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Equine Vision – Overview
In the wild, the horse is a prey / flight animal and thus good eyesight is essential in order to spot potential predators. Understanding the unique aspects of a horse’s vision is important to keeping you and your horse safe. An example: When handling horses from bright light to dark (bright outdoors to dark barn or bright outdoors to a dark horse trailer) we need to be aware that their eyes adjust much slower than ours and allow them more time for adjustment.
The eyeball of the horse is not perfectly spherical, but rather is flattened anterior to posterior (Front to back). The equine eye includes the eyeball envelope and the assisting muscles and structures surrounding the eyeball, termed the adnexa.
The wall of the eye is made up of three layers each which acts like an envelope containing specific visual components:
1. The Fibrous 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, as it does not have access to blood vessels. 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.
2. The Internal or Nervous Tunic: The vascular tunic (or uvea) 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 iris lies between the cornes and the lens, and not only gives the eye its color, but also allows for varying amounts of light to pass through its center hole, the pupil.
3. The Nervous Tunic (or retina): The nervous tunic (or retina), is made up of cells which are extensions of the brain, coming off the optic nerve. These receptors are light-sensitive, and include cones, which are less light-sensitive, but allow the eye to see color and provide visual acuity, and rod cells, which are more light sensitive, providing night vision but only seeing light and dark differences. Since only 2/3 of the eye can receive light, the receptor cells do not need to cover the entire interior of the eye, and line only the area from pupil to the optic disk. The part of the retina covered by light-sensitive cells is therefore termed the pars-optica retinae, and the blind part of the eye is termed the pars-ceaca retinae. The optic disk of the eye, however, does not contain any of these light-sensitive cells, as it is where the optic nerve leaves to the brain. It is therefore a blind spot within the eye.
Ramped Retina Misnomer: For many years it was believed that the horse's eye had a ramped retina that is shaped in such a way so that some parts of the retina are further away from the lens than others. However, recent research indicates that the horse does not have a ramped retina.
1. Anterior Chamber: The anterior chamber is the fluid-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium.
2. Choroid: The choroid, also known as the choroidea or choroid coat, is the vascular layer containing connective tissue, of the eye lying between the retina and the sclera.
3. Ciliary Body: the ring of muscle fibers that holds the lens of the eye. It also produces aqueous humor (the clear, watery fluid that moves in the eyeball).
4. Cornea: This is the clear structure at the front central part of the eye, imparting the greatest focusing power of all the ocular media.
5. Corpora Nigra: The corpora nigra is the soft brown, irregular body at the edge of the iris that shades the pupil.
6. Iridocorneal Angle: The acute angle between the iris and the cornea at the periphery of the anterior chamber of the eye.
7. Iris: The iris is the colored part of the eye.
8. Nictitating Membrane: A transparent inner eyelid in birds, reptiles, and some mammals that closes to protect and moisten.
9. Posterior Chamber:
10. Pupil: The opening in the center of the iris through which light enters the eye.
11. Retina: A delicate, multi layered, light-sensitive membrane lining the inner eyeball and connected by the optic.
12. Sclera: The tough white fibrous outer envelope of tissue covering all of the eyeball except the cornea.
13. Tapetum: The layer of reflective tissue that is on the back of the eye.
14. Optic Muscles: A group of specialized muscles that move the eye.
15. Optic Nerve: the cranial nerve that serves the retina.
Equine Vision – Eye Color: Homozygous cream dilutes ("double-dilutes") have pale blue eyes, while the blue eyes associated with white markings (bottom) are a clearer, deeper color. Although usually dark brown, the iris may be a variety of colors, including blue, hazel, amber, and green. Blue eyes are not uncommon and are associated with white markings or patterns. The white spotting patterns most often linked to blue eyes are splashed white, frame overo, and sometimes sabino. In the case of horses with white markings, one or both eyes may be blue, or part-blue. Homozygous cream dilutes, sometimes called double-dilutes, always have light blue eyes to match their pale, cream-colored coats. Heterozygous or single-dilute creams, such as palominos and buckskins, often have light brown eyes. The eyes of horses with the Champagne gene are typically greenish shades: aqua at birth, darkening to hazel with maturity. As in humans, much of the genetics and etiology behind eye color are not yet fully understood.
Equine Vision – The Adnexa (Eyelids):
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 (A clear membrane which covers the eyes cornea or lens), which lies against the eyeball. The upper eyelid is larger and can move more than the lower lid.
Equine Vision – The Haw or Equine Third Eyelid:
Unlike humans, horses also have a third eyelid (Haw or nictitating membrane) to protect the cornea. It lies on the inside corner of the eye. Technically known as the nictitating (from Latin nictare: to blink) membrane, the third eyelid is a thick fold of tissue that lies flat on the inner corner of the horse's cornea.
