Does nocturnality drive binocular vision? Octodontine rodents as a case study

At the root of the mammalian mind: The sensory organs, brain and behavior of pre-mammalian synapsids

All modern mammals are descendants of the paraphyletic non-mammaliaform Synapsida, colloquially referred to as the "mammal-like reptiles." It has long been assumed that these mammalian ancestors were essentially reptile-like in their morphology, biology, and behavior, i.e., they had a small brain, displayed simple behavior, and their sensory organs were unrefined compared to those of modern mammals. Recent works have, however, revealed that neurological, sensory, and behavioral traits previously considered typically mammalian, such as whiskers, enhanced olfaction, nocturnality, parental care, and complex social interactions evolved before the origin of Mammaliaformes, among the early-diverging "mammal-like reptiles." In contrast, an enlarged brain did not evolve immediately after the origin of mammaliaforms. As such, in terms of paleoneurology, the last "mammal-like reptiles" were not significantly different from the earliest mammaliaforms. The abundant data and literature published in the last 10 years no longer supports the "three pulses" scenario of synapsid brain evolution proposed by Rowe and colleagues in 2011, but supports the new "outside-in" model of Rodrigues and colleagues proposed in 2018, instead. As Mesozoic reptiles were becoming the dominant taxa within terrestrial ecosystems, synapsids gradually adapted to smaller body sizes and nocturnality. This resulted in a sensory revolution in synapsids as olfaction, audition, and somatosensation compensated for the loss of visual cues. This altered sensory input is aligned with changes in the brain, the most significant of which was an increase in relative brain size.

Binocular Vision and Stereopsis Across the Animal Kingdom

Most animals have at least some binocular overlap, i.e., a region of space that is viewed by both eyes. This reduces the overall visual field and raises the problem of combining two views of the world, seen from different vantage points, into a coherent whole. However, binocular vision also offers many potential advantages, including increased ability to see around obstacles and increased contrast sensitivity. One particularly interesting use for binocular vision is comparing information from both eyes to derive information about depth. There are many different ways in which this might be done, but in this review, I refer to them all under the general heading of stereopsis. This review examines the different possible uses of binocular vision and stereopsis and compares what is currently known about the neural basis of stereopsis in different taxa. Studying different animals helps us break free of preconceptions stemming from the way that stereopsis operates in human vision and provides new insights into the different possible forms of stereopsis. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A specialized reciprocal connectivity suggests a link between the mechanisms by which the superior colliculus and parabigeminal nucleus produce defensive behaviors in rodents

The parabigeminal nucleus (PBG) is the mammalian homologue to the isthmic complex of other vertebrates. Optogenetic stimulation of the PBG induces freezing and escape in mice, a result thought to be caused by a PBG projection to the central nucleus of the amygdala. However, the isthmic complex, including the PBG, has been classically considered satellite nuclei of the Superior Colliculus (SC), which upon stimulation of its medial part also triggers fear and avoidance reactions. As the PBG-SC connectivity is not well characterized, we investigated whether the topology of the PBG projection to the SC could be related to the behavioral consequences of PBG stimulation. To that end, we performed immunohistochemistry, in situ hybridization and neural tracer injections in the SC and PBG in a diurnal rodent, the Octodon degus. We found that all PBG neurons expressed both glutamatergic and cholinergic markers and were distributed in clearly defined anterior (aPBG) and posterior (pPBG) subdivisions. The pPBG is connected reciprocally and topographically to the ipsilateral SC, whereas the aPBG receives afferent axons from the ipsilateral SC and projected exclusively to the contralateral SC. This contralateral projection forms a dense field of terminals that is restricted to the medial SC, in correspondence with the SC representation of the aerial binocular field which, we also found, in O. degus prompted escape reactions upon looming stimulation. Therefore, this specialized topography allows binocular interactions in the SC region controlling responses to aerial predators, suggesting a link between the mechanisms by which the SC and PBG produce defensive behaviors.

