Article

Horse vision and an explanation for the visual behaviour originally explained by the ‘ramp retina’

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Abstract

Here we provide confirmation that the 'ramp retina' of the horse, once thought to result in head rotating visual behaviour, does not exist. We found a 9% variation in axial length of the eye between the streak region and the dorsal periphery. However, the difference was in the opposite direction to that proposed for the 'ramp retina'. Furthermore, acuity in the narrow, intense visual streak in the inferior retina is 16.5 cycles per degree compared with 2.7 cycles per degree in the periphery. Therefore, it is improbable that the horse rotates its head to focus onto the peripheral retina. Rather, the horse rotates the nose up high to observe distant objects because binocular overlap is oriented down the nose, with a blind area directly in front of the forehead.

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... Three studies had previously mapped the distribution of equine ganglion cells using whole-mounted retinae (Hebel, 1976;Harman, et al., 1999;Guo and Sugita, 2000). They all described the presence of a visual streak, which is a narrow (~1 mm) high density linear band of ganglion cells extending both nasally and temporally, 2-3 mm dorsal to the optic disc. ...
... Thoroughbreds. The other two studies (Hebel 1976;Harman et al., 1999) examined 3 horses each, but did not specify the breeds examined. A wider variety of breeds need to be examined before it can be assumed that all members of this morphologically diverse species have similar retinal ganglion cell distributions. ...
... The following equation from Harman et al. (1999) Posterior nodal distance can be calculated as 0.67 x the axial length of the eye (Harman et al., 1999). However, only eye volume was recorded in this study, not the axial lengths of the eyes. ...
Conference Paper
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There is a wide range of skull shapes within the domestic horse (Equus caballus). This is of interest because skull morphology, in particular nose length, has been shown to correlate strongly with the distribution of retinal ganglion cells in dogs. The current study examined the skull morphology and retinae of 30 horses from a variety of breeds to determine whether skull morphology was a predictive factor of ganglion cell distribution in the equine retina and therefore a potential influence on visual field. The density of ganglion cells varied significantly between individuals, with breed being a significant predictor. There was a strong positive correlation between the nasal length and the density of ganglion cells in the visual streak. There was also significant variation in the density of ganglion cells in the area centralis but this variation did not correlate with any of the skull morphology measurements. The density of ganglion cells in an individual's retina, along with its retinal magnification factor, can be used to calculate an estimate of that animal's visual acuity. The relative estimated visual acuity of the Standardbred, Thoroughbred and Arabian horses in the current study were calculated and it was demonstrated that the Standardbreds (with their longer noses) are likely to have better visual acuity in the visual streak than the Arabians (shorter noses). This may have implications for the comparative ability of horses from various breeds to detect visual stimuli, especially in the peripheral field. This, in turn, has implications for equitation especially in terms of control of flight responses.
... The visual attention test & Procedure A novel moving visual stimulus was chosen to avoid biases due to stimulus familiarity and to take into account horses' abilities to detect movements (Harman et al. 1999;Hebel 1976). Horses' vision characteristics are (i) a retina with considerably more rods than cones suggesting high skills for motion stimuli detection, (ii) a wide visual field due to lateral placement of the eyes (each eye has a horizontal visual field of up to 215°) and (iii) the privileged use of binocular visual field in order to enhance visual acuity (Hanggi and Ingersoll 2009;Harman et al. 1999;Hughes 1977). ...
... The visual attention test & Procedure A novel moving visual stimulus was chosen to avoid biases due to stimulus familiarity and to take into account horses' abilities to detect movements (Harman et al. 1999;Hebel 1976). Horses' vision characteristics are (i) a retina with considerably more rods than cones suggesting high skills for motion stimuli detection, (ii) a wide visual field due to lateral placement of the eyes (each eye has a horizontal visual field of up to 215°) and (iii) the privileged use of binocular visual field in order to enhance visual acuity (Hanggi and Ingersoll 2009;Harman et al. 1999;Hughes 1977). In experimental situations, horses' visual abilities have been shown to be adapted not only to natural objects but also to complex computer-generated stimuli as well (e.g. ...
... Using continuous focal sampling (Altmann 1974), the total amount of time spent gazing at the stimulus was recorded frame by frame (frame, 0.02 s). Horses have laterally placed eyes with a small (60-80°) binocular field of vision and almost complete (80-90%) decussation of the optic nerves, suggesting that behavioural asymmetries reflect asymmetries in hemispheric activation (Harman et al. 1999). ...
Article
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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.
... light and cones for bright light. 3,14,[16][17][18] Rod photoreceptors can reliably respond to a single photon of light and are the primary receptors used when the light levels range from virtually complete darkness on an overcast moonless night to those found at dawn and twilight. 14,19 By midmorning on a sunny day, the rods have become saturated by light, and in order for useful vision to be maintained, the eye must shift over to using primarily cones. ...
... The lateral position of the eyes in the skull affords the horse a wide panoramic view (Figs. 11-16 and 11-17), 16,24,30 and the nasal extension of the retina further enhances the horse's temporal peripheral visual field. On the basis of anatomic relationships, the horse is believed to have a total monocular visual field in the horizontal meridian (i.e., the portion of the horizon that can be seen by an eye when fixed on one point) of approximately 190 to 195 degrees, and up to 178 degrees in the vertical (superior to inferior) meridian. ...
... 68 The latter value approximates vision around its body, with only a few minor "blind spots." 16,24,30 These blind spots are small and located superior and perpendicular to the forehead, directly below the nose, in a small oval region in the superior visual field where light strikes the optic nerve itself, and the width of the animal's head directly behind it. Clearly, this extensive visual field makes it very difficult for a predator or human handler to sneak up on a horse. ...
... The lateral position of the horses' eyes, the size and curvature of the cornea, size and horizontal shape of the pupil, and angular extent of the retina provide the horse with extensive monocular vision. The binocular portion of the visual field is limited to between 65 (Crispin et al., 1990) and 80°(Harman et al., 1999 ) in front of the horse. The latter investigation concluded that this binocular overlap was located down the horse's nose and not directly ahead as was previously thought. ...
... he retina provide the horse with extensive monocular vision. The binocular portion of the visual field is limited to between 65 (Crispin et al., 1990) and 80°(Harman et al., 1999 ) in front of the horse. The latter investigation concluded that this binocular overlap was located down the horse's nose and not directly ahead as was previously thought. Harman et al. (1999) also found that a blind area existed in front of the forehead. In order to get the clearest possible picture of the visual stimuli, the image must be projected onto the area of the retina with the highest ganglion cell density. This area has been found to coincide with the area responsible for binocular vision, the temporal end of the v ...
... In order to get the clearest possible picture of the visual stimuli, the image must be projected onto the area of the retina with the highest ganglion cell density. This area has been found to coincide with the area responsible for binocular vision, the temporal end of the visual streak (Hebel, 1976; Harman et al., 1999; Guo and Sugita, 2000). Thus, the position of the head and consequently the level at which the eye is carried is important in projecting the visual image onto the most sensitive areas of the retina, particularly while the horse is in motion (Saslow, 1999), as when approaching the stimulus boxes in the present study. ...
Article
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Colour is an important feature that increases the visibility of objects and may aid recognition of con-specifics. The adaptive significance of the ability to utilize colour cues has been demonstrated in a number of species, in particular in relation to food selection. The evolution of trichromatic colour vision in primates has been associated with the advantages that it provided in the selection of yellow and orange fruits (Mollon, 1989) or tender young red foliage (Dominy and Lucas, 2001) from a background of green. The horse is a generalist herbivore that must both select nutritious plants and avoid toxic ones. Familiarity is an important factor in this selection process (Augner et al ., 1998), although it is not clear exactly what features are used to recognise previously ingested and “safe” herbage. The horse has been shown to possess dichromatic colour vision. There are two types of cone photopigment in the equine retina, with spectral peaks at 429 and 545 nanometres (Macuda, 2000).
... As noted previously, humans, and undoubtedly horses, are largely unaware of this enormous variation in light intensity, princi pally because objects are perceived in the context of their sur roundings and not in absolute terms. Several mechanisms are used to adjust to this wide range of illumination, one of which is the previously mentioned Purkinje shift wherein a duplex retina (possessing both rods and cones) uses rod photoreceptors for dim light and cones for bright light [3,14,[16][17][18]. Rod photore ceptors can reliably respond to a single photon of light and are the primary receptors used when the light levels range from virtually complete darkness on an overcast moonless night to those found at dawn and twilight [14,19]. ...
... The lateral position of the eyes in the skull affords the horse a wide panoramic view (Figures 12. 16 and 12.17) [16,24,30], and the nasal extension of the retina further enhances the horse's temporal peripheral visual field. On the basis of anatomic relationships, the horse is believed to have a total monocular visual field in the horizontal meridian (i.e., the portion of the horizon that can be seen by an eye when fixed on one point) of approximately 190-195 degrees, and up to 178 degrees in the vertical (superior to inferior) meridian. ...
