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Vision

Last revised April 19, 2017.

A horse’s eyes are not on the front of his head. They are not on the side. They are both front and side.1

An eye witness is one who has seen something important. A horse is more likely to be an ear witness. We look at something unexpected by turning our eyes and our heads. A horse listens to something unexpected by turning his ears and his head.

Mammalian predators typically have their eyes in front of their heads, with pupils that are either round or vertical slits. This adaptation gives the predator a focused binocular view of any chosen point, with poor peripheral vision. Prey species, in contrast, typically have their eyes mostly on the sides of their heads, with pupils that are horizontally elongated. This adaptation makes it harder for a predator to get to them undetected. Horses, like other grazing animals, are prey species.

The horse’s eye is the largest of any land mammal2, about eight times as large as a human’s, and even bigger than that of a bowhead whale.3 Big eyes gather more light, and are handy for the horse who must see well in dim light.

Eye Position

The eye of the horse is positioned on the side near the front of its head, project to the front, and are somewhat protuberant. This extends the total visual field to about 350 degrees.

Such a field of view is very valuable for a prey animal. But it greatly reduces the overlapping binocular visual field, which is important for depth perception and which is critical for visual acuity — the ability to resolve details.

Field of view for a horse. [Not a great diagram, because no blind spot in front of the horse is shown, and the 350 degree field of view is reduced with excessive blind spot (the white area) behind.] The horse is unable to see the area behind it, or inches in front of his head.4

The immense field of view of the horse may underlie your horse’s apparent short attention span. The more a horse can see, the less he focuses. The more he can see, the more distracted he becomes. Blinders or blinkers are very useful for horses with important jobs, like racing or pulling carts and wagons. By narrowing the field of view to just what is in front, we eliminate the distractions. Of course, this heaps more responsibility on us to ensure that there are no dangers coming that the horse can now not see.

The binocular vision directly in front of the horse’s nose benefits him when he is grazing, which is most of the time. When he’s watching the grass, his peripheral vision — which is monocular — is at work watching for danger. He’s perfectly designed for being watchful while dining.

Depth perception benefits from binocular vision, but when the horse bobs his head up and down as he moves, close objects in the monocular visual fields will bob down and up against a more static horizon, allowing a horse to infer that the object is closer. Putting a martingale on a horse, or otherwise constraining his head movement, likely interferes with his vision.

Blind Spots

With our eyes on the front of our heads, we can easily follow our finger when we place it on our nose, as in a Finger-to-Nose sobriety test. But with his eyes mostly on the sides of his head, a horse’s blind spots are quite different. If you reach to scratch your horse’s nose, or brush his face, he may not see you coming. The blind area in front of the horse’s face makes them very different than us, and you should remember this when interacting with them.

The blind spot in front of a horse extends about 3 feet from his eyes, and is affected by the amount of forelock in the way. Whiskers and a powerful nose help with knowledge of the blind spot. If your horse fails a finger-to-nose sobriety test, he may not be drunk.5

Rods and Cones

The rods in our eyes formed the earliest basis of vision in vertebrates. Rods are best for seeing in very low light, or for sensing differences in light level. But they don’t work as well as cones for detecting details of an image, and can’t detect the color red. The cones in our eyes work best in bright light, give us the details of an image, but are not as good as the rods at detecting small differences in contrast.

You may have had the experience of trying to find your horse in his pasture at night without a flashlight. Walk, walk, walk. No horse here, no horse there. Turn around, and there is your horse following you, just a few feet away from you. You hadn’t been able to find him. He saw you well, and wondered what you were doing in his place.

Horses and humans have both cones and rods, but horses have relatively more rods than humans. That is, their night vision is far better than ours, and their ability to see differences in contrast is better, but their ability to detect color (where color=combinations of red, green, blue) is not as good as ours. On a dark, moonless night in the woods, a horse can discriminate between shapes and negotiate the terrain. Add cloud cover and remove light pollution on this moonless night in the woods, and your horse will begin to have trouble navigating in unfamiliar surroundings.6 On most nights in the woods, and all nights on the plains, any glowsticks on the night ride are for the benefit of the humans. Horses evolved on the open plains, where they are perfectly adapted for seeing at any hour of the day or night.

