Where and what is the right shift factor or cerebral dominance gene? A critique of Francks et al. (2007)

Relationships of lower extremity and trunk asymmetries in elite soccer players

In light of previous research highlighting the prevalence of asymmetries in soccer players and possible links to injury risks, there is a crucial gap in the biomechanical understanding of complex relationships between lower extremity and trunk asymmetries in elite soccer players. The purpose of this study was to investigate the level, relationships, and differences among twelve different parameters of strength, morphological, and neuromuscular asymmetries in elite soccer players. Methods: Elite male soccer players (n = 25, age 21.7 ± 3.9 years) were tested in the following tests: bilateral fluid distribution, hip flexor range of motion, postural stability, isokinetic strength of knee extensors and flexors, isometric lateral trunk rotation strength, eccentric strength of knee flexors, isometric bilateral strength of hip adductors, and vertical ground reaction force in counter-movement jump-free arms, counter-movement jump, squat jump, and drop jump tests. One-way ANOVA, Pearson's coefficient (r), and partial eta squared (η p 2) were used for data analysis. Results: Significant differences in asymmetries were found in elite soccer players (F11,299 = 11.01, p < .01). The magnitude of asymmetry over 10% was in postural stability and drop jump parameters. The lowest magnitudes of asymmetries were in the fluid distribution of the lower limbs and the vertical ground reaction force during the take-off phase in squat jumps. The highest asymmetries between the dominant and non-dominant sides were found in postural stability and drop jump. A total of eleven significant correlations (p < 0.05, r = 0.41-0.63, R2 = 0.17-0.40) were detected between the analyzed asymmetries in elite soccer players. The lateral trunk rotation asymmetries were significantly correlated to vertical ground reaction force asymmetries and knee extensors. Conclusion: Long-term exposure in elite soccer leads to unilateral biomechanical loading that induces abnormal strength and morphological adaptations in favor of the dominant side while linking lower limb and trunk strength asymmetries. By unraveling these complex relationships, we strive to contribute novel methods that could inform targeted training regimens and injury prevention strategies in the elite soccer community. The data should encourage future researchers and coaches to monitor and develop trunk strength linked to lower body kinematics.

Atypical white matter microstructure in left-handed individuals

Information regarding anatomical connectivity in the human brain can be gathered using diffusion tensor imaging (DTI). Fractional anisotropy (FA) is the most commonly derived value, and reflects how strongly directional are the underlying tracts. Differences in FA are thus associated with differences in the underlying microstructure of the brain. The relationships between these differences in microstructure and functional differences in corresponding regions have also been examined. Previous studies have found an effect of handedness on functional lateralization in the brain and corresponding microstructural differences. Here, using tract-based spatial statistics to analyse DTI-derived FA values, we further investigated the structural white matter architecture in the brains of right- and left-handed males. We found significantly higher FA values for left-handed, relatively to right-handed, individuals, in all major lobes, and in the corpus callosum. In support of previous suggestions, we find that there is a difference in the microstructure of white matter in left- and right-handed males that could underpin reduced lateralization of function in left-handed individuals.

Neural cell adhesion molecules belonging to the family of leucine-rich repeat proteins

Leucine-rich repeats (LRRs) are motifs that form protein-ligand interaction domains. There are approximately 140 human genes encoding proteins with extracellular LRRs. These encode cell adhesion molecules (CAMs), proteoglycans, G-protein-coupled receptors, and other types of receptors. Here we give a brief description of 36 proteins with extracellular LRRs that all can be characterized as CAMs or putative CAMs expressed in the nervous system. The proteins are involved in multiple biological processes in the nervous system including the proliferation and survival of cells, neuritogenesis, axon guidance, fasciculation, myelination, and the formation and maintenance of synapses. Moreover, the proteins are functionally implicated in multiple diseases including cancer, hearing impairment, glaucoma, Alzheimer's disease, multiple sclerosis, Parkinson's disease, autism spectrum disorders, schizophrenia, and obsessive-compulsive disorders. Thus, LRR-containing CAMs constitute a large group of proteins of pivotal importance for the development, maintenance, and regeneration of the nervous system.

Genome-wide copy number scan identifies disruption of PCDH11X in developmental dyslexia

Developmental dyslexia (DD) is a complex heritable disorder with unexpected difficulty in learning to read and spell despite adequate intelligence, education, environment, and normal senses. We performed a whole genome copy number variations (CNV) scan on 11 dyslexic families consisting of 14 dyslexic subjects and 24 non dyslexic members using 1.8 million combined SNP and CNV markers. We found CNVs affecting protocadherin genes in six dyslexics from three families, while none among the non-dyslexic control members showed any CNV in protocadherins. We identified duplications in five cases and a deletion in one case in Xq21.3 region bearing PCDH11X. Unequal recombination between the X-transposed region (XTR) of Yp11.2 and the X chromosome might be causing these structural changes. PCDH11X, expressed in brain is implicated in cell-cell communication, verbal ability, cerebral asymmetry, and dendritic synaptic plasticity, may be regarded as a new candidate gene for dyslexia.