... On the basis of anatomic relationships, the horse is believed to have a total monocular visual field in the horizontal meridian (i.e., the portion of the horizon that can be seen by an eye when fixed on one point) of approximately 190-195 degrees, and up to 178 degrees in the vertical (superior to inferior) meridian. When the visual fields of the two eyes are combined, the total horizontal visual field is up to 350 degrees, and the horse has virtually a complete sphere of vision around its body, with only a few minor "blind spots" [16,24,30]. Recent behavioral studies have confirmed that horses can detect objects within an almost fully encompassing circle and that they are able to identify the type of object within most, but not all, of their panoramic field of view [58]. ...
Chapter
An understanding of equine vision is important for a number of reasons. Equine practitioners are frequently asked to judge the suitability of a particular horse for specific uses, ranging from the visually demanding (e.g., identification and isolation of a calf) to those that can be performed by nearly blind animals, such as a broodmare walking in an enclosed pasture without injuring herself. Additionally, effective therapy of equine ocular disease requires that the clinician possess a good working understanding of not only the normal visual capabilities and visually guided behaviors of the horse, but also how various ocular diseases alter the animal's vision and hence its utility. An understanding of equine vision also allows the clinician to provide a more accurate prognosis prior to initiating therapy and to more intelligently select from a range of potential therapeutic options for a particular disorder to maximize the probability of optimally preserving the eye's visual capabilities. After preservation of the globe and achieving a comfortable result for the patient, the clinician's therapeutic strategy must be one that best preserves vision.
... light and cones for bright light. 3,14,[16][17][18] Rod photoreceptors can reliably respond to a single photon of light and are the primary receptors used when the light levels range from virtually complete darkness on an overcast moonless night to those found at dawn and twilight. 14,19 By midmorning on a sunny day, the rods have become saturated by light, and in order for useful vision to be maintained, the eye must shift over to using primarily cones. ...
... The lateral position of the eyes in the skull affords the horse a wide panoramic view (Figs. 11-16 and 11-17), 16,24,30 and the nasal extension of the retina further enhances the horse's temporal peripheral visual field. On the basis of anatomic relationships, the horse is believed to have a total monocular visual field in the horizontal meridian (i.e., the portion of the horizon that can be seen by an eye when fixed on one point) of approximately 190 to 195 degrees, and up to 178 degrees in the vertical (superior to inferior) meridian. ...
... 68 The latter value approximates vision around its body, with only a few minor "blind spots." 16,24,30 These blind spots are small and located superior and perpendicular to the forehead, directly below the nose, in a small oval region in the superior visual field where light strikes the optic nerve itself, and the width of the animal's head directly behind it. Clearly, this extensive visual field makes it very difficult for a predator or human handler to sneak up on a horse. ...
... Research by Hall et al. [14] found that when stimuli were placed at ground level, horses were even more successful. Different from the human retina, the equine retina has no central fovea, a central pit composed of tightly packed cones within the retina [15]. Instead, horses have a "visual strip" which allows the horse to broadly see the entire horizon [15]. ...
... Different from the human retina, the equine retina has no central fovea, a central pit composed of tightly packed cones within the retina [15]. Instead, horses have a "visual strip" which allows the horse to broadly see the entire horizon [15]. To bring an object into focus, a horse will lift, lower, or tilt their head to use their visual strip; therefore, head and neck positions are important factors that affect the visual capability of the horse [15]. ...
... Instead, horses have a "visual strip" which allows the horse to broadly see the entire horizon [15]. To bring an object into focus, a horse will lift, lower, or tilt their head to use their visual strip; therefore, head and neck positions are important factors that affect the visual capability of the horse [15]. ...
Article
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Horses (Equus caballus) have been domesticated for millennia and are regularly utilized for work, sport, and companionship. Enhanced understanding of human–horse interactions can create avenues to optimize their welfare. This review explores the current research surrounding many aspects of human–horse interactions by first highlighting the horse’s sensory capabilities and how they pertain to human interactions. Evidence exists that suggests that horses can read humans in various ways through our body odours, posture, facial expressions, and attentiveness. The literature also suggests that horses are capable of remembering previous experiences when working with humans. The interrelatedness of equine cognition and affective states within the horse’s umwelt is then explored. From there, equine personality and the current literature regarding emotional transfer between humans and horses is examined. Even though horses may be capable of recognizing emotional states in humans, there remains a gap in the literature of whether horses are capable of empathizing with human emotion. The objective of this literature review is to explore aspects of the relationship between humans and horses to better understand the horse’s umwelt and thereby shed new light on potential positive approaches to enhance equine welfare with humans.
... Due to the placement of their eyes on the side of their head, zebras can use monocular and binocular vision (Barnett 2004). As a result, they can see to their side using monocular vision while they forage (Harman et al. 1999). This ability could potentially allow zebra to be passively vigilant while they feed and thus detect approaching predators. ...
... When herding with giraffe, zebra had the same options, but looking at herd mates comprised conspecifics and/ or giraffe. For zebra, binocular vision (oriented in the direction of the muzzle) provides better depth perception compared with their side-orientated monocular vision (Harman et al. 1999). As a result, it is more likely that these herbivores would use binocular vision to actively scan for approaching predators. ...
Article
Full-text available
Predation risk of individuals moving in multispecies herds may be lower due to the heightened ability of the different species to detect predators (i.e., mixed-species effect). The giraffe is the tallest land mammal, maintains high vigilance levels, and has good eyesight. As a result, heterospecific herd members could reduce their predation risk if they keyed off the giraffe’s antipredator behaviors. However, because giraffe rarely use audible alarm snorts, heterospecifics would need to eavesdrop on cues given off by the giraffe that indicate predator presence (e.g., body posture), to benefit from herding with giraffe. To test this, we compared the vigilance of zebra herding with conspecifics, with those herding with giraffe. Our results indicate that giraffe reduce zebra vigilance in zebra–giraffe herds and that in these herds, giraffe are the primary source of information regarding predation risk. In contrast, when zebra herd with conspecifics, they rely primarily on personal information gleaned from their environment, as opposed to obtaining information from conspecifics about predation risk.
... While humans are generally very good at seeing fluorescent yellow and white (hence the former's use in high-visibility clothing), the visibility of different fence colours to jockeys during races and training should be considered too. Finally, our work here has also focussed on colour, yet horses have reduced ability to see fine detail and pattern to humans (visual acuity) (Timney and Keil, 1992), albeit with a visual streak across the retina of improved acuity (Harman et al., 1999). Horses also have marked differences in their level of binocular overlap to humans, and a blind spot in front of the head (Harman et al., 1999). ...
... Finally, our work here has also focussed on colour, yet horses have reduced ability to see fine detail and pattern to humans (visual acuity) (Timney and Keil, 1992), albeit with a visual streak across the retina of improved acuity (Harman et al., 1999). Horses also have marked differences in their level of binocular overlap to humans, and a blind spot in front of the head (Harman et al., 1999). These differences may have a similarly important effect on welfare and safety, and performance in training and racing as colour. ...
Article
Full-text available
Visual information is key to how many animals interact with their environment, and much research has investigated how animals respond to colour and brightness information in the natural world. Understanding the visibility of features in anthropogenic environments, and how animals respond to these, is also important, not least for the welfare and safety of animals and the humans they co-exist with, but has received comparatively less attention. One area where this is particularly pertinent is animal sports such as horseracing. Here there is a need to understand how horses see and respond to obstacles, predominantly fences and hurdles, as this has implications for horse and rider safety, however obstacle appearance is currently designed to human perception. Using models of horse colour and luminance (perceived lightness) vision, we analysed the contrast of traditional orange markers currently used on fences from 11 UK racecourses, and compared this to potential alternative colours, while also investigating the effect of light and weather conditions on contrast. We found that for horses, orange has poor visibility and contrast against most surroundings. In comparison, yellow, blue, and white are more conspicuous, with the degree of relative contrast varying with vegetation or background type. Results were mostly consistent under different weather conditions and time of day, except for comparisons with the foreground turf in shade. We then tested the jump responses of racehorses to fences with orange, fluorescent yellow, bright blue, or white takeoff boards and midrails. Fence colour influenced both the angle of the jump and the distances jumped. Bright blue produced a larger angle of takeoff, and jumps over fluorescent yellow fences had shorter landing distances compared to orange, with bright blue fences driving a similar but non-significant trend. White was the only colour that influenced takeoff distances, with horses jumping over white fences having a larger takeoff distance. Overall, our results show that current obstacle coloration does not maximise contrast for horse vision, and that alternative colours may improve visibility and alter behavioural responses, with the ultimate goal of improving safety and welfare.
... However, complex cognitive tasks could involve the presentation of picture based stimuli e.g. shapes, which may require greater visual acuity, and therefore the use of the binocular vision (Harman et al., 1999). Binocular vision is suggested to cover approximately 80 • to the front of the horse and is typically used when the head is raised, with lateral vision utilised when the head is closer to the ground (Harman et al., 1999). ...
... shapes, which may require greater visual acuity, and therefore the use of the binocular vision (Harman et al., 1999). Binocular vision is suggested to cover approximately 80 • to the front of the horse and is typically used when the head is raised, with lateral vision utilised when the head is closer to the ground (Harman et al., 1999). As such, visual acuity may be less clear when the horse is observing at ground level (Hall, 2007), therefore the long term use of the OS for a range of cognitive tests could be compromised should the stimuli be presented at ground level. ...
