Neuroanatomical asymmetries in non-human species

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.

Electrophysiological and hemodynamic mismatch responses in rats listening to human speech syllables

Speech is a complex auditory stimulus which is processed according to several time-scales. Whereas consonant discrimination is required to resolve rapid acoustic events, voice perception relies on slower cues. Humans, right from preterm ages, are particularly efficient to encode temporal cues. To compare the capacities of preterms to those observed in other mammals, we tested anesthetized adult rats by using exactly the same paradigm as that used in preterm neonates. We simultaneously recorded neural (using ECoG) and hemodynamic responses (using fNIRS) to series of human speech syllables and investigated the brain response to a change of consonant (ba vs. ga) and to a change of voice (male vs. female). Both methods revealed concordant results, although ECoG measures were more sensitive than fNIRS. Responses to syllables were bilateral, but with marked right-hemispheric lateralization. Responses to voice changes were observed with both methods, while only ECoG was sensitive to consonant changes. These results suggest that rats more effectively processed the speech envelope than fine temporal cues in contrast with human preterm neonates, in whom the opposite effects were observed. Cross-species comparisons constitute a very valuable tool to define the singularities of the human brain and species-specific bias that may help human infants to learn their native language.

Zebrafish and medaka: model organisms for a comparative developmental approach of brain asymmetry

Comparison between related species is a successful approach to uncover conserved and divergent principles of development. Here, we studied the pattern of epithalamic asymmetry in zebrafish (Danio rerio) and medaka (Oryzias latipes), two related teleost species with 115-200 Myr of independent evolution. We found that these species share a strikingly conserved overall pattern of asymmetry in the parapineal-habenular-interpeduncular system. Nodal signalling exhibits comparable spatial and temporal asymmetric expressions in the presumptive epithalamus preceding the development of morphological asymmetries. Neuroanatomical asymmetries consist of left-sided asymmetric positioning and connectivity of the parapineal organ, enlargement of neuropil in the left habenula compared with the right habenula and segregation of left-right habenular efferents along the dorsoventral axis of the interpeduncular nucleus. Despite the overall conservation of asymmetry, we observed heterotopic changes in the topology of parapineal efferent connectivity, heterochronic shifts in the timing of developmental events underlying the establishment of asymmetry and divergent degrees of canalization of embryo laterality. We offer new tools for developmental time comparison among species and propose, for each of these transformations, novel hypotheses of ontogenic mechanisms that explain interspecies variations that can be tested experimentally. Together, these findings highlight the usefulness of zebrafish and medaka as comparative models to study the developmental mechanisms of epithalamic asymmetry in vertebrates.

Brain development, infant communication, and empathy disorders: Intrinsic factors in child mental health

Disorders of emotion, communication, and learning in early childhood are considered in light of evidence on human brain growth from embryo stages. We cite microbehavioral evidence indicating that infants are born able to express the internal activity of their brains, including dynamic “motive states” that drive learning. Infant expressions stimulate the development of imitative and reciprocal relations with corresponding dynamic brain states of caregivers. The infant's mind must have an “innate self-with-other representation” of the inter-mind correspondence and reciprocity of feelings that can be generated with an adult.

Primordial motive systems appear in subcortical and limbic systems of the embryo before the cerebral cortex. These are presumed to continue to guide the growth of a child's brain after birth. We propose that an “intrinsic motive formation” is assembled prenatally and is ready at birth to share emotion with caregivers for regulation of the child's cortical development, on which cultural cognition and learning depend.

The intrinsic potentiality for “intersubjectivity” can be disorganized if the epigenetic program for the infant's brain fails. Indeed, many psychological disorders of childhood can be traced to faults in early stages of brain development when core motive systems form.

Asymmetries of cerebral neuroanatomy

The mammalian cerebral cortex is asymmetrical. One hemisphere does not contain cortical areas or architectonic patterns, histological features, ultrastructural characteristics, or connectivities of the neurons that are not present in the other: homologous areas on the two sides may differ only in size. Asymmetry has directionality: two-thirds of human brains have plana temporale that are larger on the left. Conversely, roughly the same number of non-human brains show asymmetry in one direction as in the other. Asymmetry has magnitude: some brains show a large asymmetry, others show no asymmetry in a given area. Symmetrical areas are larger than their asymmetrical counterparts, which reflects fewer neurons in the latter. Indirect evidence points to variable asymmetry in the germinal zones in the production of symmetrical or asymmetrical cortical areas. These areas differ in their patterns of callosal connections. Fewer connections are seen in the asymmetrical cases, paralleling the smaller number of neurons. The symmetrical cases contain connections that are more widely distributed. These findings of different numbers of neurons and different proportions of callosal connections suggest that symmetrical and asymmetrical cortical areas may have different functional properties.

