Limb preferences in non-human vertebrates

Footedness in merlins: Raptors perching in a cold climate

ABSTRACTPerching or standing on one foot is commonly reported in birds but the level of consistency in using one foot over the other has been less-well documented in most species, particularly birds of prey. For birds experiencing colder temperatures, unipedal perching has been attributed to limiting heat loss through unfeathered legs and feet; individuals should spend longer periods of time perched on one foot as temperatures decrease. Using radio tracking, I collected 486 hours of observations on nine overwintering, free-living merlins (Falco columbarius) in Saskatoon, Canada. Five merlins displayed clear preferences to perch on one foot, however the direction of preference was not consistent and four birds were ambidextrous. There was a curvilinear response in the proportion of time spent in unipedal posture versus temperature, with a peak of ∼22% of the time at moderate temperatures (-10 to -19°C), but lower values at warmer and colder temperatures; the main effect of the squared term for temperature was highly influential while individual foot preference had no impact on the use of unipedal perching. Although preferential use of one foot for perching was displayed by some individuals, thermoregulation may not be the primary driver of this behaviour at colder temperatures.

Motor-sensory biases are associated with cognitive and social abilities in humans

Across vertebrates, adaptive behaviors, like feeding and avoiding predators, are linked to lateralized brain function. The presence of the behavioral manifestations of these biases are associated with increased task success. Additionally, when an individual's direction of bias aligns with the majority of the population, it is linked to social advantages. However, it remains unclear if behavioral biases in humans correlate with the same advantages. This large-scale study (N = 313-1661, analyses dependent) examines whether the strength and alignment of behavioral biases associate with cognitive and social benefits respectively in humans. To remain aligned with the animal literature, we evaluate motor-sensory biases linked to motor-sequencing and emotion detection to assess lateralization. Results reveal that moderate hand lateralization is positively associated with task success and task success is, in turn, associated with language fluency, possibly representing a cascade effect. Additionally, like other vertebrates, the majority of our human sample possess a 'standard' laterality profile (right hand bias, left visual bias). A 'reversed' profile is rare by comparison, and associates higher self-reported social difficulties and increased rate of autism and/or attention deficit hyperactivity disorder. We highlight the importance of employing a comparative theoretical framing to illuminate how and why different laterization profiles associate with diverging social and cognitive phenotypes.

Measuring paw preferences in dogs, cats and rats: Design requirements and innovations in methodology

Studying behavioural lateralization in animals holds great potential for answering important questions in laterality research and clinical neuroscience. However, comparative research encounters challenges in reliability and validity, requiring new approaches and innovative designs to overcome. Although validated tests exist for some species, there is yet no standard test to compare lateralized manual behaviours between individuals, populations, and animal species. One of the main reasons is that different fine-motor abilities and postures must be considered for each species. Given that pawedness/handedness is a universal marker for behavioural lateralization across species, this article focuses on three commonly investigated species in laterality research: dogs, cats, and rats. We will present six apparatuses (two for dogs, three for cats, and one for rats) that enable an accurate assessment of paw preference. Design requirements and specifications such as zoometric fit for different body sizes and ages, reliability, robustness of the material, maintenance during and after testing, and animal welfare are extremely important when designing a new apparatus. Given that the study of behavioural lateralization yields crucial insights into animal welfare, laterality research, and clinical neuroscience, we aim to provide a solution to these challenges by presenting design requirements and innovations in methodology across species.

Cross-hemispheric communication: Insights on lateralized brain functions

On the surface, the two hemispheres of vertebrate brains look almost perfectly symmetrical, but several motor, sensory, and cognitive systems show a deeply lateralized organization. Importantly, the two hemispheres are connected by various commissures, white matter tracts that cross the brain's midline and enable cross-hemispheric communication. Cross-hemispheric communication has been suggested to play an important role in the emergence of lateralized brain functions. Here, we review current advances in understanding cross-hemispheric communication that have been made using modern neuroscientific tools in rodents and other model species, such as genetic labeling, large-scale recordings of neuronal activity, spatiotemporally precise perturbation, and quantitative behavior analyses. These findings suggest that the emergence of lateralized brain functions cannot be fully explained by largely static factors such as genetic variation and differences in structural brain asymmetries. In addition, learning-dependent asymmetric interactions between the left and right hemispheres shape lateralized brain functions.

Hemispheric lateralization of language processing: insights from network-based symptom mapping and patient subgroups

The hemispheric laterality of language processing has become a hot topic in modern neuroscience. Although most previous studies have reported left-lateralized language processing, other studies found it to be bilateral. A previous neurocomputational model has proposed a unified framework to explain that the above discrepancy might be from healthy and patient individuals. This model posits an initial symmetry but imbalanced capacity in language processing for healthy individuals, with this imbalance contributing to language recovery disparities following different hemispheric injuries. The present study investigated this model by analyzing the lateralization patterns of language subnetworks across multiple attributes with a group of 99 patients (compared to nonlanguage processing) and examining the lateralization patterns of language subnetworks in subgroups with damage to different hemispheres. Subnetworks were identified using a whole-brain network-based lesion-symptom mapping method, and the lateralization index was quantitatively measured. We found that all the subnetworks in language processing were left-lateralized, while subnetworks in nonlanguage processing had different lateralization patterns. Moreover, diverse hemisphere-injury subgroups exhibited distinct language recovery effects. These findings provide robust support for the proposed neurocomputational model of language processing.

