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Functional hemispheric asymmetries emerge as the left and the right hemisphere are dominant for different aspects of task processing. However, the hemispheres do not work independent of each other but share information through the corpus callosum. The integration of information across the corpus callosum is dependent on its structural integrity and functionality. Several hormones, like estradiol and progesterone, can influence this function. Since earlier work has demonstrated that long-term changes in stress hormone levels are accompanied by changes in hemispheric asymmetries in several mental disorders, the aim of the current study was to investigate whether acute stress and the associated changes in stress hormone levels also affect information transfer across the corpus callosum. For this purpose, we collected EEG data from 51 participants while completing a lexical decision task and a Poffenberger paradigm twice, once after stress induction with the Trier Social Stress Test and once after a control-condition. While there were no differences in interhemispheric transfer between the stress and the non-stress condition in the Poffenberger paradigm, we observed shorter latencies to stimuli in the left visual field in the left hemisphere at the CP3-CP4 electrode pair after stress. These results suggest that the transfer of lexical material from the right to the left hemisphere was quicker under stress. Stress may increase callosal excitability and lead to more efficient signal transfer across the corpus callosum between language related areas. Future studies using pharmacological intervention are needed to further examine cooperation of the hemispheres under stress in more detail.

Brain lateralization in response to social stimuli is well known for its involvement of the right hemisphere in several vertebrate species, including humans. This study aimed to investigate the laterality of the social behavior during the mirror-images inspection in tortoises ( Testudo hermanni) . In a rectangular apparatus, in presence or in absence of two mirrors as the longer walls, we assessed: 1) the animal’s position and 2) the monocular viewing compared to the longer walls, 3) the paw used to start a movement from a resting position. Here we provide the first evidence of lateralization towards social stimuli in tortoises, a reptile that is likely to lead mostly a solitary life, but also able to show a few basic abilities in social cognition. Results revealed a preference to spend significantly more time in peripheral positions, mainly in the presence of mirrors. Moreover, a consistent left-eye preference to inspect the mirrors was observed, especially when close to them. In contrast, a significant right-eye preference appeared in absence of mirrors, when tortoises occupied the central areas. Findings show a significant preference for right-paw use in starting movements, when mirrors were present. Results are discussed with reference to other evidence of brain asymmetry.

Performance on tasks involving cognitive control such as the Stroop task is often associated with left lateralized brain activations. Based on this neuro-functional evidence, we tested whether leftward structural grey matter asymmetries would also predict inter-individual differences in combatting Stroop interference. To check for the specificity of the results, both a verbal Stroop task and a spatial one were administered to a total of 111 healthy young individuals, for whom T1-weighted magnetic resonance imaging (MRI) images were also acquired. Surface thickness and area estimations were calculated using FreeSurfer. Participants' hemispheres were registered to a symmetric template and Laterality Indices (LI) for the surface thickness and for the area at each vertex in each participant were computed. The correlation of these surface LI measures with the verbal and spatial Stroop effects (incongruent-congruent difference in trial performance) was assessed at each vertex by means of general linear models at the whole-brain level. We found a significant correlation between performance and surface area LI in an inferior posterior temporal cluster (overlapping with the so-called visual word form area, VWFA), with a more left-lateralized area in this region associated with a smaller Stroop effect only in the verbal task. These results point to an involvement of the VWFA for higher-level processes based on word reading, including the suppression of this process when required by the task, and could be interpreted in the context of cross-hemispheric rivalry.

Handedness is the most investigated form of functional hemispheric asymmetries, but its neural correlates remain unclear. Functional imaging studies suggest differences between left- and right-handers in ipsilateral activation during unilateral hand movements, but do not allow for conclusions on the temporal dimension. In the Tapley and Bryden task, subjects have to draw as many dots as possible on a paper within 20 s using either the left or the right hand. We adapted the task for use during EEG in 36 left- and 36 right-handers. Subjects performed a visually guided response task with each trial consisting of eight motor responses. We investigated the lateralized readiness potential (LRP) at the first and last response of the sequence. Overall, increasing complexity of sequences was associated with earlier and less negative LRP peaks. For the last response, right-handers showed more negative LRP peak amplitudes than left-handers. The effect of handedness on LRP peak amplitude in the first response was modulated by task complexity with a more negative LRP peak amplitude in right-handers than left-handers in simple, but not in medium or complex trials. This effect might be due to more symmetrical processing in right-handers with increasing task complexity. These findings complement previous imaging studies and add a new perspective on the relationship between laterality and schizophrenia, associated with less pronounced LRPs and a higher prevalence of left-handedness.