In humans, there are three types of cones, which detect red, green, or blue. Horses have only two types of cones, which detect blue or green, but not red. The colors seen by horses are the basic plan for most mammals that can see colors. (Bulls can’t see red either;7 it is the brightness and movement of the bullfighter’s cape that bugs them.) Like the birds and the bees, primates needed to know when fruit was ripe, and so have evolved the ability to see red.

Red and green apples in the top row show how they are seen by a human. The same apples in the bottom row show how a horse would see them.8

The vision of the horse should be considered when designing jumps. Horses will find it easiest to see a jump when it is painted in two or more contrasting colors, such as blue and green or black and white, and if the colors contrast with the footing and surrounding landscape.

The eye of the horse does not adapt to sudden changes in lighting as quickly as a human’s eye.9 So entering a dark trailer or barn on a bright day can be scary. Leaving a well-lit arena at night can be scary. Running a steeplechase where the ground alternates between shade and full sun can be scary. Entering and leaving a well-lit check point in a night endurance ride can be scary. And exiting a dark woods into a bright open field can be scary. It is a good idea to turn on your interior trailer lights before loading, and turn on the lights over the aisle of the barn before bringing the boys in. Turn on your stall lights before leading your horse out in the day. And keep in mind that a horse might not fully recover his night vision when leaving a bright place for half an hour or more.10 Horses are adapted to the pace of lighting change of sunrise and sunset, not electric lighting.

Acuity

The visual acuity (sharpness) of the horse (20/33)11 is better than a dog (20/50 to 20/75), cat (20/75 to 20/200), or rat (20/300), and in fact better than that of most other mammals,12 but it is not quite as good as a human (20/20), and far worse than a bird of prey (20/4 or 20/5), who can see an ant crawling on the ground from the roof of a 10-story building. 20/33 means that whatever we can see from 33 feet can only be seen by your horse from 20 feet. A horse must be closer than you to see the same details.

Like goats, deer, and other grazing animals, horses have horizontal pupils. As the horse lowers his head to graze, or raises his head, the eye rotates and the pupil remains parallel to the ground, a trick called cyclovergence. This elongation increases his field of view of the horizon by increasing the light entering from forward and rear, at the expense of seeing areas above. This also improves the quality of the image of the ground surface, making it easier for him to pick his way over rough surfaces.

The human eye has a fovea — a specialized small part of the retina loaded only with cones. Our acuity is highest for images that land on the fovea. Horses don’t have a fovea, but instead have a visual streak — a linear area of the retina with a high concentration of ganglion cells. Within the visual streak is a central area — the “area centralis” — where the concentration of cones is especially high. The visual field where these horse’s vision is best is not round, like the shape of our pupil, but flattened horizontally, like the shape of his pupil.

The total visual field for the horse is an image about 50% greater in size than that produced by the human eye,13 which is not to say his vision is crisper, or that things look larger, but that he takes in more at a glance.

Horses see best an area of limited height but almost unlimited width. Images landing outside our fovea are what we see “out the corner of our eye”, and this peripheral vision is likely what a horse sees of images whose light lands outside their visual streak. The vision outside the visual streak is likely blurry, but the horse is very sensitive to slight changes in light, and small displacements of moving objects.14

The view seen by the left eye of a human and a horse facing the same direction, looking over the city of Perth, Australia. In (a) is the view of the city as recorded by a camera. In (b) is the view behind human and horse, as recorded by a camera. In (c) is the view of the city as seen by a human: a small, high acuity central region surrounded by a large area of lower acuity. In (d) is the same view as seen by the horse: it runs from front to rear as a horizontal strip, with good resolution in the middle, and lower acuity top and bottom.15

Horses who want to look at something can tilt or turn their head to cause light from some object to fall on the visual streak, improving their acuity. If you are doing target training, horses seem to do better with visual tasks where the stimulus is on or near the ground.16

But the acuity of the visual streak is no match for the acuity of our fovea.

