Brain asymmetry: both sides of the story

Biological systems demonstrate asymmetry, while lateralization has been observed from humans to lower animals structurally, functionally and behaviorally. This may be derived from evolutionary, genetic, developmental, epigenetic and pathologic factors. However, brain structure and function is complex, and macroscopic or microscopic asymmetries are hard to discern from random fluctuations. In this article, we discuss brain laterality and lateralization, beginning with a brief review of the literature on brain structural and functional asymmetries. We conclude with methods to detect and quantify asymmetry, focusing on neuroproteomics, for retrieval of protein-expression patterns, as a method of diagnosis and treatment monitoring. We suggest inter-hemispheric differential proteomics as a valid method to assess the experimental and biological variations in the healthy brain, and neurologic and neuropsychiatric disorders.     

Brain asymmetry: both sides of the story

Biological systems demonstrate asymmetry, while lateralization has been observed from humans to lower animals structurally, functionally and behaviorally. This may be derived from evolutionary, genetic, developmental, epigenetic and pathologic factors. However, brain structure and function is complex, and macroscopic or microscopic asymmetries are hard to discern from random fluctuations. In this article, we discuss brain laterality and lateralization, beginning with a brief review of the literature on brain structural and functional asymmetries. We conclude with methods to detect and quantify asymmetry, focusing on neuroproteomics, for retrieval of protein-expression patterns, as a method of diagnosis and treatment monitoring. We suggest inter-hemispheric differential proteomics as a valid method to assess the experimental and biological variations in the healthy brain, and neurologic and neuropsychiatric disorders.     

fsi zebrafish show concordant reversal of laterality of viscera, neuroanatomy, and a subset of behavioral responses

Asymmetries in CNS neuroanatomy are assumed to underlie the widespread cognitive and behavioral asymmetries in vertebrates. Studies in humans have shown that the laterality of some cognitive asymmetries is independent of the laterality of the viscera; discrete mechanisms may therefore regulate visceral and neural lateralization. However, through analysis of visceral, neuroanatomical, and behavioral asymmetries in the frequent-situs-inversus (fsi) line of zebrafish, we show that the principal left-right body asymmetries are coupled to certain brain asymmetries and lateralized behaviors. fsi fish with asymmetry defects show concordant reversal of heart, gut, and neuroanatomical asymmetries in the diencephalon. Moreover, the neuroanatomical reversals in reversed fsi fish correlate with reversal of some behavioral responses in both fry and adult fsi fish. Surprisingly, two behavioral asymmetries do not reverse, suggesting that at least two separable mechanisms must influence functional lateralization in the CNS. Partial reversal of CNS asymmetries may generate new behavioral phenotypes; supporting this idea, reversed fsi fry differ markedly from their normally lateralized siblings in their behavioral response to a novel visual feature. Revealing a link between visceral and brain asymmetry and lateralized behavior, our studies help to explain the complexity of the relationship between the lateralities of visceral and neural asymmetries.     

Handedness of a Motor Program in C. elegans Is Independent of Left-Right Body Asymmetry

Complex animals display bilaterally asymmetric motor behavior, or “motor handedness,” often revealed by preferential use of limbs on one side. For example, use of right limbs is dominant in a strong majority of humans. While the mechanisms that establish bilateral asymmetry in motor function are unknown in humans, they appear to be distinct from those for other handedness asymmetries, including bilateral visceral organ asymmetry, brain laterality, and ocular dominance. We report here that a simple, genetically homogeneous animal comprised of only ∼1000 somatic cells, the nematode C. elegans, also shows a distinct motor handedness preference: on a population basis, males show a pronounced right-hand turning bias during mating. The handedness bias persists through much of adult lifespan, suggesting that, as in more complex animals, it is an intrinsic trait of each individual, which can differ from the population mean. Our observations imply that the laterality of motor handedness preference in C. elegans is driven by epigenetic factors rather than by genetic variation. The preference for right-hand turns is also seen in animals with mirror-reversed anatomical handedness and is not attributable to stochastic asymmetric loss of male sensory rays that occurs by programmed cell death. As with C. elegans, we also observed a substantial handedness bias, though not necessarily the same preference in direction, in several gonochoristic Caenorhabditis species. These findings indicate that the independence of bilaterally asymmetric motor dominance from overall anatomical asymmetry, and a population-level tendency away from ambidexterity, occur even in simple invertebrates, suggesting that these may be common features of bilaterian metazoans.     