Cerebral lateralization is an evolutionarily deep-rooted trait, ubiquitous among the vertebrates and present even in some invertebrates. Despite the advantages of cerebral lateralization in enhancing cognition and facilitating greater social cohesion, large within population laterality variation exists in many animal species. It is proposed that this variation is maintained due links with inter-individual personality trait differences. Here we explored for lateralization in Port Jackson sharks (Heterodontus portusjacksoni) using T-maze turn and rotational swimming tasks. Additionally, we explored for a link between personality traits, boldness and stress reactivity, and cerebral lateralization. Sharks demonstrated large individual and sex biased laterality variation, with females demonstrating greater lateralization than males overall. Stress reactivity, but not boldness, was found to significantly correlate with lateralization strength. Stronger lateralized individuals were more reactive to stress. Demonstrating laterality in elasmobranchs for the first time indicates ancient evolutionary roots of vertebrate lateralization approximately 240 million years old. Greater lateralization in female elasmobranchs may be related enhancing females' ability to process multiple stimuli during mating, which could increase survivability and facilitate insemination. Despite contrasting evidence in teleost fishes, the results of this study suggest that stress reactivity, and other personality traits, may be linked to variation in lateralization.

The human prefrontal cortex (PFC) processes complex sensory information for the elaboration of social behaviors. The non-invasive neuroimaging technique near-infrared spectroscopy (NIRS) identifies hemodynamic changes and concentration of oxygenated (HbO2) and deoxygenated (HHb) hemoglobin in the cerebral cortex. We studied the responses detected by NIRS in the right and left PFC activation of 28 participants (n = 14 adult young females and males) while processing social/emotional facial expressions, i.e., in conscious perception of different expressions (neutral, happy, sad, angry, disgust, and fearful) and in unconscious/masked perception of negative expressions (fearful and disgust overlapped by neutral). The power spectral analysis from concomitant ECG signals revealed the sympathetic and parasympathetic modulation of cardiac responses. We found higher HbO2 values in the right PFC of females than in males during, and in the left PFC after, following the conscious perception of the happy face. In males, the left PFC increased and the right PFC decreased HbO2 while viewing the happy expression. In both sexes, HHb values were higher during the masked presentation of disgust than fearful expression, and after the masked presentation of fearful expression than during it. Higher sympathetic and lower parasympathetic activity (LF/ HF components) occurred in females when consciously and unconsciously processing negative emotions (p

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.

The existence of left-right hemispheric differences has been suggested not only in humans but also in rodents. In recent studies, left-right anatomical and functional differences of the rodent hippocampus have been revealed. However, there is only one report investigating the left-right difference for short-term memory (STM), and the left-right difference for long-term memory (LTM) is not consistent among previous studies. Therefore, we examined the effects of unilateral hippocampal lesion and stimulation on the formation of STM and LTM in rats. Our results showed that the right, but not the left, hippocampal lesion impaired STM performance, evaluated by the alternation rate in the spontaneous alternation test and the novel-arm choice rate in the novelty preference test. In addition, electrical stimulation of the left, but not the right, hippocampus immediately before the tests impaired STM performance. On the other hand, the left, but not the right, hippocampal lesion impaired the LTM performance, evaluated by the discrimination index in the object recognition test. In addition, the stimulation of the left, but not the right, hippocampus impaired LTM performance. These results suggest that both the left and right hippocampi are involved in STM formation, and the right hippocampus has a facilitating role while the left hippocampus has a suppressing role for STM. On the other hand, LTM may be driven correctly only by the left hippocampus with appropriate level of neural activity. The left and right hippocampi of rodents may work in different mechanisms depending on the demand for STM and LTM.