The dorsal diencephalic conduction system of zebrafish as a model of vertebrate brain lateralisation

Lateralisation is an attractive and intriguing feature of the vertebrate CNS studied for decades in the different disciplines of the neurosciences. Due to the complexity of the phenomena and intrinsic limitations of the approaches used to date, it has been difficult to establish useful links between the different, and usually distant, levels of lateralisation e.g. between genetics, morphology, physiology and behaviour. Recently, the dorsal diencephalon of the teleost zebrafish has emerged as a valuable model to begin addressing this issue and as a result unravel the role of vertebrate CNS lateralisation. Zebrafish is a well-established genetic system that allows a 'bottom up' ('gene to behaviour') approach to the study of lateralisation. In fact, it is the single vertebrate system to date in which asymmetric gene expression in the brain has been directly linked to asymmetric morphology. Zebrafish offers several experimental advantages that allow the study of brain lateralisation using a wide range of experimental tools, from study of gene function through in vivo analysis of morphology and physiology to behavioural assessments. Altogether, these features will allow the establishment of operational links between lower (genetics and morphology) and upper (physiology and behaviour) levels of brain lateralisation.

Do cochlear nuclei contribute to auditory lateralization? A stereological evaluation of neuron numbers

The aim of this study was to estimate the total number of cells in the subdivisions of the cochlear nucleus (CN) of the rat with unbiased stereological methods. The total number of neurons was determined in both the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN) to compare the right and left sides. The total cell numbers were 15,280 in the left VCN, 15,400 in the right VCN, 10,260 in the left DCN, and 10,860 in the right DCN. Comparison of the right and left major subdivisions of the CN of the rat showed that there was no significant difference between the right and left sides of the rat CN. This result indicates that the CNs do not contribute to auditory lateralization in the rat in regard to cell numbers.

Lateral asymmetries in infancy: implications for the development of the hemispheres

Cerebral asymmetry of cognitive processing of stimulus information is commonly viewed as a neocortical phenomenon. However, a number of lines of evidence give innate asymmetry of brainstem motivating systems, which anticipate experience, a key role. Spontaneous asymmetries of gesture and emotion can be observed in infants, who entirely lack language and visuo-constructive skills. Motives for communication in early life may direct subsequent development of complementary cognitive systems in left and right hemispheres. In split-brain monkeys, lateralized motive sets, intentions for manipulation by one hand, can determine which hemisphere will see and learn. Evolutionary antecedents of cerebral asymmetry appear to affect motivation, social signalling and bimanual coordination, with secondary effect in perceptual processing and learning. The hemispheres of adult humans differ in links with neurochemical system that regulate motor initiatives, exploration and attention, and the approach/withdrawal balance in social encounters. Asymmetries in emotional and communicative behaviour in infancy support evidence that an Intrinsic Motive Formation emerging in the embryo human brain stem regulates asymmetries in development and in functioning of the cerebral cortex.

The asymmetry of the habenular nuclei of female and male frogs in spring and in winter

Male and female frog brains stained according to the Nissl method and cut transversely, 15 microns thick, at the level of the habenular nuclei, were investigated in spring and winter. Right and left habenular nuclei were examined. The volume and the standard deviations were calculated in each portion of the habenular nuclei investigated. The frog habenula consists of a single cell group: one on the right and two on the left side of the brain which differ, among themselves, both in the volumes of the neuropil and of the cellular ring. Functional corollaries of this striking asymmetry are still unknown. However, female and male frogs' habenular nuclei are longer and larger in spring--when frogs are sexually active--than in winter. We propose that structural brain asymmetries may be sex linked and may be induced by steroid hormonal effect in the central nervous system.

Cerebral lateralization as a source of interindividual differences in behavior

Cerebral laterality can no longer be considered an exclusively human trait, as over the last 15 years there has been an emergence of data to suggest that animal brains are also lateralized. Morphologic, chemical and behavioral indices of brain asymmetry in the rodent have been reported, and it is suggested that variations in the magnitude and direction of these indices are determined by a complex interaction of genetic, hormonal and experiential factors. Interindividual differences in cerebral laterality have been shown to covary with, or predict, individual differences in spatial behavior and stress reactivity, as well as susceptibility to stress pathology and drug sensitivity. Such findings suggest that it is possible to study individual differences in lateralized brain function through the use of animal models.

Interhemispheric connections differ between symmetrical and asymmetrical brain regions

Coronal sections from the brains of male Wistar rats that underwent corpus-callosectomy in adulthood were stained with Cresyl Violet for Nissl substance or by the Fink-Heimer method for terminal axonal degeneration. Measurements of volumetric asymmetry of neocortical region SM-I were made, and the per cent of terminal degeneration computed. As in previous studies, there was a negative correlation between asymmetry coefficient and total (right plus left) architectonic volume, indicating that symmetrical brain regions are larger than the average of the corresponding regions in asymmetrical brains. It was also found that as volumetric asymmetry increased, the per cent of axonal termination decreased, partly as a result of a decrease in the number of patches of callosal axonal termination. These results are interpreted in the light of what is known about the ontogenesis of callosal connectivity, and mechanisms for the development of architectonic asymmetry in the cerebral cortex are postulated.

