Distribution of retinal ganglion cells in five mammalian species (pig, sheep, ox, horse, dog)

Spontaneous eye blinks in horses (Equus caballus) are modulated by attention

Spontaneous eye blinks are brief closures of both eyelids. The spontaneous eye blink rate (SEBR) exceeds physiological corneal needs and is modulated by emotions and cognitive states, including vigilance and attention, in humans. In several animal species, the SEBR is modulated by stress and antipredator vigilance, which may limit the loss of visual information due to spontaneous eye closing. Here, we investigated whether the SEBR is modulated by attention in the domestic horse (Equus caballus). Our data supported previous studies indicating a tonic SEBR specific to each individual. We also found that, superimposed on a tonic SEBR, phasic changes were induced by cognitive processing. Attention downmodulated the SEBR, with the magnitude of blink inhibition proportional to the degree of attentional selectivity. On the other hand, reward anticipation upregulated the SEBR. Our data also suggested that horses possess the cognitive property of object permanence: they understand that an object that is no longer in their visual field has not ceased to exist. In conclusion, our results suggested that spontaneous eye blinks in horses are modulated by attentional cognitive processing.

Pig Models in Retinal Research and Retinal Disease

The pig has been used as a large animal model in biomedical research for many years and its use continues to increase because induced mutations phenocopy several inherited human diseases. In addition, they are continuous breeders, can be propagated by artificial insemination, have large litter sizes (on the order of mice), and can be genetically manipulated using all of the techniques that are currently available in mice. The pioneering work of Petters and colleagues set the stage for the use of the pig as a model of inherited retinal disease. In the last 10 years, the pig has become a model of choice where specific disease-causing mutations that are not phenocopied in rodents need to be studied and therapeutic approaches explored. The pig is not only used for retinal eye disease but also for the study of the cornea and lens. This review attempts to show how broad the use of the pig has become and how it has contributed to the assessment of treatments for eye disease. In the last 10 years, there have been several reviews that included the use of the pig in biomedical research (see body of the review) that included information about retinal disease. None directly discuss the use of the pig as an animal model for retinal diseases, including inherited diseases, where a single genetic mutation has been identified or for multifactorial diseases such as glaucoma and diabetic retinopathy. Although the pig is used to explore diseases of the cornea and lens, this review focuses on how and why the pig, as a large animal model, is useful for research in neural retinal disease and its treatment.

Do you see what I see? Testing horses' ability to recognise real-life objects from 2D computer projections

The use of 2-dimensional representations (e.g. photographs or digital images) of real-life physical objects has been an important tool in studies of animal cognition. Horses are reported to recognise objects and individuals (conspecifics and humans) from printed photographs, but it is unclear whether image recognition is also true for digital images, e.g. computer projections. We expected that horses trained to discriminate between two real-life objects would show the same learnt response to digital images of these objects indicating that the images were perceived as objects, or representations of such. Riding-school horses (N = 27) learnt to touch one of two objects (target object counterbalanced between horses) to instantly receive a food reward. After discrimination learning (three consecutive sessions of 8/10 correct trials), horses were immediately tested with on-screen images of the objects over 10 image trials interspersed with five real object trials. At first image presentation, all but two horses spontaneously responded to the images with the learnt behaviour by contacting one of the two images, but the number of horses touching the correct image was not different from chance (14/27 horses, p > 0.05). Only one horse touched the correct image above chance level across 10 image trials (9/10 correct responses, p = 0.021). Our findings thus question whether horses recognise real-life objects from digital images. We discuss how methodological factors and individual differences (i.e. age, welfare state) might have influenced animals' response to the images, and the importance of validating the suitability of stimuli of this kind for cognitive studies in horses.

Normative Data of Axial Length, Retinal Thickness Measurements, Visual Evoked Potentials, and Full-Field Electroretinography in Female, Wild-Type Minipigs

Purpose

The purpose of this study was to present normative data of optical coherence tomography (OCT), electrophysiological, and ocular biometry parameters and their correlation in minipigs.

Methods

Eighty-eight eyes of 44 minipigs underwent full-field electroretinogram (ERG) recording and ocular biometry. However, 10 eyes of 6 minipigs were excluded because of poor OCT image quality. The thickness of the retinal sublayers was measured on a vertical line at 5 locations with a 1 mm interval from the disc margin to the dorsal periphery and at 10 locations on the visual streak. Visual evoked potentials (VEPs) were measured in 15 eyes of 8 minipigs.