The rods and cones in a horse’s eye project onto ganglion cells which pass information on to the brain. A horse’s high sensitivity to low light and his poor visual acuity is partly a result of having a very thin layer of ganglia.17 Also of help is the tapetum lucidum, a pigmented part of the eye which reflects light back onto the retina, allowing for greater absorption when it is dark. It is the tapetum lucidum that makes a horse’s eyes shine at night when you aim a flashlight at them or take a picture with a flash. Animals that are active at night have a tapetum lucidum; humans and other primates don’t.

Accommodation

Accommodation is what the eye does when we change our focus from near to far or back. Veterinary textbooks sometimes claim that horses lack a dynamic lenticular accommodation ability,18 but a number of studies reveal that these texts are wrong.19 In other words, horses are able to focus on objects that are close, then shift their focus to objects that are far away.

Eye Movement or Head Movement

Horses can move their heads to improve their vision. Nose forward with a raised head, a horse has binocular vision for focusing on distant objects. Nose down, the horse has a binocular focus on objects in front of his feet. Horses participating in show jumping will have vertical heads after leaving a jump and moving to the next; race horses will carry their heads horizontally. A horse trudging through the woods and surprising a deer will suddenly raise his neck and shift his head to a more horizontal position, to place his binocular vision on the deer. Once he understands what he has startled, he’ll settle and return to his trudging, with a more vertical head.

But a fixed head position does not suit a horse making a jump. While he may approach the jump with a near-vertical head (to see the ground best), as he approaches the jump it moves into his blindspot. Riders focus their gaze near the top of the obstacle to be jumped,20 and the horse needs to be allowed to do the same. His intelligent impulse is to raise his head and turn it to one side enough to see the top of the approaching jump with one eye. Many riders seem to think that this motion is a sign of refusal or disobedience. In fact, it is a sign that he wants to succeed. If he is prevented from doing this with a martingale or short reins, you’ve reduced his chance of success.

Since your horse can rotate his eye to align his visual streak with the horizon, why can’t he jump with his head vertical? Because he cannot infinitely rotate his eye, and because even if he could, his upper eyelid would block his view of the jump as he approaches it. According to McGreevy et al (2010), “ these findings demonstrate that the longitudinal neck flexion of the degree desirable by popular opinion in ridden horses is not a common feature of unridden horses moving naturally. Moreover, they suggest that advertised horses in our series are generally being ridden at odds with their natural carriage and contrary to the international rules of dressage”.21

If you move your hand to the front of his nose, you put it in his blindspot. In response, your horse may suddenly raise his head and turn it away, to move his blind spot away from the object and get the object into his field of view. You can help him by showing him the object for one or the other eye to see. And if you raise your hand to rub his nose, you’ll keep his anxiety down by reaching for his neck first, and rubbing from there up to his head and down to his nose.

A horse has two ways to get an object of concern into his visual streak: move his head, or move his eye. Ocular muscles attached to the eye allow it to move within the horse’s skull, but it doesn’t roll easily or often: because the horse has no fovea and such a wide field of view, there is rarely any benefit to the horse in moving its eye up, down, left or right. You many never see it move, and will have to guess at what it is looking at.

When you are lumbering along with Mr. Horse in the woods, and a deer leaps up, your horse likely raises his head, and may stop in his tracks. While you may have seen the deer with both eyes and your binocular vision, he initially saw it with just one eye and his monocular vision. By raising his head and turning it, he is able to use his binocular vision on the deer, and better judge what it is, how far away it is, which way it is headed, etc. And he is in a better position to judge its speed and direction if he is stationary, so he may stop to get the best look. (And if the jumping animal is a predator, freezing to study it will help him hide from it.) None of this means your horse is scared by the deer. It means that his design requires him to raise and turn his head to see something with binocular vision, and to be motionless for the best analysis. Cut him some slack when he wants to see something potentially important on a trail ride.