Histological asymmetry in the primary visual cortex of the rat: implications for mechanisms of cerebral asymmetry

The present study was designed to specify the contributions of various histological parameters to hemispheric asymmetry of architectonic areas. It was found that the primary visual cortex of the rat is asymmetrical in volume, and that the asymmetry reflects side differences in the number of neurons. The implications of this finding for the understanding of mechanisms involved in the production of brain asymmetries are discussed.

Olfactory asymmetry in the rat brain

Based on cytoarchitecture, myeloarchitecture, and physiological observations in the literature, two concentric strata are identified in the olfactory bulb of the albino rat. Stimulated by a report of physiologic asymmetry between the left and right bulb of the rodent brain, we parcelled the inner and outer strata of the olfactory bulbs of 16 albino rats and measured the volume of these olfactory subdivisions. The volume of the entire olfactory bulb was found to be significantly greater in the right hemisphere. This volume asymmetry was the result of a significantly larger right outer stratum. No significant asymmetry was demonstrated for the inner stratum. These findings are discussed in the light of some physiologic and anatomic properties of the olfactory bulb.

Hypotheses on mnemonic information processing by the brain

From a neuropsychological point of view, hypotheses are offered on the possible action of the brain in the processing of mnemonic information for long-term storage (or for retrieval of long-term stored information). It is argued that strict relations between damage of circumscribed brain structures and amnesia, as they have been suggested in recent case reports, are questionable for several reasons: Firstly, the involved regions differ between cases; secondly, there is some counter-evidence from other cases in which similar neuronal damage failed to result in lasting amnesic disturbances; thirdly, it is hypothesized that even from circumscribed brain damage it is not justifiable to conclude that the lesioned structure is solely or principally responsible for the observed mnemonic changes, as the brain acts in an integrative way, that is, on the basis of a wide-spread network of neuronal information processing. On the basis of these and related arguments, hypotheses and models on mnemonic information processing in the intact and in the damaged brain are derived. With these hypotheses even the frequent observation of interindividual differences in mnemonic information processing finds a possible explanation which is in conformity to known anatomical circuits and connections and to principles of neuronal coding.

Modulation of turning preferences by learning

Rats were trained, using water reinforcement, to turn in circles (rotation) during 1 h daily test sessions. Different groups of rats were reinforced for turning either in the same or opposite direction as that elicited previously by D-amphetamine. All rats (n = 14) trained in the 'same' direction readily acquired the task whereas only 13 of 33 rats trained in the 'opposite' direction showed evidence of learning. Two days after cessation of training, the effect of D-amphetamine was greater in rats trained in the 'same' direction and decreased or reversed in rats successfully trained in the 'opposite' direction - these changes were transient, mostly disappearing a week later.

Effect of anterior-posterior lesion location on the asymmetrical behavioral and biochemical response to cortical suction ablations in the rat

Small cortical suction ablations were produced at one of several stereotaxically located sites along the anterior-posterior axis of the right or left hemisphere in the rat. Analysis of variance showed a highly significant effect of lesion location in the right hemisphere on locomotor activity. The most anterior lesions produced both the most hyperactivity and also the greatest reductions in the concentrations of norepinephrine in the ipsilateral and contralateral cortex and locus coeruleus. These results suggest that the effect of cortical lesions on spontaneous activity may be graded across the right hemisphere and that the anatomy of certain neurotransmitter pathways in the cortex may help to explain both the biochemical and behavioral findings.

Cocaine-induced rotation: sex-dependent differences between left- and right-sided rats

Cocaine elicited dose-related rotation (circling) in naïve rats. The maximum effect was greater than observed previously with other drugs. Overall, females were more sensitive to cocaine than males. However, right-biased females were more sensitive than left-biased females, whereas left-biased males were more sensitive than right-biased males. The results suggest that sex-dependent differences in brain asymmetry may be an important determinant of cocaine sensitivity.

Neuroanatomical asymmetry in the telencephalic hemispheres of the frog Rana esculenta

The frog's brain postfixed with osmium tetroxide and stained with uranyl acetate was examined for asymmetries. Examination with the light microscope of serial semithin sections of the telencephalon revealed a nucleus of dark cells located in the lateral cortex of only the left hemisphere in a region close to the 'porcion arqueada' of Pedro Ramón y Cajal. Electron microscopy of ultrathin sections suggests that the cells of this asymmetric nucleus are neurons with a coarse cytoplasmic texture, where large inclusions occur, and with broad cisternae of endoplasmic reticulum.