, suggesting a greater influence of environmental factors as age advanced. Genetic influences were higher in men and on the left hemisphere compared with the right. In multivariate models, common genetic factors were observed for global and lobar total and gray matter volumes. This study examined the genetic contribution to 53 brain global and lobar volumetric measures in older twins for the first time, and the influence of age, sex, and laterality on these genetic contributions, which are useful information for a better understanding of the process of brain aging and helping individuals to have a healthy aging.

White-matter microstructure and language lateralization in left-handers: a whole-brain MRI analysis

Most people are left-hemisphere dominant for language. However the neuroanatomy of language lateralization is not fully understood. By combining functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI), we studied whether language lateralization is associated with cerebral white-matter (WM) microstructure. Sixteen healthy, left-handed women aged 20-25 were included in the study. Left-handers were targeted in order to increase the chances of involving subjects with atypical language lateralization. Language lateralization was determined by fMRI using a verbal fluency paradigm. Tract-based spatial statistics analysis of DTI data was applied to test for WM microstructural correlates of language lateralization across the whole brain. Fractional anisotropy and mean diffusivity were used as indicators of WM microstructural organization. Right-hemispheric language dominance was associated with reduced microstructural integrity of the left superior longitudinal fasciculus and left-sided parietal lobe WM. In left-handed women, reduced integrity of the left-sided language related tracts may be closely linked to the development of right hemispheric language dominance. Our results may offer new insights into language lateralization and structure-function relationships in human language system.

Multilocus genetic models of handedness closely resemble single-locus models in explaining family data and are compatible with genome-wide association studies

Right- and left-handedness run in families, show greater concordance in monozygotic than dizygotic twins, and are well described by single-locus Mendelian models. Here we summarize a large genome-wide association study (GWAS) that finds no significant associations with handedness and is consistent with a meta-analysis of GWASs. The GWAS had 99% power to detect a single locus using the conventional criterion of P < 5 × 10(-8) for the single locus models of McManus and Annett. The strong conclusion is that handedness is not controlled by a single genetic locus. A consideration of the genetic architecture of height, primary ciliary dyskinesia, and intelligence suggests that handedness inheritance can be explained by a multilocus variant of the McManus DC model, classical effects on family and twins being barely distinguishable from the single locus model. Based on the ENGAGE meta-analysis of GWASs, we estimate at least 40 loci are involved in determining handedness.

Genetic and environmental contributions to the expression of handedness in chimpanzees (Pan troglodytes)

Most humans are right-handed and, like many behavioral traits, there is good evidence that genetic factors play a role in handedness. Many researchers have argued that non-human animal limb or hand preferences are not under genetic control but instead are determined by random, non-genetic factors. We used quantitative genetic analyses to estimate the genetic and environmental contributions to three measures of chimpanzee handedness. Results revealed significant population-level handedness for two of the three measures-the tube task and manual gestures. Furthermore, significant additive genetic effects for the direction and strength of handedness were found for all three measures, with some modulation due to early social rearing experiences. These findings challenge historical and contemporary views of the mechanisms underlying handedness in non-human animals.

Within- and between-task consistency in hand use as a means of characterizing hand preferences in captive chimpanzees (Pan troglodytes)

There remain considerable questions regarding the evidence for population-level handedness in nonhuman primates when compared with humans. One challenge in comparing human and nonhuman primate handedness involves the procedures used to characterize individual handedness. Studies of human handedness use consistency in hand use within and between tasks as a basis for hand preference classification. In contrast, studies of handedness in nonhuman primates use statistical criteria for classifying handedness. In this study, we examined within- and between-task consistency in hand use as a means of characterizing individual handedness in a sample of 300 captive chimpanzees (Pan troglodytes). Chimpanzees showed population-level right-handedness for both within- and between-tasks consistency, though the proportion of right-handed chimpanzees was lower than what has typically been reported for humans. We further found that there were small, but significant, associations in hand use between measures. There were no significant sex or colony effects on the distribution of handedness. The results are discussed in the context of theories on the evolution of handedness in nonhuman primates.