From fossils to mind

Fossil endocasts record features of brains from the past: size, shape, vasculature, and gyrification. These data, alongside experimental and comparative evidence, are needed to resolve questions about brain energetics, cognitive specializations, and developmental plasticity. Through the application of interdisciplinary techniques to the fossil record, paleoneurology has been leading major innovations. Neuroimaging is shedding light on fossil brain organization and behaviors. Inferences about the development and physiology of the brains of extinct species can be experimentally investigated through brain organoids and transgenic models based on ancient DNA. Phylogenetic comparative methods integrate data across species and associate genotypes to phenotypes, and brains to behaviors. Meanwhile, fossil and archeological discoveries continuously contribute new knowledge. Through cooperation, the scientific community can accelerate knowledge acquisition. Sharing digitized museum collections improves the availability of rare fossils and artifacts. Comparative neuroanatomical data are available through online databases, along with tools for their measurement and analysis. In the context of these advances, the paleoneurological record provides ample opportunity for future research. Biomedical and ecological sciences can benefit from paleoneurology's approach to understanding the mind as well as its novel research pipelines that establish connections between neuroanatomy, genes and behavior.

Hemispheric asymmetries and brain size in mammals

Hemispheric asymmetries differ considerably across species, but the neurophysiological base of this variation is unclear. It has been suggested that hemispheric asymmetries evolved to bypass interhemispheric conduction delay when performing time-critical tasks. This implies that large brains should be more asymmetric. We performed preregistered cross-species meta-regressions with brain mass and neuron number as predictors for limb preferences, a behavioral marker of hemispheric asymmetries, in mammals. Brain mass and neuron number showed positive associations with rightward limb preferences but negative associations with leftward limb preferences. No significant associations were found for ambilaterality. These results are only partly in line with the idea that conduction delay is the critical factor that drives the evolution of hemispheric asymmetries. They suggest that larger-brained species tend to shift towards more right-lateralized individuals. Therefore, the need for coordination of lateralized responses in social species needs to be considered in the context of the evolution of hemispheric asymmetries.

Lateralized behaviors in living humans: Application in the context of hominin brain evolution

The left and right hemispheres of our brains differ subtly in structure, and each is dominant in processing specific cognitive tasks. Our species has a unique system of distributing behavior and cognition between each cerebral hemisphere, with a preponderance of pronounced side biases and lateralized functions. This hemisphere-dependent relationship between cognitive, sensory or motor function and a set of brain structures is called hemispheric specialization. Hemispheric specialization has led to the emergence of model systems to link anatomical asymmetries to brain function and behavior. Scientific research on hemispheric specialization and lateralized functions in living humans focuses on three major domains: (1) hand preferences, (2) language, and (3) visuospatial skills and attention. In this chapter we present an overview of this research with a specific focus on living humans and the applications of this research in the context of hominin brain evolution. Our objective is to put into perspective what we know about brain-behavior relationships in living humans and how we can apply the same methods to investigate this relationship in fossil hominin species, and thus improve our understanding of the emergence and development of complex cognitive abilities.

Behavioral laterality is correlated with problem-solving performance in a songbird

Cerebral lateralization, which is often reflected in an individual's behavioral laterality (e.g., handedness and footedness), may bring animals certain benefits such as enhanced cognitive performance. Although the lateralization-cognition relationship has been widely studied in humans and other animals, current evidence supporting their relationship is ambiguous and warrants additional insights from more studies. Moreover, the lateralization-cognition relationship in non-human animals has been mostly studied in human-reared populations, and investigations of wild populations are particularly scarce. Here, we test the footedness of wild-caught male yellow-bellied tits (Pardaliparus venustulus) and investigate its association with their performance in learning to solve a toothpick-pulling problem and a drawer-opening problem. The tested birds showed an overall trend to gradually spent less time solving the problems, implying that they learned to solve the problems. Left- and right-footed individuals showed no significant differences in the latency to explore the experimental apparatuses and in the proportions that completed and did not complete the tasks. However, the left-footed individuals learned faster than the right-footed individuals in the drawer-opening experiment, indicating a potential cognitive advantage associated with left-footedness. These results contribute to the understanding of the behavioral differences between differently footed individuals and, in particular, the relationship between lateralization and cognitive ability in wild animals.

A longitudinal examination of perinatal testosterone, estradiol and vitamin D as predictors of handedness outcomes in childhood and adolescence

The developmental origins of handedness remain elusive, though very early emergence suggests individual differences manifesting in utero could play an important role. Prenatal testosterone and Vitamin D exposure are considered, yet findings and interpretations remain equivocal. We examined n = 767 offspring from a population-based pregnancy cohort (The Raine Study) for whom early biological data and childhood/adolescent handedness data were available. We tested whether 18-week maternal circulatory Vitamin D (25[OH]D), and testosterone and estradiol from umbilical cord blood sampled at birth predicted variance in direction of hand preference (right/left), along with right- and left-hand speed, and the strength and direction of relative hand skill as measured by a finger-tapping task completed at 10 (Y10) and/or 16 (Y16) years. Although higher concentrations of Vitamin D predicted more leftward and less lateralized (regardless of direction) relative hand skill profiles, taken as a whole, statistically significant findings typically did not replicate across time-point (Y10/Y16) or sex (male/female) and were rarely detected across different (bivariate/multivariate) levels of analysis. Considering the number of statistical tests and generally inconsistent findings, our results suggest that perinatal testosterone and estradiol contribute minimally, if at all, to subsequent variance in handedness. Vitamin D, however, may be of interest in future studies.