Evidence of Synaptic and Neurochemical Remodeling in the Retina of Aging Degus

Accumulation of amyloid-beta (Aβ) peptides is regarded as the hallmark of neurodegenerative alterations in the brain of Alzheimer’s disease (AD) patients. In the eye, accumulation of Aβ peptides has also been suggested to be a trigger of retinal neurodegenerative mechanisms. Some pathological aspects associated with Aβ levels in the brain are synaptic dysfunction, neurochemical remodeling and glial activation, but these changes have not been established in the retina of animals with Aβ accumulation. We have employed the Octodon degus in which Aβ peptides accumulated in the brain and retina as a function of age. This current study investigated microglial morphology, expression of PSD95, synaptophysin, Iba-1 and choline acetyltransferase (ChAT) in the retina of juvenile, young and adult degus using immunolabeling methods. Neurotransmitters glutamate and gamma-aminobutyric acid (GABA) were detected using immunogold labeling and glutamate receptor subunits were quantified using Western blotting. There was an age-related increase in presynaptic and a decrease in post-synaptic retinal proteins in the retinal plexiform layers. Immunolabeling showed changes in microglial morphology characteristic of intermediate stages of activation around the optic nerve head (ONH) and decreasing activation toward the peripheral retina. Neurotransmitter expression pattern changed at juvenile ages but was similar in adults. Collectively, the results suggest that microglial activation, synaptic remodeling and neurotransmitter changes may be consequent to, or parallel to Aβ peptide and phosphorylated tau accumulation in the retina.

Can social behaviour drive accessory olfactory bulb asymmetries? Sister species of caviomorph rodents as a case in point

In mammals, the accessory olfactory or vomeronasal system exhibits a wide variety of anatomical arrangements. In caviomorph rodents, the accessory olfactory bulb (AOB) exhibits a dichotomic conformation, in which two subdomains, the anterior (aAOB) and the posterior (pAOB), can be readily distinguished. Interestingly, different species of this group exhibit bias of different sign between the AOB subdomains (aAOB larger than pAOB or vice versa). Such species-specific biases have been related with contrasting differences in the habitat of the different species (e.g. arid vs. humid environments). Aiming to deepen these observations, we performed a morphometric comparison of the AOB subdomains between two sister species of octodontid rodents, Octodon lunatus and Octodon degus. These species are interesting for comparative purposes, as they inhabit similar landscapes but exhibit contrasting social habits. Previous reports have shown that O. degus, a highly social species, exhibits a greatly asymmetric AOB, in which the aAOB has twice the size of the pAOB and features more and larger glomeruli in its glomerular layer (GL). We found that the same as in O. degus, the far less social O. lunatus also exhibits a bias, albeit less pronounced, to a larger aAOB. In both species, this bias was also evident for the mitral/tufted cells number. But unlike in O. degus, in O. lunatus this bias was not present at the GL. In comparison with O. degus, in O. lunatus the aAOB GL was significantly reduced in volume, while the pAOB GL displayed a similar volume. We conclude that these sister species exhibit a very sharp difference in the anatomical conformation of the AOB, namely, the relative size of the GL of the aAOB subdomain, which is larger in O. degus than in O. lunatus. We discuss these results in the context of the differences in the lifestyle of these species, highlighting the differences in social behaviour as a possible factor driving to distinct AOB morphometries.

M, Mpodozis J. Anatomical Specializations Related to Foraging in the Visual System of a Nocturnal Insectivorous Bird, the Band-Winged Nightjar (Aves: Caprimulgiformes)

Nocturnal animals that rely on their visual system for foraging, mating, and navigation usually exhibit specific traits associated with living in scotopic conditions. Most nocturnal birds have several visual specializations, such as enlarged eyes and an increased orbital convergence. However, the actual role of binocular vision in nocturnal foraging is still debated. Nightjars (Aves: Caprimulgidae) are predators that actively pursue and capture flying insects in crepuscular and nocturnal environments, mainly using a conspicuous “sit-and-wait” tactic on which pursuit begins with an insect flying over the bird that sits on the ground. In this study, we describe the visual system of the band-winged nightjar (Systellura longirostris), with emphasis on anatomical features previously described as relevant for nocturnal birds. Orbit convergence, determined by 3D scanning of the skull, was 73.28°. The visual field, determined by ophthalmoscopic reflex, exhibits an area of maximum binocular overlap of 42°, and it is dorsally oriented. The eyes showed a nocturnal-like normalized corneal aperture/axial length index. Retinal ganglion cells (RGCs) were relatively scant, and distributed in an unusual oblique-band pattern, with higher concentrations in the ventrotemporal quadrant. Together, these results indicate that the band-winged nightjar exhibits a retinal specialization associated with the binocular area of their dorsal visual field, a relevant area for pursuit triggering and prey attacks. The RGC distribution observed is unusual among birds, but similar to that of some visually dependent insectivorous bats, suggesting that those features might be convergent in relation to feeding strategies.

