Equine vision

Last updated
The eye of a horse Pferdeauge.jpg
The eye of a horse

The equine eye is one of the largest of any land mammal. [1] 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.

Contents

Anatomy

The equine eye includes the eyeball and the surrounding muscles and structures, termed the adnexa.

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. [2]

The wall of the eye is made up of three layers: the internal or nervous tunic, the vascular tunic, and the fibrous tunic.

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. Creamvswhite-eyes.JPG
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. [4] 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. [5] Heterozygous or single-dilute creams, such as palominos and buckskins, often have light brown eyes. [6] The eyes of horses with the Champagne gene are typically greenish shades: aqua at birth, darkening to hazel with maturity. [7]

Horses are capable of having dichromatic (differently-colored) eyes.

As in humans, much of the genetics and etiology behind eye color are not yet fully understood.

Adnexa

The adnexa of the eye, including the third eyelid (seen in the left corner) Pferdeauge.jpg
The adnexa of the eye, including the third eyelid (seen in the left corner)

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. 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, which carries the lacrimal fluid into the nostril of the horse. [3]

The ocular muscles allow the eye to move within the skull.

Visual capacity

Visual field

The range of a horse's monocular vision, blind spots are in shaded areas Horse360.png
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. DistanceHorse.png
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. CollectedVision.png
A horse with the head held vertically will have binocular focus on objects near its feet.

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). [1] This means horses have a range of vision of about 350°, with approximately 65° of this being binocular vision and the remaining 285° monocular vision. [9]

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. [10] 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 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. [11]

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/mm2 in the visual streak compared to the 150 and 200 cells/mm2 in the peripheral area). [12] 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

A representation of how a horse possibly sees a red or a green apple (bottom) compared to how red and green apples are usually seen by most humans (top) Assorted Red and Green Apples (deuteranope view).jpg
A representation of how a horse possibly sees a red or a green apple (bottom) compared to how red and 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. [13]

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. [14] 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. [15] It is especially difficult for horses to distinguish between yellows and greens.

Sensitivity to light

Mare and foal with eyeshine from the tapetum lucidum Stutemitfohlen.jpg
Mare and foal with eyeshine from the tapetum lucidum

Horses have more rods than humans, a high proportion of rods to cones (about 20:1), [16] as well as a tapetum lucidum, giving them superior night vision. This also gives them better vision on slightly cloudy days, relative to bright, sunny days. [17] The large eye of the horse improves achromatic tasks, particularly in dim conditions, which presumably assists in the detection of predators. [18] 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." [19]

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. [ citation needed ]

Near- and far-sightedness

Many domestic horses (about a third) tend to have myopia (near-sightedness), with few being far-sighted. Wild horses, however, are usually far-sighted. [20]

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. [21] 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, [2] 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. [22]

Disorders

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:

Related Research Articles

<span class="mw-page-title-main">Visible spectrum</span> Portion of the electromagnetic spectrum that is visible to the human eye

The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light. The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well.

<span class="mw-page-title-main">Eye</span> Organ that detects light and converts it into electro-chemical impulses in neurons

Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons (neurones). In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.

<span class="mw-page-title-main">Macula</span> Oval-shaped pigmented area near the center of the retina

The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around 5.5 mm (0.22 in) and is subdivided into the umbo, foveola, foveal avascular zone, fovea, parafovea, and perifovea areas.

<span class="mw-page-title-main">Peripheral vision</span> Area of ones field of vision outside of the point of fixation

Peripheral vision, or indirect vision, is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in the "corner of one's eye". The vast majority of the area in the visual field is included in the notion of peripheral vision. "Far peripheral" vision refers to the area at the edges of the visual field, "mid-peripheral" vision refers to medium eccentricities, and "near-peripheral", sometimes referred to as "para-central" vision, exists adjacent to the center of gaze.

<span class="mw-page-title-main">Visual system</span> Body parts responsible for vision

The visual system comprises the sensory organ and parts of the central nervous system which gives organisms the sense of vision as well as enabling the formation of several non-image photo response functions. It detects and interprets information from the optical spectrum perceptible to that species to "build a representation" of the surrounding environment. The visual system carries out a number of complex tasks, including the reception of light and the formation of monocular neural representations, colour vision, the neural mechanisms underlying stereopsis and assessment of distances to and between objects, the identification of a particular object of interest, motion perception, the analysis and integration of visual information, pattern recognition, accurate motor coordination under visual guidance, and more. The neuropsychological side of visual information processing is known as visual perception, an abnormality of which is called visual impairment, and a complete absence of which is called blindness. Non-image forming visual functions, independent of visual perception, include the pupillary light reflex and circadian photoentrainment.

