Sex-dependent structural asymmetry of the medial habenular nucleus of the chicken brain
An investigation of structural asymmetry in the avian brain was conducted on the epithalamic medial habenular nucleus of the chicken. Twelve male and ten female two-day-old chickens were used for a morphometric evaluation of asymmetry. The medial habenular nucleus was measured from paraffin-wax-embedded, 8 micron-thick sections by use of a semiautomatic image analyser. The volumes of the right and left medial habenula of each animal were statistically analysed ('within animal experimental design'). The right medial habenula in males showed significant group asymmetry. In contrast, females failed to demonstrate group bias in favour of either hemisphere. However, individual females were lateralised, with either a larger right or left medial habenula. Although individuals of both sexes were lateralised, there was no significant sex difference in volume in either the right or left medial habenula. We propose that sex-linked structural asymmetry may be influenced by steroid hormonal effects in the central nervous system, and that such asymmetry could be more prevalent in the non-mammalian vertebrate brain than previously considered.
Available from: Aldo Villalon
- "Studies in amphibians and birds have shown that habenular asymmetry can be influenced by steroid hormones and thus be a subject of variability among sex. For example, left-sided asymmetry of the dorsal Hb in Rana esculenta is more pronounced in females than males, especially during the reproductive season  whereas in chicks the medial Hb shows right-sided volumetric asymmetry only in males . We found that sex-specific differences in volumetric asymmetries of the Hb are infrequent in teleosts, and thus are not a primary source of inter-species variations. "
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ABSTRACT: The vertebrate habenulae (Hb) is an evolutionary conserved dorsal diencephalic nuclear complex that relays information from limbic and striatal forebrain regions to the ventral midbrain. One key feature of this bilateral nucleus is the presence of left-right differences in size, cytoarchitecture, connectivity, neurochemistry and/or gene expression. In teleosts, habenular asymmetry has been associated with preferential innervation of left-right habenular efferents into dorso-ventral domains of the midbrain interpeduncular nucleus (IPN). However, the degree of conservation of this trait and its relation to the structural asymmetries of the Hb are currently unknown. To address these questions, we performed the first systematic comparative analysis of structural and connectional asymmetries of the Hb in teleosts. We found striking inter-species variability in the overall shape and cytoarchitecture of the Hb, and in the frequency, strength and to a lesser degree, laterality of habenular volume at the population level. Directional asymmetry of the Hb was either to the left in D. rerio, E. bicolor, O. latipes, P. reticulata, B. splendens, or to the right in F. gardneri females. In contrast, asymmetry was absent in P. scalare and F. gardneri males at the population level, although in these species the Hb displayed volumetric asymmetries at the individual level. Inter-species variability was more pronounced across orders than within a single order, and coexisted with an overall conserved laterotopic representation of left-right habenular efferents into dorso-ventral domains of the IPN. These results suggest that the circuit design involving the Hb of teleosts promotes structural flexibility depending on developmental, cognitive and/or behavioural pressures, without affecting the main midbrain connectivity output, thus unveiling a key conserved role of this connectivity trait in the function of the circuit. We propose that ontogenic plasticity in habenular morphogenesis underlies the observed inter-species variations in habenular asymmetric morphology.
Available from: Adam R Reddon
- "The Hb is asymmetrical, neuroanatomically and/or neurochemically, in a diverse array of vertebrate species ranging from jawless fish to mammals . While the data on individual variation in this asymmetry are scarce, it has been shown that individuals of the same species may differ in this asymmetry   , and these differences may be correlated with individual variation in lateralized behaviour . If the Hb is involved in inhibiting behaviours, and it projects to its target bilaterally, then the dorsal-diencephalic conduction system may be a mechanism by which one hemisphere can override a behavioural decision in the other. "
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ABSTRACT: Cerebral lateralization was once thought to be unique to humans, but is now known to be widespread among the vertebrates. Lateralization appears to confer cognitive advantages upon those that possess it. Despite the taxonomic ubiquity and described advantages of lateralization, substantial individual variation exists in all species. Individual variation in cerebral lateralization may be tied to individual variation in behaviour and the selective forces that act to maintain variation in behaviour may also act to maintain variation in lateralization. The mechanisms linking individual variation in the strength of cerebral lateralization to individual variation in behaviour remain obscure. We propose here a general hypothesis which may help to explain this link. We suggest that individuals with strong and weak lateralizations behave differently because of differences in the ability of one hemisphere to inhibit the functions of the other in each type of brain organization. We also suggest a specific mechanism involving the asymmetric epithalamic nucleus, the habenula. We conclude by discussing some predictions and potential tests of our hypothesis.
Available from: Orsola Rosa Salva
- "This response can be suppressed by lesioning the right, but not the left arcopallium (medial arcopallium and posterior nucleus of the pallial amigdala, the limbic portion of the archistriatum, homologous to the human amygdala) . It is worth noting that the limbic system is connected with habenular nuclei , that are known to be larger in the right hemisphere of the chicken brain . Moreover, hens and chicks prefer to use their left eye in order to check for the presence of aerial predators, after hearing an alarm call  . "
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ABSTRACT: Domestic chicks were tested for eye use while feeding on the floor in the presence of a dummy mask which could either look at the location where the chicks were feeding or in the opposite direction. Animals completely naïve of visual experience of human eyes and gaze showed a more intense fear response when directly looked at (as shown by higher latency to approach the food) and a preferential use of the left eye (mainly feeding structures in their right hemisphere) to monitor the dummy mask. This response, seemingly predisposed, could be reversed in chicks with experience of human eyes and gaze directed toward them, which showed higher latency to approach the food and preferential left eye use when the dummy mask looked away from them. The results are discussed in relation to evidence for a right hemisphere involvement in fear responses and detection of predators in the vertebrate brain.
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