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The equine eye is one of the largest of any land mammal.
Its visual abilities are directly related to the
animal's behavior; for example, it is active during both day and night, and it is a prey animal. Both the strengths and weaknesses of the horse's visual abilities should be taken into consideration when training the animal, as an understanding of the horse's eye can help to discover why the animal behaves the way it does in various situations.
The anatomy of the equine eye
The equine eye includes the eyeball and the surrounding muscles and structures, termed the
adnexa.
The eyeball
The eyeball of the horse is not perfectly spherical, but rather is flattened anterior to posterior. However, research has found the horse does not have a ramped retina, as was once thought.
The wall of the eye is made up of three layers: the internal or nervous tunic, the vascular tunic, and the fibrous tunic.
* The nervous tunic (or
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
) is made up of cells which are extensions of the brain, coming off the optic nerve. These receptors are light-sensitive, and include
cones
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines conn ...
, which are less light-sensitive, but allow the eye to see color and provide visual acuity, and
rod cells
Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in per ...
, which are more light-sensitive, providing night vision, but only seeing light and dark differences. Since only two-thirds 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
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...
to the
optic disk
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
. 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, so is a blind spot within the eye.
[Riegal, Ronald J. DMV, and Susan E. Hakola DMV. ''Illustrated Atlas of Clinical Equine Anatomy and Common Disorders of the Horse Vol. II''. Equistar Publication, Limited. Marysville, OH. Copyright 2000.]
* The vascular tunic (or
uvea
The uvea (; Lat. ''uva'', "grape"), also called the ''uveal layer'', ''uveal coat'', ''uveal tract'', ''vascular tunic'' or ''vascular layer'' is the pigmented middle of the three concentric layers that make up an eye.
History and etymolog ...
) is made up of the
choroid
The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye, and contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear ...
, the
ciliary body
The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...
, and the
iris
Iris most often refers to:
*Iris (anatomy), part of the eye
*Iris (mythology), a Greek goddess
* ''Iris'' (plant), a genus of flowering plants
* Iris (color), an ambiguous color term
Iris or IRIS may also refer to:
Arts and media
Fictional ent ...
. The choroid has a great deal of pigment, and is almost entirely made of blood vessels. It forms the
tapetum lucidum
The ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...
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 cornea and the
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
, and not only gives the eye its color, (''see "eye color," below'') but also allows varying amounts of light to pass through its center hole, the
pupil
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...
.
* The fibrous tunic consists of the
sclera
The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective, outer layer of the human eye containing mainly collagen and some crucial elastic fiber. In humans, and som ...
and
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...
and protects the eye. The sclera (white of the eye) is made up of
elastin
Elastin is a protein that in humans is encoded by the ''ELN'' gene. Elastin is a key component of the extracellular matrix in gnathostomes (jawed vertebrates). It is highly elastic and present in connective tissue allowing many tissues in the bod ...
and
collagen
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
. 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
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
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.
Eye 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
A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated l ...
. 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
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
cream dilutes, sometimes called
double-dilutes, always have light blue eyes to match their pale, cream-colored coats.
Heterozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
or
single-dilute creams, such as
palomino
Palomino is a genetic color in horses, consisting of a gold coat and white mane and tail; the degree of whiteness can vary from bright white to yellow. Genetically, the palomino color is created by a single allele of a dilution gene called t ...
s and
buckskins
Buckskins are clothing, usually consisting of a jacket and leggings, made from buckskin, a soft sueded leather from the hide of deer. Buckskins are often trimmed with a fringe – originally a functional detail, to allow the garment to s ...
, often have light brown eyes.
The eyes of horses with the
Champagne gene
The champagne gene is a simple dominant allele responsible for a number of rare horse coat colors. The most distinctive traits of horses with the champagne gene are the hazel eyes and pinkish, freckled skin, which are bright blue and bright pink ...
are typically greenish shades: aqua at birth, darkening to hazel with maturity.
Horses are capable of having dichromatic (differently-colored) eyes.
As in humans, much of the
genetics
Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...
and
etiology
Etiology (pronounced ; alternatively: aetiology or ætiology) is the study of causation or origination. The word is derived from the Greek (''aitiología'') "giving a reason for" (, ''aitía'', "cause"); and ('' -logía''). More completely, e ...
behind
eye color
Eye color is a polygenic phenotypic character determined by two distinct factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.
In humans, the pi ...
are not yet fully understood.
The adnexa
The
eyelid
An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyel ...
s 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
The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. All ...
) to protect the cornea. It lies on the inside corner of the eye, and closes diagonally over it.
The
lacrimal apparatus
The lacrimal apparatus is the physiological system containing the Orbit (anatomy), orbital structures for tears, tear production and drainage.Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Com ...
produces tears, providing nutrition and moisture to the eye, as well as helping to remove any debris that may have entered. The apparatus includes the lacrimal gland and the accessory lacrimal gland, which produce the tears. Blinking spreads the fluid over the eye, before it drains via the
nasolacrimal duct
The nasolacrimal duct (also called the tear duct) carries tears from the lacrimal sac of the eye into the nasal cavity. The duct begins in the eye socket between the maxillary and lacrimal bones, from where it passes downwards and backwards. The ...
, which carries the lacrimal fluid into the nostril of the horse.
The ocular muscles allow the eye to move within the skull.
Visual capacity of the horse
Visual field
Horse eyes are among the largest of any land mammal, and are positioned on the sides of the head (that is, they are positioned
laterally).
