Horse Vision

Sight Preference in Horse's/Left or Right Eye? Why?

Horses have the largest, sweetest eyes. In fact, their eyes are the largest of any land mammals. When purchasing a horse people will often say look for a large, wide apart, kind eye. This article will give you hints on how to be safer around horses on the ground while handling them and while riding. To understand how a horse sees and perceives his environment can give a greater understanding of why horse's do what they do.

Outstanding peripheral vision gives the horse early warning of predators. However, it does come with some drawbacks. For example, horses have a blind spot directly in front of their noses. A horse will always see two images, and cannot merge the images together like a human.

How a horse sees.	Human sight (same view)

• Horses have amazing peripheral vision with two blind spots. One blind spot is directly in front of his nose extending around four feet in front of it. The second blind spot is behind the tail. That blind spot extends about ten feet long, beyond the horse's tail.
• The field of view for horses can reach up to 200 degrees.
• Because of the front blind spot, the abilities of jumping horses seem all the more incredible. The horse loses sight of the obstacle when it is a few feet away and has to rely totally on the rider to tell him when to jump. Your horse jumps blind.
• Horses have very good binocular vision with a tendency towards long vision.
• Horses do not focus their eyes the way we do. Have you ever seen a horse raising and lowering its head as it looks at an object? It does that to adjust the focal length, moving until the object comes into focus on its retina. The horse's field of vision does not overlap. The right eye sees what is happening on the right side. The left sees what is happening on the left side. The images do not match up. The field of vision is completely different right and left. The eyes see independently of each other. This is called monovision.
• Even when the horse has focused as best it can, its sight is only three-fifths that of a human. In other words, when looking at an object twenty feet away, the horse sees only as much detail as a person with twenty-twenty vision would if the object were thirty-five feet away. Simply, when you are riding down the trail and see a strange object ahead, you will recognize what you are seeing long before your horse does.
• A horse sees like a human does through bifocals. While a horse is grazing his head is down. Visually he must look up to focus on the horizon. If he needs to look at something closer, the horse will raise his head to view the object through the lower portion of the eye, where he can focus more clearly.
• When you see a horse shy at a sudden movement behind him or next to him, his peripheral vision has sighted the movement but has not yet had time to focus on it.
• Your horse's vision is better than yours at night. Horses have a reflective panel on the retina that helps to gather all available light at night. The equine retina has nine times as many rod receptors as cones. These receptors are responsible for vision in low light. This suggests good night vision.
• Horses do not see color as well as humans. Equine Research Organization research indicates that horses are visually deficient in seeing green and red. Recent research into the horse's ability to see color indicates that they can distinguish between yellow and red. A horse sees mostly in shades of gray. "Equine vision expert Dr Brian Timney, of the University of Western Ontario, explains: 'Horses have little difficulty in discriminating red or blue from grey but with green and yellow, the results are mixed.'" More scientific studies are being done on the horse's ability to see color. The total story is still out on how a horse sees color.
• Behavioral data as well as physical evidence show that horses possess at least two types of cones (cones are necessary for color vision) The presence of cones indicate that horses can see some color.
• Horses have the largest eyes of any land mammal. They also have large pupils. Their eyeballs are placed toward the side of the head, which gives them a wider field of vision.
• Horses can scan their entire surroundings with only slight head movement.
• The size of the pupil improves the ability of a horse to pick up movement.
• The large size acts like your camera's wide-angle lens which is further enhanced by the placement of the visual receptors in the retina. The total effect is better peripheral (side) vision. The horse can see movement well. Scientifically it is believed that a horse does not see as clearly as a human within 4 feet.
• The horse has a blind spot directly behind him. Approaching from behind may scare your horse. He may kick or try to bolt. Let him know that you are there by making a soft sound or speak his name.
• The horse uses only one eye (monocular vision) to observe the width of its visual field. When a horse sees an object with monocular vision, he will tend to turn toward the object to better hear and see.
• When your horse uses his binocular vision (using two eyes), he has better depth perception and a more concentrated field of vision.
• When a horse switches from one eye (monocular vision) to two eyes (binocular vision) a brief visual shift sometimes occurs. This can cause an unexplained "spooking" of the horse.
• Horses have excellent night vision. If riding at night, let him choose his way.
• Horses require approximately 15 minutes for their vision to adjust when moving between differently lighted environments.
• Sudden brightness takes an equal amount of adjustment.
• Horses can also perceive depth. "Apparently, horses have many of the same depth-detecting skills that we have," says Dr Timney. "They have true stereoscopic vision, despite having lateral eyes."

In the wild horses are hunted for food by predators. In order to survive they take flight or run when threatened, kick, bite, buck, etc. Because they graze and have their heads down much of the time and because they are hunted by predators the horse's eyes are located on the sides of their head. We on, on the other hand, have our eyes in front. The placement of the horse's eyes give him a big advantage. They have excellent peripheral vision. In fact, they can see in a circle behind them. This is referred to as "mono eye," meaning one. A horse's eyes are located on either side of his head which is a big advantage for them as a prey animal as it offers a wide, circular view, meaning they can detect stalking animals sneaking up from behind. This panoramic vision is referred to as monocular vision. (mono meaning one). This mono vision enables the horse to view his surroundings on both sides, with either eye. When looking forward the horse uses binocular vision (two eyes). The binocular vision is directed down the horse's nose, not straight ahead. The horse has a blind spot in front of his forehead. When a horse is grazing, his vision is directed at the ground in front of him and if he is relaxed, his monocular vision will be at work. Should he see, hear or smell something that warrants investigation, the horse will raise his head to bring his binocular vision into force. If the object was spotted in the horse's side vision (peripheral vision), he will turn and raise his head, or even his entire body to look.