By providing moisture by producing tears as well as removing debris from the eye, it in effect, acts like a car window washer / wiper. It consists of a transparent membrane which is drawn diagonally across the eye. The third eyelid lies underneath the two outer eyelids, and it serves a number of different functions in the eye. In many horses the third eyelid is hardly noticeable in its position in the corner of the horse’s eye closest to the head’s midline. This is because it usually has the same color as the eye’s iris. However, in some horses the third eyelid is white and thus suddenly becomes obvious. In fact, some people do not like buying horses with too much white associated with their eye as they believe them to be hot headed and temperamental.
Equine Vision – Visual Field: Horses have two forms of vision:
Equine Vision – Monocular Vision: Or vision from a single eye. As discussed horses are prey animals (animals preyed upon by predator animals) and like other prey animals of prey, their eyes are set on the sides of its head this is referred to monocular vision (vision from a single eye) allowing it close to a 350 degree range of monocular vision (175 from each eye). This provides it the best chance to spot predators.
Equine Vision – Monocular Vision Blind Spots: The horse's wide range of monocular vision has two "blind spots" or areas where the animal can not see.
1. In front of the face (making a cone that comes to a point at about 3-4 feet in front of the horse)
2. Behind his head, which extends over the back and behind the tail when standing with the head facing straight forward.
Equine Vision – Binocular Vision: Or vision from both eyes. There is a trade-off to the wide range of monocular vision which is the placement of the horse's eyes decreases the possible range of binocular vision (vision using both eyes at the same time) to around 220 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.
A horse can use binocular vision to focus on distant objects by raising its head.
A horse with the head held vertical will have binocular focus on objects near its feet.
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. Riders who ride with their horses "deep," "behind the vertical," or in a rollkur 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 the horse's use of distance vision into consideration, allowing their horse to raise the head a few strides before a jump, so that the animal is able to assess the jump and the proper take-off spot.
Equine Vision - Visual Acuity and Sensitivity to Motion:
The visual acuity of the horse, or how well it is able to see details, is around 20/33. This is slightly worse than the usual 20/20 in humans, but much better than the visual acuity of dogs (20/50), cats (20/75), and rats (20/300). However, it is difficult to test an animal's visual acuity, and therefore the results may vary between studies.
The horse also has a "visual streak," or an area within the retina, linear in shape, with a high concentration of ganglion cells (up to 6100 cells/mm² in the visual streak compared to the 150 and 200 cells/mm² in the peripheral area). Horses have better acuity when the object they are looking at falls in this region. They therefore will tilt or raise their head, to help place the object within the area of the visual streak.
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.
Equine Vision - Dichromatic Color Vision: (Click here for an illustration) Horses are not color blind, but have two-color, or dichromatic vision. This means that they see two of the basic three 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 that their color vision is somewhat like red-green color blindness in humans. This means that certain colors, especially red and related colors, appear greener. It is especially difficult for horses to distinguish between yellows and greens. Dichromatic vision is the result of the animal having two types of cones in their eyes: a short-wavelength sensitive cone (S) that is optimal at 428 nm (pastel bluish-gray), and a middle-to-long wavelength sensitive cone (M/L) which sees optimally at 539 nm, more of a yellowish color. This structure may be due to the fact that horses are most active at dawn and dusk, a time when the rods of the eye are especially useful.
Equine Vision - Dichromatic Color Vision and Competition: 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. 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 have a rail down when the jump is painted with two or more contrasting colors, rather than one single color.
Equine Vision - Sensitivity To Light: Horses have more rods than humans, a high proportion of rods to cones (about 20:1), as well as Tapetum lucidum Eye Shine, which is a layer of tissue in the eye of many vertebrate animals, that lies immediately behind or sometimes within the retina. It reflects visible light back through the retina, increasing the light available to the photoreceptors. This improves vision in low-light conditions.This also gives them better vision on slightly cloudy days, relative to bright, sunny days. However, they are less able to adjust to sudden changes of light, such as when moving from a bright day into a dark barn. This should be taken into consideration during training, as certain tasks, such as loading into a trailer, may frighten a horse simply because he cannot see. It is also important in riding, as quickly moving from light to dark or vice-versa will temporarily blind the horse, and makes it difficult for him to judge what is in front of him. When riding cross-country, extra care must be given to obstacles set in the shade, as there have been cases where the horse has only had a stride in a dark area before the fence, and has misjudged the take-off and height of the obstacle, resulting in a fall.
Equine Vision - Near- and Far-sightedness: Many domestic horses (about 1/3) tend to have myopia (near sightedness), with most being far sighted. Wild horses, however, are usually far-sighted.
Equine Vision - Accommodation: Horses have relatively poor "accommodation" (the ability to turn the eye within the socket to focus on an object), as they have weak ciliary muscles. However, this does not usually place them at a disadvantage, as accommodation is often used when focusing with high acuity on things up close, and horses rarely need to do so. Instead, the horse often tilts its head slightly to focus on things without the benefit of a high degree of accommodation.