This means horses have a range of vision of about 350°, with approximately 65° of this being
binocular vision
In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an ...
and the remaining 285°
monocular vision In human species
Monocular vision vision is known as seeing and using only one eye in the human species. Depth perception in monocular vision is reduced compared to binocular vision, but still is active primarily due to accommodation of the eye a ...
.
[Sellnow, ''Happy Trails'', p. 46]
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
Rollkur or hyperflexion of the horse's neck is defined as "flexion of the horse's neck achieved through aggressive force" and is banned in International and Olympic sanctioned equestrian sports by the governing body, the International Federation f ...
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
Jumper or Jumpers may refer to:
Clothing
* Jumper (sweater), a long-sleeve article of clothing; also called a top, pullover, or sweater
**A waist-length top garment of dense wool, part of the Royal Navy uniform and the uniform of the United Stat ...
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.
Visual acuity and sensitivity to motion
The horse has a "visual streak", or an area within the retina, linear in shape, with a high concentration of ganglion cells (up to 6100 cells/mm
2 in the visual streak compared to the 150 and 200 cells/mm
2 in the peripheral area). Horses have better acuity when the objects they are looking at fall in this region. They therefore will tilt or raise their heads, to help place the objects 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.
Color vision
Horses are not
color blind
Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
, 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
Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
in humans, in which certain colors, especially red and related colors, appear more green.
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 (blue), and a middle-to-long wavelength sensitive cone (M/L) which sees optimally at 539 nm, more of a yellowish color. This structure may have arisen because horses are most active at dawn and dusk, a time when the
rods of the eye are especially useful.
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.
Sensitivity to light
Horses have more rods than humans, a high proportion of rods to cones (about 20:1), as well as a
tapetum lucidum
The ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...
, giving them superior night vision. This also gives them better vision on slightly cloudy days, relative to bright, sunny days. The large eye of the horse improves achromatic tasks, particularly in dim conditions, which presumably assists in the detection of predators.
Laboratory studies show horses are able to distinguish different shapes in low light, including levels mimicking dark, moonless nights in wooded areas. When light decreases to nearly dark, horses can not discriminate between different shapes, but remain able to negotiate around the enclosure and testing equipment in conditions where humans in the same enclosure "stumbled into walls, apparatus, pylons, and even the horse itself."
However, horses are less able to adjust to sudden changes of light than are humans, such as when moving from a bright day into a dark barn. This is a consideration during training, as certain tasks, such as loading into a trailer, may frighten a horse simply because it cannot see adequately. It is also important in riding, as quickly moving from light to dark or ''vice versa'' will temporarily make it difficult for the animal to judge what is in front of it.
Near- and far-sightedness
Many domestic horses (about a third) tend to have
myopia
Near-sightedness, also known as myopia and short-sightedness, is an eye disease where light focuses in front of, instead of on, the retina. As a result, distant objects appear blurry while close objects appear normal. Other symptoms may include ...
(near-sightedness), with few being far-sighted. Wild horses, however, are usually far-sighted.
Accommodation
Horses have relatively poor "accommodation" (change focus, done by changing the shape of the lens, to sharply see objects near and far), 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. It has been thought that, instead, the horse often tilts its head slightly to focus on things without the benefit of a high degree of accommodation,
however more recent evidence shows that the head movements are linked to the horse's use of its binocular field rather than to focus requirements.
Disorders of the eye
Any injury to the eye is potentially serious and requires immediate veterinary attention. Clinical signs of injury or disease include swelling, redness, and abnormal discharge. Untreated, even relatively minor eye injuries may develop complications that could lead to blindness. Common injuries and diseases of the eye include:
* Corneal abrasion
*
Corneal ulcer
Corneal ulcer is an inflammatory or, more seriously, infective condition of the cornea involving disruption of its epithelial layer with involvement of the corneal Stroma of cornea, stroma. It is a common condition in humans particularly in the ...
*
Keratitis
Keratitis is a condition in which the eye's cornea, the clear dome on the front surface of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves any of the following symptoms: pain, impaired e ...
*
Conjunctivitis
Conjunctivitis, also known as pink eye, is inflammation of the conjunctiva, outermost layer of the white part of the eye and the inner surface of the eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness, or itchiness may ...
*
Uveitis
Uveitis () is inflammation of the uvea, the pigmented layer of the eye between the inner retina and the outer fibrous layer composed of the sclera and cornea. The uvea consists of the middle layer of pigmented vascular structures of the eye and ...
includes recurrent uveitis and periodic ophthalmia ("
moon blindness"). Spontaneous
Equine Recurrent Uveitis
Equine recurrent uveitis (ERU) – also known as moon blindness, recurrent iridocyclitis, or periodic ophthalmia – is an acute, nongranulomatous inflammation of the uveal tract of the eye, occurring commonly in horses of all breeds, w ...
(ERU) occurs in 10-15% of the equine population, with the
Appaloosa
The Appaloosa is an American horse breed best known for its colorful spotted coat pattern. There is a wide range of body types within the breed, stemming from the influence of multiple breeds of horses throughout its history. Each horse's colo ...
breed having an eightfold higher risk than the general horse population.
Current Research, from Blindappaloosas.org
/ref>
* Habronema
* Keratoconjunctivitis sicca
Dry eye syndrome (DES), also known as keratoconjunctivitis sicca (KCS), is the condition of having dry eyes. Other associated symptoms include irritation, redness, discharge, and easily fatigued eyes. Blurred vision may also occur. Symptoms range ...
References
{{DEFAULTSORT:Equine Vision
Eye
Vision by taxon