A Cat’s-Eye View

Understanding how our cats see the world gives us insight into their behavior.

By Keith Bush

If we could view the world through the eyes of a cat, what would we see? How would our view of the world change? Would we find our visual lives diminished or enriched? Would we glimpse hitherto unknown secrets and discover clues to deeper mysteries, bringing us closer to understanding the inner workings of the feline mind?

We may not be able to look through their eyes, but by looking closely at them we can try to reach a better understanding of how they perceive the world and how that perception shapes reality.

Cover-Ups

As you draw near to your cat’s eyes in an attempt to take a closer look, don’t be surprised if the doors of perception slam shut. The eyelids act as barriers between the outside world and the delicate structures of the eye.

Each eyelid consists primarily of connective tissue with a thin layer of fur and skin on the outside and an inner lining called the palpebral conjunctiva. Contraction of a circular muscle surrounding the eye closes the lid, shutting out foreign objects or excess light. Contraction of another, opposing muscle raises the upper lid when the eyes need to be wide open.

In addition to protecting your cat’s sight, the eyelids visually express emotion.

“Fully open lids can mean they’re alert or interested,” says Rolan Tripp, DVM, founder of AnimalBehavior.Net and consultant to Petmate. “If they’re fully open with eye contact, that’s called staring.” A cat with this expression may be sending a message: I’m watching you, Bub.

“Partially open or half-closed is a sign of trust and love,” Tripp says. Slow blinks say, I can let my guard down around you. Keep scratching behind my ears.

“Completely closed lids may express appeasement,” Tripp says. Do with me what you will. “Or, he could be taking a nap.”

Cats instinctively recognize the significance of another cat’s eyelid movements and respond accordingly. Humans have to make an effort to see them as a cat would.

Cats have a whitish or pink membrane, sometimes called the third eyelid, at the inner corner of each eye. This nictating membrane normally stays concealed but may intermittently appear when a cat is drowsy and content. It should not remain partially closed, protrude or appear swollen or red, however. If it does, see a veterinarian.

When all three lids are open wide, light enters the eye as easily as sunshine enters a house with shutters open and curtain pulled back.

Windows to the Soul

A thin film of liquid, secreted by the cunjunctiva, the nictating membrane and tear glands at the corner of each eye, covers the cornea, the transparent bulge in the front of the eyeball. The tear film acts as another layer of defense. The motion of the eyelids and nictating membrane keep it evenly distributed to maintain a smooth surface through which light can pass.

The cornea itself is thinner than a dime and curved so as to refract or bend parallel rays of light toward a focal point. After the cornea, light passes through the aqueous humor, a water fluid filling the small anterior chamber between the cornea and the lens. Because these transparent, living structures contain no blood vessels, they depend on the aqueous humor to supply them with oxygen and nutrients.

Before reaching the inner chamber of the eye, light must pass through a final gateway: the iris. This muscular structure can constrict, changing the size of the pupil, the opening through which light reaches the lens. This is where you’ll see one of the most striking characteristics of your cats’ eyes.

“The pupil in a domesticated cats is a vertical slit, whereas in larger wild species it is circular,” says veterinary ophthalmologist Isabel-Ricarda Jurk, DVM, assistant professor in the department of clinical sciences at Tufts University School of Veterinary Medicine in North Grafton, MA. Lions hunt during the day, whereas the relatively small ancestors of domestic cats hunted at night in conditions of very low light. The vertical pupil, together with the horizontal eyelids, gives the cat the ability to endure the bright light of day by strictly regulating the amount of light that penetrates the lens.

Like the eyelids, irises reveal something about a cat’s emotional state.

“When you’re looking at pupil changes in the cat when there’s no change in ambient light, that has significance,” says Tripp. A large, round opening indicates excitement. “If there’s a good thing like bringing a food dish, the pupil will dilate, or if it’s a bad thing, a competitor.” Dilated pupils mean your cat has a strong feeling about something, but you’ll need other information to know if it’s anticipation or fear. On the other hand, narrow slits may mean your cat is honing in on something of interest. “When they’re focusing very intently on something, the pupils may constrict,” Tripp says.