Mechanisms of directional asymmetry in the zebrafish epithalamus

The epithalamus of zebrafish presents the best-studied case of directional asymmetry in the vertebrate brain. Epithalamic asymmetries are coupled to visceral asymmetry and include left-sided migration of a single midline structure (the parapineal organ) and asymmetric differentiation of paired bilateral nuclei (habenulae). The mechanisms underlying the establishment of epithalamic asymmetry involve the interplay between anti-symmetry and laterality signals to guide asymmetric parapineal migration. This event triggers the amplification of habenular asymmetries and the subsequent organisation of lateralised circuits in the interpeduncular nucleus. This review will summarise our current understanding on these processes and propose a sequential modular organisation of the events controlling the development of asymmetry along the parapineal–habenular–interpeduncular axis.     

Asymmetric nodal signaling in the zebrafish diencephalon positions the pineal organ

The vertebrate brain develops from a bilaterally symmetric neural tube but later displays profound anatomical and functional asymmetries. Despite considerable progress in deciphering mechanisms of visceral organ laterality, the genetic pathways regulating brain asymmetries are unknown. In zebrafish, genes implicated in laterality of the viscera (cyclops/nodal, antivin/lefty and pitx2) are coexpressed on the left side of the embryonic dorsal diencephalon, within a region corresponding to the presumptive epiphysis or pineal organ. Asymmetric gene expression in the brain requires an intact midline and Nodal-related factors. RNA-mediated rescue of mutants defective in Nodal signaling corrects tissue patterning at gastrulation, but fails to restore left-sided gene expression in the diencephalon. Such embryos develop into viable adults with seemingly normal brain morphology. However, the pineal organ, which typically emanates at a left-to-medial site from the dorsal diencephalic roof, becomes displaced in position. Thus, a conserved signaling pathway regulating visceral laterality also underlies an anatomical asymmetry of the zebrafish forebrain.     

Building an asymmetric brain: Development of the zebrafish epithalamus

The human brain exhibits notable asymmetries. Little is known about these symmetry deviations; however scientists are beginning to understand them by employing the lateralized zebrafish epithalamus as a model. The zebrafish epithalamus consists of the pineal and parapineal organs and paired habenular nuclei located bilateral to the pineal complex. While zebrafish pineal and parapineal organs arise from a common population of cells, parapineal cells undergo a separate program that allows them to migrate left of the pineal anlage. Studying the processes that lead to brain laterality in zebrafish will allow a better understanding of how human brain laterality is established.     

The parapineal mediates left-right asymmetry in the zebrafish diencephalon

The dorsal diencephalon (or epithalamus) of larval zebrafish displays distinct left-right asymmetries. The pineal complex consists of the pineal organ anlage and an unpaired, left-sided accessory organ - the parapineal. The neighboring brain nuclei, the left and right dorsal habenulae, show consistent differences in their size, density of neuropil and gene expression. Mutational analyses demonstrate a correlation between the left-right position of the parapineal and the laterality of the habenular nuclei. We show that selective ablation of the parapineal organ results in the loss of habenular asymmetry. The left-sided parapineal therefore influences the left-right identity of adjacent brain nuclei, indicating that laterality of the dorsal diencephalon arises in a step-wise fashion.     

Left-right asymmetry defect in the hippocampal circuitry impairs spatial learning and working memory in iv mice

Although left-right (L-R) asymmetry is a fundamental feature of higher-order brain function, little is known about how asymmetry defects of the brain affect animal behavior. Previously, we identified structural and functional asymmetries in the circuitry of the mouse hippocampus resulting from the asymmetrical distribution of NMDA receptor GluR ε2 (NR2B) subunits. We further examined the ε2 asymmetry in the inversus viscerum (iv) mouse, which has randomized laterality of internal organs, and found that the iv mouse hippocampus exhibits right isomerism (bilateral right-sidedness) in the synaptic distribution of the ε2 subunit, irrespective of the laterality of visceral organs. To investigate the effects of hippocampal laterality defects on higher-order brain functions, we examined the capacity of reference and working memories of iv mice using a dry maze and a delayed nonmatching-to-position (DNMTP) task, respectively. The iv mice improved dry maze performance more slowly than control mice during acquisition, whereas the asymptotic level of performance was similar between the two groups. In the DNMTP task, the iv mice showed poorer accuracy than control mice as the retention interval became longer. These results suggest that the L-R asymmetry of hippocampal circuitry is critical for the acquisition of reference memory and the retention of working memory.     