Article
Background: Large animal models of human neurological disorders are advantageous compared to rodent models due to their neuroanatomical complexity, longevity and their ability to be maintained in naturalised environments. Some large animal models spontaneously develop behaviours that closely resemble the symptoms of neural and psychiatric disorders. The horse is an example of this; the domestic form of this species consistently develops spontaneous stereotypic behaviours akin to the compulsive and impulsive behaviours observed in human neurological disorders such as Tourette's syndrome. The ability to non-invasively probe normal and abnormal equine brain function through cognitive testing may provide an extremely useful methodological tool to assess brain changes associated with certain human neurological and psychiatric conditions. New method: An automated operant system with the ability to present visual and auditory stimuli as well as dispense salient food reward was developed. To validate the system, ten horses were trained and tested using a standard cognitive task (three choice serial reaction time task (3-CSRTT)). Results: All animals achieved total learning criterion and performed six probe sessions. Learning criterion was met within 16.30±0.79 sessions over a three day period. During six probe sessions, level of performance was maintained at 80.67±0.57% (mean±SEM) accuracy. Comparison with existing method(s): This is the first mobile fully automated system developed to examine cognitive function in the horse. Conclusions: A fully-automated operant system for mobile cognitive function of a large animal model has been designed and validated. Horses pose an interesting complementary model to rodents for the examination of human neurological dysfunction.
... The tendency of the horses to look toward the end of the arena rather than to locomote there appears to be a speciesspecific characteristic. Their excellent vision likely serves as a substitute for going to the end of the arena (Harman et al., 1999;Timney and Keil, 1992). Surprisingly, despite widely varying age, differences in sex, and especially differences in training and experience in the arena, the patterns of excursions of the animals were similar. ...
... The excursions should allow a horse to investigate the arena both by sniffing and by looking. If a horse is collected during warm-up it, it will not be able to sniff or visually inspect the arena (Harman et al., 1999). We suggest that allowing a horse to sniff, look and loop may not only allow it to explore the arena but also reduce its anxiety at the end of each excursion and so build confidence that any excursion will eventually return it to its home base. ...
Article
Spontaneous locomotor behavior in a novel space reveals insights into an animal's world view or Umwelt. For example, in many animal species, spontaneous behavior in a novel environment is parsed into activities at a home base and excursions from the home base. Domestic horses (Equus ferus caballas) are frequently ridden for recreation or in performance events in an equestrian arena but there has been no description of horse behavior in an arena when they are unconstrained and "exploring" or when moving freely under saddle. The present examination exploration provides insights into horse adaptive behavior more generally as well as insights into horse performance under saddle. Thoroughbred, American Quarter Horse and mixed-breed mares and geldings of various ages, with various degrees of training under saddle, and with varying familiarity with the arena were given 30-min tests in which they were at liberty to explore an equestrian arena. Additional 30-min tests were given in which horses explored the arena containing a tethered partner, or were ridden. Despite breed, sex, age and experience, behavior was similar. A horse spent most of its time near the entrance door of the area where it looked out, paced, and rolled. Periodic excursions formed loops. The outward leg of a loop was slow, often featured sniffing the ground, and ended with a head-raised, ears-forward look toward the far end of the arena. The homeward leg of a loop was made with lowered-head and ears-back and was relatively direct and fast. Successive loops could increase or decrease in size over a test period. If a partner was tethered at the far end of the area, a horse shifted its activity toward the partner. When horses under saddle were asked to make excursions into the arena but otherwise left unconstrained, they made loops, similar to that of freely moving horses. When ridden around the arena they returned more quickly to the near end of the arena than when leaving the near end of the arena. This organized home base/excursion behavior is discussed in relation to horse social structure and to its expression while under saddle.
... There are a number of possible reasons why head lowering may induce calmness. It can significantly decrease mean arterial blood pressure (Parry et al., 1980); it replicates the position of resting (Feist and McCullough, 1976), grazing (Harman et al., 1999) and it enables the horse to have the binocular field of vision directed towards the ground and the lateral monocular fields in position to scan the lateral horizon (Harman et al., 1999). Hall et al. (2003) found that horses' performance in a visual discrimination trial was improved when the head was lowered and the stimulus presented at ground level compared to presentation of the stimulus at a height of 70 cm. ...
... There are a number of possible reasons why head lowering may induce calmness. It can significantly decrease mean arterial blood pressure (Parry et al., 1980); it replicates the position of resting (Feist and McCullough, 1976), grazing (Harman et al., 1999) and it enables the horse to have the binocular field of vision directed towards the ground and the lateral monocular fields in position to scan the lateral horizon (Harman et al., 1999). Hall et al. (2003) found that horses' performance in a visual discrimination trial was improved when the head was lowered and the stimulus presented at ground level compared to presentation of the stimulus at a height of 70 cm. ...
Conference Paper
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Globally, millions of horses are used for a range of purposes by humans with varying levels of skill in horse handling. Inappropriate handling techniques, especially those that cause flight responses or conflict behaviour, account for much of the wastage rates among horses as well as the majority of the deaths and injuries among handlers. In contrast, some techniques help to calm horses and thus facilitate training. Anecdotal evidence suggests that one such technique is lowering the height of a horse's head position. To determine the effect of head lowering, 20 horses were paired for age, sex and breed before one from each pair was allocated to Group 1 (treatment group: stimulus for head lowering applied during testing period) and the other placed into Group 2 (control group: no experimental stimulus applied during testing period). The stimulus for head lowering was downward pressure on the headcollar via the lead rope until the horse lowered its head such that its lips were approximately at mid-cannon height; as soon as this occurred the pressure was released. The testing period was 15 consecutive minutes divided into three 5-minute phases: Phase 1, in which neither group had experimental stimuli applied; Phase 2, in which Group 1 had the stimulus for head lowering applied and Group 2 had no stimuli applied; and Phase 3 that repeated the Phase 1 treatment. Behavioural responses of the head, neck and legs and the physiological responses of heart rate and heart rate variability were measured and analysed with one-way analysis of covariance. There were no significant differences between groups with any of the other responses measured, except for sniffing the ground (P=0.039), probably due to 76 the nature of the treatment. These results indicate that, under these conditions, head lowering does not result in increased calmness in horses.
... As opposed to the human retina and its central fovea, the equine retina has no central fovea but instead has what is known as a "visual strip" (24). This gives the horse the ability to broadly and most likely equally see the entire horizon, but much less above or below. ...
... Head and neck position are therefore important factors found to affect the visual abilities of horses. In 1999, Harman et al. (24). questioned whether the over-arched neck of the ridden horse in the sport of dressage would inhibit the horse's ability to see what is directly in front of it. ...
Article
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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.
... Terrestrial Cetartiodactyls are herbivore species that developed a phylogenetic adaptation to extremely contrasting habitats, from arid land to water, with consequent various degrees of differential evolution of their visual system and other senses. The horse is a perissodactyl that shares several adaptive features with most terrestrial Cetartiodactyls, including the general morphology of the visual system (Harman et al. 1999;Timney and Kiel 1999;Kendrick et al. 2001;Hall et al. 2003;Knolle et al. 2017). All Cetartiodactyls (including sea-living cetaceans) and perissodactyls developed a broad field of view mostly relying on monocular signal (table, Johnson 1901;Duke-Elder 1958), although their vision has been deemed fairly good (Piggins et al. 1996, Jacobs et al. 1998Coimbra et al. 2013Coimbra et al. , 2017. ...
... Nonetheless, neurons sensitive to ocular disparity have been found in the sheep and goat (Clarke and Witteridge 1976;Clarke et al. , 1979a. The 60° binocular field (overlap) of goats suggests that their high agility could rely on stereopsis (Howard and Rogers 1996), although no behavioral evidence seems to exist, unlike in the horse (Timney andKiel 1992, 1999), in which binocular vision reaches 80° (Harman et al. 1999) (see Table 1). ...
Article
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Cetartiodactyls include terrestrial and marine species, all generally endowed with a comparatively lateral position of their eyes and a relatively limited binocular field of vision. To this day, our understanding of the visual system in mammals beyond the few studied animal models remains limited. In the present study, we examined the primary visual cortex of Cetartiodactyls that live on land (sheep, Père David deer, giraffe); in the sea (bottlenose dolphin, Risso’s dolphin, long-finned pilot whale, Cuvier’s beaked whale, sperm whale and fin whale); or in an amphibious environment (hippopotamus). We also sampled and studied the visual cortex of the horse (a closely related perissodactyl) and two primates (chimpanzee and pig-tailed macaque) for comparison. Our histochemical and immunohistochemical results indicate that the visual cortex of Cetartiodactyls is characterized by a peculiar organization, structure, and complexity of the cortical column. We noted a general lesser lamination compared to simians, with diminished density, and an apparent simplification of the intra- and extra-columnar connections. The presence and distribution of calcium-binding proteins indicated a notable absence of parvalbumin in water species and a strong reduction of layer 4, usually enlarged in the striated cortex, seemingly replaced by a more diffuse distribution in neighboring layers. Consequently, thalamo-cortical inputs are apparently directed to the higher layers of the column. Computer analyses and statistical evaluation of the data confirmed the results and indicated a substantial correlation between eye placement and cortical structure, with a markedly segregated pattern in cetaceans compared to other mammals. Furthermore, cetacean species showed several types of cortical lamination which may reflect differences in function, possibly related to depth of foraging and consequent progressive disappearance of light, and increased importance of echolocation.