Morphometric description of the koala humerus using microcomputed tomography

The current prognosis for successful return to function in koalas with appendicular fractures is poor despite being the most common fracture type to result in successful rehabilitation. The forelimb, particularly the humerus, plays a critical role in stabilisation and support while climbing trees. Successful rehabilitation therefore requires adequate internal stabilisation to promote bone healing and faster return to function. Current knowledge of koala limb bone morphometry is lacking and would provide useful clinical insight for future orthopaedic research, particularly with regards to recommendations regarding implant size and type. In this study microcomputed tomography (micro-CT) was used to describe bone length, internal and external diameters, and cortical thickness at five transverse levels along the humerus of skeletally mature koala cadavers. Qualitative descriptions were also made regarding bone features deemed clinically relevant to potential fracture repair techniques. Mean humeral length was 114.3 mm (95% CI 107.29-121.31 mm). Mediolateral diameters were greater than craniocaudal diameters at each measurement level, and the diaphysis has a distally tapering medullary cavity. Diaphyseal cortices were relatively homogenous with slight distal thickening, and medial cortices were thickest along the entire bone. The bone protuberances of the deltoid and supinator ridges projected most of the way down the lateral surface of the bone while the medial surface remained relatively uniform. Distal to the deltoid ridge the humerus curved caudally, terminating at a craniocaudally flattened distal epiphysis. Morphometric descriptions provided in this study will serve as a useful reference for future research, guiding orthopaedic surgery and improving prognosis of koala humeral fractures.

Hand preferences in coordinated bimanual tasks in non-human primates: A systematic review and meta-analysis

The evolutionary significance of hand preferences among non-human primates and humans has been studied for decades with the aim of determining the origins of the population-level tendency. In this study, a meta-analysis was conducted to statistically integrate data on hand preferences in non-human primates performing the tube task and other bimanual tasks to determine the presence and direction of manual laterality. Significant individual-level lateralization was obtained for these bimanual tasks. In nonhuman primates, 82% of the animals analysed showed right or left-hand preference performing the tube task, this figure being 90% for other bimanual tasks. In contrast with humans, no asymmetry was found at the population level. Additionally, population-level preferences were not found in either of the tasks, although a strong manual preference was found when performing the tube task and other bimanual tasks. Species was studied as a variable moderator throughout the meta-analysis. These results highlight the importance of standardized testing methodologies across species and institutions to obtain comparable data and fill the gaps in the taxonomy.

Light-induced asymmetries in embryonic retinal gene expression are mediated by the vascular system and extracellular matrix

Left–right asymmetries in the nervous system (lateralisation) influence a broad range of behaviours, from social responses to navigation and language. The role and pathways of endogenous and environmental mechanisms in the ontogeny of lateralisation remains to be established. The domestic chick is a model of both endogenous and experience-induced lateralisation driven by light exposure. Following the endogenous rightward rotation of the embryo, the asymmetrical position in the egg results in a greater exposure of the right eye to environmental light. To identify the genetic pathways activated by asymmetric light stimulation, and their time course, we exposed embryos to different light regimes: darkness, 6 h of light and 24 h of light. We used RNA-seq to compare gene expression in the right and left retinas and telencephalon. We detected differential gene expression in right vs left retina after 6 h of light exposure. This difference was absent in the darkness condition and had already disappeared by 24 h of light exposure, suggesting that light-induced activation is a self-terminating phenomenon. This transient effect of light exposure was associated with a downregulation of the sensitive-period mediator gene DIO2 (iodothyronine deiodinase 2) in the right retina. No differences between genes expressed in the right vs. left telencephalon were detected. Gene networks associated with lateralisation were connected to vascularisation, cell motility, and the extracellular matrix. Interestingly, we know that the extracellular matrix—including the differentially expressed PDGFRB gene—is involved in morphogenesis, sensitive periods, and in the endogenous chiral mechanism of primary cilia, that drives lateralisation. Our data show a similarity between endogenous and experience-driven lateralisation, identifying functional gene networks that affect lateralisation in a specific time window.

Investigating the influence of neuter status on paw preference in dogs and cats

Motor lateralization is commonly observed through preferential paw use in dogs and cats. Previous studies have uncovered sex-related differences in paw preference, hypothesizing that these differences may be related to sex hormones. The current study aimed to compare neutered and entire individuals to further investigate whether paw preference is influenced by sex hormones. Dog and cat owners were required to fill in a questionnaire with demographic information such as sex and neuter status of their pets. They then carried out two simple paw preference tasks within their homes: a "reaching for food" task and a "reaching for a toy" task. This study revealed an overall preference among the 272 dogs and 137 cats tested to use their right paw in both tasks. In cats, the degree of paw preference (i.e., regardless of the direction) was significantly influenced by an interaction between neuter status and life stage. Also in dogs, both life stage and an interaction between neuter status and life stage tended to influence the degree of paw preference. Post-hoc power analysis revealed a lack of statistical power, suggesting that future studies using a larger sample size are needed to further investigate potential effects of neuter status on paw preference.