The lens finishes the job started by the cornea, further focusing the light to form a sharp image. In the human eye, tiny muscles pull on the lens, adjusting its shape and changing its refracting power to suit your needs, focusing on something inches from your face or 100 feet away, but that wouldn’t work for your cat.

“The elasticity of the feline lens capsule is only 5 percent of the one in humans,” Jurk says. “Rapid change in refractive power of the eye in cats occurs not by changing lens curvature like in primates but by front-to-back movement of the entire lens.” This works on the same principle as adjusting an optical zoom on a camera. “The lens can move forward by up to 0.6 mm.” A cat’s eye still can’t focus as well as a human’s, though. Maybe that’s why your cat would rather sit on the newspaper than read it.

Inner Sanctum

Having passed through the lens, light now travels through the vitreous humor, a clear gel that fills the rear chamber of the eye and accounts for most of the eye’s volume, on its way to the retina, a light-sensitive membrane at the back of the eye.

The retina contains two important kinds of cells. Rods respond to very low levels of light and don’t distinguish between colors. Cones need a higher level of light and come in different varieties to respond to different hues.

Compared to humans, cats have few cones but plenty of rods. That means they probably see color, but not very vividly. Behavioral research indicates they see blue and yellow better than red and green, but color overall doesn’t seem very important to them. To a cat, a daytime world of muddy pastels is a small price to pay for its ability to hunt prey and avoid larger predators at night.

To boost this ability, a cat has a reflective layer, the tapetum lucidum, behind the retina. Light that gets through the retina the first time bounces back, giving rods and cones a second chance to do their thing. Many animals have these, but cats’ seem to be particularly dazzling, leading to superstitions about seeing in total darkness and glowing with magical light.

“Cats eyes don’t glow in the dark,” Tripp says. “They reflect light back, which gives the impression of glowing, but they don’t have any source of emitting their own light.” It’s still a neat trick, though, and helps account for your cat’s total indifference to your sleep schedule.

Data Processing

When light bouncing around inside your cat’s eyeball actives a rod or a cone, it triggers a signal that travels along the optic nerve and makes it way to the visual cortex, the part of the brain that sorts all these bits of information into a meaningful pattern.

The feline brain is hardwired to react to detect and react to sudden movements, which could signal dinner.

“If we saw the world as cats do, anything that moved would cause our attention to be drawn to it dramatically,” Tripp says. “The bird that jumps or flaps causes the neurological attention bell to go ding-ding-ding. Instinct says, that’s prey; what you need to do is jump on it. That triggers instinctive behaviors, such as the head swing to measure the distance and the jaw chatter in preparation for biting.”

Being carnivores, cats have eyes on the fronts of their heads, unlike prey animals, which tend to have them on the sides. In exchange for a narrower field of vision, cats gain good depth perception, an important asset for a creature that leaps on prey, or from the top of a bookcase to the back of a chair. Moving the head back and forth enables the cat to judge distance even better. (Try it some time; near objects appear to move in the opposite direction relative to the background.)

This irresistible urge to pounce on small, moveable objects helps explains why cats’ tastes in toys differ from ours. While we may be drawn to flashy, brightly colored objects, they’re more excited by the way something moves; the more it looks like prey, the more it draws them.

Of course, if it’s too big, it could look like a predator. That may account for one notoriously frustrating aspect of feline behavior.

“If we think about how cats see things, we can get an idea why they go to the one person in the room who doesn’t like them,” Tripp says. “People who want to pat the cat are going to look at it and making gestures toward it. The cat is going to be repelled by people who are both staring at it and moving around and attracted to the only person who’s giving it non-threatening signals, looking away and holding still.”

What seems like idiosyncratic behavior makes perfect sense, from that point of view.