Characterization of Retinal Functionality at Different Eccentricities in a Diurnal Rodent

Although the properties of the neurons of the visual system that process central and peripheral regions of the visual field have been widely researched in the visual cortex and the LGN, they have scarcely been documented for the retina. The retina is the first step in integrating optical signals, and despite considerable efforts to functionally characterize the different types of retinal ganglion cells (RGCs), a clear account of the particular functionality of cells with central vs. peripheral fields is still wanting. Here, we use electrophysiological recordings, gathered from retinas of the diurnal rodent Octodon degus, to show that RGCs with peripheral receptive fields (RF) are larger, faster, and have shorter transient responses. This translates into higher sensitivity at high temporal frequencies and a full frequency bandwidth when compared to RGCs with more central RF. We also observed that imbalances between ON and OFF cell populations are preserved with eccentricity. Finally, the high diversity of functional types of RGCs highlights the complexity of the computational strategies implemented in the early stages of visual processing, which could inspire the development of bio-inspired artificial systems.

Topographic specializations of catecholaminergic cells and ganglion cells and distribution of calcium binding proteins in the crepuscular rock cavy (Kerodon rupestris) retina

The rock cavy (Kerodon rupestris) is a crepuscular Hystricomorpha rodent that has been used in comparative analysis of retinal targets, but its retinal organization remains to be investigated. In order to better characterize its visual system, the present study analyzed neurochemical features related to the topographic organization of catecholaminergic cells and ganglion cells, as well the distribution of calcium-binding proteins in the outer and inner retina. Retinal sections and/or wholemounts were processed using tyrosine hydroxylase (TH), GABA, calbindin, parvalbumin and calretinin immunohistochemistry or Nissl staining. Two types of TH-immunoreactive (TH-IR) cells were found which differ in soma size, dendritic arborization, intensity of TH immunoreactivity and stratification pattern in the inner plexiform layer. The topographic distribution of all TH-IR cells defines a visual streak along the horizontal meridian in the superior retina. The ganglion cells are also distributed in a visual streak and the visual acuity estimated considering their peak density is 4.13 cycles/degree. A subset of TH-IR cells express GABA or calbindin. Calretinin is abundant in most of retinal layers and coexists with calbindin in horizontal cells. Parvalbumin is less abundant and expressed by presumed amacrine cells in the INL and some ganglion cells in the GCL. The topographic distribution of TH-IR cells and ganglion cells in the rock cavy retina indicate a suitable adaptation for using a broad extension of its inferior visual field in aspects that involve resolution, adjustment to ambient light intensity and movement detection without specialized eye movements.

Observational fear learning in degus is correlated with temporal vocalization patterns

Some animals learn to fear a situation after observing another individual come to harm, and this learning is influenced by the animals' social relationship and history. An important but sometimes overlooked factor in studies of observational fear learning is that social context not only affects observers, but may also influence the behavior and communications expressed by those being observed. Here we sought to investigate whether observational fear learning in the degu (Octodon degus) is affected by social familiarity, and the degree to which vocal expressions of alarm or distress contribute. 'Demonstrator' degus underwent contextual fear conditioning in the presence of a cagemate or stranger observer. Among the 15 male pairs, observers of familiar demonstrators exhibited higher freezing rates than observers of strangers when returned to the conditioning environment one day later. Observer freezing during testing was, however, also related to the proportion of short- versus long- inter-call-intervals (ICIs) in vocalizations recorded during prior conditioning. In a regression model that included both social relationship and ICI patterns, only the latter was significant. Further investigation of vocalizations, including use of a novel, directed k-means clustering approach, suggested that temporal structure rather than tonal variations may have been responsible for communicating danger. These data offer insight into how different expressions of distress or fear may impact an observer, adding to the complexity of social context effects in studies of empathy and social cognition. The experiments also offer new data on degu alarm calls and a potentially novel methodological approach to complex vocalizations.