Head movement is far more noticeable to us than movement of the horse’s eye, and may be the more common means of bringing things into focus. But eye movement does occur, and the proof is that we sometimes see the white of a horse’s eye, sometimes don’t. A horse, can not only move its eyes, but he can move them independently, in the same way that he can move his ears independently.

Imagined Reality

In the human eye, the fovea is packed with cones — no rods — with one cone for each ganglion. The result is high acuity trichromatic color vision, and precision stereo depth perception. But the field of view provided by a stationary fovea is extremely small. So primates have developed eyes that are constantly on the move, jumping around in the socket to take in things of interest. Our sense of this, however, is a construction of reality, rather than what we are actually seeing. By looking from A to B and back, we feel that we are seeing both A and B. But at any given time, our fovea might be admiring C, and the vividness of A and B are recalled, and assembled by the brain to give us a picture of what’s out there.

In the horse looking at the horizon, nearly every point is just as clear as every other point. No mental construction effort is needed. And because points A and B are both actually being seen at the exact same time, any movement of A or B is immediately detected. In a human, we would not see such movement unless our fovea was directed at it.

Neither umwelt is “better”, though the horse could not have survived without his, and we could not have survived without ours.

Illusions

Illusions help us understand how important mental construction is to the task of seeing. The Ponzo illusion is one in which the depth cues of converging lines confuse us about the size of objects between them. You can see this illusion in the pictures below. In the first, the cars are all the same size, but the most distant one appears larger because of the converging lines of the sidewalk and street. In the second, the horizontal lines are of the same length, but appear different on the converging railroad tracks. This illusion doesn’t just affect humans. Pigeons,22 rhesus monkeys,23 chimpanzees24, and horses25 all fall for it.

As Hunter has written “Our eyes deceive us when we look down railway tracks, but our brains do not. The rails appear to converge in the distance, but we know that the rails are parallel. We know that they are the same distance apart a mile down the track as they are where we are standing, so the brain says, ‘The tracks only appear to converge because they are distant.’ But how does the brian know that the tracks are distant? The brain answers, ‘They must be distant because they appear to converge.’ (The flow of this logic must shock computer programmers, but they are accustomed to the limitations of inferior hardware.)26

Ponzo illusion.27

The Eye as an Extension of the Brain

My old model of how the eye worked was similar to that of a camera (which I don’t happen to understand either): somehow a photographic image, with some distortions, was copied through some wires to the brain, where magic happened. Sometimes the image could be projected onto a screen in my brain, so that I could vaguely recall what something looked like.

My new model is worse. Through compulsive scholarship for this book, I learn that the mammalian retina contains around 55 distinct kinds of cells. Transmission of an image to the brain works with a transformation of the image that is nothing like the original. Preprocessing in the eye is substantial. Multiple nerves carry redundant information, and some information is discarded in the eye. Rather than being an agent for the brain, the eye is really an elegant extension of it.28

When we exult that our brains are bigger than our horse’s, we need to add to each the volume taken by the eyes and their circuitry — and the nose and its circuitry, and the ears and their circuitry. Feeling small yet?

Mounting

When you were first learning to ride, your trainer likely told you to mount your horse from the left, and explained that this was traditional — that the cavalry wore their swords on their left sides because they were right handed — and that by mounting in a traditional way, you would be less likely to upset the horse. Turns out those trainers got it wrong again. Mounting a horse from the left side is a custom that dates back at least to the beginning of the fourth century. At that time, nomads in Asia used a single stirrup — a mounting stirrup — attached to the left side of the saddle.29

But there is also a tradition of mounting from the right. “The cavalry of Alexander the Great, who rode bareback with neither saddle nor stirrup to assist them, used a battle spear to pole vault aboard from the right. Samurai warriors of feudal Japan wore their two swords tucked into the obi or sash, to be handy but out of the way, and are believed to have mounted from the right. Napoleon Bonaparte, who was a left-hander and, therefore, wore his sword on the right, is said to have mounted from the right.30” Native Americans, unaffected by European culture when learning to ride and not accustomed to wearing swords, came to mount from the right. Cowboys, on the other hand, preferred the European way of mounting on the left31 — so happily we can tell the cowboys and Indians apart.

