Modulation of the development of light-initiated asymmetry in chick thalamofugal visual projections by oestradiol

The effect of androgen exposure on cerebral lateralization in the American alligator (Alligator mississippiensis)

The division of the brain manifests in lateralized physical behaviors, where specific tasks originate from one side of the body. Previous studies have shown that birds and reptiles mediate aggression in their right hemisphere and focus on opponents with their left eye. Degree of lateralization varies between sexes, likely due to androgen inhibition of lateralization in mammals, birds, and fish, but remains untested in herpetofauna. In this experiment, we investigated the effect of androgen exposure on cerebral lateralization in the American Alligator, Alligator mississippiensis. Alligator eggs were collected and incubated at female producing temperature with a subset dosed with methyltestosterone in ovo. Dosed hatchlings were randomly paired with control individuals and their interactions were recorded. The number of bites initiated by focus from each eye and the number of times an animal was bitten on each side of the body was recorded for each individual to elucidate cerebral lateralization in aggression. Control alligators had a significant bias towards left-eye bite initiation whereas androgen exposed alligators used both eyes indiscriminately. No significance was found in injury patterns. This study suggests that androgen exposure inhibits cerebral lateralization in alligator brains and corroborates right-hemisphere mediation of aggression, something previously unstudied in crocodilians.

Ecdysone signaling determines lateral polarity and remodels neurites to form Drosophila's left-right brain asymmetry

Left-right (LR) asymmetry of the brain is fundamental to its higher-order functions. The Drosophila brain's asymmetrical body (AB) consists of a structural pair arborized from AB neurons and is larger on the right side than the left. We find that the AB initially forms LR symmetrically and then develops LR asymmetrically by neurite remodeling that is specific to the left AB and is dynamin dependent. Additionally, neuronal ecdysone signaling inhibition randomizes AB laterality, suggesting that ecdysone signaling determines AB's LR polarity. Given that AB's LR asymmetry relates to memory formation, our research establishes AB as a valuable model for studying LR asymmetry and higher-order brain function relationships.

A function for the bicameral mind

Why do the left and right sides of the brain have different functions? Having a lateralized brain, in which each hemisphere processes sensory inputs differently and carries out different functions, is common in vertebrates, and it has now been reported for invertebrates too. Experiments with several animal species have shown that having a lateralized brain can enhance the capacity to perform two tasks at the same time. Thus, the different specializations of the left and right sides of the brain seem to increase brain efficiency. Other advantages may involve control of action that, in Bilateria, may be confounded by separate and independent sensory processing and motor outputs on the left and right sides. Also, the opportunity for increased perceptual training associated with preferential use of only one sensory or motoric organ may result in a time advantage for the dominant side. Although brain efficiency of individuals can be achieved without the need for alignment of lateralization in the population, lateral biases (such as preferences in the use of a laterally-placed eye) usually occur at the population level, with most individuals showing a similar direction of bias. Why is this the case? Not only humans, but also most non-human animals, show a similar pattern of population bias (i.e., directional asymmetry). For instance, in several vertebrate species (from fish to mammals) most individuals react faster when a predator approaches from their left side, although some individuals (a minority usually ranging from 10 to 35%) escape faster from predators arriving from their right side. Invoking individual efficiency (lateralization may increase fitness), evolutionary chance or simply genetic inheritance cannot explain this widespread pattern. Using mathematical theory of games, it has been argued that the population structure of lateralization (with either antisymmetry or directional asymmetry) may result from the type of interactions asymmetric organisms face with each other.

Effect of yolk corticosterone on begging in the yellow-legged gull

Behavioral lateralization is widespread across vertebrates. The development of lateralization is affected by both genetic and environmental factors. In birds, maternal substances in the egg can affect offspring lateralization via activational and/or organizational effects. Corticosterone affects the development of brain asymmetry, suggesting that variation in yolk corticosterone concentration may also influence post-natal behavioral lateralization, a hypothesis that has never been tested so far. In the yellow-legged gull (Larus michahellis), we increased yolk corticosterone concentration within physiological limits and analyzed the direction of lateralization of hatchlings in reverting from supine to prone position ('RTP' response) and in pecking at dummy parental bills to solicit food provisioning ('begging' response). We found that corticosterone treatment negatively affected the frequency of begging and it may cause a slight leftward lateralization. However, the direction of lateralization of the RTP response was not affected by corticosterone administration. Thus, our study shows a maternal effect mediated by corticosterone on a behavioral trait involved in parent-offspring communication during food provisioning events. The findings on lateralization are not conclusive due to the weak effect size but provide information for further ecological and evolutionary studies, investigating mechanisms underlying the development of lateralization.