Selective binocular vision loss in two subterranean caviomorph rodents: Spalacopus cyanus and Ctenomys talarum

To what extent can the mammalian visual system be shaped by visual behavior? Here we analyze the shape of the visual fields, the densities and distribution of cells in the retinal ganglion-cell layer and the organization of the visual projections in two species of facultative non-strictly subterranean rodents, Spalacopus cyanus and Ctenomys talarum, aiming to compare these traits with those of phylogenetically closely related species possessing contrasting diurnal/nocturnal visual habits. S. cyanus shows a definite zone of frontal binocular overlap and a corresponding area centralis, but a highly reduced amount of ipsilateral retinal projections. The situation in C. talarum is more extreme as it lacks of a fronto-ventral area of binocular superposition, has no recognizable area centralis and shows no ipsilateral retinal projections except to the suprachiasmatic nucleus. In both species, the extension of the monocular visual field and of the dorsal region of binocular overlap as well as the whole set of contralateral visual projections, appear well-developed. We conclude that these subterranean rodents exhibit, paradoxically, diurnal instead of nocturnal visual specializations, but at the same time suffer a specific regression of the anatomical substrate for stereopsis. We discuss these findings in light of the visual ecology of subterranean lifestyles.

Congruent representation of visual and acoustic space in the superior colliculus of the echolocating bat Phyllostomus discolor

The midbrain superior colliculus (SC) commonly features a retinotopic representation of visual space in its superficial layers, which is congruent with maps formed by multisensory neurons and motor neurons in its deep layers. Information flow between layers is suggested to enable the SC to mediate goal-directed orienting movements. While most mammals strongly rely on vision for orienting, some species such as echolocating bats have developed alternative strategies, which raises the question how sensory maps are organized in these animals. We probed the visual system of the echolocating bat Phyllostomus discolor and found that binocular high acuity vision is frontally oriented and thus aligned with the biosonar system, whereas monocular visual fields cover a large area of peripheral space. For the first time in echolocating bats, we could show that in contrast with other mammals, visual processing is restricted to the superficial layers of the SC. The topographic representation of visual space, however, followed the general mammalian pattern. In addition, we found a clear topographic representation of sound azimuth in the deeper collicular layers, which was congruent with the superficial visual space map and with a previously documented map of orienting movements. Especially for bats navigating at high speed in densely structured environments, it is vitally important to transfer and coordinate spatial information between sensors and motor systems. Here, we demonstrate first evidence for the existence of congruent maps of sensory space in the bat SC that might serve to generate a unified representation of the environment to guide motor actions.

Adaptation to nocturnality - learning from avian genomes

The recent availability of multiple avian genomes has laid the foundation for a huge variety of comparative genomics analyses including scans for changes and signatures of selection that arose from adaptions to new ecological niches. Nocturnal adaptation in birds, unlike in mammals, is comparatively recent, a fact that makes birds good candidates for identifying early genetic changes that support adaptation to dim-light environments. In this review, we give examples of comparative genomics analyses that could shed light on mechanisms of adaptation to nocturnality. We present advantages and disadvantages of both "data-driven" and "hypothesis-driven" approaches that lead to the discovery of candidate genes and genetic changes promoting nocturnality. We anticipate that the accessibility of multiple genomes from the Genome 10K Project will allow a better understanding of evolutionary mechanisms and adaptation in general.

Effects of Habitat and Social Complexity on Brain Size, Brain Asymmetry and Dentate Gyrus Morphology in Two Octodontid Rodents

Navigational and social challenges due to habitat conditions and sociality are known to influence dentate gyrus (DG) morphology, yet the relative importance of these factors remains unclear. Thus, we studied three natural populations of O. lunatus (Los Molles) and Octodon degus (El Salitre and Rinconada), two caviomorph species that differ in the extent of sociality and with contrasting vegetation cover of habitat used. The brains and DG of male and female breeding degus with simultaneous information on their physical and social environments were examined. The extent of sociality was quantified from total group size and range area overlap. O. degus at El Salitre was more social than at Rinconada and than O. lunatus from Los Molles. The use of transects to quantify cover of vegetation (and other physical objects in the habitat) and measures of the spatial behavior of animals indicated animal navigation based on unique cues or global landmarks is more cognitively challenging to O. lunatus. During lactation, female O. lunatus had larger brains than males. Relative DG volume was similar across sexes and populations. The right hemisphere of male and female O. lunatus had more cells than the left hemisphere, with DG directional asymmetry not found in O. degus. Degu population differences in brain size and DG cell number seemed more responsive to differences in habitat than to differences in sociality. Yet, large-sized O. degus (but not O. lunatus) that ranged over larger areas and were members of larger social groups had more DG cells per hemisphere. Thus, within-population variation in DG cell number by hemisphere was consistent with a joint influence of habitat and sociality in O. degus at El Salitre.