Nongenetic factors associated with human handedness and footedness in Japanese twin children

OBJECTIVE

The aim of this study is to clarify the factors related to the handedness and footedness of twins using two of the largest databases on Japanese twins available.

METHODS

The first group consisted of 1,131 twin pairs, all school children either 11 or 12 years old (S group), and the second group consisted of 951 twin pairs of different ages (1-15 years) in several maternal associations (M group). All data were gathered using a questionnaire. Factors associated with the handedness or footedness of twin individuals were analyzed by univariate and multivariate logistic analyses.

RESULTS

Multivariate logistic analysis showed that for handedness, birth year (OR=1.02) and neonatal asphyxia (OR=1.62) were selected in the S group, and sex (OR=1.34), the age of twins (OR=1.56), parity (OR=1.31), gestational age (OR=1.58), and family history (OR=1.82) were selected in the M group. for footedness, birth complications (OR=1.37) were selected in the S group, and sex (OR=1.33), the age of twins (OR=1.69), gestational age (OR=1.83), and family history (OR=2.49) were selected in the M group. Factors associated with handedness and footedness specific to twins, such as zygosity, placentation, birth order within twin pairs and the sex of the cotwin, were not found, although being a twin might have some effects.

CONCLUSION

It was concluded that factors that affect handedness or footedness in general, such as sex, birth year, age, parity, neonatal asphyxia, gestational age, birth complications, and family history, seem to have stronger effects on handedness and footedness than being a twin.

Abnormal functional motor lateralization in healthy siblings of patients with schizophrenia

Earlier neuroimaging studies of motor function in schizophrenia have demonstrated reduced functional lateralization in the motor network during motor tasks. Here, we used event-related functional magnetic resonance imaging during a visually guided motor task in 18 clinically unaffected siblings of patients with schizophrenia and 24 matched controls to investigate if abnormal functional lateralization is related to genetic risk for this brain disorder. Whereas activity associated with motor task performance was mainly contralateral with only a marginal ipsilateral component in healthy participants, unaffected siblings had strong bilateral activity with significantly greater response in ipsilateral and contralateral premotor areas as well as in contralateral subcortical motor regions relative to controls. Reduced lateralization in siblings was also identified with a measure of laterality quotient. These findings suggest that abnormal functional lateralization of motor circuitry is related to genetic risk of schizophrenia.

Handedness in Twins: Joint Analysis of Data From 35 Samples

Simultaneous analysis of handedness data from 35 samples of twins (with a combined sample size of 21,127 twin pairs) found a small but significant additive genetic effect accounting for 25.47% of the variance (95% confidence interval [CI] 15.69–29.51%). No common environmental influences were detected (C = 0.00; 95% CI 0.00–7.67%), with the majority of the variance, 74.53%, explained by factors unique to the individual (95% CI 70.49–78.67%). No significant heterogeneity was observed within studies that used similar methods to assess handedness, or across studies that used different methods. At an individual level the majority of studies had insufficient power to reject a purely unique environmental model due to insufficient power to detect familial aggregation. This lack of power is seldom mentioned within studies, and has contributed to the misconception that twin studies of handedness are not informative.

Sex hormone activity in alcohol addiction: integrating organizational and activational effects

There are well-known sex differences in the epidemiology and etiopathology of alcohol dependence. Male gender is a crucial risk factor for the onset of alcohol addiction. A directly modifying role of testosterone in alcohol addiction-related behavior is well established. Sex hormones exert both permanent (organizational) and transient (activational) effects on the human brain. The sensitive period for these effects lasts throughout life. In this article, we present a novel early sex hormone activity model of alcohol addiction. We propose that early exposure to sex hormones triggers structural (organizational) neuroadaptations. These neuroadaptations affect cellular and behavioral responses to adult sex hormones, sensitize the brain's reward system to the reinforcing properties of alcohol and modulate alcohol addictive behavior later in life. This review outlines clinical findings related to the early sex hormone activity model of alcohol addiction (handedness, the second-to-fourth-finger length ratio, and the androgen receptor and aromatase) and includes clinical and preclinical literature regarding the activational effects of sex hormones in alcohol drinking behavior. Furthermore, we discuss the role of the hypothalamic-pituitary-adrenal and -gonadal axes and the opioid system in mediating the relationship between sex hormone activity and alcohol dependence. We conclude that a combination of exposure to sex hormones in utero and during early development contributes to the risk of alcohol addiction later in life. The early sex hormone activity model of alcohol addiction may prove to be a valuable tool in the development of preventive and therapeutic strategies.