... The equine visual field is very different from the human visual field. Horses have a blind area directly in front of the middle of the forehead that extends caudoventrally around the body to encompass the limbs [38]. Therefore, horses cannot see their hooves as they cross the poles and are reliant on a feed-forward mechanism based on visual perception during the approach to judge how much the hooves should be elevated. ...
... In spite of these limitations in their visual field, horses learn to adjust their limb movements to negotiate obstacles in their path safely. The flexor musculature, which raises the limbs in the swing phase, has high responsiveness to visual stimuli whereas the extensor musculature, which provides support and propulsion during stance, is more responsive to proprioceptive input [38]. When a person steps over an obstacle set to 10% of body height, the foot is raised primarily by knee flexion with small contributions from hip and ankle flexors [35]. ...
Article
Reasons for Performing StudyTrotting over poles is used therapeutically to restore full ranges of limb joint motion. The mechanics of trotting over poles have not yet been described, hence quantitative evidence for the presumed therapeutic effects is lacking. Objectives To compare limb kinematics in horses trotting over level ground, over low poles and over high poles to determine changes in joint angulations and hoof flight arcs. Study DesignRepeated measures experimental study in sound horses. Methods Standard motion analysis procedures with skin-fixed reflective markers were used to measure swing phase kinematics from eight horses trotting on level ground, over low (11 cm) and high (20 cm) poles spaced 1.05 ± 0.05 m apart. Spatiotemporal variables and peak swing phase joint flexion angles were compared using repeated measures ANOVA (P<0.05) with Bonferroni correction for pairwise post-hoc testing. ResultsPeak heights of the fore and hind hooves increased significantly and progressively from no poles (fore: 13.8 ± 3.8 cm; hind: 10.8 ± 2.4 cm) to low poles (fore: 30.9 ± 4.9 cm; hind: 24.9 ± 3.7 cm) and to high poles (fore: 41.0 ± 3.9 cm; hind: 32.7 ± 4.0 cm). All joints of the fore and hindlimbs contributed to the increase in hoof height through increased swing phase flexion. The hooves cleared the poles due to increases in joint flexion rather than by raising the body higher during the suspension phases of the stride. Conclusions The increases in swing phase joint flexions indicate that trotting over poles is effective for activating and strengthening the flexor musculature. Unlike the use of proprioceptive stimulation devices in which the effects decrease over time due to habituation, the horse is required to elevate the hooves to ensure clearance whenever poles are present. The need to raise the limbs sufficiently to clear the poles and place the hooves accurately requires visuomotor coordination which may be useful in the rehabilitation of neurological cases.
... Horses understand gazing to some extent (walk towards the focus of attention of a person; body orientation or gaze?Total vision range: about 340° (reviewed Murphy et al. 2009) Each eye: vertical 178° (Huges 1977), horizontal 200° (Harman et al. 1999) Dolphins: lateral eye position YET alternate looking for pointing at humans (Xitco et al. 2001Xitco et al. , 2004) ...
... Brown (1969) strengthened this argument by revealing a streak in the retina of the red-eared turtle (Pseudemys scripta elegans) and to suggested that animals low to the ground would almost always view a horizon (provided by the ground near to them and either the sky or vegetative/environmental cover), where the streak could enhance both visual resolution and attention over an extended area. Yet, a visual streak persists in some species whose head is high relative to the ground, such as the ostrich Struthio camelus (Boire et al., 2001), African elephant L. africana (Stone and Halasz, 1989), horse E. f. caballus (Harman et al., 1999), and giraffe Giraffa camelopardalis (Coimbra et al., 2013). Hughes (1977) took the association between the horizon and the visual streak a step further by proposing the "terrain hypothesis," which states not only that the presence of a visual streak is associated with a horizon-dominated environment, but also that the line of vision should not be obstructed by vegetation. ...
Chapter
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Visual sensory demands vary substantially across vertebrates. Different visual sensory components have evolved to meet these sensory demands and enhance visual behavioral performance. One of these components is the retinal specialization, which is a portion of the retina with generally high ganglion cell densities, which increase spatial resolving power. Retinal speciali- zations are relevant from a functional perspective because animals can align these “acute zones” with objects of interest within a localized region of their visual space, consequently affecting different behavioral dimensions. In this chapter, we reviewed the different types of retinal specializations found in vertebrates (retinal area, fovea, visual streak, radial anisotropy, area gigantocellularis) by discussing the different hypotheses proposed over decades to explain their function. Empirical tests on the functional properties of these different retinal specializations have been limited, which constrains our ability to understand the functional evolution of the vertebrate eye. We derive specific predictions from each of the hypotheses put forward to identify their degree of overlap. Finally, we provide some future directions as to how to test these functional hypotheses by integrating physiological and behavioral approaches. Testing these functional hypotheses will enhance our understanding of the relationship between the eye and the physical environment, and ultimately the visual ecology of vertebrates.
... Here we present EquiFACS, a Facial Action Coding System for the domestic horse (Equus caballus). Until now, there has been no methodology available that documented all of the facial Horses are predominantly visual animals, with reasonable visual acuity that, at 23 cycles per degree, is better than domestic cats and dogs [23][24][25]. While horses' use of head and body posture in signaling has been described in observational literature (e.g. ...
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Although previous studies of horses have investigated their facial expressions in specific contexts, e.g. pain, until now there has been no methodology available that documents all the possible facial movements of the horse and provides a way to record all potential facial configurations. This is essential for an objective description of horse facial expressions across a range of contexts that reflect different emotional states. Facial Action Coding Systems (FACS) provide a systematic methodology of identifying and coding facial expressions on the basis of underlying facial musculature and muscle movement. FACS are anatomically based and document all possible facial movements rather than a configuration of movements associated with a particular situation. Consequently, FACS can be applied as a tool for a wide range of research questions. We developed FACS for the domestic horse (Equus caballus) through anatomical investigation of the underlying musculature and subsequent analysis of naturally occurring behaviour captured on high quality video. Discrete facial movements were identified and described in terms of the underlying muscle contractions, in correspondence with previous FACS systems. The reliability of others to be able to learn this system (EquiFACS) and consistently code behavioural sequences was high-and this included people with no previous experience of horses. A wide range of facial movements were identified, including many that are also seen in primates and other domestic animals (dogs and cats). EquiFACS provides a method that can now be used to document the facial movements associated with different social contexts and thus to address questions relevant to understanding social cognition and comparative psychology, as well as informing current veterinary and animal welfare practices.
... Using continuous focal sampling [49], the total amount of time spent gazing at the stimulus was recorded frame by frame (frame, 0.02s). Horses have laterally placed eyes with a small (60-80˚) binocular field of vision and almost complete (80-90%) decussation of the optic nerves, suggesting that behavioural asymmetries reflect asymmetries in hemispheric activation [50]. Binocular gaze towards the stimulus was defined when the horse faced the head (mid line of the head, between both eyes) and both eyes were oriented forwards towards the visual stimulus. ...
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Attention is a central process of cognition and influences the execution of daily tasks. In humans, different types of work require different attentional skills and sport performance is associated with the ability to attention shift. Attention towards humans varies in dogs used for different types of work. Whether this variation is due to the recruitment of individuals suitable for specific types of work, or to the characteristics of the work, remains unclear. In the present study, we hypothesized that domestic horses ( Equus caballus ) trained for different types of work would also demonstrate different attentional characteristics but we also explored other possible factors of influence such as age, sex and breed. We exposed more than sixty horses, working in 4 different disciplines, and living in two types of housing conditions, to a visual attention test (VAT) performed in the home environment. Individual attentional characteristics in the test were not significantly influenced by age, sex, breed or conditions of life but were strongly related to the type of work. Riding school horses showed longer sequences and less fragmented attention than all other horses, including sport horses living in the same conditions. Interestingly, sport performance was correlated with attention fragmentation during the test in eventing horses, which may need more attention shifting during the competitions. Working conditions may influence attention characteristics indirectly through welfare, or directly through selection and training. Our study opens new lines of thought on the determinants of animal cognition and its plasticity and constitutes a further step towards understanding the interrelationship between working conditions and cognition.
... Amongst vertebrates, horses are especially interesting as they have laterally placed eyes and almost complete decussation of the optic fibres 20 . Indeed horse's perceptual laterality has been widely evidenced in this species. ...
Article
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Lateralization of brain functions has been suggested to provide individuals with advantages, such as an increase of neural efficiency. The right hemisphere is likely to be specialized for processing attention for details and the left hemisphere for categorization of stimuli. Thus attentional processes actually may underlie lateralization. In the present study, we hypothesized that the attentional state of horses could be reflected in the lateralization of brain responses. We used i) a recently developed attention test to measure horses' visual attentional responses towards a standardized stimulus and ii) a recently developed portable EEG telemetric tool to measure brain responses. A particular emphasis was given to the types of waves (EEG power profile) and their side of production when horses were either attentive towards a visual stimulus or quiet standing. The results confirmed that a higher attentional state is associated with a higher proportion of gamma waves. There was moreover an interaction between the attentional state, the hemisphere and the EEG profile: attention towards the visual stimulus was associated with a significant increase of gamma wave proportion in the right hemisphere while "inattention" was associated with more alpha and beta waves in the left hemisphere. These first results are highly promising and contribute to the large debate on functional lateralization.
