Equine Vision: Impact on Trail Behavior (Book Excerpt)

It's all in the eye of the beholder. That cliché has been around for years, but when we consider it in light of the human eye compared to the equine eye, the saying takes on a whole different meaning.

Humans and horses literally see things differently, and this difference can sometimes lead to problems on the trail. No doubt you've ridden on windy days when suddenly your horse became agitated and excited. A piece of paper flew across the road or trail and your horse jumped or stopped dead in its tracks. This might well be the same horse you rode on this trail yesterday without a hint of skittishness.

Why the sudden change in behavior? Because of the way your horse sees. Once you learn how the equine eye functions, you'll better understand your horse's actions. And you'll have insight managing your trail horse when it becomes skittish for no apparent reason. Let's consider the windy day-flying paper scenario. You didn't give a second thought to that piece of paper skittering across the trail. Your horse, though, saw a strange, out-of-focus object moving across its path. The inability of your horse's eye to send a clear message to the brain brought on apprehension and fear.

The fear of unidentified moving objects comes from being a prey animal. For centuries equines served as tasty meals for big cats and other predators. Some would lie in wait or up in a tree for a horse to pass by and then spring to the attack. In other, rarer cases, the predator simply outran the horse and dragged it down for the kill.

Whatever the case, the horse knew sudden movement meant danger.

However, Mother Nature did not leave the horse defenseless against predators. She blessed the horse with an unusually broad field of vision, keen hearing, the ability to lash out with front and back hooves and, most importantly, the ability to outdistance all but the fastest of pursuers.

Unique Eye

The equine eye is unique in that it has both monocular and binocular vision. With monocular vision, the horse can see objects with one eye. As a result the brain often is receiving two images at the same time, sort of like a human attempting to watch side-by-side television sets simultaneously and absorb the action on both. With binocular vision, however, the horse can see the same object with both eyes at the same time and only one message is conveyed to the brain. A horse's eyes are wide apart and on each side of its head to accommodate monocular vision better--very important to a prey animal.

Humans have only binocular vision. We fall into the predator rather than the prey category. As such, our eyes are in the front of the head, and only a short distance apart, just like other predators--lions, tigers, dogs, and cats. A predator's eyes are designed to focus on the prey it's attempting to chase down.

Therefore, we can quickly focus on objects while the horse, with its broader range of vision, has trouble bringing things into focus.


The human eye is a lot like an auto-focus camera, making use of disc-like lenses that are attached to powerful ciliary muscles. These muscles quickly flex or relax to adjust the lens shape as needed. If you want to look at something far away, you simply stare at the object and, in a flash, the lens focuses on it and you see it with clarity. If the object is close, the same thing happens, only the focal point is much nearer.

If you want to look at something to either the right or left, you simply turn your head and the ciliary muscles do the rest, sending a sharp image to the retina, a mass of nerve receptors on your eyeball's back wall. The retina then transmits the image to the brain--all in the proverbial blink of an eye.

Compared with a human's ciliary muscles, the horse's are underdeveloped and do a poor job of bringing objects into focus. There's also a big difference in the retina. In humans the retina is a smooth, concave surface. The equine retina is more concave in some places than in others, and some sections are nearer the cornea than others. (The cornea is that transparent structure at the front of the eye that covers the iris and pupil and admits light to the interior.)

Difficulty Focusing

Because the horse can't focus on objects as we do, it compensates by lifting and lowering or weaving its head from side to side. It may also try to get either farther away from or closer to an object to bring it into focus.

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.

This means, of course, that when you're riding down the trail and see a strange object ahead, you'll recognize what you're seeing long before your horse does.

Monocular Vision

Because its eyes are large--the largest of any land mammal--and on each side of its skull, the horse has a very broad field of vision. A horse actually has a vision field of more than 350 degrees, compared to the less than 180 degrees in the human.

Only about 65 degrees of the horse's field of vision are binocular--seen with both eyes--while the remaining 285 degrees are monocular--seen with one eye.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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