Expression of sex hormone receptors in the brain of male and female newly hatched chicks

Chromosomal sex and steroid hormones play a determining role in brain sexual differentiation during chick embryonic development. Hormone effects on the brain are associated with the expression pattern of their intracellular receptors, which is sexually dimorphic in many species. We determined by Western blot the content of progesterone, estrogen, and androgen receptors (PR-A and PR-B, ERα, and AR, respectively) in the cortex, cerebellum, tectum, and hypothalamus of female and male newly hatched chicks. Males presented a higher content of PR-B in the tectum whereas females exhibited a higher content of PR-A in the hypothalamus. ERα was only detected as a band of 66kDa, and it showed a higher content in the cerebellum and tectum of females as compared to these regions in males. Besides, males exhibited a higher content of AR in the tectum than females. Our study suggests that newly hatched chicks show a sexual dimorphism in the expression of sex hormone receptors in brain regions involved in sexual behavior such as the hypothalamus, and in non-sexual behavior such as the optic tectum and the cerebellum.

Asymmetry in food handling behavior of a tree-dwelling rodent (Sciurus vulgaris)

Asymmetry in motor patterns is present in a wide variety of animals. Many lateralized behaviors seem to depend on brain asymmetry, as it is the case of different tasks associated to food handling by several bird and mammal species. Here, we analyzed asymmetry in handling behavior of pine cones by red squirrels (Sciurus vulgaris). Red squirrels devote most of their daily activity to feeding, thus this species constitutes an appropriate model for studying asymmetry in food processing. We aimed to explore 1) the potential lateralization in handling of pine cones by squirrels, 2) the dominant pattern for this behavior (left- vs. right-handed), and 3) whether this pattern varies among populations and depending on the pine tree species available. Results revealed that red squirrels handle pine cones in an asymmetrical way, and that direction of asymmetry varies among populations and seems to be determined more by local influences rather than by the pine tree species.

Does testosterone affect lateralization of brain and behaviour? A meta-analysis in humans and other animal species

Lateralization of brain and behaviour has been the topic of research for many years in neuropsychology, but the factors guiding its development remain elusive. Based on sex differences in human lateralization, four hypotheses have been postulated that suggest a role for androgens, specifically testosterone. With the discovery that lateralization is a fundamental principle in the organization of brain and behaviour among vertebrates, it has now become possible to experimentally test such hypotheses in animal models. The use of different taxa, humans, other mammalian species and birds (with oestradiol and not testosterone involved in sexual differentiation in birds) facilitates to differentiate between the hypotheses. We used meta-analyses for analysing papers that provided sufficient information, and a semi-quantitative approach based on all relevant studies that we extracted from the literature. We tested the predictions of these hypotheses regarding strength and direction of lateralization for motor output, language and visuospatial cognition in these three taxa. We tested for sex differences and early organizational effects of testosterone (both correlative and experimental studies). We found sex differences in the direction of lateralization for non-human mammals (motor biases similar to humans) and in direction and strength in birds (visual cognitive tasks). However, the prediction that prenatal testosterone exposure affects the direction of lateralization was not supported for humans. In birds and non-human mammals, opposite trends were found, with the effect in non-human mammals being opposite to the expectation based on sex differences. None of the four hypotheses was sufficiently supported and more studies, testing a wider array of functions in different taxa while reporting the data more completely are needed.

Lateralization of social cognition in the domestic chicken (Gallus gallus)

In this paper, we report on the ongoing work in our laboratories on the effect of lateralization produced by light exposure in the egg on social cognition in the domestic chick (Gallus gallus). The domestic chick possesses a lateralized visual system. This has effects on the chick's perception towards and interaction with its environment. This includes its ability to live successfully within a social group. We show that there is a tendency for right brain hemisphere dominance when performing social cognitive actions. As such, chicks show a left hemispatial bias for approaching a signalled target object, tend to perceive gaze and faces of human-like masks more effectively when using their left eye, are able to inhibit a pecking response more effectively when viewing a neighbour tasting a bitter substance with their left eye, and are better able to perform a transitive inference task when exposed to light in the egg and when forced to use their left eye only compared to dark-hatched or right eye chicks. Some of these effects were sex specific, with male chicks tending to show an increased effect of lateralization on their behaviours. These data are discussed in terms of overall social cognition in group living.

