THROUGH A CAT'S EYE

Understanding Feline Vision

by Joelle Steele

In ancient Egypt where the feline animal was much revered, women used kohl sticks to line their eyelids into the shape of a cat's eye. That same culture also believed that the cat's eye was one of the eyes of their god Horus. Over the years, the cat's eye has formed the basis of many a superstitious belief. Hell's fires were reflected in them, they were the "mirror" of the spirit realm, the mineral chrysoberyl was nicknamed the "cat's eye" and was used to ward off witches, and in some cultures it is still believed that cats' eyes enable them to see supernatural entities.

WHAT CATS SEE

It's no small wonder that we find the eyes of our feline companions so captivating and so compelling, especially when you consider how beautiful they are. Part of this beauty is attributable to the fact that cats have the largest eyes of any mammal, relative to the size of their bodies. If your eyes were of a similar proportion to your body they would be at least six inches in diameter! In addition, because their eyes are in the front of the head, rather than on the sides, they always appear to be looking directly at you -- and probably are!

The size and placement of a cat's eyes are not for purely aesthetic reasons. Cats are natural hunters and their sense of sight is important to their survival. Those large eyes are designed for seeing at night and they are located where they provide both a three dimensional view of the world combined with acute depth perception, the latter being important for a cat who needs to judge the distance before making that leap from the garage roof onto the narrow fence below.

Their peripheral vision is achieved like that of humans but is far better than ours. They have approximately 120 degrees of binocular vision and 80 degrees of monocular vision per side. They see things a little differently from one eye to the other like we do, and those perceptions are merged into a final picture, sometimes as the cat moves its head from side to side to gain the best visual vantage. That 280 degree field of vision allows them to readily perceive even the slightest movement -- a necessity for a hungry hunter in the wild.

EYE STRUCTURE

The eye of the cat is structurally similar to that of the human eye. The sphere-shaped organ consists of a large cornea, the transparent protective part, which is surrounded by a very thin rim of white tissue called the sclera. Beneath the cornea is the iris, the colored part of the eye, which regulates the opening and closing of the pupil much like the aperture of a camera.

When light enters the pupil, it passes through a large lens inside the eye which acts as a focusing mechanism and projects the image being viewed onto the retina, the light sensitive membrane in the back part of the eye. The lens is not highly developed in the cat and they cannot adjust the focus so it is believed that their vision is probably somewhat blurry in the mid-to-close field of vision. They can, however, compensate for this to some degree by narrowing their pupils down to tiny slits, thereby sharpening the image a little. We probably look very fuzzy to our feline companions up close because their optimum range of vision is from six to twenty feet.

The cat eye is covered by an outer eyelid and by a third eyelid known as the "haw" or, more correctly, the nictitating membrane. This eyelid moves from the inside corner of the eye across to the outside edge providing an additional protective layer which helps lubricate the eye by dispersing the tears and removing foreign particles such as dust.

NIGHT VISION & SEEING COLORS

There are two issues which come up time and time again when people talk about what a cat can and cannot see: seeing in the dark and seeing colors. It is the retina which controls both. It is composed of photoceptor nerve cells called rods and cones. These cells send a message via the optic nerve to the brain telling the cat what it is seeing.

The cones provide color vision and the rods react to light and dark. The cat has far more rods than cones which is why he is able to see so well in dim light. Contrary to popular belief, cats cannot see in total darkness any better than we can, but they have a rod to cone ratio of about 25:1 whereas the human eye has a ratio of about 4:1. This explains their much greater maneuverability in semi-darkness as does the fact that in reduced light their pupils open to their fullest extent.

Cat eyes have horizontal and vertical controls courtesy of the eyelids which move up and down at right angles to the pupil. This enables a cat to be more sensitive to whatever light might be available in a darkened environment. In fact, in dim light they can see about five to seven times better than we can. This means that they require only about one-sixth as much light as we do to see well at night. We also know that cats can see in the ultraviolet range of the spectrum and it is believed that they may be capable of seeing other light rays which humans can only detect with scientific measuring devices.

Another contributor to a cat's night vision is the presence of the tapetum lucidum, located behind the lens and in front of the retina. It's name translates most commonly as "bright carpet," but also can be interpreted as "bright covering." It is responsible for the glow in a cat's eyes which appears when they are struck by a beam of light. Actually, the eyes are not glowing at all, but are simply reflecting back the light which is being shined into them. The tapetum lucidum is composed of ten reflective layers of zinc and proteins which magnify every bit of light coming through the cornea and into the pupil.