Results

All minipigs were female with a mean age and axial length of 13.83 ± 10.56 months and 20.33 ± 0.88 mm, respectively. The implicit time of the a-wave and b-wave in scotopic 3.0 ERGs was longer than that in photopic 3.0 ERG. The implicit time of the n2-wave and p2-wave in VEP was 25.67 ± 7.41 ms and 52.96 ± 10.38 ms, respectively. The total retinal layer (TRL) and nerve fiber layer (NFL) became thinner near the periphery. The inner retinal sublayers near the visual streak were thicker than those at other locations. Central TRL and NFL thickness on visual streak was 223.06 ± 23.19 µm and 74.03 ± 13.93 µm, respectively. The temporal TRL and NFL on the visual streak were thicker than those on the nasal side.

Conclusions

The normative electrophysiological and OCT parameters used in our study can be used as reference data in further pig studies.

Translational Relevance

This study presents normative data of minipigs, which are adequate animal models for preclinical studies.

Use of the Retinal Vascular Histology to Validate an Optical Coherence Tomography Angiography Technique

Purpose

To determine the fidelity of optical coherence tomography angiography (OCTA) techniques by direct comparison of the retinal capillary network images obtained from the same region as imaged by OCTA and high-resolution confocal microscope.

Method

Ten porcine eyes were perfused with red blood cells for OCTA image acquisition from the area centralis and then perfusion-fixed, and the vessels were labeled for confocal imaging. Two approaches involving post-processing of two-dimensional projection images and vessel tracking on three dimensional image stacks were used to obtain quantitative measurements. Data collected include vessel density, length of visible vessel track, count of visible branch points, vessel track depth, vessel diameter, angle of vessel descent, and angle of dive for comparison and analysis.

Results

Comparing vascular images acquired from OCTA and confocal microscopy, we found (1) a good representation of the larger caliber retinal vessels, (2) an underrepresentation of retinal microvessels smaller than 10 µm and branch points in all four retinal vascular plexuses, particularly the intermediate capillary plexus, (3) reduced visibility associated with an increase in the angle of descent, (4) a tendency to loss visibility of vessel track at a branch point or during a sharp dive, and (5) a reduction in visibility with increase in retinal depth on OCTA images.

Conclusions

Current OCTA techniques can visualize the retinal capillary network, but some types of capillaries cannot be detected by OCTA, particularly in the middle to deeper layers.

Translational Relevance

The information indicates the limitation in clinical use and scopes for improvement in the current OCTA technologies.

Ganglion cells and displaced amacrine cells density in the retina of the collared peccary (Pecari tajacu)

In the present study, we investigated the topographical distribution of ganglion cells and displaced amacrine cells in the retina of the collared peccary (Pecari tajacu), a diurnal neotropical mammal of the suborder Suina (Order Artiodactyla) widely distributed across central and mainly South America. Retinas were prepared and processed following the Nissl staining method. The number and distribution of retinal ganglion cells and displaced amacrine cells were determined in six flat-mounted retinas from three animals. The average density of ganglion cells was 351.822 ± 31.434 GC/mm². The peccary shows a well-developed visual streak. The average peak density was 6,767 GC/mm² and located within the visual range and displaced temporally as an area temporalis. Displaced amacrine cells have an average density of 300 DAC/mm², but the density was not homogeneous along the retina, closer to the center of the retina the number of cells decreases and when approaching the periphery the density increases, in addition, amacrine cells do not form retinal specialization like ganglion cells. Outside the area temporalis, amacrine cells reach up to 80% in the ganglion cell layer. However, in the region of the area temporalis, the proportion of amacrine cells drops to 32%. Thus, three retinal specializations were found in peccary’s retina by ganglion cells: visual streak, area temporalis and dorsotemporal extension. The topography of the ganglion cells layer in the retina of the peccary resembles other species of Order Artiodactyla already described and is directly related to its evolutionary history and ecology of the species.