Left and right in the amphibian world: which way to develop and where to turn?

The last decade has seen a dramatic increase in studies on the development, function and evolution of asymmetries in vertebrates, including amphibians. Here we discuss current knowledge of behavioral and anatomical asymmetries in amphibians. Behavioral laterality in the response of both adult and larval anurans to presumed predators and competitors is strong and may be related, respectively, to laterality in the telencephalon of adults and the Mauthner neurons of tadpoles. These behavior lateralities, however, do not seem to correlate with visceral asymmetries in the same animals. We briefly compare what is known about the evolution and development of asymmetry in the structure and function of amphibians with what is known about asymmetries in other chordate and non-chordate groups. Available data suggest that the majority of asymmetries in amphibians fall into two independent groups: (1) related to situs viscerum and (2) of a neurobehavioral nature. We find little evidence linking these two groups, which implies different developmental regulatory pathways and independent evolutionary histories for visceral and telencephalic lateralizations. Studies of animals other than standard model species are essential to test hypotheses about the evolution of laterality in amphibians and other chordates.     

Asymmetrical subcortical plasticity entails cognitive progression in older individuals

Structural brain asymmetries have been associated with cognition. However, it is not known to what extent neuropsychological parameters and structural laterality covary with aging. Seventy‐five subjects drawn from a larger normal aging cohort were evaluated in terms of MRI and neuropsychological parameters at two moments (M1 and M2), 18 months apart. In this time frame, asymmetry as measured by structural laterality index (ΔLI) was stable regarding both direction and magnitude in all areas. However, a significantly higher dispersion for this variation was observed in subcortical over cortical areas. Subjects with extreme increase in rightward lateralization of the caudate revealed increased M1 to M2 Stroop interference scores, but also a worsening of general cognition (MMSE). In contrast, subjects showing extreme increase in leftward lateralization of the thalamus presented higher increase in Stroop interference scores. In conclusion, while a decline in cognitive function was observed in the entire sample, regional brain asymmetries were relatively stable. Neuropsychological trajectories were associated with laterality changes in subcortical regions.     

Cortical asymmetries at different spatial hierarchies relate to phonological processing ability

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

Local tissue interactions across the dorsal midline of the forebrain establish CNS laterality

The mechanisms that establish behavioral, cognitive, and neuroanatomical asymmetries are poorly understood. In this study, we analyze the events that regulate development of asymmetric nuclei in the dorsal forebrain. The unilateral parapineal organ has a bilateral origin, and some parapineal precursors migrate across the midline to form this left-sided nucleus. The parapineal subsequently innervates the left habenula, which derives from ventral epithalamic cells adjacent to the parapineal precursors. Ablation of cells in the left ventral epithalamus can reverse laterality in wild-type embryos and impose the direction of CNS asymmetry in embryos in which laterality is usually randomized. Unilateral modulation of Nodal activity by Lefty1 can also impose the direction of CNS laterality in embryos with bilateral expression of Nodal pathway genes. From these data, we propose that laterality is determined by a competitive interaction between the left and right epithalamus and that Nodal signaling biases the outcome of this competition.     

Three ways to make two sides: genetic models of asymmetric nervous system development

Many anatomical and functional features of nervous systems are asymmetric about the left-right axis. These asymmetries can exhibit either random or invariant laterality at the population level. Recent studies in fish and worms provide insight into the developmental mechanisms used to create both types of asymmetry. These studies reveal diverse and molecularly complex strategies for developing asymmetric nervous systems.     