Handedness and drinking behaviour

OBJECTIVES

A number of papers have investigated whether there is an association between handedness and alcohol consumption hypothesizing that alcoholism may be a consequence of atypical cerebral lateralization or a response to the stress involved in being a minority in a right-handed world. Research to date has mostly used small clinical samples, some without a comparison group. This paper exams this issue using a large population-based random sample.

DESIGN

A large multi-country data set of nationally representative samples of the non-institutional population aged 50 years and older from 12 European countries was used (N= 27,428). methods: Logistic regression was used to model the frequency with which individuals self-report the frequency of alcohol consumption. A series of models with differing numbers of potential confounders are estimated. The predictors of frequent and infrequent drinking are investigated separately.

RESULTS

After controlling for a number of confounders it is shown that left-handers do drink more often. However, this is due to them being less likely to drink rarely (less than once a month) or not at all.

CONCLUSIONS

The evidence suggests that while there is an association between left-handedness and frequency of alcohol consumption there is no reason to believe that it is associated with excessive alcohol consumption or risky drinking.

Left-handedness is Correlated with CSF Monoamine Metabolite and Plasma Cortisol Concentrations, and with Impaired Sociality, in Free-ranging Adult Male Rhesus Macaques (Macaca mulatta)

In this research we examined biological and behavioural correlates of handedness in free-ranging adult male rhesus macaques (Macaca mulatta). Specifically, we examined relationships between handedness and cerebrospinal fluid (CSF) concentrations of the monoamine metabolites 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), and 3-methoxy-4-hydroxyphenylglycol (MHPG), plasma concentrations of the hormones cortisol and adrenocorticotropin (ACTH), and multiple indices of social behaviour, including occurrences of proximity to other animals, grooming, submission, and aggression. We determined handedness through systematic observation of animals reaching for food in their unrestricted home environment. The frequency of right- versus left-hand use was significantly positively correlated with CSF 5-HIAA, CSF MHPG, and plasma cortisol concentrations, and with social proximity and the frequency and duration of bouts in which animals received grooming. The frequency of right- versus left-hand use was significantly negatively correlated with the frequency of submissive behaviour, and with the frequency and intensity of bouts in which animals received aggression. We conclude that handedness is associated with an array of biological and behavioural processes in free-ranging adult male rhesus macaques and that left-handedness may be used to identify individuals at increased risk for impaired functioning of the serotonin, norepinephrine, and hypothalamic-pituitary-adrenal systems, and for social isolation and susceptibility to violent attack.

Hand preference and hand skill in families with schizophrenia

Direction and degree of handedness in humans are variable between individuals and thought to be in part inherited. Several studies have shown an increase in non-right handedness among patients with schizophrenia, and some have included unaffected relatives. The present study was designed to determine whether reduced right handedness is more frequent among individuals with schizophrenia as compared with their well relatives and whether it clusters within families having multiple ill members. A total of 259 families comprising 418 individuals diagnosed with schizophrenia or schizoaffective disorder, 54 individuals with other psychoses, 145 family members with depression and other minor diagnoses, and 288 unaffected individuals were included. Hand preference was assessed by the Annett Scale and right relative to left hand skill measured using the Tapley-Bryden test. For all assessments of hand preference and hand skill, females were significantly more lateralised towards the right than males. Those individuals with schizophrenia or schizoaffective disorder had significantly less right hand preference than their unaffected relatives when measured as a quantitative index of items from the Annett Scale (p = .019), but not categorically (right, left or mixed). In contrast, there was no difference in hand skill between diagnostic groups. Hand preference was significantly correlated among male-male affected sibling pairs (p = .01) and similar results were found for hand skill among the total group of affected pairs (p = .001). Although these results only partially support a relationship between handedness and schizophrenia, they nevertheless draw attention to sex differences in hand preference and the familial aspects of hand preference in this disorder. More direct approaches to the genetics of cerebral dominance and psychosis are required.