... Fryxell, 2004). In addition, given the position of the eyes of many herbivores, it is likely that they may be able to obtain social information about which patch to join without having to lift their heads (see Harman, Moore, Hoskins, & Keller, 1999). If this is the case, then this increases the ability of scroungers to continuously monitor the foraging of other group members. ...
Article
Group-living animals use social information when making patch-joining/ scrounging decisions. However, the extent to which they use finder’s share (i.e. amount of food eaten in a patch before other individuals arrive) as a cue when making these decisions is unknown. It is likely that the removal of finder’s share decreases patch attractiveness to scroungers. However, it is unclear how large a finder’s share must be to reduce attractiveness, or how this varies with food availability. To answer these questions, we recorded the patch-joining decisions of dominant goats (Capra hircus) when presented with a choice between an artificial patch where finder’s share had been removed by a subordinate patch holder (producer), and one where the patch holder had just started eating. We used time spent feeding by a patch holder (10, 30, 60 and 120 sec) as an index of finder’s share size, and tested this using three food availabilities (40 g, 100 g, 300 g). At low (40 g) and intermediate (100 g) food availabilities, scrounging goats avoided the finder’s share patch once the patch holder had fed for >30 seconds (i.e. 25% and 17% of the food removed respectively). However, at the highest food availability (300 g), these goats continued to join the finder’s share patch even after the patch holder had fed for 120 s (18% removed). Ultimately, our results indicate that goats weigh up both food availability and the finder’s share when making scrounging decisions. Nevertheless, finder’s share removal was less important in patches with more food.
... A comunicação entre cavalos frequentemente acontece sem a utilização de vocalizações, possivelmente para evitar a atração de predadores (McGreevy, 2004), e valendo-se principalmente de sua condição natural de serem animais predominantemente visuais, com uma acuidade visual melhor do que cães e gatos domésticos (Timney e Keil, 1992;Miller e Murphy, 1995;Harman et al., 1999). Os equinos conseguem identificar diversos objetos a partir de vários ângulos de sua visão monocular (imagem formada por apenas um olho), demonstrando boa capacidade visual, independentemente de estarem ou não usando a visão binocular (imagem formada pelas informações luminosas captadas pelos dois olhos) (Hanggi e Ingersoll, 2012). ...
Article
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As expressões faciais têm como finalidade estabelecer a comunicação entre os equinos. Apesar disso, também podem ser utilizadas como produto de uma situação específica e, desse modo, refletir o estado dos cavalos. Com isso, denota-se a relevância dessa linguagem corporal como indicador comportamental para avaliar diversos estados mentais positivos e negativos dos equinos. O conhecimento geral acerca das expressões faciais, entretanto, ainda é incipiente e os estudos apresentam divergências metodológicas que precisam ser compreendidas antes da replicação de tais métodos, assim como é de extrema relevância esclarecer as limitações de seu uso. Esta revisão, portanto, foi elaborada com o objetivo de apresentar e discutir os dados disponíveis na literatura no que tange às expressões faciais em cavalos, com destaque para os métodos de avaliação e as limitações no uso das características faciais. Espera-se que esta revisão possa nortear estudos futuros no desenvolvimento científico da área e contribuir para a identificação das principais lacunas.
... É possível que o campo de visão monocular dos equinos seja de 190 a 195 graus no eixo horizontal e 178 graus no eixo vertical quando o cavalo estiver olhando para um ponto fixo. Quando as imagens monoculares produzidas pelos dois olhos são combinadas, o campo visual horizontal tem uma amplitude de até 350 graus, restando apenas alguns pontos cegos (Harman et al., 1999;Mills e Nankervis, 2005;Miller e Murphy, 2010). ...
Article
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São diversas as circunstâncias em que os cavalos são acometidos por emoções negativas como medo ou ansiedade, podendo resultar em reações defensivas explosivas, que possivelmente podem derivar em acidentes. Estas respostas de defesa fazem parte de um conjunto de comportamentos classificados como inatos, os quais foram desenvolvidos e aprimorados ao longo do processo evolutivo do Equus caballus na tentativa de sobreviver às investidas de caça dos predadores, contribuindo para a preservação da espécie. Apesar de o cavalo doméstico, que convive habitualmente com o ser humano, já não ter contato regular com predadores naturais, os mecanismos defensivos e as emoções negativas que envolvem tal situação ainda permanecem ativos, produzindo respostas comportamentais específicas. Torna-se relevante, portanto, o estudo aprofundado sobre quais elementos desencadeiam tais reações defensivas nos cavalos a fim de que as causas desses comportamentos sejam evitadas, reduzindo, assim, o número de acidentes. Por isso, o objetivo dessa revisão foi investigar como os equinos percebem seus predadores, as possíveis hipóteses evolutivas, bem como suas implicações práticas. Este tema, quando melhor esclarecido, poderá contribuir para o aperfeiçoamento de instalações e manejos relacionados aos equinos, com a intenção de reduzir ou ainda evitar tais reações de medo, que implicam no empobrecimento do bem-estar animal.
... On the other hand, the horses used preferentially their left eye to investigate the sound source (i.e. the loudspeaker) when hearing the human voices broadcast during the negative interactions, suggesting a right hemisphere main involvement in processing such voices (80-90% of the optic fibres decussation in horses' brain 55 ). This finding is consistent with the right hemisphere specializations for the perception and the expression of intense/negative emotions and the control of rapid responses previously described for several vertebrate species 1,56 . ...
Article
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Brain lateralization is a phenomenon widely reported in the animal kingdom and sensory laterality has been shown to be an indicator of the appraisal of the stimulus valence by an individual. This can prove a useful tool to investigate how animals perceive intra- or hetero-specific signals. The human-animal relationship provides an interesting framework for testing the impact of the valence of interactions on emotional memories. In the present study, we tested whether horses could associate individual human voices with past positive or negative experiences. Both behavioural and electroencephalographic measures allowed examining laterality patterns in addition to the behavioural reactions. The results show that horses reacted to voices associated with past positive experiences with increased attention/arousal (gamma oscillations in the right hemisphere) and indicators of a positive emotional state (left hemisphere activation and ears held forward), and to those associated with past negative experiences with negative affective states (right hemisphere activation and ears held backwards). The responses were further influenced by the animals’ management conditions (e.g. box or pasture). Overall, these results, associating brain and behaviour analysis, clearly demonstrate that horses’ representation of human voices is modulated by the valence of prior horse-human interactions.
... Feral horses usually form long-term stable groups (Berger, 1977) and do not frequently face each other in foraging situations, although mutual grooming and mutual swatting of flies are performed in counter-directed positions (i.e. two individuals facing opposite directions) (Feist, Mccullough, & Dean, 1976;Heleski, Shelle, Nielsen, & Zanella, 2002). Horses are a good species for studying behavioural lateralisation because they have a relatively small field of binocular vision, estimated at 80°, and their left and right eyes have largely independent views of their left and right sides, respectively (Harman, Moore, Hoskins, & Keller, 1999). Several studies have reported laterality in behaviours including agonistic and affiliative behaviours in counter-directed positions, vigilance to auditory stimuli and relative spatial positioning in mother-infant relationships in horses in co-directed positions (i.e. two individuals facing the same direction) (Austin & Rogers, 2012, 2014Farmer, Krueger, & Byrne, 2010;Farmer, Krüger, Byrne, & Marr, 2018;Karenina et al., 2017;. ...
Article
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Behavioural lateralisation is an effective way for animals to manage daily tasks by specialising behaviour to either side of the body. Many types of lateralisation are linked to the function of each brain hemisphere. Lateralisation of monitoring behaviour in mother–infant relationships occurs in a wide range of mammals, where infants frequently use their left eye to monitor their mother. However, few studies have focused on this type of spatial relationships among adults in daily life, such as during foraging. The present study focused on monitoring adult feral horse behaviour using quantitative analysis of spatial relationships, using drone technology. We found that horses form a localised spatial relationship with their nearest neighbour. Specifically, the nearest neighbour was located to the left rear of a target individual significantly more frequently than to the right rear. Furthermore, the nearest neighbour was less frequently located behind a target individual. We propose that this relationship is caused by a left‐eye preference, because information via the left eye predominantly proceeds to the right hemisphere, which is dominant for social processing.
... Sniffing the arena likely helps it to determine what other horses may have been there. A horse has one of the largest eyes of all animals and excellent vision and so it need not go to the far end of the arena to visually investigate it [12]. Its ears forward posture allows it to investigate sounds both inside and outside the arena. ...
... Vision provides a larger contribution to stabilization of balance than vestibular or proprioceptive input (Hansson et al., 2010) and peripheral vision contributes more than central vision to maintaining a stable standing posture (Berencsi et al., 2005). Horses have an extensive peripheral (monocular) visual field (Harman et al., 1999) which may favor stability by providing panoramic visual information. Visual deprivation appeared to cause some horses to become anxious, which is not unexpected in a prey species and it took longer to collect sufficient trials of long enough duration in these animals. ...