Behavioral markers of laterality reflecting underlying neurobiological asymmetries between the cerebral hemispheres are related to differential susceptibility to mood disorders. In the present study, we investigated the strength and consistency of a novel behavioral lateralization, head-turning asymmetry, and its relation to behavioral despair in adult female Wistar rats. Head-turning biases were determined in a test where water-deprived rats had to turn their head to right or left to gain access to a water dispenser. This procedure was administered 4 times over 8 days. Four days after the head-turning test, rats were subjected to two forced swim tests separated by 24 h to examine the relationship between head-turning asymmetry and behavioral despair. Rats were administered one more head-turning test session after the second swim test to determine whether behavioral despair induction altered head-turning direction preferences. Results revealed significant correlations among head-turning test sessions indicating head-turning direction preference as measured with our method is a consistent behavioral lateralization. Although most rats were strongly lateralized, there was no bias in either direction at the population level. Importantly, we found that while rats with a left head-turning bias showed a significant increase in the duration of immobility from the first to the second swim test, right-biased rats performed similarly in the two swim tests. Behavioral despair induction did not change head-turning direction preferences. The present findings show that head-turning asymmetries are predictive of mood disorders in rats and may serve as the basis to elucidate the mechanisms relating hemispheric asymmetries to depression in humans.

Gender differences in spatial memory favoring men are frequently reported, and the involvement of the hippocampus in these functions is well-established. However, little is known of whether this behavioral gender difference is mirrored in a gender difference in hippocampal function. Here we assessed hippocampal activity, using functional MRI, while 24 men and women moved through three-dimensional virtual mazes (navigation phase) of varying length, and at the end-point estimated the direction of the starting-point (pointing phase). Men were indeed more accurate than women at estimating direction, and this was especially true in longer mazes. Both genders activated the posterior hippocampus throughout the whole task. During the navigation phase, men showed a larger activation in the right hippocampus than women, while in the pointing phase, women showed a larger activation in the left hippocampus than men. Right-lateralized activation during the navigation phase was associated with greater task performance, and may reflect a spatial strategy that is beneficial in this task. Left-sided activation during the pointing phase might reflect a less efficient post hoc verbal recapitulation of the route. This study is the first to identify neural correlates of the commonly observed male advantage in recalling one's original position, and points to hippocampal lateralization as a possible explanation for this behavioral gender difference.

Left–right antennal asymmetry has been reported in honeybees. We studied primitive social bees to investigate the evolutionary origins of the asymmetry. Three species of Australian native, stingless bees (Trigona carbonaria, Trigona hockingsi and Austroplebeia australis) were trained to discriminate two odours, lemon (+)/vanilla (−), using the Proboscis Extension Reflex (PER). Recall of the olfactory memory at 1 h after training was better when the odour was presented on the right than on the left side of the bee. In contrast, recall at 5 h after training was better when the odour was presented on the left than on the right side of the bee. An additional experiment with T. hockingsi bees, fed with sugar 1 h before recall and tested at 5 h, produced similar results, showing that the shift in lateralized recall was due to the lapse of time per se and not to changes in motivation to feed. Stingless bees show the same laterality as honeybees, suggesting that asymmetry evolved prior to the evolutionary divergence of these species.

The corpus callosum is the largest white matter structure in the human brain, connecting cortical regions of both hemispheres. Complete and partial callosotomies or callosal lesion studies have granted more insight into the function of the corpus callosum, namely the facilitation of communication between the cerebral hemispheres. How the corpus callosum mediates this information transfer is still a topic of debate. Some pose that the corpus callosum maintains independent processing between the two hemispheres, whereas others say that the corpus callosum shares information between hemispheres. These theories of inhibition and excitation are further explored by reviewing recent behavioural studies and morphological findings to gain more information about callosal function. Additional information regarding callosal function in relation to altered morphology and dysfunction in disorders is reviewed to add to the discussion of callosal involvement in interhemispheric transfer. Both the excitatory and inhibitory theories seem likely candidates to describe callosal function, however evidence also exists for both functions within the same corpus callosum. For future research it would be beneficial to investigate the functional role of the callosal sub regions to get a better understanding of function and use more appropriate experimental methods to determine functional connectivity when looking at interhemispheric transfer.