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.

Neuroanatomical asymmetry patterns in individuals with schizophrenia and their non-psychotic siblings

Neuroanatomical endophenotypes may reveal insights into the processes by which genetic factors increase the risk of developing schizophrenia. To determine whether patterns of neuroanatomical asymmetries may be useful as schizophrenia-related endophenotypes, we compared patterns of structural asymmetries in patients with schizophrenia, healthy controls, and their respective siblings. The surfaces of the left and right amygdala, hippocampus, thalamus, caudate nucleus, putamen, globus pallidus, and nucleus accumbens were assessed in 40 pairs of healthy comparison controls (CON) and their siblings (CON-SIB) and 25 pairs of patients with schizophrenia (SCZ) and their siblings (SCZ-SIB) in magnetic resonance (MR) images using large deformation diffeomorphic metric mapping (LDDMM) and parallel transport techniques. The within-subject asymmetry deformation of each structure was first measured via LDDMM, and then translated to a global template via parallel transport for evaluation of the patterns of asymmetry both within and across siblings. Our results revealed that asymmetries observed in CON subjects occurred in the amygdala and the anterior segment of the hippocampus with more pronounced expansion deformation in the right-sided structures (R>L asymmetry) but not in the basal ganglia and thalamus. Disturbance in this pattern of asymmetries was observed in both SCZ and SCZ-SIB subjects. More specifically, exaggerations and reductions in the normative pattern of asymmetries were observed in the amygdala-hippocampus formation, basal ganglia, and thalamus. These altered patterns of asymmetries are present in subjects with schizophrenia and their siblings, and therefore may represent a schizophrenia-related endophenotype.

The effect of handedness on academic ability: a multivariate linear mixed model approach

In recent years questions have arisen about whether there are any links between handedness and academic abilities as well as other factors. In this study we investigate the effects of gender, writing hand, relative hand skill, and UK region on mathematics and reading test scores by applying a multivariate linear mixed-effects model. A data sample based on 11,847 11-year-old pupils across the UK from the National Child Development Study was considered for the analysis. Our results show that pupils who write with one hand while having better skill with their other hand (i.e., inconsistent writing hand and superior hand) obtained lower test scores in both reading and mathematics than pupils with consistent writing hand and superior hand. Furthermore, we confirm previous findings that degree of relative hand skill has a significant effect on both reading and maths scores and that this association is not linear. We also found higher scores of reading in children from the south of England, and of mathematics in children from the south of England and Scotland, when compared to other UK regions.

Scalp hair-whorl orientation of Japanese individuals is random; hence, the trait's distribution is not genetically determined

Because the features of clockwise versus anti-clockwise orientation of hair-whorl coiling developed on a person's scalp is (partially, albeit significantly) correlated with that individual's right- versus left-hand-use preference (i.e., handedness) in the US and British subjects, these traits have been recently suggested to be determined biologically and through a common genetic mechanism. Here I report results of a serendipitously made observation with the Japanese population that helps to scrutinize validity of partial correlation between these attributes and to ascertain whether the underlying gene's frequency variations exist in different gene pools. Surprisingly, the whorl orientation in the Japanese individuals was found to be random, although their handedness variation is similar to that of the US population. Therefore, the whorl orientation trait is not genetically determined in the Japanese population. This result supports the idea that separate decisions must be made during embryogenesis for developing handedness and hair-whorl features at least in Japanese individuals. A recent study found the lack of association between whorl orientation and handedness in the German population, yet previous studies suggested that their scalp hair orientation is genetically determined. Therefore, pronounced genetic variation for the hair-whorl trait exists between individuals of different geographical regions. As hand preference exhibits "complex correlation" with brain hemispheric functional specialization, implications of these findings are discussed here with the goal to define biology of brain hemispheric laterality determination.

