When dogs look into our eyes, it’s a gift. Making eye contact with humans or with other animals is not a common behavior—it can be seen as an act of aggression between dogs—but when our dogs feel relaxed in our presence, or when they want to communicate something to us—“Look what I found!”—they gaze at us with a soft expression that says “You’re my friend and I love you.”
Our dogs’ beautiful eyes are more than a means of communication, however. They are complex organs of sight that rely on a number of features to adjust the amount of light entering the eye, focus on objects at varying distances, visually follow moving objects such as tennis balls or bunnies, produce the images relayed to the brain, and protect the eye from injury.
Eyes seem delicate but they’re protected by bone, muscles and mechanisms of movement that help to prevent injury. The eyeball rests in a bony hollow called the orbit where it’s cushioned by fat. The orbit also contains muscles, nerves, blood vessels, and lacrimal glands, which produce tears.
The white of the eye, the sclera, is fibrous connective tissue that plays a supporting role, helping the eyeball to maintain its shape. (Fun fact: The sclera isn’t always white. In some dogs it is spotted.) It surrounds the cornea, the large, clear area at the front of the eye. The cornea protects the front of the eye, lets light in, and focuses light on the retina.
Covering the sclera and lining the inside of the eyelids is a thin, protective membrane called the conjunctiva. It secretes fluids that help to lubricate the eye and prevents dust, other irritants, and microbes from entering the eye, making it the first line of defense against corneal infection. Lots of little blood vessels in the conjunctiva provide nutrients to the eye and the lids. That’s why your pupper’s eyes get so red if the conjunctiva becomes irritated.
In the center of the eye is the dark, circular pupil, which in turn is surrounded by the iris, the colored part of the eye. A dog’s eyes are usually brown or blue, but some dogs have what are called odd eyes: one brown and one blue. Odd eyes are usually seen in breeds such as Australian Shepherds, Great Danes, Shetland Sheepdogs, and Siberian Huskies. Chesapeake Bay Retrievers have unusual eye color as well: yellowish to amber.
But the iris is more than decorative pigmentation; it’s actually a muscle with the job of making the pupil larger or smaller to regulate the amount of light that enters the eye. In darkness, the pupil enlarges to let light in; in bright light, it becomes smaller, reducing the amount of light entering the eye.
How the Eye Works
There’s a lot more to vision than just looking at something. Vision as a sense is related to numerous factors, including visual field, acuity, depth perception, and color perception, says veterinary ophthalmologist Shelby Reinstein, DVM, DACVO.
As far as mechanics, a dog’s eye works much like a human eye.
“The light rays pass through the clear window of the eye—the cornea—and are then focused by the lens onto the back of the eye,” she says. “This area contains a thin, specialized layer of cells called the retina, which is much like the film in a camera. The retina has unique cells called rods and cones.”
The rods and cones convert the light ray energy into electrical energy, taking the vision signal up to the brain via the optic nerve. The cones are also involved in a dog’s perception of color. You may have heard that dogs don’t see color, but that’s a common misconception, Reinstein says.
“Dogs have dichromatic vision, meaning they can see two main colors: blue and yellow. In comparison, humans have trichromatic vision, seeing three main colors: red, green, and blue.”
Do different dogs see things differently? It seems as if sighthounds—known for their ability to rapidly detect motion—would have the best vision, but that’s not necessarily the case, at least not in the way you might imagine.
A number of physical qualities combine to make sighthounds such as Afghan Hounds, Borzoi, Greyhounds, Salukis, and Whippets the exceptional hunters that they are: a deep chest with great lung capacity; a light, thin body; a long stride; and great agility and speed.
“The shape of their head and position of their eyes is also ideal for chasing prey, and some recent studies show these breeds may have higher levels of specialized retinal cells—retinal ganglion cells in the visual streak—providing them with improved movement detection,” Reinstein says.
The position of the eyes and the shape of the head can affect visual field (the entire area that can be seen when the eye is looking forward) and depth perception, producing differences in eyesight among breeds. Flat-faced (brachycephalic) dogs have eyes that are placed more to the side. That increases their monocular field of view, or what they can see with each eye, but it reduces the amount of overlap in the center, known as binocular perception, reducing their depth perception, Reinstein says.
While dogs can outsniff us any day of the week, their eyesight isn’t quite as good. Normal eyesight for humans is 20/20, while for dogs it is about 20/75, making them nearsighted. “However, some breeds, such as Labradors, have a better visual acuity closer to humans of 20/20,” Reinstein says.
The lens, located behind the iris, is important in a dog’s ability to focus on objects that are up close or far away. Using the small ciliary muscles, it can change its shape to focus light on the retina. When the muscles contract, the lens becomes thicker, useful for focusing on items such as a toy that’s nearby. When the ciliary muscles relax, the lens thins, allowing focus on distant objects.