Development and function of lateralization in the avian brain

The avian brain is functionally lateralized. Different strategies of choice (within and between modalities) are adopted by each hemisphere. Visual lateralization has been studied most but attention to auditory, olfactory and magnetic cues is also lateralized. The left hemisphere (LH) focuses on cues that reliably separate pertinent stimuli from distracting stimuli (e.g. food from pebbles, odour cues from attractive visual cues, magnetic cues from other cues indicating location), whereas the right hemisphere (RH) has broad attention and is easily distracted by novel stimuli. The RH also controls fear and escape responses, as in reaction to predators. Exposure of the embryo to light just before hatching, when the posture adopted occludes the left eye (LE) but not the right eye (RE), leads to the development of asymmetry in the visual projections to the pallium and enhances the ability of the RE/LH to inhibit attention to distracting visual cues and of the LH to inhibit the RH, but has no effect on the RH's interest in novelty. Exposure to light before hatching has both short- and long-term consequences that are important for species-typical behaviour and survival. For example, on a food search task with a predator presented overhead, dark-incubated chicks perform poorly on both aspects of the task, whereas light-exposed chicks have no difficulty. Steroid hormone levels prior to hatching modulate light-dependent development of asymmetry in the visual projections and consequently affect neural competence for parallel processing and response inhibition. Differences between lateralization in the chick and pigeon are discussed.

Corticosterone treatment of the chick embryo affects light-stimulated development of the thalamofugal visual pathway

By injecting a single 60 microg dose of corticosterone into the eggs of domestic chicks on day 18 of incubation, we have shown that elevated levels of this hormone affect the development of asymmetry in the visual projections from the thalamus to the Wulst regions in the left and right hemispheres of the forebrain. In vehicle-treated (control) embryos this visual pathway develops asymmetry in response to light stimulation during the final stages of incubation, when the embryo is oriented so that its left eye is occluded by its body and its right eye can be stimulated by light entering through the egg shell. Pre-hatching exposure to light leads to more projections from the left side of the thalamus to the right Wulst than from the right side of the thalamus to the left Wulst, as confirmed here by injection of the tracers Fluorogold and Rhodamine into the left and right Wulst followed by counting the number of labelled cell bodies in the thalamus (asymmetry greater in males than females). The chicks injected with corticosterone pre-hatching did not develop any group bias for asymmetry in response to light exposure before hatching. They were random with respect to presence/absence of lateralization and, when present, the lateralization was not as strong as in the controls and its direction was random. The corticosterone-treated group had fewer projections from the left side of the thalamus to the right Wulst than did the controls. The results are considered with respect to maternal deposits of the hormone in the yolk and pre-hatching stress of the embryo.

Light experience and the development of behavioural lateralisation in chicks

In late-stage embryos of domestic fowl, exposure of the right eye to light entering through the shell induces asymmetry of the thalamofugal visual pathway, together with differences in performance according to whether the right or left eye (RE, LE) is in use (Behav. Brain Res. 38 (1990) 211). Nevertheless, at least some of the main specialisations of the right and left eye systems (RES, LES) are not dependent on such exposure. Higher ability of LES to assess and respond to novelty is present in dark-incubated (Da) chicks. This is probably also true of RES ability to control response, and specifically to inhibit shift to an alternative response (i.e. to a novel stimulus). We imprinted chicks on red table-tennis balls with a horizontal, white strip on their equator. At test, they chose between this and a ball with a vertical, white strip. Da chicks showed clear choice with the LE, but not with the RE. Unexpectedly, light-incubated (Li) chicks failed to show LE/RE differences in choice. Exploratory pecks at a novel feature were greatly reduced in Li. Two effects of light exposure on RES are likely. The first is greater use of RES in the home-cage, affecting what is learned about the companion ball. This may make RES more competent in assessing ball properties, and so explain the enhanced choice by RE, that abolished the RE/LE difference in Li. Secondly, the ability of RES to inhibit shift to an alternative response is enhanced. Light exposure and being female similarly opposed shift to the novel feature, but probably via different mechanisms. The effects of exposure are discussed as an example of the generation of a range of behavioural phenotypes, which are sustained within a single population by varying or frequency-dependent selection.