Schizotypal personality traits and atypical lateralization in motor and language functions

Atypical cerebral lateralization in motor and language functions in regard to schizotypal personality traits in healthy populations, as well as among schizophrenic patients, has attracted attention because these traits may represent a risk factor for schizophrenia. Although the relationship between handedness and schizotypal personality has been widely examined, few studies have adopted an experimental approach. This study consisted of three experiments focusing on motor and language functional lateralization in regard to schizotypal personality in the absence of mental illness: line-drawing, finger tapping, and a semantic go/no-go task. The results suggested that positive schizotypal personality might be related to functional non-lateralization in regard to at least some functions (e.g., spatial motor control and semantic processing in the present study). Subjects with high schizotypal personality traits performed equally with their right and left-hands in the line-drawing task and they reacted equally with their right and left-hands in a semantic go/no-go task involving semantic auditory stimuli presented in both ears. However, those low in schizotypal personality traits showed typical lateralization in response to these tasks. We discuss the implications of these findings for schizotypal atypical lateralization.

Genetic influences on handedness: data from 25,732 Australian and Dutch twin families

Handedness refers to a consistent asymmetry in skill or preferential use between the hands and is related to lateralization within the brain of other functions such as language. Previous twin studies of handedness have yielded inconsistent results resulting from a general lack of statistical power to find significant effects. Here we present analyses from a large international collaborative study of handedness (assessed by writing/drawing or self report) in Australian and Dutch twins and their siblings (54,270 individuals from 25,732 families). Maximum likelihood analyses incorporating the effects of known covariates (sex, year of birth and birth weight) revealed no evidence of hormonal transfer, mirror imaging or twin specific effects. There were also no differences in prevalence between zygosity groups or between twins and their singleton siblings. Consistent with previous meta-analyses, additive genetic effects accounted for about a quarter (23.64%) of the variance (95%CI 20.17, 27.09%) with the remainder accounted for by non-shared environmental influences. The implications of these findings for handedness both as a primary phenotype and as a covariate in linkage and association analyses are discussed.

What's left in asymmetry?

Left-right patterning is a fascinating problem of morphogenesis, linking evolutionary and cellular signaling mechanisms across many levels of organization. In the past 15 years, enormous progress has been made in elucidating the molecular details of this process in embryos of several model species. While many outside the field seem to believe that the fundamental aspects of this pathway are now solved, workers on asymmetry are faced with considerable uncertainties over the details of specific mechanisms, a lack of conceptual unity of mechanisms across phyla, and important questions that are not being pursued in any of the popular model systems. Here, we suggest that data from clinical syndromes, cryptic asymmetries, and bilateral gynandromorphs, while not figuring prominently in the mainstream work on LR asymmetry, point to crucial and fundamental gaps of knowledge about asymmetry. We identify 12 big questions that provide exciting opportunities for fundamental new advances in this field.

Increased fronto-temporal perfusion in bipolar disorder

OBJECTIVES

Previous imaging reports showed over-activation of fronto-limbic structures in bipolar patients, particularly in response to emotional stimuli. In this study, for the first time, we used perfusion weighted imaging (PWI) to analyze lobar cerebral blood volume (CBV) in bipolar disorder to further explore the vascular component to its pathophysiology.

METHODS

Fourteen patients with DSM-IV bipolar disorder (mean age+/-SD=49.00+/-12.30 years; 6 males, 8 females) and 29 normal controls (mean age+/-SD=45.07+/-10.30 years; 13 males, 16 females) were studied. PWI images were obtained following intravenous injection of paramagnetic contrast agent (Gadolinium-DTPA), with a 1.5 T Siemens magnet using an echo-planar sequence. The contrast of enhancement (CE), was calculated pixel by pixel as the ratio of the maximum signal intensity drop during the passage of contrast agent (Sm) by the baseline pre-bolus signal intensity (So) (CE=Sm/So*100) for frontal, temporal, parietal, and occipital lobes, bilaterally, on two axial images. Higher CE values correspond to lower CBV and viceversa.

RESULTS

Bipolar patients had significantly lower CE values in left frontal and temporal lobes (p=0.01 and p=0.03, respectively) and significantly inverse laterality index for frontal lobe (p=0.017) compared to normal controls. No significant correlations between CE measure and age or clinical variables were found (p>0.05).