Novelty and surprises differentially modify the left and right sides of the brain. Here we show that repeated brief exposures to the novelty of a non-home environment during infancy and early adulthood lead to long-lasting changes in adulthood in the global bi-lateralization organization of the brain as indexed by a transiently detectable right-sided orientating bias upon the initial encounter with the novel environment. Most surprisingly, we show that in the same individuals, the short-term effect of the combined neonatal and adulthood novelty exposures on functional brain asymmetry measured at young adulthood (5 months of age) is distinctively different from the long-term effect measured at late adulthood (15 months of age). These results suggest that long-lasting, cumulative effects of early life experience on brain and behavior organization are not necessarily permanent, but continue to unfold, presumably via interactions with a multitude of unmonitored intervening life events.

Asymmetries in brain and behaviour have been demonstrated in numerous species representing all major vertebrate taxa, and may be a universal feature of the vertebrate nervous system. While descriptions of lateralization at the behavioural and neuroanatomical level are widespread, examples of correlation between asymmetries in behaviour and neural structures remain relatively scarce. In the past few years, the habenular nucleus has emerged as a potential site for the neural basis of some lateralized behaviours. Here we investigate the relation between continuous individual variation in asymmetry of the habenulae and behaviour in the detour task in the convict cichlid (Amatitlania nigrofasciata). We found that both male and female convicts show a significant population-level bias towards relatively larger left habenulae. We also show that habenular asymmetry is correlated with behavioural lateralization in both males and females, but in opposite directions. This adds to previous studies showing both in convict cichlids and other vertebrates an interaction between sex and lateralized behaviour. The results of this study increase our understanding of the role of the habenula in lateralized behaviour and highlight the importance of a comparative approach to understanding the development and evolution of habenular asymmetry.

Functional laterality appears to be present in many brain functions in man and animals. The existence of paired neural circuits which act differentially to modulate a specific behavioural function seems to be an evolutionary successful strategy in animal evolution. In spite of many examples described in mammals, birds and other vertebrates and invertebrates, still its intrinsic mechanism is not completely understood. In this work the participation of the baso-lateral amygdala (BLA) on lateralized motivated exploratory behaviour and the possible influence of histamine neurons in these mechanisms were studied in rats. Different groups of animals under xylacine-ketamine anesthesia were implanted with microinjection guide cannulae into the right or left BLA. 72. h after implantation, animals were tested in hole-board cage (OVM) with a novelty object positioned in the center of the arena, as a model of exploration of a non-conflictive environment, and 24. h later they were tested in the Elevated Asymmetric Plus Maze (APM) as a model of conflictive exploration. In the day of the experiment, lidocaine was applied into the left, or right BLA in order to block the electrical activity of BLA neurons. Saline in the contralateral BLA was considered control. Results showed that exploratory activity in the OVM was significantly inhibited when lidocaine was microinjected into the left BLA, and no changes were observed when lidocaine was applied into the right BLA. When histamine was microinjected into the right BLA and lidocaine into the contralateral BLA, head-dipping, rearing, and focalized exploration behaviour were significantly inhibited. In the APM, lidocaine treatment increased equally the exploration of the " single wall" and " high and low walls" arms of the labyrinth, independently if blocking of electrical activity of the BLA neurons was performed in the left or right amygdala. Histamine treatment inhibited significantly exploration of the lesser fear-inducing arms of the labyrinth but its effect was more pronounced when histamine microinjection was in the left BLA. In conclusion, present evidence support the lateralized participation of the amygdala on exploratory behaviour and histamine neurons appear to mediate part of these differential modulations. Fil: Alvarez Toro, Edgardo Orozimbo. Universidad Nacional de Cuyo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto de Medicina y Biología Experimental de Cuyo; Argentina Fil: Banzan, Arturo Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto de Medicina y Biología Experimental de Cuyo; Argentina. Universidad Nacional de Cuyo; Argentina

International audience; As non-human primates are phylogenetically close to humans, they are ideal models to investigate the precursors of human brain hemispheric specialisation. However, in spite of hundreds of reports investigating hand preference, empirically based theories generating valuable predictions are still lacking, mainly because of a disappointing deficiency in comparability between studies and even more so between species. Therefore, we propose here to adapt, for the first time, the Quantifying Hand Preference (QHP) task, devised for humans, to non-human primates. This test could be a very useful standard measure of hand preference for simple reaching in non-human primates because of its simplicity to set up and of the parameters (e.g., subject's posture; position of the item) it can control. Our test subjects were 42 baboons of various ages and both sexes. Our results highlight the crucial influence of item position on hand preference for simple reaching, even when the item is positioned close to the subjects' body midline. Both sex and age influence baboons' handedness index but this effect varies according to the position of the item to be grasped. We discuss our results within the theoretical framework concerning hemispheric specialisation for object manipulation and with the perspective of replicating this experiment with other non-human primate species and genera.