Why the preference for mounting from the left side? It likely has to do with the prevalence of right handedness. Those who are right handed may find it easier to lift their right leg over the saddle, pivoting on their left leg in the process.

Does it matter? In fact, when a horse is approached from the left, the right side of its brain does much of the processing, and that is the side that reacts most quickly to new or scary stuff. The right side of the brain is best at discerning if an approaching rider is known or not, friendly or menacing. It is quite possible that horses prefer a rider to approach from the left, to give it the greatest and fastest certainty that the approaching person is safe. But because a horse flees farther when startled from the left side than from the right,32 you might prefer to avoid this situation by always mounting on the right. Assuming you are not carrying your sword, have stirrups on both sides, and are not trying to be mistaken for a cowboy, mounting on the right might be a good idea.

But never mind your horse: if you are left-handed, give mounting on the horse’s right side a try: it might be easier for you.

But better than always mounting from the same side would be to develop the skill to mount from either side, and to alternate between them. This was Xenophon’s recommendation in 360 BCE33, and it still seems like a good one. Ambidexterity will leave you ready to climb aboard if you have some odd situation or your horse has happened to sidle up to the wrong side of a stump, and it will better distribute the strain you put on the tree in your saddle, your stirrup leathers, and your horse’s back. Switching sides will take a bit of practice for both you and your horse.

Myths about Horse Vision

The myth of apparent size. Many believe that because a horse’s eye is larger than ours, that somehow the things they see are larger to them.

  • “Your horse’s eyeball is the largest orb found in any land mammal, and has a correspondingly oversized retina. The effect of this large retina is that it magnifies everything he sees—to him, up-close objects look 50 percent larger than they appear to you.34
  • “A horse’s eyeball is the largest orb found in any land mammal. This means it also has an oversized retina. The result? Objects seen by the horse are magnified. To a horse, an object might seem a full 50% larger than it does to a human. Thus, a large dog barking or snapping at its heels might appear as large as a small pony.35

This is not so. A horse’s perception depends on a combination of brain and image sent from the eye. While an object is physically larger when it falls on a horse’s retina than when it falls on our own (because the horse’s eye is larger), nothing is “seen” until the brain has digested these signals. So horses see objects at their correct size.36

The myth that a horse must see something with both eyes. Many believe that if a horse has only seen something with one eye, that it will be “new” when seen with the other eye. Something familiar to one eye will be entirely novel to the other.

  • “Because your horse must see an image with both sides of his brain before determining whether it’s friend or foe, viewing objects with one eye doesn’t adequately acclimate him to them.37
  • “In order to maintain its own inner comfort zone, a horse must see an object with both eyes or, at least, have processed the same image with each eye separately.38

This is silly. The horse doesn’t have two brains. It has two halves of one brain, and they are nicely connected to each other with a corpus callosum and other wiring which sends information where it is needed. Experiments prove that the need for a horse to see something with both eyes is pure myth.39 No matter which eye a horse uses to view an object, it recognizes it instantly with the other eye.

However, for simply seeing something, horses show more lateralization than humans. In humans, about 53% of optic nerve fibers pass through the opposite optic tract. But in horses, between 81% and 88% of the nerves from one side pass over to the opposite side.40 So when approaching that wheelbarrow, if your horse is restrained from turning his head, and sees it only with one eye, then the other side of his brain gets 81-88% of that signal, and for the other side to deal with it well, information from the first side must go through the corpus callosum to the other side. If the left eye is the one (and only one) to see it, the right side of his brain gets to deal with it first, and he’ll likely be a bit more anxious than if the right eye was the only one to see it.

Myths of head position.

