Wednesday, January 16, 2013

How Horse's See The World

How come we have two eyes but only see one of everything? And really, have you ever heard the phrase "two are better than one"?

Having two eyes is certainly better than having just one because two eyes provide us with stereo vision and depth perception - two things that just eye can not give us.

With 2 ½ inches separating our two eyes, each eye views an object from a slightly different angle.

For instance, if you hold up a finger and look at it with just your right eye, the image is different from the image of when you look at it with just your left eye.

The right eye sees more of the right side of your finger while the left eye sees more of the left side of your finger.

If you placed the two different images on top of one another, they would not match and our vision would be out of focus.

But, the fact is that our brain sorts out these varying visual messages from our two eyes - then it combines the images and recreates one three-dimensional image.

This is referred to as "binocular vision." Just like your look through two lenses in binoculars, humans view the world through two lenses.

The eyes of many other animals are placed differently than ours. Many birds have an eye on each side of their head. Each eye sees a completely separate area stretching out on the left or the right.

Viewing the world through two eyes provides us with depth perception. When you look at your finger through just one eye, it looks a lot flatter.

So now, have you ever wondered how your horse's vision differs from yours? How he or she sees the world?

Well, while we see in binocular vision, a horse sees in both binocular and monocular vision with a much greater percentage being monocular.

Fact is that our horses see better than us in some ways, and worse than us in others. Because our horse's eyes are positioned farther apart than ours are, he has a much wider field of vision.

Unlike our parents, a horse doesn't have eyes in the back of his head - but it sometimes seems that way. In fact, a horse can see nearly 360 degrees. Of course, with the exception of a blind spot directly behind him and one directly in front.

The equine eye is the largest of any land mammal. Its visual abilities are directly related to the animal's behavior and the fact that the horse is a flight animal.

Like most prey animals, the horse's eyes are set on the sides of its head, allowing it close to a 350° range of monocular vision.

And yes, it is true that horses have the largest eyes of any land mammal - and are lateral-eyed, meaning their eyes are positioned on the sides of their heads. This means horses have a range of vision of more than 350°, with approximately 65° of this being binocular vision and the remaining 285° monocular vision.

What this does is provide a horse with the best chance to spot predators. That's one of nature's defensive gifts to horse, of course the others being speed of flight and an ability to see at night.

While most of us know that horse have a wonderful ability to outrun those they see as predators, many might not realize that horses have superior night vision. Because of this, they also have better vision on slightly cloudy days, relative to bright, sunny days.

However, horses are less able to adjust to sudden changes of light than are humans, such as when moving from a bright day into a dark barn.

Both the strengths and weaknesses of the horse's visual abilities should be taken into consideration when training horses.

And yes, a horse's superior ability to see at night is a consideration that should be taken into account during training, as certain tasks such as loading into a trailer, may frighten a horse simply because it cannot see adequately.

It is also important in riding, as quickly moving from light to dark or vice versa will temporarily make it difficult for the animal to judge what is in front of it.

And as for color? Well, horses are not color blind. They have dichromatic vision. This means they see two of the basic three color wavelengths of visible light, compared to the three-color (trichromic vision) of most humans.

In other words, horses naturally see the blue and green colors of the spectrum and the color variations based upon them, but cannot distinguish red.

Research indicates their color vision is somewhat like red-green color blindness in humans, in which certain colors, especially red and related colors, appear more green.

The horse's limited ability to see color is sometimes taken into consideration when designing obstacles for the horse to jump, since the animal will have a harder time distinguishing between the obstacle and the ground if the two are only a few shades off.

Because of this, most people paint their jump rails a different color from the footing or the surrounding landscape so the horse may better judge the obstacle on the approach.

As for being near or far sighted? Almost all domestic horses tend to be near-sighted, with few being far-sighted. Interesting enough to note is that wild horses are usually all far-sighted.