Sensory Abilities of Horses and Their Importance for Equitation Science

Vision, hearing, olfaction, taste, and touch comprise the sensory modalities of most vertebrates. With these senses, the animal receives information about its environment. How this information is organized, interpreted, and experienced is known as perception. The study of the sensory abilities of animals and their implications for behavior is central not only to ethology but also to animal welfare. Sensory ability, perception, and behavior are closely linked. Horses and humans share the five most common sensory modalities, however, their ranges and capacities differ, so that horses are unlikely to perceive their surroundings in a similar manner to humans. Understanding equine perceptual abilities and their differences is important when horses and human interact, as these abilities are pivotal for the response of the horse to any changes in its surroundings. This review aims to provide an overview of the current knowledge on the sensory abilities of horses. The information is discussed within an evolutionary context and also includes a practical perspective, outlining potential ways to mitigate risks of injuries and enhance positive horse-human interactions. The equine sensory apparatus includes panoramic visual capacities with acuities similar to those of red-green color-blind humans as well as aural abilities that, in some respects exceed human hearing and a highly developed sense of smell, all of which influence how horses react in various situations. Equine sensitivity to touch has been studied surprisingly sparingly despite tactile stimulation being the major interface of horse training. We discuss the potential use of sensory enrichment/positive sensory stimulation to improve the welfare of horses in various situations e.g. using odors, touch or sound to enrich the environment or to appease horses. In addition, equine perception is affected by factors such as breed, individuality, age, and in some cases even color, emphasizing that different horses may need different types of management. Understanding the sensory abilities of horses is central to the emerging discipline of equitation science, which comprises the gamut of horse-human interactions. Therefore, sensory abilities continue to warrant scientific focus, with more research to enable us to understand different horses and their various needs.

Origins of Functional Organization in the Visual Cortex

How are the complex maps for orientation selectivity (OS) created in the primary visual cortex (V1)? Rodents and rabbits have a random distribution of OS preferences across V1 while in cats, ferrets, and all primates cells with similar OS preferences cluster together into relatively wide cortical columns. Given other clear similarities in the organization of the visual pathways, why is it that maps coding OS preferences are so radically different? Prominent models have been created of cortical OS mapping that incorporate Hebbian plasticity, intracortical interactions, and the properties of growing axons. However, these models suggest that the maps arise primarily through intracortical interactions. Here we focus on several other features of the visual system and brain that may influence V1 structure. These are: eye divergence, the total number of cells in V1, the thalamocortical networks, the topography of the retina and phylogeny. We outline the evidence for and against these factors contributing to map formation. One promising theory is that the central-to-peripheral ratio (CP ratio) of retinal cell density can be used to predict whether or not a species has pinwheel maps. Animals with high CP ratios (>7) have orientation columns while those with low CP ratios (<4) have random OS maps. The CP ratio is related to the total number of cells in cortex, which also appears to be a reasonable contributing factor. However, while these factors correlate with map structure to some extent, there is a gray area where certain species do not fit elegantly into the theory. A problem with the existing literature is that OS maps have been investigated in only a small number of mammals, from a small fraction of the mammalian phylogenetic tree. We suggest four species (agouti, fruit bat, sheep, and wallaby) that have a range of interesting characteristics, which sit at intermediate locations between primates and rodents, that make them good targets for filling in the missing gaps in the literature. We make predictions about the map structures of these species based on the organization of their brains and visual systems and, in doing so, set possible paths for future research.

Short-Time Ocular Ischemia Induces Vascular Endothelial Dysfunction and Ganglion Cell Loss in the Pig Retina

Visual impairment and blindness are often caused by retinal ischemia-reperfusion (I/R) injury. We aimed to characterize a new model of I/R in pigs, in which the intraocular pathways were not manipulated by invasive methods on the ocular system. After 12 min of ischemia followed by 20 h of reperfusion, reactivity of retinal arterioles was measured in vitro by video microscopy. Dihydroethidium (DHE) staining, qPCR, immunohistochemistry, quantification of neurons in the retinal ganglion cell layer, and histological examination was performed. Retinal arterioles of I/R-treated pigs displayed marked attenuation in response to the endothelium-dependent vasodilator, bradykinin, compared to sham-treated pigs. DHE staining intensity and messenger RNA levels for HIF-1α, VEGF-A, NOX2, and iNOS were elevated in retinal arterioles following I/R. Immunoreactivity to HIF-1α, VEGF-A, NOX2, and iNOS was enhanced in retinal arteriole endothelium after I/R. Moreover, I/R evoked a substantial decrease in Brn3a-positive retinal ganglion cells and noticeable retinal thickening. In conclusion, the results of the present study demonstrate that short-time ocular ischemia impairs endothelial function and integrity of retinal blood vessels and induces structural changes in the retina. HIF-1α, VEGF-A, iNOS, and NOX2-derived reactive oxygen species appear to be involved in the pathophysiology.