CONCLUSION

This study found increased left frontal and temporal CBV in bipolar disorder. Fronto-temporal hyper-perfusion may sustain over-activation of these structures during emotion modulation, which have been observed in patients with bipolar illness.

Of mice and men - and lopsided birds

The article by Zucca and Sovrano (2008, this issue) represents part of a new wave of studies of lateralization in nonhuman species. This work is often in conflict with earlier studies of human cerebral asymmetry and handedness, and the associated claim that these asymmetries are uniquely human, and perhaps even a result of the "speciation event" that led to modern humans. It is now apparent that there are close parallels between human and nonhuman asymmetries, suggesting that they have ancient roots. I argue that asymmetries must be seen in the context of a bilaterally symmetrical body plan, and that there is a balance to be struck between the adaptive advantages of symmetry and asymmetry. In human evolution, systematic asymmetries were incorporated into activities that probably are unique to our species, but the precursors of these asymmetries are increasingly evident in other species, including frogs, fish, birds, and mammals - especially primates.

Artificial selection on laterality in the teleost fish Girardinus falcatus

We performed five generations of artificial selection on laterality of eye preference in Girardinus falcatus using a detour test. Two lines were selected for right turning when encountering a potential predator, two for left turning, one for no turning bias and one unselected line was used as control. We observed a prompt response to directional selection in all lines and the response was approximately symmetrical in left and right turning lines. However, the response to selection ceased after the first or the second generation and unexpectedly in all lines the average laterality score slowly decreased in subsequent generations. After selection was suspended for three generations, no significant variation in mean laterality was observed in most cases, indicating that natural selection was not actively opposing artificial selection during the experiment. After five generations, selected lines maintained substantial additive variance as evidenced by the possibility of rapidly reversing the direction of laterality bias in just one generation of counter-selection.

The association between scalp hair-whorl direction, handedness and hemispheric language dominance: is there a common genetic basis of lateralization?

The hemispheres of the human brain are functionally asymmetric. The left hemisphere tends to be dominant for language and superior in the control of manual dexterity. The mechanisms underlying these asymmetries are not known. Genetic as well as environmental factors are discussed. Recently, atypical anticlockwise hair-whorl direction has been related to an increased probability for non-right-handedness and atypical hemispheric language dominance. These findings are fascinating and important since hair-whorl direction is a structural marker of lateralization and could provide a readily observable anatomical clue to functional brain lateralization. Based on data on handedness and hair-whorl direction, Amar Klar proposed a genetic model ("random-recessive model") in that a single gene with two alleles controls both handedness and hair-whorl orientation (Klar, A.J.S., 2003. Human handedness and scalp hair-whorl direction develop from a common genetic mechanism. Genetics 165, 269-276). The present study was designed to further investigate the relationship between scalp hair-whorl direction with handedness and hemispheric language dominance. 1212 subjects were investigated for scalp hair-whorl direction and handedness. Additionally, we determined hemispheric language dominance (as assessed by a word generation task) in a subgroup of 212 subjects using functional transcranial Doppler sonography (fTCD). As for the single attributes - hair-whorl direction, handedness, and language dominance - we reproduced previously published results. However, we found no association between hair-whorl direction and either language dominance or handedness. These results strongly argue against a common genetic basis of handedness or language lateralization with scalp hair-whorl direction. Inspection of hair patterns will not help us to determine language dominance.

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.

Molecular approaches to brain asymmetry and handedness

In the human brain, distinct functions tend to be localized in the left or right hemispheres, with language ability usually localized predominantly in the left and spatial recognition in the right. Furthermore, humans are perhaps the only mammals who have preferential handedness, with more than 90% of the population more skillful at using the right hand, which is controlled by the left hemisphere. How is a distinct function consistently localized in one side of the human brain? Because of the convergence of molecular and neurological analysis, we are beginning to consider the puzzle of brain asymmetry and handedness at a molecular level.