Possible evolutionary origins of cognitive brain lateralization

Despite the substantial literature on the functional architecture of the asymmetries of the human brain, which has been accumulating for more than 130 years since Dax and Broca's early reports, the biological foundations of cerebral asymmetries are still poorly understood. Recent advances in comparative cognitive neurosciences have made available new animal models that have started to provide unexpected insights into the evolutionary origins and neuronal mechanisms of cerebral asymmetries. Animal model-systems, particularly those provided by the avian brain, highlight the interrelations of genetic, hormonal and environmental events to produce neural and behavioural asymmetries. Novel evidences showing that functional and structural lateralization of the brain is widespread among vertebrates (including fish, reptiles and amphibians) have accumulated rapidly. Perceptual asymmetries, in particular, seem to be ubiquitous in everyday behaviour of most species of animals with laterally placed eyes; in organisms with wider binocular overlap (e.g., amphibians), they appear to be retained for initial detection of stimuli in the extreme lateral fields. We speculate that adjustment of head position and eye movements may play a similar role in mammals with frontal vision as does the choice for right or left lateral visual fields in animals with laterally placed eyes. A first attempt to trace back the origins of brain asymmetry to early vertebrates is presented, based on the hypothesis that functional incompatibility between the logical demands associated with very basic cognitive functions is central to the phenomenon of cerebral lateralization.

Light experience and lateralization of the two visual pathways in the chick

Using retrograde labelling with the fluorescent tracer rhodamine B isocyanate (RITC), we have examined the organisation of the thalamofugal and tectofugal visual projections to the forebrain of the young chick. In addition, we have investigated the influence of light exposure prior to hatching on the development of the tectofugal visual projections. Our results for the thalamofugal projections confirm those found previously; viz., that there are more projections from the left side of the thalamus to the right hyperstriatum of the forebrain than from the right side of the thalamus to the left hyperstriatum in males and females. The organisation of the tectofugal visual projections to the rotundal nuclei was more symmetrical (males only examined) although there was a trend towards a greater number of projections from the left optic tectum to its ipsilateral nucleus rotundus than from the right optic tectum to its ipsilateral nucleus rotundus. There are numerous projections from the optic tecta to their contralateral rotundal nuclei but, in contrast to reports for the pigeon, no marked asymmetry was present in these. The ratio of contralateral to ipsilateral projections revealed significant asymmetry for projections from the ventral regions of the optic tecta and symmetry from the dorsal regions. Thus both visual pathways of the chick have asymmetrical organisation but the asymmetry is much greater in the thalamofugal pathway. The slight asymmetry in the tectofugal projections may be determined by exposing the embryo to light just before hatching, as known to be the case for thalamofugal projections.

Effects of dark rearing and light exposure on memory for a passive avoidance task in day-old chicks

Light exposure during embryogenesis is necessary for functional and morphological maturation in the domestic chick. In the present study, dark incubation was demonstrated to induce a weak amnestic effect on retention for a passive avoidance task and a diminution in discriminative memory ability in day-old chicks. Putative explanations based on possible motor, attentional, or visual impairment were excluded. Light exposure of dark-reared eggs, specifically during embryonic days E19 to E20, alleviated the retention and discrimination deficits. The processes which might mediate between prehatch light stimulation and posthatch behavioral effects are discussed.

Early Experiential Effects on Laterality: Research on Chicks has Relevance to Other Species

The influence of early experience on the development of lateralisation of hemispheric function was further investigated, using the chick as a model. A range of functions are lateralised in the chick and these correlate with asymmetry in the organisation of the visual projections. Chicks using the right eye and, therefore, primarily the left hemisphere are able to switch from pecking randomly at grain and pebbles to pecking mainly at grain, whereas those using the left eye and primarily the right hemisphere continue to peck at random. Exposure to light during the last days of incubation establishes this lateralisation in males, as a consequence of the embryo being oriented in the egg so that the left eye only is occluded. Males incubated in the dark peck at random when using either the right or left eye. Irrespective of light experience, females perform the same as dark-incubated males: they are not influenced by light exposure. Monocular performance of the pebble-grain task is compared to binocular performance, and the sensitive period for the influence of light is delineated. The interactive effects of sex hormone levels on the differentiation of lateralisation are discussed and also the relevance of the results to other species, including humans.