A novel test for evaluating horses' spontaneous visual attention is predictive of attention in operant learning tasks

Attention is described as the ability to process selectively one aspect of the environment over others. In this study, we characterized horses' spontaneous attention by designing a novel visual attention test (VAT) that is easy to apply in the animal's home environment. The test was repeated over three consecutive days and repeated again 6 months later in order to assess inter-individual variations and intra-individual stability. Different patterns of attention have been revealed: 'overall' attention when the horse merely gazed at the stimulus and 'fixed' attention characterized by fixity and orientation of at least the visual and auditory organs towards the stimulus. The individual attention characteristics remained consistent over time (after 6 months, Spearman correlation test, P < 0.05). The validity of this novel test as a predictor of individual attentional skills was assessed by comparing the results, for the same horses, with those obtained in both a 'classical' experimental attention test the 'five-choice serial reaction time task' (5-CSRTT) and a work situation (lunge working context). Our results revealed that (i) individual variations remained consistent across tests and (ii) the VAT attention measures were not only predictive of attentional skills but also of learning abilities. Differences appeared however between the first day of testing and the following test days: attention structure on the second day was predictive of learning abilities, attention performances in the 5-CSRRT and at work. The VAT appears as a promising easy-to-use tool to assess animals' attention characteristics and the impact of different factors of variation on attention.

The eye of the Barbary sheep or aoudad (Ammotragus lervia): reference values for selected ophthalmic diagnostic tests, morphologic and biometric observations

The purpose of this study was to describe the normal ocular anatomy and establish reference values for ophthalmic tests in the Barbary sheep or aoudad (Ammotragus lervia). Aoudad eyes are large and laterally positioned in the head with several specialized anatomic features attributed to evolutionary adaptations for grazing. Normal values for commonly used ophthalmic tests were established, Schirmer tear test (STT) - 27.22 ± 3.6 mm/min; Predominant ocular surface bacterial microbiota - Staphylococcus sp.; Corneal esthesiometry- 1.3 ± 0.4 cm; Intraocular pressure by rebound tonometry- 19.47 ± 3.9 mmHg; Corneal thickness- 630.07 ± 20.67 µm, B-mode ultrasonography of the globe-axial eye globe length 29.94 ± 0.96 mm, anterior chamber depth 5.03 ± 0.17 mm, lens thickness 9.4 ± 0.33 mm, vitreous chamber depth 14.1 ± 0.53 mm; Corneal diameter-horizontal corneal diameter 25.05 ± 2.18 mm, vertical corneal diameter 17.95 ± 1.68 mm; Horizontal palpebral fissure length- 34.8 ± 3.12 mm. Knowledge of these normal anatomic variations, biometric findings and normal parameters for ocular diagnostic tests may assist veterinary ophthalmologists in the diagnosis of ocular diseases in this and other similar species.

Spatial relationships among the cellular tapetum, visual streak and rod density in dogs

The dog visual system is well suited to dim light conditions due to rod-dominated retina and the reflective tapetum. The topographical distributions of rods and thickness of the tapetum of the dog were quantified in retinal whole mounts stained with thionine, and spatial relationships among the tapetum, rod density and visual streak of high ganglion cell density were elucidated. The relationship between the retina and tapetum was analyzed in parasagittal sections stained with thionine or hematoxylin-eosin. The tapetum was thick in its center, and the thickest part consisted of 9 to 12 tapetal cell layers. Rod density ranged from 200,000 to 540,000/mm². Maximum rod density was found in the area dorsal to the visual streak, and the density in that area was significantly higher than the rod density in the visual streak and accorded spatially with the thickest part of the tapetum. The horizontal visual streak was found over the horizontal line through the optic disc in the temporal half and extended slightly into the nasal half. The central area of the highest density of ganglion cells was approximately located midway between the nasal and temporal ends of the visual streak. The visual streak was located within the tapetal area, but ventrally to the thick part of the tapetum.