Modulation of behavioural laterality in wild New Caledonian crows (Corvus moneduloides): Vocalization, age and function

The New Caledonian crow (Corvus moneduloides) is known for displaying a unique set of tool-related behaviours, with the bird's bill acting as an individually consistently lateralized effector. However, we still fail to understand how such laterality develops, is modulated or even if its expression is consistent across other behavioural categories. Creating the first ethogram for this species allowed us to examine laterality and vocalisations in a population of wild, free-flying New Caledonian crows using detailed analyses of close-up video footage. We revealed the existence of an overall strong left-sided bias during object manipulation only and which was driven by the adult crows of our focal population, the stabilization of individual preferences occurring during the birds' juvenile years. Individually, at least one crow showed consistent side biases to the right and left within different behavioural categories. Our findings highlight previously unknown variability in behavioural laterality in this species, thus advocating for further investigation. Specifically, we argue that a better understanding of the New Caledonian crow's biology and ecology is required if one wishes to pursue the promising comparative road that laterality could be connected to the evolution of tool-making.

Food handling shapes the laterality of paw use in the Chinese red panda (Ailurus styani)

Limb preference, associated with behavioural laterality and hemispheric lateralization, has long received high attention from both ethologists and psychologists. Chinese red pandas in Shanghai Zoo perform natural unimanual actions in two different ways during food handling, making it an appropriate subject to investigate the effect of food handling on pawedness. In this study, we estimated the direction and the strength of pawedness in spontaneous feeding behaviours in zoo-housed Chinese red pandas. All the subjects exclusively used the preferred paw when handling fruit pieces throughout the observational period, with three of them preferred the left paw while the other two the right paw. By contrast, both paws were found to be used alternately in consuming bamboo. Our results indicate that different food manipulations have effects on pawedness in the Chinese red panda where more complex handling could probably enhance its pawedness.

When left is right: The effects of paw preference training on behaviour in mice

Spontaneous limb preferences exist in numerous species. To investigate the underlying mechanisms of these preferences, different methods, such as training, have been developed to shift preferences artificially. However, studies that systematically examine the effects of shifting preferences on behaviour and physiology are largely missing. Therefore, the aim of this study was to assess the impact of shifting paw preferences via training on spontaneous home cage behaviour, as well as anxiety-like behaviour and exploratory locomotion (Elevated plus maze test, Dark light test, Open field test, Free exploration test), learning performance (Labyrinth-maze) and stress hormones (fecal corticosterone metabolites) in laboratory mice (Mus musculus f. domestica). For this, we assessed spontaneous paw preferences of C57BL/6J females (N_ambilateral = 23, N_left = 23, N_right = 25). Subsequently, half of the individuals from each category were trained once a week for four weeks in a food-reaching task to use either their left or right paw, respectively, resulting in six groups: AL, AR, LL, LR, RL, RR. After training, a battery of behavioural tests was performed and spontaneous preferences were assessed again. Our results indicate that most mice were successfully trained and the effect of training was present days after training. However, a significant difference of preferences between RL and LL mice during training suggests a rather low training success of RL mice. Additionally, preferences of L mice differed from those of A and R mice after training, indicating differential long-term effects of training in these groups. Furthermore, left paw training led to higher levels of self-grooming, possibly as a displacement behaviour, and more time spent in the light compartment of the Dark light test. However, overall, there was no systematic influence of training on behavioural measures and stress hormones. Different explanations for this lack of influence, such as the link between training and hemispheric functioning or the intensity and ecological relevance of the training, are discussed.

Is There an Association between Paw Preference and Emotionality in Pet Dogs?

Research with humans and other animals has suggested that preferential limb use is linked to emotionality. A better understanding of this still under-explored area has the potential to establish limb preference as a marker of emotional vulnerability and risk for affective disorders. This study explored the potential relationship between paw preference and emotionality in pet dogs. We examined which paw the dogs preferentially used to hold a Kong™ and to perform two different locomotion tests. Dogs’ emotionality was assessed using a validated psychometric test (the Positive and Negative Activation Scale—PANAS). Significant positive correlations were found for dogs’ paw use between the different locomotion tasks, suggesting that dogs may show a more general paw preference that is stable across different types of locomotion. In comparison, the correlations between the Kong™ Test and locomotion tests were only partially significant, likely due to potential limitations of the Kong™ Test and/or test-specific biomechanical requirements. No significant correlations were identified between paw preference tests and PANAS scores. These results are in contrast to previous reports of an association between dog paw preference and emotionality; animal limb preference might be task-specific and have variable task-consistency, which raises methodological questions about the use of paw preference as a marker for emotional functioning.

Laterality in Horse Training: Psychological and Physical Balance and Coordination and Strength Rather Than Straightness

Simple Summary

For centuries, straightening a horse has been considered a key element in achieving its responsiveness and suppleness and has been a traditional goal in training. However, body asymmetry (natural crookedness), motor laterality (the preference for limbs on one side) and sensory laterality (the preference for sensory organs on one side) are naturally occurring phenomena. In humans, the forced correction of these imbalances, for example, forcing left-handed children to write with their right hands, has been shown to lead to psychological imbalance. In view of this, lateral asymmetries in horses should be accepted, and training should focus on psychological and physical balance, coordination and equal strength on both sides, instead of enforcing “straightness”. To explore this, we conducted a review of the literature on motor and sensory laterality in horses and found that the evidence suggests that enforcing straightness may be stressful and may even be counterproductive by causing psychological and physical imbalance relative to a horse, making it tense and uncooperative. In general, body asymmetry has been shown to have little impact on performance, but increases in motor and sensory laterality can indicate insufficiencies in housing, handling and training. We, therefore, propose that laterality should be recognized as a welfare indicator and that straightness in a horse should be achieved by conducting training focused on balance, coordination and equal strength on both sides.