<span class="mw-page-title-main">Cone cell</span> Photoreceptor cells responsible for color vision made to function in bright light

Cone cells, or cones, are photoreceptor cells in the retinas of vertebrates' eyes. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones function best in relatively bright light, called the photopic region, as opposed to rod cells, which work better in dim light, or the scotopic region. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye, with the highest concentration being towards the macula.

Dichromacy is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats require only two primary colors to be able to represent their visible gamut. By comparison, trichromats need three primary colors, and tetrachromats need four. Likewise, every color in a dichromat's gamut can be evoked with monochromatic light. By comparison, every color in a trichromat's gamut can be evoked with a combination of monochromatic light and white light.

<span class="mw-page-title-main">Visual acuity</span> Clarity of vision

Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye influence the sharpness of an image on its retina. Neural factors include the health and functioning of the retina, of the neural pathways to the brain, and of the interpretative faculty of the brain.

<span class="mw-page-title-main">Fovea centralis</span> Small pit in the retina of the eye responsible for all central vision

The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.

This is a partial list of human eye diseases and disorders.

<span class="mw-page-title-main">Human eye</span> Sensory organ of vision

The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm.

<span class="mw-page-title-main">Eye examination</span> Series of tests assessing vision and pertaining to the eyes

An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, ophthalmologist, or an orthoptist. Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.

<span class="mw-page-title-main">Nyctalopia</span> Condition making it difficult or impossible to see in relatively low light

Nyctalopia, also called night-blindness, is a condition making it difficult or impossible to see in relatively low light. It is a symptom of several eye diseases. Night blindness may exist from birth, or be caused by injury or malnutrition. It can be described as insufficient adaptation to darkness.

<span class="mw-page-title-main">Infant visual development</span>

Infant vision concerns the development of visual ability in human infants from birth through the first years of life. The aspects of human vision which develop following birth include visual acuity, tracking, color perception, depth perception, and object recognition.

<span class="mw-page-title-main">Cat senses</span> Senses of Felis catus

Cat senses are adaptations that allow cats to be highly efficient predators. Cats are good at detecting movement in low light, have an acute sense of hearing and smell, and their sense of touch is enhanced by long whiskers that protrude from their heads and bodies. These senses evolved to allow cats to hunt effectively at dawn and dusk.

<span class="mw-page-title-main">Bird vision</span> Senses for birds

Vision is the most important sense for birds, since good eyesight is essential for safe flight. Birds have a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". Birds are theropod dinosaurs, and the avian eye resembles that of other reptiles, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, bird's eyes are protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.

<span class="mw-page-title-main">Mammalian eye</span>

Mammals normally have a pair of eyes. Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.

Visual perception is the ability to interpret the surrounding environment through photopic vision, color vision, scotopic vision, and mesopic vision, using light in the visible spectrum reflected by objects in the environment. This is different from visual acuity, which refers to how clearly a person sees. A person can have problems with visual perceptual processing even if they have 20/20 vision.

<span class="mw-page-title-main">Vision in fish</span>

Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish retinas generally have both rod cells and cone cells, and most species have colour vision. Some fish can see ultraviolet and some are sensitive to polarised light.

<span class="mw-page-title-main">Eagle eye</span>

The eagle eye is among the sharpest in the animal kingdom, with an eyesight estimated at 4 to 8 times stronger than that of the average human. Although an eagle may only weigh 10 pounds (4.5 kg), its eyes are roughly the same size as those of a human. Eagle weight varies: a small eagle could weigh 700 grams (1.5 lb), while a larger one could weigh 6.5 kilograms (14 lb); an eagle of about 10 kilograms (22 lb) weight could have eyes as big as that of a human who weighs 200 pounds (91 kg). Although the size of the eagle eye is about the same as that of a human being, the back side shape of the eagle eye is flatter. Their eyes are stated to be larger in size than their brain, by weight. Color vision with resolution and clarity are the most prominent features of eagles' eyes, hence sharp-sighted people are sometimes referred to as "eagle-eyed". Eagles can identify five distinctly colored squirrels and locate their prey even if hidden.