Parental and perinatal factors influencing the development of handedness in captive chimpanzees

It has been proposed that human right handedness is determined by genetic factors associated with the emergence of language, whereas non-human primate handedness is determined by random, non-genetic factors. These different mechanisms account for differences in the distribution of handedness between human and non-human primates. Here we report evidence that genetic factors play a role in the determination of handedness in chimpanzees. We further report that differential rearing has no influence on the expression of handedness in related individuals. Contrary to many theories of the origin of handedness, these results indicate that genetic factors have a significant influence on handedness in chimpanzees.

Comparative and familial analysis of handedness in great apes

Historically, population-level handedness has been considered a hallmark of human evolution. Whether nonhuman primates exhibit population-level handedness remains a topic of considerable debate. This paper summarizes published data on handedness in great apes. Comparative analysis indicated that chimpanzees and bonobos show population-level right handedness, whereas gorillas and orangutans do not. All ape species showed evidence of population-level handedness when considering specific tasks. Familial analyses in chimpanzees indicated that offspring and maternal (but not paternal) handedness was significantly positively correlated, but this finding was contingent upon the classification criteria used to evaluate hand preference. Overall, the proportion of right handedness is lower in great apes compared with humans, and various methodological and theoretical explanations for this discrepancy are discussed.

Of minds and embryos: left-right asymmetry and the serotonergic controls of pre-neural morphogenesis

Serotonin is a clinically important neurotransmitter regulating diverse aspects of cognitive function, sleep, mood, and appetite. Increasingly, it is becoming appreciated that serotonin signaling among non-neuronal cells is a novel patterning mechanism existing throughout diverse phyla. Here, we review the evidence implicating serotonergic signaling in embryonic morphogenesis, including gastrulation, craniofacial and bone patterning, and the generation of left-right asymmetry. We propose two models suggesting movement of neurotransmitter molecules as a novel mechanism for how bioelectrical events may couple to downstream signaling cascades and gene activation networks. The discovery of serotonin-dependent patterning events occurring long before the development of the nervous system opens exciting new avenues for future research in evolutionary, developmental, and clinical biology.

Association between scalp hair-whorl direction and hemispheric language dominance

Asymmetry is a common phenomenon in higher organisms. In humans, the cortical representation of language exhibits a high degree of asymmetry with a prevalence of about 90% of left hemispheric dominance, the underlying mechanisms of which are largely unknown. Another sign that exhibits a form of lateralization is the scalp hair-whorl direction, which is either clockwise or anti-clockwise. The scalp hair-whorl develops from the same germ layer as the nervous system, the ectoderm, between the 10th and 16th week in utero and has been shown to be associated with various neurodevelopmental disorders. Here, we use an established fMRI paradigm to examine the association of a solely biological marker of asymmetry, hair-whorl direction and language lateralization. We show that the mechanism that influences hair-whorl direction and handedness [Klar, A.J.S., 2003. Human handedness and scalp hair-whorl direction develop from a common genetic mechanism. Genetics 1651, 269-276.] also affects cerebral language dominance.

Opposite effects of androgen receptor CAG repeat length on increased risk of left-handedness in males and females

Prenatal exposure to testosterone has been hypothesised to effect lateralization by influencing cell death in the foetal brain. Testosterone binds to the X chromosome linked androgen receptor, which contains a polymorphic polyglutamine CAG repeat, the length of which is positively correlated with testosterone levels in males, and negatively correlated in females. To determine whether the length of the androgen receptor mediates the effects of testosterone on laterality, we examined the association between the number of CAG repeats in the androgen receptor gene and handedness for writing. Association was tested by adding regression terms for the length of the androgen receptor alleles to a multi-factorial-threshold model of liability to left-handedness. In females we found the risk of left-handedness was greater in those with a greater number of repeats (p=0.04), this finding was replicated in a second independent sample of female twins (p=0.014). The length of the androgen receptor explained 6% of the total variance and 24% of the genetic variance in females. In males the risk of left-handedness was greater in those with fewer repeats (p=0.02), with variation in receptor length explaining 10% of the total variance and 24% of the genetic variance. Thus, consistent with Witelson's theory of testosterone action, in all three samples the likelihood of left handedness increased in those individuals with variants of the androgen receptor associated with lower testosterone levels.