Abstract

For centuries, a goal of training in many equestrian disciplines has been to straighten the horse, which is considered a key element in achieving its responsiveness and suppleness. However, laterality is a naturally occurring phenomenon in horses and encompasses body asymmetry, motor laterality and sensory laterality. Furthermore, forcibly counterbalancing motor laterality has been considered a cause of psychological imbalance in humans. Perhaps asymmetry and laterality should rather be accepted, with a focus on training psychological and physical balance, coordination and equal strength on both sides instead of enforcing “straightness”. To explore this, we conducted a review of the literature on the function and causes of motor and sensory laterality in horses, especially in horses when trained on the ground or under a rider. The literature reveals that body asymmetry is innate but does not prevent the horse from performing at a high level under a rider. Motor laterality is equally distributed in feral horses, while in domestic horses, age, breed, training and carrying a rider may cause left leg preferences. Most horses initially observe novel persons and potentially threatening objects or situations with their left sensory organs. Pronounced preferences for the use of left sensory organs or limbs indicate that the horse is experiencing increased emotionality or stress, and long-term insufficiencies in welfare, housing or training may result in left shifts in motor and sensory laterality and pessimistic mentalities. Therefore, increasing laterality can be regarded as an indicator for insufficiencies in housing, handling and training. We propose that laterality be recognized as a welfare indicator and that straightening the horse should be achieved by conducting training focused on balance, coordination and equal strength on both sides.

Extra food provisioning does not affect behavioural lateralization in nestling lesser kestrels

Costs and benefits of brain lateralization may depend on environmental conditions. Growing evidence indicates that the development of brain functional asymmetries is adaptively shaped by the environmental conditions experienced during early life. Food availability early in life could act as a proxy of the environmental conditions encountered during adulthood, but its potential modulatory effect on lateralization has received little attention. We increased food supply from egg laying to early nestling rearing in a wild population of lesser kestrels Falco naumanni, a sexually dimorphic raptor, and quantified the lateralization of preening behaviour (head turning direction). As more lateralized individuals may perform better in highly competitive contexts, we expected that extra food provisioning, by reducing the level of intra-brood competition for food, would reduce the strength of lateralization. We found that extra food provisioning improved nestling growth, but it did not significantly affect the strength or direction of nestling lateralization. In addition, maternal body condition did not explain variation in nestling lateralization. Independently of extra food provisioning, the direction of lateralization differed between the sexes, with female nestlings turning more often towards their right. Our findings indicate that early food availability does not modulate behavioural lateralization in a motor task, suggesting limited phenotypic plasticity in this trait.

Insights Into Human and Nonhuman Primate Handedness From Measuring Both Hands

Handedness is part of our everyday lives, but where does it come from? Researchers studying nonhuman primates and young children have approached this question from different perspectives—evolutionary and developmental, respectively. Their work converges on the conclusion that measurement matters in the science of handedness. Coming to a consensus on assessment will guide future research into the origins of handedness. A candidate behavior for promoting multidisciplinary comparison is role-differentiated bimanual manipulation.

Heritability in corpus callosum morphology and its association with tool use skill in chimpanzees (Pan troglodytes): Reproducibility in two genetically isolated populations

The corpus callosum (CC) is the major white matter tract connecting the left and right cerebral hemispheres. It has been hypothesized that individual variation in CC morphology is negatively associated with forebrain volume (FBV) and this accounts for variation in behavioral and brain asymmetries as well as sex differences. To test this hypothesis, CC surface area and thickness as well as FBV was quantified in 221 chimpanzees with known pedigrees. CC surface area, thickness and FBV were significantly heritable and phenotypically associated with each other; however, no significant genetic association was found between FBV, CC surface area and thickness. The CC surface area and thickness measures were also found to be significantly heritable in both chimpanzee cohorts as were phenotypic associations with variation in asymmetries in tool use skill, suggesting that these findings are reproducible. Finally, significant phenotypic and genetic associations were found between hand use skill and region-specific variation in CC surface area and thickness. These findings suggest that common genes may underlie individual differences in chimpanzee tool use skill and interhemispheric connectivity as manifest by variation in surface area and thickness within the anterior region of the CC.