PCSK6 VNTR Polymorphism Is Associated with Degree of Handedness but Not Direction of Handedness

Although the left and right human cerebral hemispheres differ both functionally and anatomically, the mechanisms that underlie the establishment of these hemispheric specializations, as well as their physiological and behavioral implications, remain largely unknown. Since cerebral asymmetry is strongly correlated with handedness, and handedness is assumed to be influenced by a number of genetic and environmental factors, we performed an association study of LRRTM1 rs6733871 and a number of polymorphisms in PCSK6 and different aspects of handedness assessed with the Edinburgh handedness inventory in a sample of unrelated healthy adults (n = 1113). An intronic 33bp variable-number tandem repeat (VNTR) polymorphism in PCSK6 (rs10523972) shows a significant association (significance threshold: p<0.0025, adjusted for multiple comparisons) with a handedness category comparison (P = 0.0005) and degree of handedness (P = 0.001). These results provide further evidence for the role of PCSK6 as candidate for involvement in the biological mechanisms that underlie the establishment of normal brain lateralization and thus handedness and support the assumption that the degree of handedness, instead the direction, may be the more appropriate indicator of cerebral organization.

The protocadherin 11X/Y (PCDH11X/Y) gene pair as determinant of cerebral asymmetry in modern Homo sapiens

Annett's right-shift theory proposes that human cerebral dominance (the functional and anatomical asymmetry or torque along the antero-posterior axis) and handedness are determined by a single “right-shift” gene. Familial transmission of handedness and specific deviations of cerebral dominance in sex chromosome aneuploidies implicate a locus within an X–Y homologous region of the sex chromosomes. The Xq21.3/Yp11.2 human-specific region of homology includes the protocadherin 11X/Y (PCDH11X/Y) gene pair, which encode cell adhesion molecules subject to accelerated evolution following the separation of the human and chimpanzee lineages six million years ago. PCDH11X and PCDH11Y, differentially regulated by retinoic acid, are highly expressed in the ventricular zone, subplate, and cortical plate of the developing cerebral cortex. Both proteins interact with β-catenin, a protein that plays a role in determining axis formation and regulating cortical size. In this way, the PCDH11X/Y gene pair determines cerebral asymmetry by initiating the right shift in Homo sapiens.

The cross-cultural nature of the relationship between schizotypy and mixed-handedness

Positive schizotypy is associated with non-right-handedness, which includes left- and mixed-handedness. However, because the underlying mechanisms might be different, it is important to examine whether both left- and mixed-handedness are associated with a high incidence of schizotypy. During 2009-2010, we used both the Perceptual Aberration Scale and Schizotypal Personality Questionnaire to assess 1315 undergraduate students in Taiwan for schizotypy and the Annett handedness questionnaire to assess handedness. Among the three-way classifications based on Annett's grouping, the fully left-handed group appeared to have the lowest score of positive schizotypy; next was the fully right-handed group and then the mixed-handed. Among the three-way classifications driven from cluster analysis, mixed-handers showed highest score of positive schizotypy. When handedness was treated continuously, both direction (e.g., Hand Preference Index) and consistency (e.g., Either hand use score) indicators were significantly correlated with schizotypy. The results of regression analyses showed that the quadratic handedness measure were negatively associated with schizotypy. The results remained similar after correcting for social pressure on left-handedness. In conclusion, the relationship between schizotypy and mixed-handedness appears to be cross-cultural. The dichotomous classification of handedness, right- vs. non-right-handedness, appears to be insufficient. Additional studies on the distinct mechanisms of mixed- and left-handedness are warranted.

Non-replication of an association of Apolipoprotein E2 with sinistrality

A recent report found that left-handed adolescents were more than three times more likely to have an Apolipoprotein (APOE) ϵ2 allele. This study was unable to replicate this association in young adults (N=166). A meta-analysis of nine other datasets (N=360 to 7559, Power > 0.999) including that of National Alzheimer's Coordinating Center also failed to find an over-representation of ϵ2 among left-handers indicating that this earlier outcome was most likely a statistical artefact.

Complex tasks force hand laterality and technological behaviour in naturalistically housed chimpanzees: inferences in hominin evolution

Clear hand laterality patterns in humans are widely accepted. However, humans only elicit a significant hand laterality pattern when performing complementary role differentiation (CRD) tasks. Meanwhile, hand laterality in chimpanzees is weaker and controversial. Here we have reevaluated our results on hand laterality in chimpanzees housed in naturalistic environments at Fundació Mona (Spain) and Chimfunshi Wild Orphanage (Zambia). Our results show that the difference between hand laterality in humans and chimpanzees is not as great as once thought. Furthermore, we found a link between hand laterality and task complexity and also an even more interesting connection: CRD tasks elicited not only the hand laterality but also the use of tools. This paper aims to turn attention to the importance of this threefold connection in human evolution: the link between CRD tasks, hand laterality, and tool use, which has important evolutionary implications that may explain the development of complex behaviour in early hominins.

