Equine Eye Vets Veterinary Eye Care for Horses
We often are often asked whether animals see things in the same way that human do. This is, needless to say, a difficult question to answer since we cannot ask animals what they see. In fact there is no certainty that even in human beings we all see things in the same way!
It is possible to infer various things about animal vision based on their behavior and the structure and function of the elements which make up the visual pathways in animals.
Although most domestic animals use a variety of senses which are less important to humans (smell and hearing in particular) we can be pretty certain that vision is an important sense which is well developed in animals.
The domestic animals we keep as pets fall into the categories of predators (cats and dogs) and prey species (horses) – at least in the wild. Cats and dogs have eye that have developed to enable acute vision and excellent vision in low light conditions. This allows these animals to track down and catch their fast moving prey day or night. Cats have the most sensitive eyes among the domestic species. The sensitivity of the eye is low light is due to a high proportion of rod photoreceptors in the retina which are the cells that are best able to capture low levels of light. This is aided by the tapetum – the reflective layer behind the retina which increases the light gathering power of the eye. The tapetum is the structure which gives animals the eye shine when seen in headlights at night. Humans lack a tapetum and have much less sensitive eye in dim light or at night. In some species such as horses the large eye size also confers some ability to gather light in dim conditions – important to be able to see movements of a predator at night.
Most species benefit from having eyes which can detect rapid motion (for hunted species this allows them to notice movements of a potential predator and take evasive action). A high preponderance of rod photoreceptors improves movement detection in low light levels and in the peripheral visual fields. The placement of the eye in the head (lateral), modifications of the pupil (horizontal elongated pupil) coupled with high rod numbers in the retina allow animals such as horses to pick up movement over a wide area of their environment (beneficial to a hunted animal).
The other type of photoreceptor in the retina is the cone – this cell functions primarily in bright light and are the cells which occur in greatest numbers in the human fovea – the area largely responsible for the excellent visual acuity enjoyed by primates. Most animals lack this considerable concentration of cones in one area, although most species do have a band of increased cone density in the more central retina. Most of the domestic species do not change the focusing power of the lens to the same extent as in humans. For these reasons (amongst others) the visual acuity of most domestic animals is not expected to be as good as in humans.
Although most animals have emmetropic vision (whereby rays of light entering the eye are focused to a sharp image at the plane of the retina) we occasionally see animals (just like people) which are short-sighted (myopic) or even hyperopic (far-sighted). Although these abnormalities may not be as critical as in humans (who need to be able to read and perform other tasks requiring acute vision) we do see dogs and horses with abnormal visual refraction (which we can measure with retinoscopy) who demonstrate strange visual behavior which might benefit from some visual correction (contact lenses?).
The differences in the types of cone photoreceptors in the retina between primates and the domestic species probably accounts for differences in the colors that we expect animals to see. Humans have three types of cone – responsive to red, green and blue hues of color. Dogs have fewer cones, no foveal concentration of cones and overall fewer cones than humans. The cones that dogs do have are responsive to red and blue hues – they lack green cones). This would be akin to certain types of color blindness in people whereby dogs will confuse red and green colors. Maybe dogs can see primarily in blue and yellow. Nocturnal species which have high rod concentrations in the retina see poorly if at all in color since the rods are poorly responsive to different colors. Cats, although possessing three types of color responsive cones only appear to see in two hues based on behavioral studies. The same may be said of horses. Many other more exotic species (some reptiles and many birds) probably see well in color.
Glossary of Anatomic Terms
Cornea - the clear, optically transparent window at the front of the eye. The anterior part of the fibrous coat of the eye which provides 2/3 of the focusing power of the eye
Sclera - the white part of the eye which surrounds the cornea and extends back to the posterior pole of the eye forming the posterior fibrous coat of the eye.
Conjunctiva - the thin vascular layer which lies over the sclera and is reflected back at the conjunctival fornix to line the eyelids and cover the third eyelid.
Third Eyelid - conjunctival covered movable structure ("T" shaped cartilage and third eyelid gland) located between the lower eyelid and the eyeball in the medial part of the orbit. The third eyelid moves across the eye as a protective mechanism. The third eyelid gland produces part of the tear film which bathes the eye and conjunctiva.
Anterior Chamber - the fluid filled space behind the cornea and in front of the iris and lens. The anterior chamber is filled with clear fluid (aqueous humor) which enters the anterior chamber through the pupil and drains out of the chamber through the iridocorneal angle at the edge of the corneal between the cornea and iris.
Posterior Chamber - a very small space between the posterior surface of the iris and the ciliary body and the lens and lens supportive zonule - aqueous humor is secreted into the posterior chamber by cells of the ciliary body
Ciliary Body - A muscular and vascular layer of tissue lined by cells which secrete aqueous humor into the posterior chamber. The muscles of the ciliary body are involved in accommodation (fine focusing by moving or changing the shape of the lens) in some species.
Iris - The pigmented structure arising at the edge of the anterior chamber which lies against the anterior capsule of the lens. The pupil is a hole in the center of the iris. Pupil size is changed by contraction and relaxation of muscle in the iris which change its shape.
Iridocorneal Angle - area between the edge of the iris and the cornea through which aqueous humor drains from the eye into the venous circulation.
Aqueous Humor - Clear fluid with low protein content found in the posterior and anterior chamber of the eye. This fluid functions to supply nutrients to the internal ocular structures and carry away wast products of metabolism and as such resembles transparent blood. It is also involved in maintaining the intra-ocular pressure by a balance between the rate of production in the eye and the rate at which it drains back into the veins.
Lens - clear spheroidal structure located behind the iris and in front of the vitreous humor. The lens acts to fine focus light onto the retina providing about 1/3 of the focusing power of the eye. It is supported through 360 degrees from the ciliary body.
Lens Zonule - the fine collagenous strands which support the lens around its equator from the ciliary body
Vitreous Humor - the largest volume of the eye in most species located behind the lens and ciliary body and inside the posterior layers of the eye (retina, choroid an sclera). the vitreous is a viscous gel comprised mostly of water.
Retina - the light sensitive layer of the eye (commonly compared to the retina of a camera. Nerve fibers from the retina leave the eye in the optic nerve through a sieve like structure in the sclera near the posterior pole of the eye.
Choroid - the outer posterior vascular coat of the eye (comprised of large numbers of blood vessels which provide part of the nutrition required by the metabolically active retina).
Optic Nerve - the second cranial nerve which connects the eye to the brain and carries information from the retina to the brain for visual processing
Orbit - the space within the skull in which the eye is located. It is mostly bounded by bone and muscle of the skull as well at the eyelids