Handedness in ADHD: Meta-Analyses

Meta-analyses have shown that several neurodevelopmental and psychiatric disorders, such as autism spectrum disorder and schizophrenia, are associated with a higher prevalence of atypical (left-, non-right-, or mixed-) handedness. One neurodevelopmental disorder for which this association is unclear is attention deficit hyperactivity disorder (ADHD). Here, some empirical studies have found evidence for a higher prevalence of atypical handedness in individuals with ADHD compared to neurotypical individuals. However, other studies failed to establish such an association. Therefore, meta-analytic integration is critical to estimate whether or not there is an association between handedness and ADHD. We report the results of three meta-analyses (left-, mixed-, and non-right-handedness) comparing handedness in individuals with ADHD to controls (typically developing individuals). The results show evidence of a trend towards elevated levels of atypical handedness when it comes to differences in left- and mixed-handedness (p = 0.09 and p = 0.07, respectively), but do show clear evidence of elevated levels of non-right-handedness between individuals with ADHD and controls (p = 0.02). These findings are discussed in the context of the hypothesis that ADHD is a disorder in which mostly right-hemispheric brain networks are affected. Since right-handedness represents a dominance of the left motor cortex for fine motor behavior, such as writing, as well as a left-hemispheric dominance for language functions, and about 90% of individuals are right-handers, this hypothesis might explain why there is not stronger evidence for an association of left-handedness with ADHD. We suggest that the mechanisms involved in the pathogenesis of ADHD might show an overlap with the mechanisms involved in handedness strength, but not handedness direction.

Limb Preference in Animals: New Insights into the Evolution of Manual Laterality in Hominids

Until the 1990s, the notion of brain lateralization—the division of labor between the two hemispheres—and its more visible behavioral manifestation, handedness, remained fiercely defined as a human specific trait. Since then, many studies have evidenced lateralized functions in a wide range of species, including both vertebrates and invertebrates. In this review, we highlight the great contribution of comparative research to the understanding of human handedness’ evolutionary and developmental pathways, by distinguishing animal forelimb asymmetries for functionally different actions—i.e., potentially depending on different hemispheric specializations. Firstly, lateralization for the manipulation of inanimate objects has been associated with genetic and ontogenetic factors, with specific brain regions’ activity, and with morphological limb specializations. These could have emerged under selective pressures notably related to the animal locomotion and social styles. Secondly, lateralization for actions directed to living targets (to self or conspecifics) seems to be in relationship with the brain lateralization for emotion processing. Thirdly, findings on primates’ hand preferences for communicative gestures accounts for a link between gestural laterality and a left-hemispheric specialization for intentional communication and language. Throughout this review, we highlight the value of functional neuroimaging and developmental approaches to shed light on the mechanisms underlying human handedness.

Common principles in the lateralisation of auditory cortex structure and function for vocal communication in primates and rodents

This review summarises recent findings on the lateralisation of communicative sound processing in the auditory cortex (AC) of humans, non-human primates, and rodents. Functional imaging in humans has demonstrated a left hemispheric preference for some acoustic features of speech, but it is unclear to which degree this is caused by bottom-up acoustic feature selectivity or top-down modulation from language areas. Although non-human primates show a less pronounced functional lateralisation in AC, the properties of AC fields and behavioral asymmetries are qualitatively similar. Rodent studies demonstrate microstructural circuits that might underlie bottom-up acoustic feature selectivity in both hemispheres. Functionally, the left AC in the mouse appears to be specifically tuned to communication calls, whereas the right AC may have a more 'generalist' role. Rodents also show anatomical AC lateralisation, such as differences in size and connectivity. Several of these functional and anatomical characteristics are also lateralized in human AC. Thus, complex vocal communication processing shares common features among rodents and primates. We argue that a synthesis of results from humans, non-human primates, and rodents is necessary to identify the neural circuitry of vocal communication processing. However, data from different species and methods are often difficult to compare. Recent advances may enable better integration of methods across species. Efforts to standardise data formats and analysis tools would benefit comparative research and enable synergies between psychological and biological research in the area of vocal communication processing.

Analogous laterality in trunk movements in captive African elephants: A pilot study

Lateralization of hand use in primates has been extensively studied in a variety of contexts, and starts to be investigated in other species and organs in order to understand the evolution of the laterality according to different tasks. In elephants, the orientation of the movements of the trunk has been observed mainly in feeding and social contexts, in free conditions. However, little is known about the influence of task complexity on trunk laterality. In this study, we compared the lateralization of the trunk in two conditions: standardized and free. We offered granules to six African elephants on each side of an opened trapdoor to create a constraining environment and reported the different behaviours employed and their orientation. In addition, we observed the same individuals in free conditions and noted the lateralization of the use of their trunk. We revealed a common right side preference in all our elephants, both in standardized and free conditions. This side bias was stronger in our constraining task, adding evidence for the task complexity theory. We finally described laterality in new behaviours in the literature on elephants, such as pinching, gathering or exploration with the trunk.

It Is Not Just in the Genes

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

Chinese red panda (Ailurus styani) has stable individual-level laterality of anogenital rubbing

Behavioural laterality was widely discovered in both vertebrates and invertebrates. However, reports of behavioural laterality in scent-marking are scarce and focused on limb preference during scent-marking. In this study, we observed another scent-marking behaviour, anogenital rubbing, which involved whole-body movement, in zoo-housed Chinese red pandas. We recorded the moving direction of the buttocks when initiating anogenital rubbing. Our results showed that three of our subjects were explicitly left-biased when initiating anogenital rubbing and the other one showed more left initiation although there was no statistical significance. Besides, the laterality was consistent across the whole observational period. This is the first report of laterality in anogenital rubbing, perhaps indicating hemispheric specialization for chemical communication in the Chinese red panda.