And now for a digression into the world of silly. Listen to the experts on head position and its effects on horse vision:

  • “Horses usually have to be taught good head position.41” This one is too preposterous! Should we teach them to walk, too?
  • “The horse has to lower its head to see faraway objects. The horse has to raise its head to see close objects.42” Nope. Look at the picture of the grazing horse earlier in this chapter.
  • Head down is “a way to tell the horse that you want him to relax”.43 No. If he wants to relax, he’ll do so, and take a relaxed position. Forcing him to look relaxed won’t change anything on the inside.
  • “Binocular vision is directed down the horse’s nose, not straight ahead.44” No. Binocular vision occurs in the area where the input from both eyes converges. A horse with its head down to graze will have rotated his eye so that it is still scanning the horizon. Watch him: his pupils will always be horizontal, and remain that way whether he raises or lowers his head.

The myth that a horse with blue eyes is more likely to develop eye disease. This myth is mostly false. Bergstrom et al (2014) report “Horses with blue or heterochromic irises are more likely to develop ocular squamous cell carcinoma (SCC) than horses with brown irises, but are not more likely to have adnexal, corneal or intraocular/orbital disease or to be presented for evaluation of ophthalmic disease.45” The increased risk of SCC is believed to be a result of the lack of pigment in the skin surrounding the eyes of many blue eyed horses. The pigment (melanin) normally present makes the skin dark brown and protects against UV rays.

Interesting fact: Horses with longer noses or with more convex nasal profiles have a greater density of ganglion cells in the visual streak, giving them better vision. All things being equal, Thoroughbreds and Standardbreds are likely to have better vision than Arabians.46

Summary

Comparing humans and horses:

  • Acuity: horses quite good, humans better.
  • Accommodation: horses OK, humans better.
  • Color vision: horses have it, but don’t see red. Humans see red.
  • Judging distance, and depth perception: horses and humans about equally good.
  • Night vision: horses fabulous. Humans stink.
  • Field of view: horses can see about 350 degrees of a horizontal band. Human field of view is small and circular.

For more information

  • Murphy, Jack, Carol Hall, and Sean Arkins. “What horses and humans see: a comparative review.” International Journal of Zoology 2009.

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1 image source: Ransom, Jason I., and Brian S. Cade. “Quantifying Equid Behavior–A Research Ethogram for Free-Roaming Feral Horses.” U.S. Geological Survey Techniques and Methods 2-A9, 23 p. (2009) http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1018&context=usgspubs

2 Soemmerring DW. A comment on the horizontal sections of eyes in man and animals. Anderson SR, Munk O, eds. Schepelern HD, transl. Copenhagen: Bogtrykkeriet Forum; 1971.; Knill LM, Eagleton RD, Harver E (1977). “Physical optics of the equine eye”. Am J Vet Res. 38 (6): 735–737. PMID 879572. Relative to body size, some mammals have larger eyes than the horse.

3 Andersen, S. R., and O. Munk. “A comment on the horizontal sections of eyes in man and animals.” Acta Ophthalmologica, Supplement 10 (1971): 77-84.

4 Image source: Banks, M., W. Sprague, G. Love, J. Parnell, J. Schmoll. “Horses and Sheep and their Amazing Eye Movements”. University of California Television (UCTV) https://www.youtube.com/watch?v=LOoe2XVaFrU

5 Image source: Murphy, Jack, Carol Hall, and Sean Arkins. “What horses and humans see: a comparative review.” International Journal of Zoology 2009 (2009). See also “Equine vision”. WIkipedia. https://en.wikipedia.org/wiki/Equine_vision

6 Hanggi, Evelyn B., and Jerry F. Ingersoll. “Stimulus discrimination by horses under scotopic conditions.” Behavioural processes 82.1 (2009): 45-50.

7 Stratton, George M. “The Color Red, and the Anger of Cattle.” Psychological Review 30.4 (1923): 321.

8 Photo source: “Equine vision” in Wikipedia. https://en.wikipedia.org/wiki/Equine_vision

9 Williams, Moyra. Horse psychology. Barnes, 1969.; Wouters, L., A. Moor, and Y. Moens. “Rod and cone components in the electroretinogram of the horse.” Zentralblatt für Veterinärmedizin Reihe A27.4 (1980): 330-338.