Article
PurposeTo determine the feasibility of using a handheld spectral-domain optical coherence tomography (SD-OCT) instrument to characterize normal corneal, retinal, and optic nerve head anatomy in vivo in standing horses.Methods Clinically normal horses under sedation, palpebral nerve blockage, and pharmacologically induced mydriasis were imaged with a SD-OCT instrument (Envisu SD-OCT, Bioptigen, Inc., Morrisville, NC). Radial volumes from the cornea (axial, superior, inferior, nasal, and temporal), and rectangular volumes from the retina (dorsal, ventral, nasal, and temporal) and optic nerve head were acquired. Manual measurements of the corneal layers within the five regions, retinal and nerve fiber layer thickness in the four different regions adjacent to the ONH, and vertical and horizontal axis of the optic nerve head (ONH) and optic cup (OC) were obtained using the same device.ResultsTotal corneal thickness (mean ± SD) measurements were 800 ± 50, 937 ± 61, 956 ± 61, 912 ± 65, and 884 ± 68 μm for the axial, superior, inferior, nasal, and temporal regions, respectively. The highest total retinal and nerve fiber layer thickness (mean ± SD), at the level of the ONH, was found nasally 459 ± 115 and 377 ± 116 μm, respectively, followed by the temporal, dorsal, and ventral quadrants. The dimensions of the ONH and OC (mean ± SD) were 3.682 ± 0.276 and 2.175 ± 0.502 mm for the horizontal, and 3.012 ± 0.278 and 2.035 ± 0.488 mm for the vertical axis.Conclusions The SD-OCT instrument employed in this study may be used on sedated horses and allows the acquisition of high-resolution images, and thickness measurements involving the cornea, retina, and optic nerve.
Article
Ophthalmic examination in the horse is generally limited to crude assessment of vision and screening for ocular lesions. The refractive state of equine eyes and the potential impact on vision and performance requires further investigation. To assess the refractive state of a large, mixed-breed sample of horses and ponies in the United Kingdom (UK). The refractive state of both eyes of 333 horses and ponies was determined by streak retinoscopy, and the effect of age, height, gender, breed and management regime on the refractive state assessed. Emmetropia was found in 557 of 666 (83.63%) of eyes; 228/333 (68.5%) of the horses/ponies were emmetropic in both eyes. Refractive errors of greater than 1.50 D (in either direction) were found in 2.7% of the eyes tested. Ametropic eyes included hyperopia (54%) and myopia (46%). Anisometropia was found in 30.3% of horses and ponies. Breed of horse/pony was the only factor that affected refractive state (in the left eye only, P < 0.05) with Thoroughbred crosses having a tendency toward myopia and Warmbloods/Shires toward hyperopia. The retinoscopic survey found emmetropia to be the predominant refractive state of the equine eye with no evidence of an overall trend toward myopia or hyperopia. However, individual and breed-related differences were found. Such factors should be considered in the selection of horses for sport and leisure, and when evaluating their performance potential. More comprehensive visual testing would be valuable in identifying underlying causes of behavioral problems.
Article
The total number, distribution and size of retinal ganglion cells of buffalo (Bos bubalis) were described in four wholemounts of the retina stained by the Nissl method. The ganglion cells were observed in the area of visual streak, dorsotemporal, dorsonasal and ventral parts of the retina. Soma size was measured from samples taken from these different retinal regions. Ganglion cells were concentrated along the horizontal visual streak, which lies dorsal to the optic disc. The mean value of the total number of ganglion cells was 1,527,965. The peak ganglion cell density totaled 4,617/mm2 at the temporal end of the streak. The distribution of ganglion cells varied in different parts of the retina where it was highest in the dorsotemporal part, average in the dorsonasal part and lowest in the ventral part.
Article
Human beings mounted and they mount horses for different purposes. They have trained and used the horses in different ways. They have done that frequently and they do that frequently with the more or less differentiated expressed conviction: The equid is predisposed for the use under the saddle by his anatomical, physiological and psychic constitution. Therefore the rider does not "burden" the horse much if he acts correctly and respects the nature of the animal. Contrarily, pain, anxiety, suffering, misadjusted behavior and damage, known from the past and nowadays described by veterinarians, justify the critical view of the facts: Use under the saddle is not without consequences for the horse in general. In particular, extreme or special demands provoke considerable suffering and damage. In some cases such demands, for example, the extreme demands on the horse used as drawing power in past centuries, result in premature unfitness for the use or in premature death. These facts mean that humans create a stable base for the existence and the wellbeing of the horse through the different ways of using the animal. However, the egoistic interests of the human being also provide the inclination to reduce the health and the comfort of the equid and, consequently, to diminish a part of the animal's natural lifetime by paying greater attention to man's profit.
Article
This study sought to determine whether the retinal organization of the white rhinoceros, a large African herbivore with lips specialized for grazing in open savannahs, relates to its foraging ecology and habitat. Using stereology and retinal wholemounts, we estimated a total of 353,000 retinal ganglion cells. Their density distribution reveals an unusual topographic organization of a temporal (2,000 cells/mm(2) ) and a nasal (1,800 cells/mm(2) ) area embedded within a well-defined horizontal visual streak (800 cells/mm(2) ), which is remarkably similar to the retinal organization in the black rhinoceros. Alpha ganglion cells comprise 3.5% (12,300) of the total population of ganglion cells and show a remarkably similar distribution pattern with maximum densities also occurring in the temporal (44 cells/mm(2) ) and nasal (40 cells/mm(2) ) areas. We found higher proportions of alpha cells in the dorsal and ventral retinas. Given their role in the detection of brisk transient stimuli, these higher proportions may facilitate the detection of approaching objects from the front and behind while grazing with the head at 45°. Using maximum density of total ganglion cells and eye size (29 mm, axial length), we estimated upper limits of spatial resolving power of 7 cycles/deg (temporal area), 6.6 cycles/deg (nasal area) and 4.4 cycles/deg (horizontal streak). The resolution of the temporal area potentially assists with grazing, while the resolution of the streak may be used for panoramic surveillance of the horizon. The nasal area may assist with detection of approaching objects from behind, potentially representing an adaptation compensating for limited neck and head mobility. This article is protected by copyright. All rights reserved.
Article
Since about twenty years a growing number of sport horses are ridden in an extreme way of overbending (atlantooccipital flexion) during training in general and while preparing before the start in a competition. Most often overbending involves a deep neck (cervicothoracal flexion),but occasionally it is also seen with an elevated neck (cervicothoracal extension).Orthodox interpreters of equitation turn against the practice of extreme overbending, these days usually indicated by the terms "Rollkur", "Hyperflexion" oder "LDR (Low, deep, round)". In their view this method neglects essential principles of "classical" training of a riding horse. More over it compromises the well being and the health of the horse. In contrast the proponents of overbending defense this method as a way to improve physical training of the horse. Since the beginning of the last decade considerable veterinary research has been conducted about the physical and the psychic consequences of extreme overbending to sort out these differences. However, the veterinary work and interpretations still have not resulted in a uniform image: Several veterinarians see a compromise of health especially by extensively bended vertebral joints, and by hindering movements, breathing and optical perception. Further, they see impairment of the well being, reaching to pain and suffering. Other veterinarians negate such consequences, if overbending is practised correctly. This means: Several results and interpretations of veterinary research in this field are in the same way contradicting as are the convictions of the practitioners in the saddle. The veterinarians were (so far) not able to clarify the physical and the psychic consequences of extreme overbending in a way that results in a scientific judgement, which could obligatorily regulate the behaviour of the trainers. From the view of functional morphology and from the view of orthopaedics it is unlikely that the extreme deviation from the natural disposition of the horse has no consequences. For these views and for the orientation of training the horse based on its natural dispositions the absence of consequences would be a fact that could have, and that should have, far going consequences. To find a solution for the described problem, further veterinary investigations are required, representative, valid and reliable investigations, harmonised between the different research groups. Hypothetically stated: The longer and more flexible necks, achieved by selection in breeding, can be flexed to a further extent without consequences than short and firm necks. If the care for the health and the wellbeing of the horse would primarily lead the practices of the riders, they should demonstrate the harmlessness of such a method of training before using this method, according to the principle "in dubio pro equo".
Article
Since about twenty years sport horses often are ridden extremely overbended(atlantooccipitale flexion), mostly with a deep neck(cervicothoracale flexion), but by some riders or in some situations with an elevated neck(cervicothoracale extension). These head-neck-positions are possible by the natural flexibility of the neck and the head. The described positions differ from the position, the horses choose while moving without restrictions by a rider. Especially corresponding to the way of moving with high demands , the horses take the position of the neck in a medium hight and the nose-forehead-line before the vertical. They properly do that to promote the effect of head and neck for the balance, for breathing and for the perception. Especially the overbending practised in an extreme extent and for a longer time differs from the rules of the national and the international riding organisations. Orthodox interpreters of the riding doctrine turn against the practice of extreme overbending, in these days usually denominated by the terms "Rollkur", "Hyperflexion" oder "LDR (Low, deep, round)". In their view this method not only relinquishes essential principles of training a riding horse but also endangers the well being and the health of the horse. The riding organisations do not take a definite position against overbending. Obviously they are primarly interested not to see diminuished the image of the riders they are looking for and the sport, which they organize and which they are financed by. Still the different experiments, investigations and statements done by veterinarians about the physical and the psychic consequences of the extreme overbending do not show a clear picture. Some veterinarians see the endangering of the health of extensively bended vertebra joints, the hindering of the movements, breathing and perception, just as well detractions from the well beeing, reaching to pain and suffering. Some veterinarians negate the consequences, if the overbending is practised well. From the view of functional morphology and from the view of orthopaedics it is unlikely, that the extreme deviation from the natural disposition of the horse works without consequences. For these views and for the orientation of the training of the horse on his natural dispositions the absence of consequences would be an essential fact. To answer these questions valid and reliable results of further investigations are required.
