Friday, July 16, 2010

Vision correction in horses?

I know there are more important equine ailments that need research, but could some horses need vision correction? I've often wonder if Kylie is near-sighted (myopic) or even hyperopic (far-sighted). I searched the internet and did not come up with much. I guess it would be hard to give and eye exam to horses as they could not respond to the numerous better or worse scenarios as the optometrist shows them slides through corrective glass. I did find a very old article in TIME from 1927 about a racehorse owner that had some of his horses fitted with glasses. There was a horse in my barn that wore contact lenses, but I believe they were tinted because he was sensitive to light.

Some interesting information on the way a horse sees...

Horses' vision is flat visions unlike the three-dimensional vision humans have. It is used more for identifying movement at far distances. The horse’s eyes “have an incredible ability to detect motion. The horse can see a small bird flutter in a tree across a canyon. Movement may mean danger and danger, of course, means move the other way at top speed.”

The horse focuses his eyes in a unique way. He uses the irregular surface of the retina by lifting, lowering or weaving his head, gathering in all the light rays to produce the best image. Quite often, when his head movements seem senseless to you, he's simply getting a better look at something that interests him. It's probably about the same thing a person does when he turns his head to see what's beside him, rather than straining his eyeballs to look out of the corner of his eye. Racehorse trainers resolve the problem by the use of blinkers, confining the equine's eyes to the straight and narrow and "keeping his eyes on the ball."
There is a trade-off to a wide range of monocular vision: The placement of the horse's eyes decreases the possible range of binocular vision (vision using both eyes at the same time) to around 65 degrees 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 a human. The horse uses its binocular vision by looking straight at an object, raising its head when a horse 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 neck somewhat arched.

A horse will raise or lower its head to increase its range of binocular vision. A horse's visual field is lowered when it is asked to go "on the bit" with the head 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.

There is another unique aspect to the equine eye. The eyeballs operate in a manner similar to bifocal glasses. This makes sense since the horse is a grazing creature. When its head is lowered and it is looking through the upper portion of the eyes, it can continually survey the horizon for the approach of predators. If there is something up close that demands its attention, the horse will raise its head and examine the object through the lower portion of the eye.

The horses' extravagantly big eyeball (largest of any land mammal's) magnifies everything fifty percent larger than we perceive it. That enables him to see distant objects in clearer detail than we can (an advantage for a prey animal needing to spot predators far away).

The amount of light that is reflected by objects also dramatically affects the way that the horse sees them. A horse may have gone by the same picnic table hundreds of times without incident. If that same picnic table were to be freshly painted you could expect the horse to react to it the same way he might if it were a crouching tiger. To the horse, the freshly painted picnic table looks like a completely different object than the dull brown picnic table it used to be. And, depending on his personality, he may need to identify and inspect the "new" picnic table thoroughly, before dismissing it as harmless. Or in Kylie's case the same table sometimes is a harmless picnic table and others it is a crouching tiger lokking for its next meal.

Obviously, freshly painted objects reflect much more light than weathered items. (The horse might also detect a strong smell that we are not aware of). At shows, poles and standards may be newly painted and very shiny. If your horse is not accustomed to the amount of reflection he sees from newly painted shiny obstacles, he is likely to shy at them or even refuse to go near them. A glossy blue jump standard appears to the horse as something completely unlike the weathered, unpainted or brown ones you might be using at home.

Horses certainly would not have survived long as a species if it were natural for them to happily and curiously trot up to strange, new shapes. For all he knows, those never-before-seen clumps of tied corn stalks, enchantingly placed at the sides of a jump and further enhanced by pyramids of small pumpkins could be nests of horse-eating wolverines waiting in ambush. Why on earth should he be pleased to encounter them?

Great article about how a horse sees:

The American College of Veterinary Ophthalmologists (ACVO) official site. The ACVO is the organization in the USA which certifies veterinary ophthalmologists. The directory provides the location of ACVO certified veterinary ophthalmologists worldwide.

The International Equine Ophthalmology Consortium is an organization with a worldwide membership of veterinary ophthalmologists with a special interest in eye diseases in horses.


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