Sight (Vision)
The equine eye is one of the largest of any land mammal.
The horse’s eyeball is not perfectly spherical. It is flattened, anterior to posterior. Research has found that the horse does not have a ramped retina. (Ramped Retina -different regions of the retina were at different distances from the lens.)
The wall of the eye is made up of three layers: fibrous tunic, the vascular tunic, and the internal or nervous tunic.
The fibrous tunic consists of the sclera and cornea and protects the eye. The sclera (white of the eye) is made up of elastin and collagen. The cornea (clear covering on the front of the eye) is made up of connective tissue and bathed in lacrimal fluid and aqueous humor, which provides it nutrition, because it does not have access to blood vessels.
The vascular tunic is made up of the choroid, the ciliary body, and the iris. The choroid has a great deal of pigment, and is almost entirely made of blood vessels. It forms the tapetum lucidum when it crosses over the fundus of the eye, causing the yellowish-green eye shine when light is directed into the animal's eyes at night. The tapetum lucidum reflects light back onto the retina, allowing for greater absorption in dark conditions.
The nervous tunic (or retina) is made up of cells which are extensions of the brain, coming off the optic nerve.
The lens of the eye lies posterior to the iris, and is held suspended by the ciliary suspensory ligament and the ciliary muscle, which allows for "accommodation" of the eye: it allows the lens to change shape to focus on different objects. The lens is made up of onion-like layers of tissue.
If you're looking at your computer screen, your head is straight ahead, looking at the screen. But you notice you can still see off to your sides, you just can't focus in. How we use our peripheral vision is if something moves quickly to the right, we'll turn our head and focus in on it with our binocular vision. Now imagine that range almost all the way behind you and you could still see things behind you, that's how a horse sees. That would be awesome if you have kids, right?
The eyelids are made up of three layers of tissue: a thin layer of skin, which is covered in hair, a layer of muscles which allow the lid to open and close, and the palpebral conjunctiva, which lies against the eyeball. The opening between the two lids forms the palpebral tissue. The upper eyelid is larger and can move more than the lower lid. Unlike humans, horses also have a third eyelid (nictitating membrane) to protect the cornea. It lies on the inside corner of the eye, and closes diagonally over it.
The lacrimal apparatus produces tears, providing nutrition and moisture to the eye, as well as helping to remove any debris that may have entered. Blinking spreads the fluid over the eye, before it drains via the nasolacrimal duct, which carries the lacrimal fluid into the nostril of the horse.
The ocular muscles allow the eye to move within the skull.
Visual field
The range of a horse's monocular vision, blind spots are in shaded areas.
A horse can use binocular vision to focus on distant objects by raising its head.
A horse with the head held vertically will have binocular focus on objects near its feet.
So since Horses eyes are among the largest of any land mammal, and are positioned on the sides of the head, that means horses have a range of vision of about 350°, with approximately 65° of this being binocular vision and the remaining 285° monocular vision.
This provides a horse with the best chance to spot predators. The horse's wide range of monocular vision has two "blind spots," or areas where the animal cannot see: in front of the face, making a cone that comes to a point at about 90–120 cm (3–4 ft) in front of the horse, and right behind its head, which extends over the back and behind the tail when standing with the head facing straight forward. Therefore, as a horse jumps an obstacle, it briefly disappears from sight right before the horse takes off.
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 a human. The 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. 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. Riders who ride with their horses "deep", "behind the vertical", or in a rollkur (hyperflexion of the horse's neck) frame decrease the range of the horse's distance vision even more, focusing only a few feet ahead of the front feet. 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. Such 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.
This is a representation of how a horse possibly sees a red or a green apple (bottom) compared to how red or green apples are usually seen by most humans (top)
Horses are not color blind, they have two-color, or dichromatic vision. This means they distinguish colors in two wavelength regions 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 that 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 different. Therefore, most people paint their jump rails a different color from the footing or the surrounding landscape so that the horse may better judge the obstacle on the approach. Studies have shown that horses are less likely to knock a rail down when the jump is painted with two or more contrasting colors, rather than one single color. It is especially difficult for horses to distinguish between yellows and greens.
About a third of horses tend to have myopia (near-sightedness), with few being far-sighted. Wild horses, however, are usually far-sighted.
