Functional implications of species differences in the size and morphology of the isthmo optic nucleus (ION) in birds

Mapping the avian visual tectofugal pathway using 3D reconstruction

Image processing in amniotes is usually accomplished by the thalamofugal and/or tectofugal visual systems. In laterally eyed birds, the tectofugal system dominates with functions such as color and motion processing, spatial orientation, stimulus identification, and localization. This makes it a critical system for complex avian behavior. Here, the brains of chicks, Gallus gallus, were used to produce serial brain sections in either coronal, sagittal, or horizontal planes and stained with either Nissl and Gallyas silver myelin or Luxol fast blue stain and cresyl echt violet (CEV). The emerging techniques of diffusible iodine-based contrast-enhanced computed tomography (diceCT) coupled with serial histochemistry in three planes were used to generate a comprehensive three-dimensional (3D) model of the avian tectofugal visual system. This enabled the 3D reconstruction of tectofugal circuits, including the three primary neuronal projections. Specifically, major components of the system included four regions of the retina, layers of the optic tectum, subdivisions of the nucleus rotundus in the thalamus, the entopallium in the forebrain, and supplementary components connecting into or out of this major avian visual sensory system. The resulting 3D model enabled a better understanding of the structural components and connectivity of this complex system by providing a complete spatial organization that occupied several distinct brain regions. We demonstrate how pairing diceCT with traditional histochemistry is an effective means to improve the understanding of, and thereby should generate insights into, anatomical and functional properties of complicated neural pathways, and we recommend this approach to clarify enigmatic properties of these pathways.

The relative sizes of nuclei in the oculomotor complex vary by order and behaviour in birds

Eye movements are a critical component of visually guided behaviours, allowing organisms to scan the environment and bring stimuli of interest to regions of acuity in the retina. Although the control and modulation of eye movements by cranial nerve nuclei are highly conserved across vertebrates, species variation in visually guided behaviour and eye morphology could lead to variation in the size of oculomotor nuclei. Here, we test for differences in the size and neuron numbers of the oculomotor nuclei among birds that vary in behaviour and eye morphology. Using unbiased stereology, we measured the volumes and numbers of neurons of the oculomotor (nIII), trochlear (nIV), abducens (nVI), and Edinger-Westphal (EW) nuclei across 71 bird species and analysed these with phylogeny-informed statistics. Owls had relatively smaller nIII, nIV, nVI and EW nuclei than other birds, which reflects their limited degrees of eye movements. In contrast, nVI was relatively larger in falcons and hawks, likely reflecting how these predatory species must shift focus between the central and temporal foveae during foraging and prey capture. Unexpectedly, songbirds had an enlarged EW and relatively more nVI neurons, which might reflect accommodation and horizontal eye movements. Finally, the one merganser we measured also has an enlarged EW, which is associated with the high accommodative power needed for pursuit diving. Overall, these differences reflect species and clade level variation in behaviour, but more data are needed on eye movements in birds across species to better understand the relationships among behaviour, retinal anatomy, and brain anatomy.

Comparative Genomics Provides Insights into Adaptive Evolution in Tactile-Foraging Birds

Tactile-foraging birds have evolved an enlarged principal sensory nucleus (PrV) but smaller brain regions related to the visual system, which reflects the difference in sensory dependence. The “trade-off” may exist between different senses in tactile foragers, as well as between corresponding sensory-processing areas in the brain. We explored the mechanism underlying the adaptive evolution of sensory systems in three tactile foragers (kiwi, mallard, and crested ibis). The results showed that olfaction-related genes in kiwi and mallard and hearing-related genes in crested ibis were expanded, indicating they may also have sensitive olfaction or hearing, respectively. However, some genes required for visual development were positively selected or had convergent amino acid substitutions in all three tactile branches, and it seems to show the possibility of visual degradation. In addition, we may provide a new visual-degradation candidate gene PDLIM1 who suffered dense convergent amino acid substitutions within the ZM domain. At last, two genes responsible for regulating the proliferation and differentiation of neuronal progenitor cells may play roles in determining the relative sizes of sensory areas in brain. This exploration offers insight into the relationship between specialized tactile-forging behavior and the evolution of sensory abilities and brain structures.

Pretectal projections to the oculomotor cerebellum in hummingbirds (Calypte anna), zebra finches (Taeniopygia guttata), and pigeons (Columba livia)

In birds, optic flow is processed by a retinal-recipient nucleus in the pretectum, the nucleus lentiformis mesencephali (LM), which then projects to the cerebellum, a key site for sensorimotor integration. Previous studies have shown that the LM is hypertrophied in hummingbirds, and that LM cell response properties differ between hummingbirds and other birds. Given these differences in anatomy and physiology, we ask here if there are also species differences in the connectivity of the LM. The LM is separated into lateral and medial subdivisions, which project to the oculomotor cerebellum and the vestibulocerebellum. In pigeons, the projection to the vestibulocerebellum largely arises from the lateral LM; the projection to the oculomotor cerebellum largely arises from the medial LM. Here, using retrograde tracing, we demonstrate differences in the distribution of projections in these pathways between Anna's hummingbirds (Calypte anna), zebra finches (Taeniopygia guttata), and pigeons (Columba livia). In all three species, the projections to the vestibulocerebellum were largely from lateral LM. In contrast, projections to the oculomotor cerebellum in hummingbirds and zebra finches do not originate in the medial LM (as in pigeons) but instead largely arise from pretectal structures just medial, the nucleus laminaris precommissuralis and nucleus principalis precommissuralis. These species differences in projection patterns provide further evidence that optic flow circuits differ among bird species with distinct modes of flight.