Photoreceptors in the retina sense light and make images sharp. The part of the retina known as the area centralis is highly sensitive, packed with thousands of photoreceptors, each attached to a nerve fiber. The bundles of nerve fibers form the optic nerve, which carries images—in the form of electrical impulses—to the brain.
One way in which canine eyes differ from human eyes is in their possession of a structure called the tapetum lucidum, located in the retina. It’s what causes animal eyes to glow green when light shines on them in the dark. The tapetum reflects the light rays back through the retina, improving the animal’s ability to see in dim light, Reinstein says. Most mammals have a tapetum, the exceptions being humans, pigs, and squirrels.
You may think of Doggles or Rex Specs when you think canine eye protection—and we’ll talk about those—but it starts at a more basic level: the eyelids.
The upper and lower eyelids cover the eye, helping to shield it from injury not only with their thin folds of skin but also with reflexive blinks to repel anything that might be approaching the eye or in response to pain or irritation. Blinking also distributes tears over the eye’s surface, keeping it moist and washing away small, irritating particles.
In addition to their regular eyelids, dogs have what’s called a nictitating membrane, or third eyelid, which also helps to protect the eyeball. It extends across the eye as needed, sort of like a little windshield wiper.
What about pet sunglasses? Are they a fashion statement or eyecare necessity? Sometimes they’re a little bit of both. There’s no doubt that dogs look cool in their eyewear, but it can also serve as protection from wind-blown dust and debris and reduce glare in bright sunlight. Dogs are always sticking their noses into bushes or other places, and their eyes lead the way. Whether they’re hiking, boating, playing fetch, kicking up sand on the beach, roughhousing with other dogs, or irritating a cat, protective eyewear can have a place. It may also be a plus for dogs with large, protruding eyeballs, such as Boston Terriers, Cavalier King Charles Spaniels, French Bulldogs, and Pugs. And normal aging can affect a dog’s ability to regulate the amount of light entering the eye. Dogs who squint or close their eyes on a sunny day may be helped by canine sunnies.
Common eye problems in dogs include dry eye, corneal ulcers, cataracts, and glaucoma. Young dogs may be affected by developmental eyelid issues such as entropion (an inward-turning eyelid) or extra eyelashes (known as distichia). The latter might seem attractive to us but is merely irritating to dogs. Trauma from dog fights or being hit by a car can cause damage to eyes as well.
“The most common eye conditions in dogs that are breed-related are cataracts and glaucoma,” Reinstein says. “Many breeds of dogs have an increased risk of forming cataracts as adult dogs, including Cocker Spaniels, Boston Terriers, and Poodles. Glaucoma is known to be inherited in certain breeds such as Cocker Spaniels, Basset Hounds, Boston Terriers, and Shiba Inu, to name a few.”
When does your doggo need to see the eye doc? The veterinarian should perform a brief eye exam every time your dog is seen but may refer you to a veterinary ophthalmologist for an in-depth exam if signs of disease are noted or if your dog needs eye surgery such as cataract removal or corneal grafting.
Eyes that are cloudy or hazy may indicate an eye emergency such as glaucoma. Vision changes might not be apparent until vision loss is advanced, but if you notice your pet tripping on steps, bumping into things, or missing that piece of popcorn you toss when he usually nabs it in mid-air, those are clues that his eyesight should be checked by an expert.
Ophthalmologists can’t show your dog an eye chart and have her read the letters S Q I R L so how do they examine a dog’s eyes and check vision?
A visit with a veterinary ophthalmologist includes testing of both light reflexes and vision reflexes, along with measuring eye pressure—tonometry—and tear production and checking for corneal ulcers with a green dye test, known as a fluorescein stain.
“To check your pet’s vision, a test is performed called the ‘menace response,’” Reinstein says. “This is when the examiner’s hand is used to make a quick, menacing gesture at one eye of the dog, while the other is covered, and looking for the dog to blink. This response is learned, so it is not present in animals less than about 4 months of age.”
A veterinary ophthalmologist also uses a hand-held slit lamp biomicroscope—the same thing you put your chin in when you see the ophthalmologist. This allows eye doctors to detect small abnormalities such as corneal ulcers or cataracts. They also use specialized lenses and headsets to examine the retina.
It’s a good idea to check your dog’s eyes at home on a regular basis to make sure they’re bright, shiny and healthy. Call your vet for an appointment if your dog is squinting, pawing or rubbing at the eye, is tearing or has eye discharge, if eyelids are swollen, or if eyes are excessively red. Those aren’t “wait and see” signs. They indicate that your pup is in pain, so he needs to be seen ASAP.