Breastfeeding predicts handedness

This study examined whether being breastfed predicts handedness. Two large representative samples from Britain (n=13,421) and Ireland (n=8,426) were used. Multivariate probit models are estimated, reporting the marginal effects of covariates on the probability of a child being right-handed. For both datasets, children who have been breastfed for a minimum period (between 1 month and 6 weeks) are significantly less likely to be left-handed. The results are robust to a variety of controls. Hence breastfeeding is an environmental factor that predicts handedness in a non-linear way, although the mechanism is unknown.

Decreased prevalence of left-handedness among females with male co-twins: evidence suggesting prenatal testosterone transfer in humans?

Studies of singletons suggest that right-handed individuals may have higher levels of testosterone than do left-handed individuals. Prenatal testosterone levels are hypothesised to be especially related to handedness formation. In humans, female members from opposite-sex twin pairs may experience elevated level of prenatal exposure to testosterone in their intrauterine environment shared with a male. We tested for differences in rates of left-handedness/right-handedness in female twins from same-sex and opposite-sex twin pairs. Our sample consisted of 4736 subjects, about 70% of all Finnish twins born in 1983-1987, with information on measured pregnancy and birth related factors. Circulating testosterone and estradiol levels at age 14 were available on 771 and 744 of these twins, respectively. We found significantly (p=.006) lower prevalence of left-handedness in females from opposite-sex pairs (5.3%) compared to females from same-sex pairs (8.6%). The circulating levels of neither testosterone nor estradiol related to handedness in either females or males. Nor were there differences in circulating testosterone or estradiol levels between females from opposite-sex and same-sex twin pairs. Birth and pregnancy related factors for which we had information were unrelated to handedness. Our results are difficult to fully explain by postnatal factors, but they offer support to theory that relates testosterone to formation of handedness, and in a population-based sample, are suggestive of effects of prenatal testosterone transfer.

The relationship between hand preference, hand performance, and general cognitive ability

The idea that handedness indicates something about a person's cognitive ability and personality is a perennial issue. A variety of models have been put forward to explain this relationship and predict a range of outcomes from higher levels of cognitive ability in left-handers or moderate right-handers to lower levels of achievement in left- or mixed-handers. We tested these models using a sample (n = 895) drawn from the BRAINnet database (www.brainnet.net). Participants completed a general cognitive ability (GCA) scale and a test of hand preference/performance. Moderate right-handers, as indexed by their performance measures, had higher GCA scores compared with strong left- or right-handers. The performance measure also showed lower levels of GCA for left-handers compared with right-handers. The hand preference data showed little or no association with cognitive ability-perhaps because this measure clusters individuals toward the extremes of the handedness distribution. While adding support to Annett's heterozygous advantage model, which predicts a cognitive disadvantage for strong left- or right-handers, the data also confirm recent research showing a GCA disadvantage for left-handers. Although this study demonstrates that handedness is related to cognitive ability, the effects are subtle and might only be identified in large-scale studies with sensitive measures of hand performance.

Antithetical asymmetry in schizophrenia and bipolar affective disorder: a line bisection study

OBJECTIVE

Evolutionary theories link the pathogenesis of psychosis with anomalous brain asymmetry. Research shows that aberrant lateralization is linked to schizophrenia with elevated rates of left-handedness and reversal of normal cerebral asymmetries. However, lateralization is underexamined in bipolar affective disorder (BPAD) and the available literature suggests the possibility of greater lateralization, which is diametrically opposite to what is observed in schizophrenia. For the first time, we report concurrent analyses of asymmetry in BPAD and schizophrenia using a line bisection task.

METHODS

We examined 164 subjects (31 patients with BPAD in remission, 30 patients with schizophrenia, and 103 healthy controls) using a two-hand line bisection task with established methodology. Raters with good inter-rater reliability (intraclass correlation coefficient > 0.8) measured deviation from the center. Task performance was compared using analysis of covariance with age, sex, and education as covariates.

RESULTS

Study groups did not differ significantly on age, sex, and handedness (p > 0.06). Patients (both schizophrenia and BPAD) had significantly more errors in identifying the center than controls (p < 0.001). Patients with schizophrenia bisected fewer lines at center than controls and BPAD subjects (p < 0.001). Using their right hand, schizophrenia patients had significant rightward deviation and BPAD patients had leftward deviation (p = 0.001). A significant interaction between diagnosis and direction of deviation (p = 0.01) was noted, with significant rightward deviation in schizophrenia and a trend toward leftward deviation in BPAD.