The centrifugal visual system of a palaeognathous bird, the Chilean Tinamou (Nothoprocta perdicaria)

The avian centrifugal visual system, which projects from the brain to the retina, has been intensively studied in several Neognathous birds that have a distinct isthmo-optic nucleus (ION). However, birds of the order Palaeognathae seem to lack a proper ION in histologically stained brain sections. We had previously reported in the palaeognathous Chilean Tinamou (Nothoprocta perdicaria) that intraocular injections of Cholera Toxin B subunit retrogradely label a considerable number of neurons, which form a diffuse isthmo-optic complex (IOC). In order to better understand how this IOC-based centrifugal visual system is organized, we have studied its major components by means of in vivo and in vitro tracing experiments. Our results show that the IOC, though structurally less organized than an ION, possesses a dense core region consisting of multipolar neurons. It receives afferents from neurons in L10a of the optic tectum, which are distributed with a wider interneuronal spacing than in Neognathae. The tecto-IOC terminals are delicate and divergent, unlike the prominent convergent tecto-ION terminals in Neognathae. The centrifugal IOC terminals in the retina are exclusively divergent, resembling the terminals from "ectopic" centrifugal neurons in Neognathae. We conclude that the Tinamou's IOC participates in a comparable general IOC-retina-TeO-IOC circuitry as the neognathous ION. However, the connections between the components are structurally different and their divergent character suggests a lower spatial resolution. Our findings call for further comparative studies in a broad range of species for advancing our understanding of the evolution, plasticity and functional roles of the avian centrifugal visual system.

The effect of unilateral disruption of the centrifugal visual system on normal eye development in chicks raised under constant light conditions

The centrifugal visual system (CVS) comprises a visually driven isthmic feedback projection to the retina. While its function has remained elusive, we have previously shown that, under otherwise normal conditions, unilateral disconnection of centrifugal neurons in the chick affected eye development, inducing a reduced rate of axial elongation that resulted in a unilateral hyperopia in the eye contralateral to the lesion. Here, we further investigate the role of centrifugal neurons in ocular development in chicks reared in an abnormal visual environment, namely constant light. The baseline ocular phenotype of constant light-reared chicks (n = 8) with intact centrifugal neurons was assessed over a 3-week post-hatch time period and, subsequently, compared to chicks raised in normal diurnal lighting (n = 8). Lesions of the isthmo-optic tract or sham surgeries were performed in another seventeen chicks, all raised under constant light. Ocular phenotyping was performed over a 21-day postoperative period to assess changes in refractive state (streak retinoscopy) and ocular component dimensions (A-scan ultrasonography). A pathway-tracing paradigm was employed to quantify lesion success. Chicks raised in constant light conditions with an intact CVS developed shallower anterior chambers combined with elongated vitreous chambers relative to chicks raised in normal diurnal lighting. Seven days following surgery to disrupt centrifugal neurons, a significant positive correlation between refractive error asymmetry between the eyes and lesion success was evident, characterized by hyperopia in the eye contralateral to the lesion. By 21 days post-surgery, these contralateral eyes had become emmetropic, while ipsilateral eyes had developed relative axial hyperopia. Our results provide further support for the hypothesis that the centrifugal visual system can modulate eye development.

Integrating brain, behavior, and phylogeny to understand the evolution of sensory systems in birds

The comparative anatomy of sensory systems has played a major role in developing theories and principles central to evolutionary neuroscience. This includes the central tenet of many comparative studies, the principle of proper mass, which states that the size of a neural structure reflects its processing capacity. The size of structures within the sensory system is not, however, the only salient variable in sensory evolution. Further, the evolution of the brain and behavior are intimately tied to phylogenetic history, requiring studies to integrate neuroanatomy with behavior and phylogeny to gain a more holistic view of brain evolution. Birds have proven to be a useful group for these studies because of widespread interest in their phylogenetic relationships and a wealth of information on the functional organization of most of their sensory pathways. In this review, we examine the principle of proper mass in relation differences in the sensory capabilities among birds. We discuss how neuroanatomy, behavior, and phylogeny can be integrated to understand the evolution of sensory systems in birds providing evidence from visual, auditory, and somatosensory systems. We also consider the concept of a "trade-off," whereby one sensory system (or subpathway within a sensory system), may be expanded in size, at the expense of others, which are reduced in size.

Mosaic and concerted evolution in the visual system of birds

Two main models have been proposed to explain how the relative size of neural structures varies through evolution. In the mosaic evolution model, individual brain structures vary in size independently of each other, whereas in the concerted evolution model developmental constraints result in different parts of the brain varying in size in a coordinated manner. Several studies have shown variation of the relative size of individual nuclei in the vertebrate brain, but it is currently not known if nuclei belonging to the same functional pathway vary independently of each other or in a concerted manner. The visual system of birds offers an ideal opportunity to specifically test which of the two models apply to an entire sensory pathway. Here, we examine the relative size of 9 different visual nuclei across 98 species of birds. This includes data on interspecific variation in the cytoarchitecture and relative size of the isthmal nuclei, which has not been previously reported. We also use a combination of statistical analyses, phylogenetically corrected principal component analysis and evolutionary rates of change on the absolute and relative size of the nine nuclei, to test if visual nuclei evolved in a concerted or mosaic manner. Our results strongly indicate a combination of mosaic and concerted evolution (in the relative size of nine nuclei) within the avian visual system. Specifically, the relative size of the isthmal nuclei and parts of the tectofugal pathway covary across species in a concerted fashion, whereas the relative volume of the other visual nuclei measured vary independently of one another, such as that predicted by the mosaic model. Our results suggest the covariation of different neural structures depends not only on the functional connectivity of each nucleus, but also on the diversity of afferents and efferents of each nucleus.