10 Wouters, L., A. Moor, and Y. Moens. “Rod and cone components in the electroretinogram of the horse.” Zentralblatt für Veterinärmedizin Reihe A 27, no. 4 (1980): 330-338.

11 Sources differ on the exact acuity. Some suggest it is as poor as 20/60, while others suggest it is as good as 20/30.

12 Timney, Brian, and Kathy Keil. “Visual acuity in the horse.” Vision research32.12 (1992): 2289-2293.; For somewhat different results, see Miller, Paul E., and Flickering Lights. “Vision in animals-What do dogs and cats see.” In The 25th Annual Waltham/OSU Symposium. Small Animal Ophthalmology, pp. 27-28. 2001.

13 Farrall, H., and M. C. Handscombe. “Follow‐up report of a case of surgical aphakia with an analysis of equine visual function.” Equine Veterinary Journal22, no. S10 (1990): 91-93.

14 Ehrenhofer, Marion CA, Cornelia A. Deeg, Sven Reese, Hans‐Georg Liebich, Manfred Stangassinger, and Bernd Kaspers. “Normal structure and age‐related changes of the equine retina.” Veterinary ophthalmology 5, no. 1 (2002): 39-47.

15 Image from Harman, A. M., Moore, S., Hoskins, R., & Keller, P. (1999). Horse vision and an explanation for the visual behaviour originally explained by the ‘ramp retina’. Equine veterinary journal31(5), 384-390.

16 Hall, Carol A., Helen J. Cassaday, and Andrew M. Derrington. “The effect of stimulus height on visual discrimination in horses.” Journal of animal science81.7 (2003): 1715-1720.

17 Prince, J.H., Diesem, C.D., Eglitis, I., Ruskell, G.L. 1960. Anatomy and Histology of the Eye and Orbit in Domestic Animals. Charles C. Thomas. Springfield, IL, pp 130-139.

18 For example, see Nicolas, Eugène. Veterinary and comparative ophthalmology. Brown, 1914.; Duke-Elder, Stewart. System of Ophthalmology Vol. 1 The Eye in Evolution. Henry Kimpton, 1958.

19 Harman, A. M., Moore, S., Hoskins, R., & Keller, P. (1999). Horse vision and an explanation for the visual behaviour originally explained by the ‘ramp retina’. Equine veterinary journal31(5), 384-390.; Sivak, J. G., & Allen, D. B. (1975). An evaluation of the “ramp” retina of the horse eye. Vision research15(12), 1353IN8-1356.

20 Laurent, Michael, Rémi Dinh Phung, and Hubert Ripoll. “What visual information is used by riders in jumping?.” Human Movement Science 8.5 (1989): 481-501.

21 McGreevy, Paul D., Alison Harman, Andrew McLean, and Lesley Hawson. “Over-flexing the horse’s neck: A modern equestrian obsession?.” Journal of Veterinary Behavior: Clinical Applications and Research 5, no. 4 (2010): 180-186.

22 Fujita, Kazuo, Donald S. Blough, and Patricia M. Blough. “Pigeons see the Ponzo illusion.” Animal Learning & Behavior 19.3 (1991): 283-293.

23 Fujita, Kazuo. “Perception of the Ponzo illusion by rhesus monkeys, chimpanzees, and humans: Similarity and difference in the three primate species.” Perception & Psychophysics 59.2 (1997): 284-292.

24 Fujita, Kazuo. “Perception of the Ponzo illusion by rhesus monkeys, chimpanzees, and humans: Similarity and difference in the three primate species.” Perception & Psychophysics 59.2 (1997): 284-292.

25 Timney, Brian, and Kathy Keil. “Horses are sensitive to pictorial depth cues.” Perception 25.9 (1996): 1121-1128.; Timney, Brian, and Todd Macuda. “Vision and hearing in horses.” Journal of the American Veterinary Medical Association 218.10 (2001): 1567-1574.