Individuals in the early stage of Parkinson's disease exhibit cognitive impairments as a result of hemisphere damage. The mesocortical dopamine system, particularly the medial prefrontal cortex (mPFC), is implicated in cognitive functions and is characterized by an asymmetric organization. Oxytocinase activity (OX) is also asymmetrically distributed in the mPFC of normal rats and is involved in cognitive functions. OX was measured in the left and right mPFC of rats with left or right hemi-parkinsonism, induced by intrastriatal injections of 6-hydroxydopamine, and compared with sham controls. These results demonstrated that the striking basal left predominance of OX observed in both the left and the right sham controls was radically disrupted in lesioned animals. The bilateral distribution in lesioned animals was altered differently depending on the injured hemisphere. These results may reflect changes in the enzyme substrates and consequently in the functions in which they are involved. These results may account, in part, for the cognitive abnormalities observed in hemi-parkinsonism.

Neonatal cerebral hypoxia-ischemia (HI) is an important cause of neurological deficits. The Levine-Rice model of unilateral HI is a useful experimental tool, but the resulting brain damage is mainly restricted to one hemisphere. Since the rat presents morphological and biochemical asymmetries between brain hemispheres, behavioral outcome from this model is probably dependent on which hemisphere is damaged. We here investigated the effects of sex and lesioned hemisphere on the outcome of open field, plus maze, inhibitory avoidance and water maze tasks in adult rats previously submitted to neonatal unilateral HI. Females were more active than males in some of studied parameters and males presented better spatial learning. Hypoxia-ischemia caused spatial deficits independently of sex or damaged hemisphere. Right-HI increased locomotion only in males and caused working memory in females and on aversive learning in both males and females. Morphological analysis showed that right-HI animals presented greater reduction of ipsilateral striatum area, with females being more affected. Interestingly, males showed greater hippocampal volume. These results show that task performance and cerebral damage extension are lateralized and sex-dependent, and that the right hemisphere, irrespective of sex, is more vulnerable to neonatal cerebral hypoxia-ischemia.

Lateralized recall of olfactory memory in honeybees was tested, following conditioning of the proboscis extension reflex (PER), at 1 or 6 h after training. After training with lemon (+)/vanilla (−) or cineol (+)/eugenol (−) recall at 1 h was better when the odour was presented to the right side of the bee than when it was presented to the left side. In contrast, recall at 6 h was better when the odour was presented to the left than to the right side. This confirmed previous evidence of shorter-term recall via the right antenna and long-term memory recall via the left antenna. However, when trained with either a familiar appetitive odour (rose) as a negative stimulus, or with a naturally aversive odour (isoamyl acetate, IAA) as a positive stimulus, bees showed suppression of the response from both the right and the left side at 1 h after training (likely due to retroactive inhibition) and at 6 h responded to both odours on both sides. We argued that at 6 h, when access to memory has completed the shift from the right to the left side, memory of these familiar odours in the left side of the brain would be present as both positive (rose)/negative (IAA) (as a result of long-term memory either biologically encoded or acquired well before testing) and negative (rose)/positive (IAA) (as a result of the long-term memory of training) stimuli, thus producing response competition. As a direct test of this hypothesis, bees were first trained with unfamiliar lemon (+)/vanilla (−) and then (16 h later) re-trained with vanilla (+)/lemon (−); as predicted, 6 h after re-training bees responded to both odours on both the left and right side.

