The equine eye
is the largest of all land mammalswhales, seals and the ostrich are
the only other animals that have larger eyes. How well do horses see? Can
they see colours, depth, motion and how well do they see in the dark? These
are tricky questions to answer seeing as we can't get this information straight
from the horse's mouth!
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 'monocular' ('mono' meaning 'one') which enables them to view their surroundings on both sides, with either eye. Their 'binocular' vision (with both eyes) is directed down their nose and not straight ahead and the horse actually has a blind spot in front of its 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 something that warrants investigation, the horse will raise his head to bring the binocular vision into force. If the object was spotted in the horse's side vision, he will turn and raise his head, or even whole body to look.
A horse's large eye is an advantage as it enables him to detect the slightest motion which is why windy days make most horses uneasy...too many moving things! The horse moves its head in order to bring the object into its binocular field, which also gives better depth perception. This offers a better view, as while the horse has both monocular and binocular vision, he probably can't utilize both at the same time, hence he raises his head to switch to both (binocular) eyes. Say you are riding along, your horse nice and relaxed...he is probably using his monocular vision. Suddenly he spots an unusual object ahead and instantly raises his head and pricks his ears. This allows him to look down his nose and employ his binocular vision. If the object is on the ground, the horse will lower his head, again in order to look down his nose and use both eyes for a clear view.
Another reason horses move their heads up and down is that their visual field is narrow so objects seen the clearest are the ones that fall within this narrow areathe horse tilts his head in order to get as much of an object as possible to cast an image onto the eye. So if a horse needs to look down his nose to see where he is going, what happens when he is 'on the bit' as in showing or dressage? A horse who is flexed at the poll will have his head vertical (at right angles) to the ground and cannot see straight in front of him, only down his nose towards the ground. Recent research found this blind spot in front of the horse is about the width of his body and a horse 'on the bit' must rely on the rider for direction as he is almost working blind! If you watch showjumpers negotiating a course, you will notice they lift their heads when approaching the fence to get a better idea of height and depth with their binocular vision. Try walking quickly towards a wall with one eye closed, then open both eyes and you will see that the view with both eyes gives you better depth perception.
Alison Harmon from the University of Western Australia, who has been involved in research on equine vision, once witnessed a nasty accident involving two dressage horses practising a freestyle routine. They were cantering around the arena, on the bit, and collided head on...their vision was directed down their noses towards the ground so they simply didn't see each other!
Horses were believed to have poor vision and be short sighted but they actually have very good binocular vision with a tendency towards long vision. It's logical to think that given a horse's wide vision range, they are able to see a rider on their backs however a rider is in the horse's blind spot. If you can't see either of the horse's eyes when mounted, then he can't see you!
How much detail can horses see? Using a method of placing rewards behind a trapdoor, a reseach team 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.
Horses are mostly day animals although they will continue to graze at night which suggests they do have some night vision. Horse's eyes are sensitive to weak light, so they can see fairly well at dusk, but they don't have the ability to adjust their eyes to darkness quickly, which is why they will often refuse to enter a dark building or float from bright sunshine. One cross country jump at the Sydney Olympics situated in dim shade caused some problems and a few falls. Colour Vision It was once commonly thought horses were colour blind but in fact they do have the ability to see some colour. The eyes contain light-sensitive cells and there are two types of cells called rods and cones. Humans have three different types of cones which means we can see all colours. Cats also have three types of cones but they are weak compared to ours so they can only see in pastel colours. To a cat, a green lawn appears as a whitish one. Dogs only have two types of cones and see colour similar to a human who is red-green colour blind. Horses have only two types of cones as well, so their ability to see colour is also limited.
To learn how horses saw various colours, the research team had to find a way to test how horses could tell the difference between actual colours rather than them just picking a colour that appeared bright–for example, red looks bright while blue looks dark. To do this, they asked the horse to select a colour on a grey background that they could vary from light to dark. They discovered horses could always pick out red and blue regardless of what the background was like. However, horses would only reliably select yellow and green when these were brighter than the background. If the brightness of the colours was equal to the grey background, some horses couldn't pick these colours as easily. A few could tell the difference between green and yellow while others could not, so the results for these were mixed.
Given this information, maybe where some horses are concerned that old saying should be amended to, "the grass is always a wishy-washy greenish-grey on the other side of the fence"! Stallions in the wild have often been observed to prefer mares of a particular colour and it's thought this is probably related to their mother's coat colour. Knowing how a horse views his world makes a big difference when it comes to handling. And given how well horses usually perform when we are restricting their vision by riding 'on the bit' should instill a responsibility to 'steer' thoughtfully.
It also displays how much trust horses show by allowing us to sometimes be their eyes!
© 2006 Horsewyse Magazine