Chapter
Success in working with livestock is not related to the intelligence or lack of it on the part of the animals. The process of getting an animal from point A to point B can be a distressful, reactive, trial and error attempt in which the human finally lucks out at getting the job done, or it can be a stress-free experience for the human and the animals. Which type of experience it is will depend on the human's understanding of how an animal perceives its world, how animals learn, the relative importance of its social peers, and what motivates it to move. With that knowledge, appropriate techniques can be used to humanely create an outcome that is both effective and efficient.
Article
The anatomical characteristics of each of the many species today employed in biomedical research are very important when selecting the correct animal model(s), especially for conducting translational research. In previous papers, these features have been considered for fish (D'Angelo et al. Ann. Anat, 2016, 205:75), the most common laboratory rodents, rabbits, and pigs (Lossi et al. 2016). I here follow this line of discussion by dealing with the importance of proper knowledge of ferrets, goats, sheep, and horses' main anatomical features in translational research.
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A complete, thorough ocular examination is a basic and essential aspect of equine ophthalmology. In this chapter, ophthalmic examination of the horse is discussed, with emphasis on techniques, tools, and instruments, and basic diagnostic modalities. Examination of the equine eye includes obtaining a detailed history and signalment, inspecting the patient in a well-lit environment, examining the ocular structures in a darkened environment, facilitating the examination with restraint, sedation, and local nerve blocks, and collecting relevant diagnostic samples or data.
Chapter
The posterior segment encompasses the vitreous, retina, choroid, sclera, and optic nerve. Evaluation of these structures is an important part of the ophthalmic examination. Congenital lesions and acquired conditions within the posterior segment can have a dramatic impact on vision. The normal appearance of the vitreous and fundus are covered in detail in this chapter. In addition, the conditions that affect the posterior segment, their treatments, impact on vision, and prognoses are described. Vitrectomy and retinal reattachment surgery are also discussed.
Article
Inattentional Blindness (IB) is a phenomenon which has been widely researched in humans. If attention is highly focused, humans may become blind to other environmental cues. To further understand the perception of horses, we applied the well-tested experimental design of human IB studies to the first study of this kind in horses. Since the use of cognitive tasks in human IB-studies ensures that the subject's attention is focused in a certain direction, we wanted to show if a food reward is strong enough to induce IB in the horse while confronted with a novel object. Therefore, we tested 24 horses, divided in an experimental (n = 12) and a control group (n = 12). The horses had to walk through the same corridor (10 m) for 12 consecutive trials. In the experimental group, animals were trained to focus on a food reward presented at the end of the corridor. Their duration to pass the run decreased significantly, indicating that an expectation focused on the feed was induced. In the test trials 11 and 12, a startle stimulus was presented to both groups. The response to the novel object was scored from 1 (no reaction) to 5 (flight reaction). We found a significant difference between the groups in the second test run with the experimental group showing lower reactions to the novel object (p = 0.018). The horses in the experimental group where not completely “blind” to the object, but the food reward clearly influenced their reaction. The results of the study suggest that while the occurrence of IB in horses has yet to be proven, a suitable distractor can lower horses’ startle response to a novel object. By further investigation into IB, new insight into the perception of horses can emerge and improve the understanding between horses and their handlers.
Article
Performing a brain computerized tomography scan (CT scan) on a foal requires specific equipment and anesthesia for large animals. However, the information obtained may demonstrate lesions responsible for the neurological deficits. Especially, CT scan findings may help to understand a mechanism of cerebral ischemia. Indeed, categories of cerebral ischemia are divided in three types: territorial infarctions (downstream of the territory of an artery), watershed infarctions (slow-flow at the junction of two arterial territories), and lacunar infarctions (small-vessel occlusions). Hypersensitivity reactions and type I anaphylactic IgE antibody reactions are severe potential adverse effects of sulfonamide administration, which occur in about three percent of cases. In horses, anaphylaxis is often clinically expressed as hypotension and collapse. Cardiovascular collapse may lead to multiorgan slow-flow leading to infarction with multiorgan failure and death. We report the case of a filly that suffered a presumed watershed cerebral infarction after antibiotic injection, indicated on a brain CT scan. This was attributed to a cerebral slow-flow during cardiovascular collapse, at the posterior junction of the right cerebral arteries. No abnormalities were initially identified on the CT scan; however, a review of the imaging by a radiologist specialized in cerebrovascular diseases detected a limited right occipital cortico-subcortical lesion in the visual cortex, interpreted as an ischemic scar in the watershed area related to hemodynamic infarction. This case highlights that detection of brain lesions by CT scan might require specialized knowledge and careful reading for interpretation particularly in the case of limited lesions.
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This chapter discusses equine ophthalmic examination, including assessment of ocular function, inspection of the complex anatomy of the globe and surrounding structures, and ocular diagnostic tests and sampling procedures. Techniques routinely used for medication delivery and treatment of ophthalmic disease are described, as are data detailing disease prevalence in various populations of horses. Several instruments are used during a complete ophthalmic examination of the standing equine patient. Ophthalmologists use an entire suite of instruments, including a Finnoff transilluminator, a slit‐lamp biomicroscope, a direct ophthalmoscope, and a binocular or monocular indirect ophthalmoscopy set. Cranial nerve (CN) evaluation is performed before any sedation is induced. These CNs are assessed by observing globe and eyelid position and mobility, and by testing the menace response, pupillary light, and dazzle reflexes, as well as the sensation of ocular and adnexal structures.
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Evaluation of the posterior segment of the equine eye is an essential part of an ophthalmic, pre‐purchase, and general physical examination. Ocular posterior segment abnormalities can be congenital or acquired and include both primary ophthalmic and systemic diseases. The significance of posterior segment disease to an individual horse and the equine industry is difficult to quantitate. The posterior segment of the eye includes the vitreous, retina, choroid, sclera, and optic nerve. Retinal dysplasia is a very rare congenital condition in horses, resulting in rosettes and multifocal retinal disorganization. Surgical management of equine recurrent uveitis with vitrectomy has been described. Complications include cataract formation, phthisis bulbi, and retinal detachment. The pathogenesis of various retinal and optic nerve conditions will be able to be more fully ascertained. Attempts to reattach equine retinas will allow refinement of the surgical technique, leading to improved visual outcomes.
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An understanding of equine vision allows the clinician to provide a more accurate prognosis prior to initiating therapy and to more intelligently select from a range of potential therapeutic options for a particular disorder to maximize the probability of optimally preserving the eye's visual capabilities. This chapter provides the clinician with a review of the normal visual abilities of the horse and describes how select ocular abnormalities may alter the animal's vision and behavior. The most critical aspect of vision, however, is the ability to identify an object (a wolf, for example) as separate from its surroundings (dense vegetation). Because this distinction is so important for survival, animals (including humans) with normal vision can “see” an object if it differs sufficiently from its surroundings in any one of six different aspects: luminance (brightness), motion, depth, texture (which is related to visual acuity), orientation, or color.
Article
To investigate the distribution of retinal ganglion cells (RGCs) and visual acuity in alpacas (Vicugna pacos) through Brn-3a immunofluorescent labeling. Five eyes from four healthy alpacas with normal ophthalmic examination findings were included in the study. The axial length of the globes was measured before fixation. All five retinas were treated with Brn-3a antibodies to label RGCs. Images taken with a fluorescent microscope were used for RGC counting. RGC density maps were reconstructed by computer software. Visual acuity was estimated based on the results of peak RGC density and ocular anatomical parameters. The reconstructed retinal maps from Brn-3a labeling showed a horizontal streak across the retinal meridian superior to the optic nerve head with a temporal, upward extension. The highest RGC densities were in the temporal retinas. The maximal visual acuity was located in the temporal retina and was estimated to range between 12.5 and 13.4 cycles per degree. Alpacas have a horizontal streak across the retinal meridian superior to the optic disk with a temporal, upward extension based on the Brn-3a labeling of RGCs. The maximal visual acuity was located in the temporal retina. The reconstructed retinal maps indicate the RGC topography of alpacas is similar to that of other herbivores, but is different from that of dromedary camels.