Patients who had undergone a unilateral trans-sylvian selective amygdalohippocampectomy as treatment for chronic intractable epilepsy were tested in a virtual Morris Water Maze (MWM) task where they were required to locate a hidden platform as a measure of spatial learning. These individuals' performance on spatial tasks was compared to age-matched healthy controls and drug-matched healthy controls. Training occurred in two different maze environments, one with conventional cues such as windows and doors, and another with abstract cues, such as colours and patterns. Participants searched for a hidden platform in the virtual pool, guided by either the conventional or abstract cues. There was a significant impairment in the surgery group compared to the control groups in all environments, however in the abstract environment only the patients with right-sided lesions were significantly worse than the controls. There was no difference between the groups on a control egocentric navigation task. These results suggest that people who have had right-sided surgery are impaired in spatial tasks, and that the level of impairment on the spatial task may be dependent on the characteristics of the cues such as how easily the cues are verbalised. These results support the notion of the functional lateralization of specific elements of spatial memory and functional lateralization, and may shed light on previous inconsistencies in this area of research.

During feeding behaviour, dogs were suddenly presented with 2D stimuli depicting the silhouette of a dog, a cat or a snake simultaneously into the left and right visual hemifields. A bias to turn the head towards the left rather than the right side was observed with the cat and snake stimulus but not with the dog stimulus. Latencies to react following stimulus presentation were lower for left than for right head turning, whereas times needed to resume feeding behaviour were higher after left rather than after right head turning. When stimuli were presented only to the left or right visual hemifields, dogs proved to be more responsive to left side presentation, irrespective of the type of stimulus. However, cat and snake stimuli produced shorter latencies to react and longer times to resume feeding following left rather than right monocular visual hemifield presentation. Results demonstrate striking lateralization in dogs, with the right side of the brain more responsive to threatening and alarming stimuli. Possible implications for animal welfare are discussed.

Ectotherms have been shown being lateralized as well as mammals and birds. This is particularly evident in visual lateralization, i.e. the different use of the eyes, leading to use a specific eye to observe specific kind of stimuli and to process them with the correspondent contralateral hemisphere. Several lower vertebrates are facilitated in this from the lateral position of the eyes, enabling them to carry out more tasks simultaneously, controlled by different eyes and relative hemispheres. Predatory responses seem usually mediated by the right eye/left hemisphere in fishes, amphibians and some sauropsids, but there are no strong evidences of this in lizards. Eighteen wild males of the Common wall lizard Podarcis muralis were tested individually in captivity to ascertain whether they are lateralized to look at prey with a specific eye. The lizards were gently induced entering a 30-cm long central arm of a T-maze which led to a 44.5-cm long arm cross-arm at whose extremities there were two identical prey, Tenebrio molitor larvae, familiar to the lizards. We recorded what direction the lizards chose to reach the prey and the frequency and duration of head turning, indicative of looking either prey with the left or the right eye. We found that individuals show being lateralized at individual level. The preferred direction taken to reach the prey is the right for the majority of those (4 of 5) showing an evident preference, indicating also a possible form of laterality at population level. In addition, lizards maintained the same head side of the direction taken turned for more time towards the prey than the opposite head side, revealing an eye preference for observing this kind of cue. Our study demonstrates how males of Podarcis muralis have a visual lateralization to capture prey. Furthermore, it is another support to the hypothesis of vertebrate lateralization derivation from a common ancestor.

Successful motor behavior depends on optimal information processing and planning. In the present study, the neural response of the motor system to conflict of visuomotor discrepancy (mirror-reversed vision) and complexity (task difficulty and hand laterality) was evaluated during the performance of bimanual actions. EEG coherence, expressing interregional communication, showed that conflict of visuomotor incongruence resulted in activation changes in both hemispheres, whereas conflict of task complexity evoked adjustments primarily in the left hemisphere. Furthermore, interhemispheric coherence was modified due to both forms of conflict. This demonstrates that conflict demands elicit distinct processes across the motor system. The data further illustrate that functional couplings are dynamically modulated during bimanual behaviour, suggesting that interactions between brain regions provide higher-order links for information processing and integration in view of complex motor skills.