Exposure to different wavelengths of light and the development of structural and functional asymmetries in the chicken

The thalamofugal visual projections of the chick are known to develop in response to stimulation by light prior to hatching, and asymmetry in the number of projections develops as a consequence of the embryo being oriented in the egg so that it occuludes its left eye. The right eye only is stimulated by light and this causes the visual projections connected to the right eye to develop in advance of those connected to the left. We have now found that exposure of embryos, from day 19 of incubation to hatching, to red (peak transmission at 670 nm) or green (peak at 500 nm) light is as effective as broad-spectrum (white) light in establishing asymmetry in these projections. The intensities of the light to which the embryos were exposed in each case were equivalent, achieved in part by removing the air sac end of the egg shell. The thalamofugal visual projections, therefore, develop in response to light stimulation but appear to have no wavelength specificity. This result is consistent with the apparent lack of involvement of the thalamofugal visual pathway in colour vision. However, functional asymmetry, tested as left-right eye differences in categorising grain from pebbles, was found to be less marked in the chicks that had been exposed to green light compared to those that had been exposed to 'while' light, and it was absent in those exposed to red light or incubated in the dark. Thus, there is wavelength specificity for the development of the behavioural asymmetry, which suggests involvement of colour-coded neurons outside the thalamofugal visual pathway, probably in the tectofugal pathway. Exposure of the embryos to red and green light alternating at 30 min intervals is as effective as "white' light for establishing both the structural and functional asymmetry.

Diencephalic asymmetries

Structural asymmetry in diencephalic regions has been reported in a number of studies since the pioneering observations by Kemali and Braitenberg, Atlas of the frog's brain. Springer Verlag: 1969. Anatomical differences between the left and right habenulae have been identified in many lower vertebrate species. While there are few reports of structural asymmetry in the dorsal thalamus, there is evidence that asymmetrical thalamofugal projections can be induced in the visual system of chicks by lateralized sensory stimulation prior to hatching. Finally, there have been consistent reports of differences between and right sides of the hypothalamus in their sensitivity to the effects of circulating gonadal hormones in rats. In most cases, these asymmetries are sex-linked and correspond to a lateralization of function. Although the significance of these diencephalic asymmetries is still enigmatic, their existence indicates that asymmetry is not a phylogenetically recent feature of the brain, and the left-right differences in the brain may be mediated by a common ontogenetic mechanism and may underlie the development of highly specialized functions.

Behavioral, structural and neurochemical asymmetries in the avian brain: a model system for studying visual development and processing

The emphasis of this review is on the visual systems and lateralized visually guided behavior in several avian species. Lateral asymmetry is known to be present in the tectofugal visual projections to the forebrain of the pigeon and in the thalamofugal visual projections to the forebrain of the chicken. These structural asymmetries are discussed in the context of the behavioral and neurochemical asymmetries. While recognizing the need to investigate the organization of both of the visual pathways within one avian species; this review reasons inductively that the lateralized organization of the two visual pathways leads to binocular input to the right hemisphere via the thalamofugal visual system and to the left hemisphere via the tectofugal visual system. For each system, input to the other hemisphere is primarily monocular. This specialization of the hemispheres for visual processing has predictable effects on behavior. The role of asymmetrical light stimulation of the eyes of the embryo in determining the lateralizations in the visual pathways and some behaviors is discussed, as are other lateralizations generated or altered by imprinting and passive avoidance learning.

Menstrual cycle affects functional cerebral asymmetries

Cerebral lateralization in females is probably modulated by the menstrual cycle such that a part of hemispheric asymmetries are diminished with an increase of the steroids estrogen and progesterone during the follicular and luteal phase and enhanced with steroid decreases during the menstrual phase. However, previous data were contradictory with regard to the hemispheric side of modulation and could not analyze which steroid is mainly responsible for cycle dependent lateralization shifts. Therefore, in the present study estrogen and progesterone changes were assessed separately and related to changes in cerebral asymmetry. Plasma levels of estrogen and progesterone were measured once during luteal and once during menstrual cycle phase while 20 females subjects performed a verbal and a figural lateralized matching task. The results showed a significant cycle phase x lateralization interaction for the right hemisphere dominated figural comparison task but not for the left hemispheric lexical condition. Although the lateralization was modulated by the menstrual cycle, a within-subject regression analysis demonstrated that the asymmetry shift was not under direct influence of estrogen or progesterone. Thus, the present study provides further empirical support for cycle dependent alterations in lateralization but makes it unlikely that this effect is directly caused by estrogen or progesterone plasma level variations.