Canine retina has a primate fovea-like bouquet of cone photoreceptors which is affected by inherited macular degenerations

Retinal areas of specialization confer vertebrates with the ability to scrutinize corresponding regions of their visual field with greater resolution. A highly specialized area found in haplorhine primates (including humans) is the fovea centralis which is defined by a high density of cone photoreceptors connected individually to interneurons, and retinal ganglion cells (RGCs) that are offset to form a pit lacking retinal capillaries and inner retinal neurons at its center. In dogs, a local increase in RGC density is found in a topographically comparable retinal area defined as the area centralis. While the canine retina is devoid of a foveal pit, no detailed examination of the photoreceptors within the area centralis has been reported. Using both in vivo and ex vivo imaging, we identified a retinal region with a primate fovea-like cone photoreceptor density but without the excavation of the inner retina. Similar anatomical structure observed in rare human subjects has been named fovea-plana. In addition, dogs with mutations in two different genes, that cause macular degeneration in humans, developed earliest disease at the newly-identified canine fovea-like area. Our results challenge the dogma that within the phylogenetic tree of mammals, haplorhine primates with a fovea are the sole lineage in which the retina has a central bouquet of cones. Furthermore, a predilection for naturally-occurring retinal degenerations to alter this cone-enriched area fills the void for a clinically-relevant animal model of human macular degenerations.

The fibrous tapetum of the horse eye

The tapetum lucidum is a light-reflective tissue in the eyes of many animals. Many ungulates have a fibrous tapetum. The horse has one of the largest eyes of any living animal and also has excellent vision in low-light environments. This study aimed to clarify the macroscopic tapetal shape, relationship between the tapetal thickness and the degree of pigmentation of the retinal pigment epithelium (RPE), spatial relationship between the visual streak and the tapetum, and wavelength of the light reflected from the tapetum in the horse. Macroscopically, weak light revealed the tapetum as a horizontal band located dorsal to and away from the optic disc. The tapetum expanded dorsally as the illumination increased. The tapetal tissue consisted of lamellae of collagen fibrils running parallel to the retinal surface; these spread over almost the entire ocular fundus and were thicker in the horizontal band dorsal to the disc. Only the horizontal band of the tapetum was covered by unpigmented RPE, suggesting that this band reflects light and is responsible for mesopic and scotopic vision. The visual streak was located in the ventral part of the horizontal band, ventral to the thickest part of the tapetum. The wavelength of the light reflected from the horizontal band of the tapetum was estimated from the diameter and interfibrous distance of the collagen fibrils to be approximately 468 nm. Therefore, the light reflected from the tapetum should be more effectively absorbed by rods than by cones, and should not interfere with photopic vision.

Blue-cone horizontal cells in the retinae of horses and other equidae

The morphology of horizontal cells chiefly of the horse, but also of asses, mules, and a zebra, has been examined by Lucifer yellow injections into lightly fixed retinae and by immunocytochemistry. In common with other mammals, equids have a B-type horizontal cell, i.e., a cell with dendrites synapsing with cones and possessing a single axon synapsing with rods. Most mammalian retinae have a further type of horizontal cell, the A-type, also synapsing with cones but without an axon. The second type of horizontal cell in equids also has no axon; otherwise, it is most unusual. Compared with other mammalian A-type cells, it has a vary large dendritic field, both absolutely and relative to the dendritic fields of B-type cells. The dendrites are fine and sparsely branching. Their most striking feature is that they bear a low density of irregularly spaced synaptic terminal aggregates, suggesting their cone contacts are selective. Immunolabelling of S (blue)-cones in horse retina showed that they comprise, depending on retinal location, 10-25% of the cone population. For a single horse A-type cell, it is shown that 44 of its 45 terminal aggregates are congruent with the pedicles of S-cones. Immunostaining with a calbindin antibody demonstrated that each type of horizontal cell forms an independent regular mosaic. The density ratio of B- to A-type cells varied between 5 and 10. This is the first demonstration in a mammalian retina of a horizontal cell type with a direct input exclusively from S-cones.