A growing body of literature suggests that sex differences exist in both rodents and humans in terms of the central processing of stress and emotion, and an important factor in this regard may involve differential hemispheric specialization. The amygdala has been shown to be functionally asymmetrical in both rats and humans and its involvement in stress and emotion processing is well documented. Given that amygdala function is importantly modulated by dopamine (DA), the present study examined the effects of left vs. right unilateral DAergic depletion targeting the basolateral amygdala in male and female rats. We examined behaviour in the elevated plus maze over two consecutive days, and plasma adrenocorticotropic hormone (ACTH) levels in response to a separate 30min restraint stress. Overall, females showed significantly more exploration of open arms of the plus maze than males, while not differing in general activity reflected in closed arm entries. Significant SexxHemisphere interactions were observed for all measures related to open arm exploration, as right amygdala DA depletion produced an anxiolytic effect in males, increasing open arm exploration, but reduced this behaviour in females. Moreover, open arm exploration was greatly reduced on the second maze exposure in males, but unchanged in females. A SexxHemisphere interaction was also found for plasma ACTH levels. It is suggested that the role of amygdala DA on stress-related behaviour and physiology reflects inherent differences in amygdala function dependent on both sex and hemisphere.

Cerebral lateralization, the partitioning of cognitive tasks to one cerebral hemisphere, is a widespread phenomenon among vertebrates. Despite this diversity, every species studied to date shows substantial individual variation in the strength of lateralization. The neural basis of this trait is unclear, although asymmetries in cerebral structures have been investigated for over a century. The habenular nuclei, for example, have been shown to present striking neuroanatomical and/or neurochemical asymmetries in species ranging from jawless fish to mammals. In teleost fish, these nuclei are relatively symmetrical in most species. Those teleosts that do have asymmetrical habenular nuclei, show varying patterns of asymmetry in different species. Here we investigate the relationship between individual variation of asymmetry in the habenula of a South American cichlid fish, Geophagus brasiliensis, and behaviour in a commonly used test for visual laterality in fish, the detour task. We show that the strength of asymmetry in the habenula is correlated with strength of behavioural lateralization in the detour task. Both the strength and direction of habenular asymmetry are correlated with individual differences in growth rate. We suggest that this relationship results from processes linking growth rate and sexual differentiation to frequency-dependent variation in life-history strategies. To our knowledge, this is the first study to demonstrate a relationship at the individual level between neural asymmetry and lateralized behaviour in a fish.

Language dominance has repeatedly been demonstrated by means of functional transcranial Doppler (fTCD) ultrasonography. However, one strongly lateralizing paradigm might be an overestimation of overall language lateralization. We hypothesized that significant differences between hemodynamic patterns could be shown by using multiple language functions (such as productive and receptive tasks). Particularly, we expected more left-hemispheric activity in frontal and productive tasks and we expected less clear left-hemispheric activity in temporal regions and receptive tasks. Thirty healthy volunteers were included in the study. The lateralization index (LI) was measured with transcranial Doppler ultrasonography for each subject during four language tasks: a cued word generation task (WF), a sentence construction task (SENT), a reading task (READ), and a semantic decision task (SEMANT). Left-hemispheric dominance was found in 90%, 80%, 73.3% and 66.7% of the subjects for respectively WF, SENT, READ and SEMANT. A repeated measures analysis of variance revealed significant differences in LIs between the different language tasks (p.01). Our results showed that productive and syntactic tasks (WF and SENT) lateralize stronger than receptive tasks (READ and SEMANT). The use of a variety of language tasks in the determination of language lateralization therefore appears to be essential in providing a more comprehensive view of language functioning.

Previous studies suggest that laterality of the viscera, morphological asymmetries of the brain, and lateralization of cognitive functions have a common genetic origin. To test this hypothesis, we conducted an artificial selection experiment for behavioural lateralization of eye use in two strains (TL and GT) of zebrafish ( Danio rerio ), maintaining one selected line in each strain for five generations. In addition, we investigated, using molecular markers, whether there was any correlation among directionality in eye preference, diencephalic left–right asymmetries in the brain and positioning of the viscera. After one generation of selection, the right- and left-eye lines of both strains showed a significant difference in the behavioural trait. This difference was maintained for all the five generations even though we observed a progressive decline in the response to artificial selection in subsequent generations for both strains. Overall, anatomical evidence suggests that selection for right-eye use significantly increased the frequency of reversed asymmetry in the epithalamus while selection for left-eye use decreased it. However, the response was irregular since not all samples conformed to this pattern. The association between the direction of behavioural selection and pancreas position was less clear-cut, although the concordance between visceral and brain asymmetries exceeded 90% in both strains.