Laterality preferences at rest and predatory behaviour of the Gyrfalcon (Falco rusticolus): An alpha predator of the sky

Brain lateralization is generally considered adaptive for an individual and it can be ascertained, for example, by measuring the preferential use of limbs. Avian models have been extensively used to investigate the evolution and the advantages of brain lateralization. Birds of prey are a good model to study motor laterality, however to date they have been studied almost exclusively in the context of predatory behaviour. In this study, we tested lateralization in Gyrfalcon (Falco rusticolus) across multiple contexts, and collected the following measures:(1) standing leg preference when sleeping, (2) wing preference to position the head while sleeping and (3) leg preference to grasp food. At the population level, we found left-leg lateralization while sleeping and no preference for placing the head under the left or the right wing. In the context of the predatory behaviour, we found a trend towards using the left leg to grasp food. Across the behaviours observed, we did not find evidence of lateralization at an individual level, as most of the subjects were ambidextrous. This study highlights the importance of the behavioural context when investigating side-bias and hemispheric laterality.

Polygenic scores for handedness and their association with asymmetries in brain structure

Handedness is the most widely investigated motor preference in humans. The genetics of handedness and especially the link between genetic variation, brain structure, and right-left preference have not been investigated in detail. Recently, several well-powered genome-wide association studies (GWAS) on handedness have been published, significantly advancing the understanding of the genetic determinants of left and right-handedness. In the present study, we estimated polygenic scores (PGS) of handedness-based on the GWAS by de Kovel and Francks (Sci Rep 9: 5986, 2019) in an independent validation cohort (n = 296). PGS reflect the sum effect of trait-associated alleles across many genetic loci. For the first time, we could show that these GWAS-based PGS are significantly associated with individual handedness lateralization quotients in an independent validation cohort. Additionally, we investigated whether handedness-derived polygenic scores are associated with asymmetries in gray matter macrostructure across the whole brain determined using magnetic resonance imaging. None of these associations reached significance after correction for multiple comparisons. Our results implicate that PGS obtained from large-scale handedness GWAS are significantly associated with individual handedness in smaller validation samples with more detailed phenotypic assessment.

How Asymmetries Evolved: Hearts, Brains, and Molecules

Humans belong to the vast clade of species known as the bilateria, with a bilaterally symmetrical body plan. Over the course of evolution, exceptions to symmetry have arisen. Among chordates, the internal organs have been arranged asymmetrically in order to create more efficient functioning and packaging. The brain has also assumed asymmetries, although these generally trade off against the pressure toward symmetry, itself a reflection of the symmetry of limbs and sense organs. In humans, at least, brain asymmetries occur in independent networks, including those involved in language and manual manipulation biased to the left hemisphere, and emotion and face perception biased to the right. Similar asymmetries occur in other species, notably the great apes. A number of asymmetries are correlated with conditions such as dyslexia, autism, and schizophrenia, and have largely independent genetic associations. The origin of asymmetry itself, though, appears to be unitary, and in the case of the internal organs, at least, may depend ultimately on asymmetry at the molecular level.

Paw preferences in mice and rats: Meta-analysis

Mice and rats are among the most common animal model species in both basic and clinical neuroscience. Despite their ubiquity as model species, many clinically relevant brain-behaviour relationships in rodents are not well understood. In particular, data on hemispheric asymmetries, an important organizational principle in the vertebrate brain, are conflicting as existing studies are often statistically underpowered due to small sample sizes. Paw preference is one of the most frequently investigated forms of hemispheric asymmetries on the behavioural level. Here, we used meta-analysis to statistically integrate findings on paw preferences in rats and mice. For both species, results indicate significant hemispheric asymmetries on the individual level. In mice, 81 % of animals showed a preference for either the left or the right paw, while 84 % of rats showed this preference. However, contrary to what has been reported in humans, population level asymmetries were not observed. These results are particularly significant as they point out that paying attention to potential individual hemispheric differences is important in both basic and clinical neuroscience.

Brain Size Associated with Foot Preferences in Australian Parrots

Since foot preference of cockatoos and parrots to hold and manipulate food and other objects has been associated with better ability to perform certain tasks, we predicted that either strength or direction of foot preference would correlate with brain size. Our study of 25 psittacine species of Australia found that species with larger absolute brain mass have stronger foot preferences and that percent left-footedness is correlated positively with brain mass. In a sub-sample of 11 species, we found an association between foot preference and size of the nidopallial region of the telencephalon, an area equivalent to the mammalian cortex and including regions with executive function and other higher-level functions. Our analysis showed that percent left-foot use correlates positively and significantly with size of the nidopallium relative to the whole brain, but not with the relative size of the optic tecta. Psittacine species with stronger left-foot preferences have larger brains, with the nidopallium making up a greater proportion of those brains. Our results are the first to show an association between brain size and asymmetrical limb use by parrots and cockatoos. Our results support the hypothesis that limb preference enhances brain capacity and higher (nidopallial) functioning.

Laterality in Responses to Acoustic Stimuli in Giant Pandas

Simple Summary

Functional lateralization in the auditory system has been widely studied. Accordingly, behavioral laterality responses affected by acoustic stimuli have been observed in many vertebrate species. In this study, we assessed giant pandas’ behavioral responses to different acoustic stimuli in order to examine cerebral lateralization. We concluded that adult giant pandas showed a left-hemisphere bias in response to positive acoustic stimuli. Furthermore, we found the specific valence of cerebral lateralization for different categories of acoustic stimuli, of which some were relevant to the lateralization while others were not relevant. Our findings support an evolutionary strategy that giant pandas process auditory signals similar to other mammals.