Microconnectomics of the pretectum and ventral thalamus in the chicken (Gallus gallus)

The avian pretectal and ventrothalamic nuclei, encompassing the griseum tectale (GT), n. lentiformis mesencephali (LM), and n. geniculatus lateralis pars ventralis (GLv), are prominent retinorecipient structures related to optic flow operations and visuomotor control. Hence, a close coordination of these neural circuits is to be expected. Yet the connectivity among these nuclei is poorly known. Here, using intracellular labeling and in situ hybridization, we investigated the detailed morphology, connectivity, and neurochemical identity of neurons in these nuclei. Two different cell types exist in the GT: one that generates an axonal projection to the optic tectum (TeO), LM, GLv, and n. intercalatus thalami (ICT), and a second population that only projects to the LM and GLv. In situ hybridization revealed that most neurons in the GT express the vesicular glutamate transporter (VGluT2) mRNA, indicating a glutamatergic identity. In the LM, three morphological cell types were defined, two of which project axons towards dorsal targets. The LM neurons showed strong VGluT2 expression. Finally, the cells located in the GLv project to the TeO, LM, GT, n. principalis precommisuralis (PPC), and ICT. All neurons in the GLv showed strong expression of the vesicular inhibitory amino acid transporter (VIAAT) mRNA, suggesting a GABAergic identity. Our results show that the pretectal and ventrothalamic nuclei are highly interconnected, especially by glutamatergic and GABAergic neurons from the GT and GLv, respectively. This complex morphology and connectivity might be required to organize orienting visuomotor behaviors and coordinate the specific optic flow patterns that they induce. J. Comp. Neurol. 524:2208-2229, 2016. © 2015 Wiley Periodicals, Inc.

Olfaction, navigation, and the origin of isocortex

There are remarkable similarities between the brains of mammals and birds in terms of microcircuit architecture, despite obvious differences in gross morphology and development. While in reptiles and birds the most expanding component (the dorsal ventricular ridge) displays an overall nuclear shape and derives from the lateral and ventral pallium, in mammals a dorsal pallial, six-layered isocortex shows the most remarkable elaboration. Regardless of discussions about possible homologies between mammalian and avian brains, a main question remains in explaining the emergence of the mammalian isocortex, because it represents a unique phenotype across amniotes. In this article, we propose that the origin of the isocortex was driven by behavioral adaptations involving olfactory driven goal-directed and navigating behaviors. These adaptations were linked with increasing sensory development, which provided selective pressure for the expansion of the dorsal pallium. The latter appeared as an interface in olfactory-hippocampal networks, contributing somatosensory information for navigating behavior. Sensory input from other modalities like vision and audition were subsequently recruited into this expanding region, contributing to multimodal associative networks.

Alzheimer's Disease-Related Protein Expression in the Retina of Octodon degus

New studies show that the retina also undergoes pathological changes during the development of Alzheimer's disease (AD). While transgenic mouse models used in these previous studies have offered insight into this phenomenon, they do not model human sporadic AD, which is the most common form. Recently, the Octodon degus has been established as a sporadic model of AD. Degus display age-related cognitive impairment associated with Aβ aggregates and phosphorylated tau in the brain. Our aim for this study was to examine the expression of AD-related proteins in young, adult and old degus retina using enzyme-linked or fluorescence immunohistochemistry and to quantify the expression using slot blot and western blot assays. Aβ4G8 and Aβ6E10 detected Aβ peptides in some of the young animals but the expression was higher in the adults. Aβ peptides were observed in the inner and outer segment of the photoreceptors, the nerve fiber layer (NFL) and ganglion cell layer (GCL). Expression was higher in the central retinal region than in the retinal periphery. Using an anti-oligomer antibody we detected Aβ oligomer expression in the young, adult and old retina. Immunohistochemical labeling showed small discrete labeling of oligomers in the GCL that did not resemble plaques. Congo red staining did not result in green birefringence in any of the animals analyzed except for one old (84 months) animal. We also investigated expression of tau and phosphorylated tau. Expression was seen at all ages studied and in adults it was more consistently observed in the NFL-GCL. Hyperphosphorylated tau detected with AT8 antibody was significantly higher in the adult retina and it was localized to the GCL. We confirm for the first time that Aβ peptides and phosphorylated tau are expressed in the retina of degus. This is consistent with the proposal that AD biomarkers are present in the eye.