References

  1. 1 2 Hartley, C; Grundon, RA (2016). "Chapter 5: Diseases and surgery of the globe and orbit". In Gilger, BC (ed.). Equine Ophthalmology (3rd ed.). John Wiley & Sons. p. 151. ISBN   9781119047742.
  2. 1 2 Sivak JG, Allen D (1975). "An evaluation of the ramp retina on the horse eye". Vision Res. 15 (12): 1353–1356. doi:10.1016/0042-6989(75)90189-3. PMID   1210017. S2CID   31898878.
  3. 1 2 3 4 5 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.
  4. ↑ "Choosing an American Paint Horse" PetPlace.com web site accessed July 20, 2007 at http://www.petplace.com/horses/choosing-an-american-paint-horse/page1.aspx Note American Paint Horse is a breed wherein most representatives are of pinto coloring
  5. ↑ "Cream dilution (CrD." Australian Equine Genetics Research Centre, web page accessed July 20, 2007 at http://www.aegrc.uq.edu.au/index.html?page=30056 Archived 2007-08-29 at the Wayback Machine
  6. ↑ Locke, MM; LS Ruth; LV Millon; MCT Penedo; JC Murray; AT Bowling (2001). "The cream dilution gene, responsible for the palomino and buckskin coat colors, maps to horse chromosome 21". Animal Genetics. 32 (6): 340–343. doi:10.1046/j.1365-2052.2001.00806.x. PMID   11736803. The eyes and skin of palominos and buckskins are often slightly lighter than their non-dilute equivalents.
  7. ↑ "Genetics of Champagne Coloring." The Horse online edition, accessed May 31, 2007 at http://www.thehorse.com/viewarticle.aspx?ID=9686
  8. ↑ Miller, Robert W. Western Horse Behavior and Training. Main Street Books, 1975. ISBN   0-385-08181-2 ISBN   978-0-385-08181-8
  9. ↑ Sellnow, Happy Trails, p. 46
  10. ↑ "Animal Eye Care. "About animal vision." Accessed March 11, 2010". Archived from the original on April 28, 2015. Retrieved July 11, 2007.
  11. ↑ "Horsewyse: How horses see. Date Accessed 7/11/07". Archived from the original on 2016-10-27. Retrieved 2007-07-11.
  12. ↑ Harman AM, Moore S, Hoskins R, Keller P. Horse vision and the explanation of visual behaviour originally explained by the ‘ramp retina’. Equine Vet J 1999; 31(5):384–390.
  13. ↑ McDonnell, Sue. "In Living Color." The Horse, Online edition, June 1, 2007. Web site accessed July 27, 2007 at http://www.thehorse.com/viewarticle.aspx?ID=9670
  14. ↑ Carroll J, Murphy CJ, Neitz M, Ver Hoeve JN, Neitz J (2001). "Photopigment basis for dichromatic color vision in the horse". Journal of Vision. 1 (2): 80–87. doi: 10.1167/1.2.2 . PMID   12678603 . Retrieved July 27, 2007.
  15. ↑ Stachurska A, Pieta M, Nesteruk E (2002). "Which obstacles are most problematic for jumping horses?". Appl Anim Behav Sci. 77 (3): 197–207. doi:10.1016/S0168-1591(02)00042-4.
  16. ↑ Wouters L, De Moor A (1979). "Ultrastructure of the pigment epithelium and the photoreceptors in the retina of the horse". Am J Vet Res. 40 (8): 1066–1071. PMID   525910.
  17. ↑ Saslow C (1999). "Factors affecting stimulus visibility for horses". Appl Anim Behav Sci. 61 (4): 273–284. doi:10.1016/S0168-1591(98)00205-6.
  18. ↑ Roth, LS; Balkenius, A; Kelber, A (2008). Roth, L.S.; Balkenius, A.; Kelber, A. (eds.). "The absolute threshold of colour vision in the horse". PLOS ONE. 3 (11): e3711. doi: 10.1371/journal.pone.0003711 . PMC   2577923 . PMID   19002261.
  19. ↑ "Shedding Light on Equine Night Vision" The Horse online edition, October 12, 2009
  20. ↑ Giffin, James M and Tom Gore. Horse Owner's Veterinary Handbook, Second Edition. Howell Book House. New York, NY. Copyright 1998.
  21. ↑ Prince JH, Diesem CD, Eglitis I, Ruskell GL. "Anatomy and histology of the eye and orbit in domestic animals." Springfield, IL: CC Thomas; 1960.
  22. ↑ Harman AM, Moore S, Hoskins R, Keller P (1999). "Horse vision and an explanation for the visual behaviour originally explained by the 'ramp retina'". Equine Veterinary Journal. 31 (5): 384–90. doi:10.1111/j.2042-3306.1999.tb03837.x. PMID   10505953.
  23. ↑ Current Research, from Blindappaloosas.org
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.