Lateralized tool use in wild New Caledonian crows

Many vertebrate species exhibit sensory and motor asymmetries. Laterality studies of tool use have focused on primates, where hemispheric asymmetries, manifested behaviourally in hand preferences, are thought to be associated with complex motor tasks. Here we report strong individual lateralization for tool use in birds. New Caledonian crows, Corvus moneduloides, hold stick tools with their bills while foraging, often with the nonworking end laterally positioned on one side of the head and the working end possibly positioned in the binocular field. We observed four wild crows to determine whether tools were consistently held on one side. All crows showed a significant preference (two right, two left). This preference was independent of any asymmetry in tool manufacture and held for artificial holes similarly accessible for tools held on either side. This is the first demonstration of lateralized tool use in a nonprimate. In addition, all 173 tools used unilaterally were held only on a crow's preferred side. Such pronounced individual laterality for tool use in natural conditions has previously been reported only in humans and chimpanzees.     

Small heat shock proteins are necessary for heart migration and laterality determination in zebrafish

Small heat shock proteins (sHsps) regulate cellular functions not only under stress, but also during normal development, when they are expressed in organ-specific patterns. Here we demonstrate that two small heat shock proteins expressed in embryonic zebrafish heart, hspb7 and hspb12, have roles in the development of left–right asymmetry. In zebrafish, laterality is determined by the motility of cilia in Kupffer's vesicle (KV), where hspb7 is expressed; knockdown of hspb7 causes laterality defects by disrupting the motility of these cilia. In embryos with reduced hspb7, the axonemes of KV cilia have a 9+0 structure, while control embyros have a predominately 9+2 structure. Reduction of either hspb7 or hspb12 alters the expression pattern of genes that propagate the signals that establish left–right asymmetry: the nodal-related gene southpaw (spaw) in the lateral plate mesoderm, and its downstream targets pitx2, lefty1 and lefty2. Partial depletion of hspb7 causes concordant heart, brain and visceral laterality defects, indicating that loss of KV cilia motility leads to coordinated but randomized laterality. Reducing hspb12 leads to similar alterations in the expression of downstream laterality genes, but at a lower penetrance. Simultaneous reduction of hspb7 and hspb12 randomizes heart, brain and visceral laterality, suggesting that these two genes have partially redundant functions in the establishment of left–right asymmetry. In addition, both hspb7 and hspb12 are expressed in the precardiac mesoderm and in the yolk syncytial layer, which supports the migration and fusion of mesodermal cardiac precursors. In embryos in which the reduction of hspb7 or hspb12 was limited to the yolk, migration defects predominated, suggesting that the yolk expression of these genes rather than heart expression is responsible for the migration defects.     

Limits to length asymmetry detection in starlings: implications for biological signalling

Fluctuating asymmetry has received considerable recent attention in evolutionary biology as these small developmental asymmetries can be related to biological fitness and, hence, could be used as a visual cue (or signal) of quality among individuals. The ability of signal receivers to detect and respond to small asymmetries is a fundamental assumption of the symmetry-signalling hypothesis, but has not been experimentally investigated. In this study I have investigated the perceptual threshold to detect and respond to paired-bar length asymmetry in a common bird, the European starling Sturnus vulgaris, by means of operant-learning techniques. The threshold indicates how large the length asymmetry must be to be reliably discriminated from symmetry; birds could not detect an asymmetry of 1.25%. In nature, many asymmetries can be smaller than 1.25%, hence this initial study suggests that caution should be used when trying to invoke symmetry-signalling in natural populations.     

Tool behavior and the origins of laterality

The cerebral cortex of modern humans is exceptional in that it is characterized by certain functional asymmetries for which no clear homologue is known in the non-human primates. These asymmetries consist at least in part in the presence in the left cerebral hemisphere of certain mechanisms which mediate language as well as skilled manual activity. Right-handedness is the most obvious overt manifestation of this cerebral asymmetry. It is argued in this paper that the lateralized representation of these mechanisms is an evolutionary consequence of the requirement for asymmetric employment of the forelimbs in the making and using of tools during hominid evolution. The adaptiveness of such an asymmetric arrangement is shown to follow from a few assumptions pertaining to brain organization and evolution. The colateralization of language mechanisms in modern humans to the left hemisphere is held to be a consequence of the coupling of these linguistic mechanisms to the motoric mechanisms already lateralized to the left hemisphere at an earlier point in hominid evolution. Finally, it is argued that this explanation of the evolution of laterality is more parsimonious in relation to known facts about human evolution than competing hypotheses.