As for color, it was long believed that cats saw the world only in black and white and shades of grey. Now we know that cats do see colors. Their range of color perception is more limited that ours, but they can see the colors they do perceive far more brilliantly than we do. For the record, they can distinguish between grey and red, grey and green, grey and blue, grey and yellow, red and green, red and blue, green and blue, and yellow and blue.

EYE COLOR

Eye color is due to the pigment in the iris. Cat's eyes come in many colors, the most common being yellow and green. But, as you have probably seen, some cats have orange, blue, or purple eyes, sometimes multi-colored eyes, and sometimes varying eye color. A cat may have eyes which appear bright golden yellow one day and deep green the next, much like the hazel eyes of humans which can range in color from green to blue to grey, all within the course of a single day.

Sometimes a cat is born with eyes of different colors. This is commonly seen in Persians where one eye is blue and the other is orange or gold. This can be due to congenital deafness on the blue side, and blue eyes in white cats almost always goes hand in hand with deafness.

DISEASES & DISORDERS

Because the eye is such a complex organ, it is prone to a number of ailments and diseases. If, at any time, you even suspect that your cat has an eye problem, you should seek veterinarian assistance immediately.

Symptoms of eye problems include clear or colored discharge, redness of the sclera, excessive tearing, squinting, tenderness to touch, sensitivity to light, film over the cornea, cloudiness inside the eye, unevenly dilated pupils, tremors of the eye, itching, crusting, swelling or bulging, and in general, anything that does not look normal.

Some symptoms may be signs of brain injury or neurological damage. Others may be the result of infections or even allergic reactions. Foreign bodies can become trapped behind the eyelid or the nictitating membrane. Some cats experience tumors on the eyelids which can spread if not attended to.

One common problem which is most often seen with Persians is that of tearing. As a consequence of breeding, the space for tears to collect at the corner of the eye is often too small. Sometimes the tear duct is not even in the correct place for optimum drainage or the tear ducts are too narrow to hold the tears back. The end result is an overflowing of tears which often stain the fur under the eyes. Unless there is an infection present as well, there is no treatment for this condition.

Another common problem is conjunctivitis, more commonly called "pink eye." This condition is an inflammation of the lining membrane of the inside of the eyelids. Cats with pink eye have red eyes, discharge, and itching. It is not usually painful unless the cat has pawed so much in trying to scratch the itchy eye that he has broken the skin which has in turn become infected. Some forms of conjunctivitis can be treated at home with a dilute solution of boric acid. But, to be on the safe side you should get a complete veterinary diagnosis before attempting any home remedies.

The cornea is a sensitive part of the eye. It is subject to abrasion from scratches made by another cat, from a dry object such as a twig grazing the protective cellular layer, or from a foreign body imbedded beneath the eyelid. Ulcers can be very danger and require immediate veterinarian intervention. Ulcers appear as dull spots or depressions on the surface of the cornea. Keratitis is an inflammation of the cornea in which the normally transparent cornea begins to become hazy or cloudy. This may result in partial or complete blindness if left untreated.

Other disease and disorders may also lead to blindness. A cataract is a spot on the lens which blocks out light. They are rare in cats and are usually the result of an old injury or infection. Other cause include diabetes and genetic predispositions to cataracts. Cataracts should not be confused with nuclear sclerosis, a condition commonly seen in older cats which does not interfere with their vision. It occurs when the lens loses water due to age and a bluish haze appears behind the cornea. It is not a cause for alarm and requires no treatment.

Congenital glaucoma is rare but not unknown in the purebreds such as Persians and Siamese to name a few. More commonly found is secondary glaucoma which results as a complication from other diseases of the lens or corneal regions of the eye. This is a painful disease because it is characterized by an increase in fluid pressure within the eyeball and a hardening of the eyeball as a result. A cat with glaucoma has a fixed stare, a hazy cornea, and a dilated pupil. If left untreated the eye may swell up and protrude and permanent loss of vision may occur.

There are a few known retinal diseases which are usually characterized by a loss of sight, particularly night vision. Most of these diseases lead to blindness and there are few effective treatments. However, even the totally blind cat can get along quite well and live a perfectly happy and relatively normal existence if kept indoors, in familiar surroundings, with loving human companions.