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Dog Vision, How It Differs from Ours

Dogs are susceptible to many different eye problems, some of them similar to eye problems in humans. But there are definite differences in how a dog’s eyes function and how our own eyes work.

To understand how different vision disorders affect your dog, you need to learn how their vision differs from a humans, and how that may contribute to the disease. Let’s take a look at some of the differences, and from there we can discuss various eye and vision problems our dogs may experience.

Makeup of a Dog’s Eye

Looking at the picture of a dog eye, you may not see any difference at all, unless you noticed that third eyelid.

The third eyelid, also known as the nictitating membrane, serves some important functions beyond just protecting the cornea. About a third of the tear production comes from this third eyelid. It also contains lymphoid tissue, which produces antibodies, and thus helps prevent infection of the eye. Finally, it serves to clear any debris or mucus from the cornea, keeping it as clean as possible.

It is important to note that you will not normally see this third eye, except possibly when your dog is sleeping. If you see it in other circumstances, it could indicate a problem that needs to be addressed.

So the eyelids, including the third eyelid, protect the cornea. The cornea is a transparent lining that covers the front of the eyeball, including the iris and the pupil. The iris acts somewhat like aperture stops of a camera, narrowing or widening to control the amount of light let into the eyeball interior. It is normally some shade of brown, but some dogs have blue irises. The light admitted goes through a lens, which focuses that light to the back of the eye, and the retina. The retina converts the light in a chemical reaction to an electrical signal to be sent by the optic nerve to the dog’s brain, where it is interpreted as sight.

The Retina, More Differences

The retina in both humans and dogs contains two types of photo-receptors, the rods and the cones. Rods are responsible for perception of shades of light and for seeing motion. The cones are responsible for the ability to see colors and for more acute sight.

Both the dog and human eyes are rod dominant, meaning many more rods exist than cones. However, that is where the similarity stops. Dogs have only 10% the cone density of humans, which affects both their ability to perceive color and their visual acuity.

In a dog, the mixture of rods and cones is similar throughout the retina, where in the human, an area in the central portion of the retina (called the macula) the concentration of cones is 100%. Actually, the dog also has an area more densely populated with cones, called the visual “streak”. This horizontal band varies in width depending on the breed. Long-nosed dogs tend to have wider streaks than short-nosed. In fact the pug’s streak is almost circular vice being in a rectangular band. This “streak” is the reason you often see your dog tilting her head, the better to see with!

It has been estimated that a dog’s vision is about 20-75, meaning that what a normal human can see well at 75 feet would have to be about 20 feet away for the same clarity in a dog. Yes, most dogs are near sighted! Some breeds, however, have been bred for better vision, and have a visual acuity more like the human’s. An example is the Labrador Retriever.

Finally, the makeup of the cones shows another difference between what a dog sees and what we do. The human retina contains three different types of cones, each sensitive to a specific part of the color spectrum (red, green, and blue). The dog, however, only has two types of cones, those sensitive to blue and those sensitive to yellow. Since all the colors we recognize are some mixture of the colors recognized by our cones, it is obvious that the human can recognize far more colors than the dog can. In fact, the dogs color perception is very much like a human suffering from red-green color blindness. They can see shades of blue very well, but reds and greens all appear as a shade of yellow. See the color wheels below for an idea of how different colors look. On the left is a normal color wheel, and on the right is what the dog sees.

Normal color wheel A dogs color wheel

Putting all the differences in the makeup of the retina is probably easiest by showing a simulated comparison between what we see versus what our dog sees. Below is a pair pictures of me and my dog Roxy (amazing likenesses if I do say so!). On the left is what we as humans see in that picture. On the right is how Roxy sees it.

Rodney and his dog, Roxy Rodney and his dog, Roxy, as seen by Roxy

Other Differences in Dog Vision

Have you ever seen an eerie glow coming from your dog’s eyes? This is because behind the retina is another thin film with a mirror like finish, which reflects the light back to give the rods one more chance at capturing the visual. This feature, along with the larger pupil size to allow more light in, makes the dog perfectly adapted to seeing in low light situations, such as during night hunts.

And lastly, observe the setting of your dog’s eyes. Notice that proportionally they are much wider spread than yours? And also notice that instead of pointing straight ahead most dogs are actually pointed outwards slightly. Combined, these features give the dog a much greater peripheral vision than humans, and the dog can detect motion almost behind her! But this comes at a cost. Depth perception is the result of vision from both eyes overlapping, and this overlap is much less in a dog than in a human.

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