CONCLUSIONS

Study findings suggest attenuation of normal pseudoneglect in schizophrenia and accentuation of normal pseudoneglect in BPAD, indicating lesser lateralization in schizophrenia and possibly greater lateralization in BPAD. From an evolutionary perspective, schizophrenia and BPAD might have antithetical origins.

Cerebral asymmetries: complementary and independent processes

Most people are right-handed and left-cerebrally dominant for speech, leading historically to the general notion of left-hemispheric dominance, and more recently to genetic models proposing a single lateralizing gene. This hypothetical gene can account for higher incidence of right-handers in those with left cerebral dominance for speech. It remains unclear how this dominance relates to the right-cerebral dominance for some nonverbal functions such as spatial or emotional processing. Here we use functional magnetic resonance imaging with a sample of 155 subjects to measure asymmetrical activation induced by speech production in the frontal lobes, by face processing in the temporal lobes, and by spatial processing in the parietal lobes. Left-frontal, right-temporal, and right-parietal dominance were all intercorrelated, suggesting that right-cerebral biases may be at least in part complementary to the left-hemispheric dominance for language. However, handedness and parietal asymmetry for spatial processing were uncorrelated, implying independent lateralizing processes, one producing a leftward bias most closely associated with handedness, and the other a rightward bias most closely associated with spatial attention.

Reduced laterality as a trait marker of schizophrenia--evidence from structural and functional neuroimaging

Laterality is a characteristic principle of the organization of the brain systems for language, and reduced hemispheric asymmetry has been considered a risk factor for schizophrenia. Here we sought support for the risk factor hypothesis by investigating whether reduced asymmetry of temporal lobe structure and function is also present in unaffected relatives. Sixteen schizophrenia patients, 16 age-matched first-degree relatives, and 15 healthy controls underwent high-resolution three-dimensional anatomical imaging and functional magnetic resonance imaging during auditory stimulation. Both the overall auditory cortex and planum temporale volumes and the lateralization to the left hemisphere were markedly reduced in patients. The decrease of lateralization correlated with increased severity of symptoms. In addition, both the overall functional activation in response to auditory stimulation and its asymmetry were reduced in the patients. Relatives had intermediate values between patients and controls on both structural and functional measures. This study provides added support for the idea that reduced hemispheric asymmetry is a biological risk factor for schizophrenia.

Incidence of atypical handedness in epilepsy and its association with clinical factors

The incidence of atypical handedness (left-handedness and ambidexterity) in patients with epilepsy, particularly its association with major clinical factors, is not well established. We evaluated a full range of clinical variables in 478 patients with epilepsy from the United States and Korea. With the Edinburgh Handedness Inventory, handedness was established as both a categorical variable (right-handed, left-handed, ambidextrous) and a continuous variable. Seizures were classified as complex or simple partial, primary generalized, or generalized tonic-clonic. The relationship between handedness and a range of clinical findings was explored. The overall incidence of atypical handedness in our patients was higher than in the general population (13.6%) and significantly higher in the U.S. patient group (17.6%) than in the Korean patients (8.8%). Handedness was not associated with sex; age; seizure type; age at onset; type, side, or site of EEG or brain imaging abnormalities; family history of seizures; refractory epilepsy; or history of epilepsy surgery.

The protocadherin 11X/Y gene pair as a putative determinant of cerebral dominance in Homo sapiens

The cerebral torque, a bias from right frontal to left occipital across the anterior–posterior axis is arguably the defining feature of the human brain, and the foundation for language. What is its genetic basis? Handedness and anatomical data suggest that this torque is specific to humans relative to the extant great apes. Asymmetry deficits associated with sex chromosome aneuploidies implicate loci on both the X and Y chromosomes. A block from the Xq21.3 band was duplicated to the Y chromosome 6 million years ago (close to, and a possible cause of the chimpanzee/hominin separation) containing the human-specific gene pair PCDH11X/Y. PCDH11Y has been subject to positive selection through hominin evolution including 18 amino-acid changes to the longest isoform of the protein. The PCDH11X protein has been subject to five substitutions including two cysteines in the ectodomain. The gene pair can account for sex differences, for example, in cerebral asymmetry and language.

Understanding the genetics of behavioural and psychiatric traits will only be achieved through a realistic assessment of their complexity

Francks et al. (2007) performed a recent study in which the first putative genetic effect on human handedness was identified (the imprinted locus LRRTM1 on human chromosome 2). In this issue of Laterality, Tim Crow and colleagues present a critique of that study. The present paper presents a personal response to that critique which argues that Francks et al. (2007) published a substantial body of evidence implicating LRRTM1 in handedness and schizophrenia. Progress will now be achieved by others trying to validate, refute, or extend those findings, rather than by further armchair discussion.