Article
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We have estimated the total number, distribution and peak density of retinal ganglion cells (RGCs) in retinal wholemounts of several species of microchiropteran (echolocating) bats. The estimates are based on counts of Nissl-stained, presumed RGCs. The total number of presumed RGCs varies among the species: from about 4,500 in Rhinolophus rouxi to about 120,000 in Macroderma gigas. In addition, in two species (Nyctophilus gouldi and M. gigas), the estimates are based on counts of positively identified RGCs retrogradely labelled with the enzyme horseradish peroxidase injected into the retinorecipient nuclei. In these two species, the numbers and distributions of retrogradely labelled RGCs and Nissl-stained presumed RGCs are very similar. In all six species studied, the peak-density regions of presumed (or positively identified) RGCs are located in the inferotemporal retinae, and the RGC isodensity lines tend to be horizontally elongated. However, the RGC densities in the high-density regions are only 2-4 times greater than those in the low-density regions in the superior retinae. The somal sizes of RGCs vary from 5 to 16 micron in diameter and are unimodally distributed. There is no indication of the existence of distinct morphological classes of RGCs. The axial lengths of microchiropteran eyes vary from 1.8 mm in R. rouxi to 7.0 mm in M. gigas. For all species the posterior nodal distance (PND) was assumed to be 0.52 of the axial length of the eye. This assumption is based on the analysis of published data concerning schematic eyes of nocturnal vertebrates. These derived values of the PNDs allowed us to calculate the retinal magnification factors and the number of RGCs per degree of visual angle. From these, the upper limits of visual acuity were derived on the basis of the assumptions of the sampling theorem. The estimated upper limits of visual acuity of the six species of echolocating bats vary from about 0.35 cycles/degree in R. rouxi to about 2 cycles/degree in M. gigas. This range is quite similar to the range of visual acuities in murid rodents.
Article
This treatise on comparative ophthalmology is written both for the layman and the specialist. Part 1 outlines the essentials of the vertebrate (human) eye, the histology and physiology of the vertebrate retina, and discusses scotopic and photopic vision. To this is added an account of the embryological and evolutionary genesis of the eye. Part 2 discusses the following topics: adaptations to arhythmic activity as seen in photomechanical retinal changes and in pupil mobility; adaptations to diurnal activity; adaptations to nocturnal activity; adaptations to space and motion; adaptations to media and substrates including aquatic and aerial vision; and adaptations to photic quality including color vision in animals, dermal color-changes, and coloration of the eye. Part 3 traces the history of the eye from the lowest to the highest living vertebrates. There is a 24-page bibliography and an index and glossary. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Article
In order to ascertain shape and location of the central area, the distribution of ganglion cells was measured in whole mounts of the retina from pig, sheep, ox, horse, and dog. Although exact comparison of corresponding points of measurement in different animals was not possible, the measurements allowed the mapping of retinal ganglion cell density, typical for the particular species. In all ungulates a streak of high cell density extends along a straight horizontal line, dorsal to the optic disc. As a rule a maximum of ganglion cell density is found close to the temporal end of the visual streak. In the dog a well demarcated oval portion of the streak continues into a short temporal (variable) and a long nasal linear arm. The functional significance of these findings is discussed.
Article
Using a rapid freezing and sectioning technique, the distance between the lens and retina of the horse eye was measured. There is no indication of a ramp retina that could serve accommodation. The pupil axis of the eye coincides with the maximum lens to retina distance. The changes in the lens-retina distance are greater below the axis than above it. Calculations were made of refractive power of the horse eye from measurements of curvature and refractive indices of the ocular tissues. These calculations agree both qualitatively and quantitatively with retinoscopic measurements on live horses. Both show that the refractive state shifts in the direction of hyperopia above and below the axis and that this shift is greater below the axis than above it. Some dynamic accommodative ability in the living eye was observed.
Article
We assessed the ease with which horses could learn visual discriminations and measured their resolution acuity. We trained three horses to press their noses against one of two large wooden panels to receive a small food reward. Following training on a series of two-choice discrimination tasks, resolution acuity was measured. Although there was some variability between animals, the best acuity obtained was 23.3 c deg-1. Within the margin of error imposed by limited anatomical data, the obtained values are consistent with predictions based on retinal ganglion cell density estimates and posterior nodal distance/axial length ratios. They suggest that the resolution acuity of the horse is limited by ganglion cell density in the temporal portion of the narrow visual streak.
Article
The refraction of 12 street cats' eyes and of 11 caged cats was measured by retinoscopy, and the anteroposterior (axial) length of the eyeball was measured by ultrasound. While 87.5 per cent of eyes in street cats were found hypermetropic (average + 1.14 D), among cats caged for periods of 8.5-14.0 months under conditions of near vision. 68.2 per cent were myopic (average, -0.62 D). The anteroposterior length of the eyeball was practically equal in both groups (20.43 mm): it was also practically equal for myopic and hypermetropic eyes. The site of the refractive changes is discussed.
Article
The common marmoset (Callithrix jacchus) is a small, diurnal, New World monkey amenable to vision research. In this paper we describe the visual optics and cone photoreceptor topography of the normal adult marmoset. Paraxial optical ray-tracing shows that the marmoset eye is well represented as a scaled-down version of the human eye. The density of foveal and perifoveal cone photoreceptors in the marmoset is as high, and in peripheral retina higher, than those reported in humans and macaques. The foveal acuity predicted by the Nyquist limits set by the cone mosaic (30 c/deg) is in agreement with behavioral measures of visual acuity. Foveal depth of focus is remarkably small (< 0.2 D) for an eye of this size (axial length about 11 mm). Estimates of the amplitude of accommodation using infrared photorefraction indicate that the marmoset is capable of more than 20 D of accommodation.
Article
We have previously shown that the mature adult quokka, aged between 8 and 15 years, has a distinct cell topography in the retinal pigment epithelium (RPE). We reported that the adult cell densities were high in central temporal retina and low in a peripheral band, adjacent to the ora serrata, a region with a concentration of multinucleate cells. In the present paper, we have studied the development of these features in order to understand how they mature, as well as to gain insight into regional specializations of the RPE. Retinal area, cell density and the extent of multinucleation were analysed using whole-mounted retinae from animals aged post-natal day (P) 2 to 15. The retina continues to grow in area throughout life, however, RPE cell number does not change. The features of the mature adult RPE develop at different times over the entire lifespan of the animal. In peripheral retina, cell density decreases throughout life and the band of low cell density becomes progressively wider and more distinct with age with an increasing proportion of multinucleate cells. By contrast, RPE cell density in equatorial retina remains, throughout life, at the level observed in 1-year-old animals. A specialization of high cell density in temporal central RPE was discernible in animals older than 2 years, with the cell density of this region increasing steadily beyond this age. Central regions of other quadrants demonstrate a constant and relatively uniform density with age. The RPE in the marsupial quokka is a dynamic tissue, demonstrating topographic changes throughout life.
Article
The retinal pigment epithelium (RPE) of the quokka wallaby. Setonix brachyurus, grows and changes throughout life. To investigate factors that determine changes in the quokka RPE, we have examined topography of this tissue in experimentally enlarged eyes. Unilateral eyelid suture was conducted at the time of normal eye opening, postnatal day (P) 110, and animals were examined at 1 or 1 1/2 years of age. The numbers and densities of RPE cells and the extent of multinucleation were compared with those in normal animals. Eyelid suture resulted in a 9.8% and 17.4% increase in retinal area at 1 and 1 1/2 years, respectively; a significant degree of myopia was associated with this enlargement. Cell density topography in experimental eyes was not the same as in controls. Cells from central retina were disproportionately larger in the experimental than control eyes. However, the RPE cell topography in sutured eyes was not the same as that of aged retinae of a similar size. Notably, in sutured eyes there was no development of the high or highest cell densities seen in equatorial and temporal central RPE in aged retinae, respectively. Furthermore, the degree of cell enlargement in peripheral regions was slight compared with that observed in similar-sized, aged retinae. There was no increase in RPE cell number; rather, average cell area increased accompanied by no change or a slight decrease in RPE thickness. Consequently, overall volume of cells did not change significantly. The large number of multinucleate cells normally seen in aged animals was not observed in experimentally enlarged eyes, implying that an increase in cell volume may be the trigger for multinucleation.
Article
To determine how regional cell density of this tissue changes with age, the authors examined the topography of the human retinal pigment epithelium (RPE) in wholemounted tissue obtained from eyes aged 12 to 89 years, donated for corneas. The RPE, with choroid attached, was wholemounted and stained with cresyl violet. From these preparations, the authors analyzed retinal area, RPE cell number, and cell density. Retinal pigment epithelial cell number is highly variable between persons but does not appear to be age related. Retinal area increases until approximately 30 years of age, but beyond this age individual variation masks further enlargement. The distinctive topography of the RPE changes markedly with age. There is a modification from the relatively homogeneous cell density distribution in the youngest retinas examined toward a more heterogeneous pattern in older retinas. From mid-adolescence, a band of larger cells appears at the extreme periphery, adjacent to the ora serrata, which gradually widens so that by 90 years of age, it occupies the outermost 30% of the retinal area. Cell density is highest in the central temporal retina, adjacent to the macula in the neural retina, throughout life. Cell density values in this region increase slightly with age, and the difference between this and surrounding regions becomes more marked with age. With no marked change in total cell number, peripheral RPE in humans enlarges in area throughout life, but the RPE in more central regions decreases in area.
The eye in evolution
  • S Duke-Elder
Duke-Elder, S. (1958) The eye in evolution. In: System of Ophthalmology, Vol. 1