Because of the horse's large eyes, he is able to detect the slightest movement in surroundings. That is one of the reasons horses shy often. It is also one of the reasons that horses are spooky on windy days. That falling leaf or flapping plastic bag could be something that is going to prey upon the horse. At least, that is how the horse sees it. The horse moves its head in order to bring the object into his binocular field, which also gives better depth perception. This offers a better view. He cannot use both his monocular vision and binocular vision at the same time.

For example, you are riding along and your horse is relaxed and using is monocular (peripheral) vision. Something catches his attention and he raises his head, perks up his ears, maybe even snorts or shys. He is now using his binocular vision (both eyes), looking down his nose. He is trying to survey the situation with both eyes to determine if he is safe or should be concerned. If the object is on the ground, the horse will lower his head, look down his nose, use both eyes for a clear view, and sometimes will snake around the object, not wanting to get too close to it. Horses move their heads up and down because their visual field is narrow. Objects seen the clearest are the ones that fall within this narrow field of vision. The horse tilts his head in order to get as much of an object as possible to cast an image onto the eye.

"How much detail can horses see? Using a method of placing rewards behind a trapdoor, a research team tested how much detail a horse could see by placing stripes on the door. The horse was trained to choose the striped door over the plain one for the food reward. They varied the thickness of the stripes until they were so fine, the horses could not distinguish the striped door from the grey. From the results, they discovered that horses see as well as we do...perhaps better! Using the Snellen scale to compare horse vision with our own, indicates that horses actually see well at a distance. The Snellen scale for humans is 20/20, meaning that a person can read the same line on an eye chart from 20 feet that the 'standard' person reads from the same distance. Using this Snellen scale, horses rate 20/30 while as a matter of interest (and by comparison) a dog is 20/50, a cat 20/75 while rats rate 20/300."

Having the understanding of the monocular vision and the binocular vision and knowing the horse is programmed to take flight because he is a prey animal, we can see that a horse does not like to go into a dark place like a trailer. We can also understand why the horses eyes transition slowly when we turn the lights on in the barn when the horse has been in the dark. The slightest movement is picked up by the horse's peripheral (monocular) vision. That may be one reason that your horse shys on the trail or wherever he feels threatened. Flying birds, falling leaves, etc. come from above and that takes the horse time to focus and process. A white rock will often cause a horse to shy. Why? The way a horse's eye sees color may cause him to determine that rock may be a threat. Shadows can also cause your horse to shy. His visual acuity, the way he moves from monovision to binocular vision, together with his instincts to get out of harms way help to make your horse a 800+ pound moving object that can do you harm. Try to think and see like your horse and it makes his behavior understandable. He is not being bad, he is being a horse.

"A key to understanding the instinctive behavior of horses and mules is to know how they see, hear, smell, and feel things. As prey animals, horses and mules are very perceptive--they had to be to survive in the wild. However, along with that acuity comes some limitations.

Vision

Horses and mules move their eyes independently, allowing them to see objects in two different directions at once. Their eyes protrude slightly from the sides of the head, allowing panoramic vision with a visual field that measures about 350 degrees. This visual field is predominantly monocular--or seen with one eye at a time. The monocular portion of the field measures about 285 degrees. Monocular vision is relatively flat and is used for detecting distant motion. Horses and mules also have a binocular visual field--an area of about 65 degrees that is seen with both eyes at once. In contrast, the human field of vision, which measures less than 180 degrees, is mostly binocular. Binocular vision is three dimensional and contributes to depth perception.

Because their eyes are on the sides of their heads, horses and mules have blind spots in their binocular vision (figure 1-10). They cannot see the tips of their own noses or anything directly beneath their heads, limiting the ability to see anything directly in front. They cannot see objects closer than 4 feet (1.2 meters) with binocular vision. They also don't automatically see something behind that is narrower than their body. Horses and mules can't see forward and sideways at the same time.

In order to focus their vision, horses and mules must move their heads, and they can do so with amazing speed. They can focus their vision more quickly than can humans. Usually when stock lift their heads, they are looking at something in the distance. They lower their heads to focus on low, close objects. This visual arrangement allows horses and mules to graze and watch for danger at the same time, but may affect their depth perception. Occasionally stock run into, fall over, or step on low-lying objects that they did not see or recognize as a hazard, such as posts, wires, holes, signs, rocks, and waterbars. Stepping into an animal's burrow can cause a horse or mule to trip, fall down, or break a leg. Common burrow dwellers include ground squirrels, badgers, and prairie dogs..."

Temple Grandin can shed further light on how a horse sees, thinks, and behaves. She describes horses as "seeing in pictures." My husband and I had the good fortune to hear Temple Grandin lecture at the Smithsonian Institution. She explained how her autism has helped her understand how animals think and perceive their environment. Another good site on horse vision How Horses See by Evelyn B. Hanggi, M.S., Ph.D.

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