26 Creasey, Mike. “Light: Science and Magic. An Introduction to Photographic Lighting.” (2009): 26-26.

27 Image sources: Wikipedia. “Ponzo Illusion” https://en.wikipedia.org/wiki/Ponzo_illusion; https://in.pinterest.com/pin/510525307735105810/

28 Leblanc, Michel-Antoine. The Mind of the Horse. Harvard University Press, 2013. pp.136-137.

29 Cartier, Michel. “Considérations sur l’histoire du harnachement et de l’équitation en Chine.” Anthropozoologica 18 (1993): 29-43.; Lazaris, S. (2007, October). Considérations sur l’apparition du fer à clous: contribution à l’histoire du cheval dans l’Antiquité tardive. In La veterinaria antica e medievale (pp. 259-291).

30 “Why cowboys get on the left side of a horse”. Pets on Mom.me http://animals.mom.me/cowboys-left-side-horse-4252.html

31 “Why cowboys get on the left side of a horse”. Pets on Mom.me http://animals.mom.me/cowboys-left-side-horse-4252.html

32 Austin, N. P., and L. J. Rogers. “Asymmetry of flight and escape turning responses in horses.” Laterality 12.5 (2007): 464-474.

33 Xenophon writes “To meet the case in which the horseman may chance to be leading his horse with the left hand and carrying his spear in the right, it would be good, we think, for every one to practice vaulting on to his seat from the right side also. In fact, he has nothing else to learn except to do with his right limbs what he has previously done with the left, and vice versa. And the reason we approve of this method of mounting I that it enables the soldier at one and the same instant to get astride of his horse and to find himself prepared at all points, supposing he should have to enter the lists of battle on a sudden.” Morgan, M. H. “Xenophon: The Art of Horsemanship.” JA Allen and Company Limited, London, UK (1993).

34 Hayes, Karen. “Understand Your Horse’s Eyesight”. Horse& Rider. http://horseandrider.com/article/horseseye-view-27407

35 Sellnow, Les. “The Equine Eye”. Oct 15, 2001. http://www.thehorse.com/articles/10774/the-equine-eye

36 See Leblanc, Michel-Antoine. The Mind of the Horse. Harvard University Press, 2013, p. 126.

37 Hayes, Karen. “Understand Your Horse’s Eyesight”. Horse& Rider. http://horseandrider.com/article/horseseye-view-27407

38 Sellnow, Les. “The Equine Eye”. Oct 15, 2001. http://www.thehorse.com/articles/10774/the-equine-eye

39 Hanggi, Evelyn B. “Interocular transfer of learning in horses (Equus caballus).” Journal of Equine Veterinary Science 19.8 (1999): 518-524. Horses in her study “demonstrated high levels of interocular transfer.” The author concludes that “contrary to beliefs held by many people, horses are capable of interhemispheric transfer of visual information.”

40 Cummings, J. F., and A. Lahunta. “An experimental study of the retinal projections in the horse and sheep.” Annals of the New York Academy of Sciences 167.1 (1969): 293-318.; Levine, Jonathan M., Gwendolyn J. Levine, Anton G. Hoffman, and Gerald Bratton. “Comparative anatomy of the horse, ox, and dog: the brain and associated vessels.” Neurology 3, no. 3 (2008).

41 “Head Position Matters” Equisearch Aug 31, 2005. http://www.equisearch.com/article/head-position-matters

42 Icelandic Horse Connection. “Icelandic Horse and Horse Vision”. http://iceryder.net/vision.html

43 “Keeping Your Horse’s Head Down”. Holistic Horse. http://holistichorse.com/health-care/keeping-your-horses-head-down/

44 Johnson, Debora “Horse Vision” http://horsehints.org/HorseVision.htm

45 Bergstrom, B. E., Labelle, A. L., Pryde, M. E., Hamor, R. E., & Myrna, K. E. (2014). Prevalence of ophthalmic disease in blue‐eyed horses. Equine Veterinary Education26(8), 438-440.

46 Evans, K. E., & McGreevy, P. D. (2007). The distribution of ganglion cells in the equine retina and its relationship to skull morphology. Anatomia, histologia, embryologia36(2), 151-156.

 

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