Your Horse's Amazing Eyes
Wondering exactly what (and how) your horse sees with those big, expressive eyes? Very differently from the way you do--and the way he sees often affects the way he acts.
The structure of the horse’s eye is well suited to this behaviour, and it is probably a good idea to look at this a bit more closely. Specifically, we are interested in the retina - that part of the eye onto which light is focused and which encodes the light signals for use by the brain. Packed like bundled sticks onto the inner surface of the retina are cells that are receptive to light. These come in two types, rods and cones, each of which are receptive in different ways. Rods are connected to nerves in groups, giving an "overview" type of sight, but they are not colour sensitive. Cone cells are connected individually to nerves so providing a greater amount of clarity and detail, and contain the required pigments to be colour sensitive. Fully nocturnal animals that have a very high proportion of rod cells are unlikely to be able to see colour and need to rely much more on their sense of smell.
(1) - Iris; (2) - Cornea; (3) - Anterior Chamber; (4)- Lens; (5) - Ciliary Body; (6) - Vitreous Humour; (7) - Retina; (8) - Choroid; (9) - Sclera; (10) - Optic Nerve; (11) - Optic Disc.
A general rule can be applied when comparing the vision of different animals, and that is that the higher a percentage of rod cells there are, the better will be the animal’s night-sight. As you might expect, the horse’s eye has a greater percentage of rod cells than does our own. However, there are sufficient cone cells for us to believe the horse to have colour vision, despite those people who suggest that this is not so. Bernhard Grzimek, in his article 'On the Psychology of the Horse' gives details of his research into colour recognition in horses, and offers good proof of the horse's ability to recognise colours. Ability to see colour is not the same as its importance to the animal concerned.
Horses extravagantly big eyeball magnifies everything fifty percent larger than we perceive it. That enables it to see distant objects in clearer detail than we can (an advantage for a prey animal needing to spot predators far away).
The size of the eye is another strong indication that the horse is at least partly nocturnal.
The horse has an eye that is larger than that of much larger mammals such as the elephant and the whale, and therefore sight is obviously very important to the horse. In the past some people believed that horses see everything enlarged, probably because of the size of the eye, and that it was one of the reasons horses shy. We now know that this is not so. A larger eye is better able to receive information in poor light but does not produce a bigger image. Those familiar with computer imaging using a scanner might think of it as providing better resolution
Horse can see almost 360 degrees around himself because his eyes are on the sides of his head. That's why he notices objects or movements behind him that you (with eyes on the front of your head) can't even see without turning around. But his "rear view" vision is less distinct than his vision from about his shoulder forward - so he naturally wants to skitter away from unfamiliar things behind him or turn to see them better.
Such feature many animals being potential victims of predators possess very much: it allows to notice occurrence of the enemy at once. However eye-sockets of a horse nevertheless are a little revolved forward, that allows to use and binocular sight - its corner is equal approximately to 30-70 degrees. The "Blind" zone absolutely small: the horse does not see only that is at it at once behind a nape, it is necessary a forehead and under a chin. However, to see, that there it is created, enough the slightest turn of a head.
Horse has a second blind spot, too: about 6 feet directly behind his tail. Hearing something coming from that vulnerable angle, he may swing his body to one side so he can see what's approaching - or just kick in self-defense. For that reason, if his quarters are toward you as you approach his stall, keep the door closed and quietly but firmly push them over so he can see you before you go in.
Black - "Blind" zone;
It has a second blind spot, too: about 6 feet directly behind his tail. Hearing something coming from that vulnerable angle, horse may swing his body to one side so it can see what's approaching - or just kick in self-defense. For that reason, if his quarters are toward you as you approach his stall, keep the door closed and quietly but firmly push them over so he can see you before you go in.
He sees most things with one eye - monocular vision - instead of with both eyes simultaneously (binocular vision - the way you see the world - which he uses for just a small area in front of his head). That's why he may spook at something that he's already walked past and reacted to once: He's seeing it for the first time - with hiso ther eye. (How to know whether he's looking with one eye or two? As he tries to focus with both eyes on something, he pricks both ears straight ahead.)
Horses has a completely different method of depth perception. Because it can't always use two eyes (binocular vision is what enables you to to perceive depth), it first gauges the relative distance of objects by comparing how big they appear with how big it knows they are. He knows humans are a certain approximate size, for instance, so a human who looks small to his monocular vision is a greater distance away. (That's why, if he sees something w ith one eye that doesn't fit his idea of what's normal, he turns his head for a more accurate binocular fix.)
When we ask a horse to go forward, with a straight body and in a straight line - the horse must trust the rider to judge whether a glimpsed object or noise coming from behind it is safe. But if the required element of trust is not present, then it is quite reasonable for the horse to try to change its line and body angle. If, when it instinctively attempts to do this, it meets resistance and discomfort from a bit, there is a clash of interests. In this instance, the evolutionary design needs to express itself so that the horse can get a good look at the problem and, if it is prevented, then some degree of anxiety is extremely likely.
Having realized how sees a horse much easier it to understand.