The horse's wide range of monocular vision has two "blind spots," areas where the animal cannot see.   One is right in front of the face which comes to a point at about 3–4 ft in front of the horse, and the other is right behind its head which extends over the back and behind the tail when standing with the head facing straight forward.

And imagine this for a moment, as a horse jumps an obstacle - the object briefly disappears from sight right before the horse takes off. That is called amazing!

Most people don't realize that a horse's big nose actually gets in the way of vision.

Have you ever noticed how a horse stretches out his nose to meet another horse or to check out what you have in your hand?

Friends, he's not just acting defensively by keeping a distance from you. And he's certainly not "respecting" your space.

Fact is that he's arching his neck and pointing his nose so he can both smell the object and focus on it. Other than that blind spot directly in front of him, a horse's close-up vision is excellent.

The wide range of monocular vision has a trade-off:

The placement of the horse's eyes decreases the possible range of binocular vision to around 65° on a horizontal plane, occurring in a triangular shape primarily in front of the horse's face. Therefore, the horse has a smaller field of depth perception than we humans do.

A horse uses its binocular vision by looking straight at an object, raising its head when it looks at a distant predator or focuses on an obstacle to jump.

To use binocular vision on a closer object near the ground, such as a snake or threat to its feet, the horse drops its nose and looks downward with its neck somewhat arched.

A horse will raise or lower its head to increase its range of binocular vision.

This has everything to do with his distance vision. And yes, it is a different story from what he sees close up.

Horses depend on sound, smell, and his excellent memory for shapes and movement to help him make sense of things he sees in the distance.

A horse's visual field is lowered when their head is held perpendicular to the ground. This makes the horse's binocular vision focus less on distant objects and more on the immediate ground in front of the horse, suitable for arena distances, but less adaptive to a cross-country setting.

An example: Riders of jumpers take their horses' use of distance vision into consideration, allowing their horses to raise their heads a few strides before a jump, so the animals are able to assess the jumps and the proper take-off spots.

The horse is very sensitive to motion, as motion is usually the first alert that a predator is approaching.

Motion is usually first detected in their periphery, where they have poor visual acuity, and horses will usually act defensive and run if something suddenly moves into their peripheral field of vision.

What does that mean? Well, horses are mentalists!

It's very true. While some say horse are dumb, I've always believed the opposite in that they are much smarter than we are in most cases.

The reason I say this is because as an Instructor in the Marine Corps, I saw what it took to get Marines to preform. Repetition, memory, concentration, and an ability to react instinctively to training responses. And yes, it's tough to learn.

Fact is that horse do all of that and more. A horse can't literally see the halter you're hiding behind your back when you enter the pasture, though through experience and a keen power of observation, he can recognize your characteristic halter-hiding walk and movement.

Subsequently he responds in one of two ways: He either responds has he has been taught, or as he sees fit to avoid an unpleasantness. And friends, to discern between the two takes a great deal of intelligence.

Because the picture that each eye sees doesn't overlap as much as ours does, his depth perception - the ability to judge distances - just isn't as good as ours. So he may get frightened of something that's quite a distance from him.

Like us, a horse's visual abilities are directly related to their behavior. In their case, the fact that the horse is a flight animal.

Both the strengths and weaknesses of the horse's visual abilities should be taken into consideration when dealing with horses. An understanding of the horse's eye can help us figure out why horses behave the way they do in various situations.

When a horse sees something unfamiliar, he'll raise his head suddenly, scanning the horizon with nostrils flared and ears turned like satellite dishes trying to catch familiar sounds. And yes, we should recognize their actions to help us understand how they see the world around them.

Some folks might say that horse can't see very well, that their vision is not very good at all. But really, we should all accept the fact that seeing is not only done with our eyes. And for horses, well that's even more true.

The way I see it is that with the help of their other senses of sound, and smell, and instinct, their ability to "see" their world around them is truly excellent for their needs.

Story by Tom Correa

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