Abstract

Cerebral lateralization is a common feature present in many vertebrates and is often observed in response to various sensory stimuli. Numerous studies have proposed that some vertebrate species have a right hemisphere or left hemisphere dominance in response to specific types of acoustic stimuli. We investigated lateralization of eight giant pandas (Ailuropoda melanoleuca) by using a head turning paradigm and twenty-eight acoustic stimuli with different emotional valences which included twenty-four conspecific and four non-conspecific acoustic stimuli (white noise, thunder, and vocalization of a predator). There was no significant difference in auditory laterality in responses to conspecific or non-conspecific sounds. However, the left cerebral hemisphere processed the positive stimuli, whereas neither of the two hemispheres exhibited a preference for processing the negative stimuli. Furthermore, the right hemisphere was faster than the left hemisphere in processing emotional stimuli and conspecific stimuli. These findings demonstrate that giant pandas exhibit lateralization in response to different acoustic stimuli, which provides evidence of hemispheric asymmetry in this species.

Functional Lateralization of the Mirror Neuron System in Monkey and Humans

To date, both in monkeys and humans, very few studies have addressed the issue of the lateralization of the cortical parietal and premotor areas involved in the organization of voluntary movements and in-action understanding. In this review, we will first analyze studies in the monkey, describing the functional properties of neurons of the parieto-frontal circuits, involved in the organization of reaching-grasping actions, in terms of unilateral or bilateral control. We will concentrate, in particular, on the properties of the mirror neuron system (MNS). Then, we will consider the evidence about the mirror neuron mechanism in humans, describing studies in which action perception, as well as action execution, produces unilateral or bilateral brain activation. Finally, we will report some investigations demonstrating plastic changes of the MNS following specific unilateral brain damage, discussing how this plasticity can be related to the rehabilitation outcome

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

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

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.

No evidence that footedness in pheasants influences cognitive performance in tasks assessing colour discrimination and spatial ability

The differential specialization of each side of the brain facilitates the parallel processing of information and has been documented in a wide range of animals. Animals that are more lateralized as indicated by consistent preferential limb use are commonly reported to exhibit superior cognitive ability as well as other behavioural advantages. We assayed the lateralization of 135 young pheasants (Phasianus colchicus), indicated by their footedness in a spontaneous stepping task, and related this measure to individual performance in either 3 assays of visual or spatial learning and memory. We found no evidence that pronounced footedness enhances cognitive ability in any of the tasks. We also found no evidence that an intermediate footedness relates to better cognitive performance. This lack of relationship is surprising because previous work revealed that pheasants have a slight population bias towards right footedness, and when released into the wild, individuals with higher degrees of footedness were more likely to die. One explanation for why extreme lateralization is constrained was that it led to poorer cognitive performance, or that optimal cognitive performance was associated with some intermediate level of lateralization. This stabilizing selection could explain the pattern of moderate lateralization that is seen in most non-human species that have been studied. However, we found no evidence in this study to support this explanation.

Does Brain Lateralization Affect the Performance in Binary Choice Tasks? A Study in the Animal Model Danio rerio

Researchers in behavioral neuroscience commonly observe the behavior of animal subjects in the presence of two alternative stimuli. However, this type of binary choice introduces a potential confound related to side biases. Understanding whether subjects exhibit this bias, and the origin of it (pre-existent or acquired throughout the experimental sessions), is particularly important to interpreting the results. Here, we tested the hypothesis according to which brain lateralization may influence the emergence of side biases in a well-known model of neuroscience, the zebrafish. As a measure of lateralization, individuals were observed in their spontaneous tendencies to monitor a potential predator with either the left or the right eye. Subjects also underwent an operant conditioning task requiring discrimination between two colors placed on the left–right axis. Although the low performance exhibited in the operant conditioning task prevents firm conclusions from being drawn, a positive correlation was found between the direction of lateralization and the tendency to select the stimulus presented on one specific side (e.g., right). The choice for this preferred side did not change throughout the experimental sessions, meaning that this side bias was not the result of the prolonged training. Overall, our study calls for a wider investigation of pre-existing lateralization biases in animal models to set up methodological counterstrategies to test individuals that do not properly work in a binary choice task with stimuli arranged on the left–right axis.

The relationship between problem-solving ability and laterality in cats

The association between hemispheric asymmetries and cognitive ability is one of the key areas of comparative laterality research. In several animal species, individual limb preferences correlate with perceptual, cognitive, or motor abilities, possibly by increasing dexterity of one limb and minimizing response conflicts between hemispheres. Despite this wealth of research, the association between laterality and cognitive abilities in the cat (Felis catus) is not well understood. Therefore, it was the aim of the present study to investigate the relationship between laterality and problem-solving ability in cats. To this end, strength and direction of paw preferences in 41 cats were measured using two novel food reaching tasks in which the animals needed to open a lid in order to reach the food reward. We found that cats that showed a clear preference for one paw were able to open more lids succesfully than ambilateral animals. Moreover, cats that preferred to interact with the test apparatus with their paw from the beginning, opened more lids than cats the first tried to interact with the test apparatus using their heads. Results also suggested a predictive validity of the first paw usage for general paw usage. It was also shown that the cats' individual paw preferences were stable and task-independent. These results yield further support to the idea that lateralization may enhance cognitive abilities.