Through the Eyes of Your Canine

By Sarah Probst Information Specialist University of Illinois College of Veterinary Medicine

Owners who want to better understand their canine companions must recognize that dogs see the world from a different visual perspective. The differences begin with the structure of the eye.

"We have a good idea what canines see because we know the make-up of the retina of a dog's eye," says Dr. Ralph Hamor, a veterinarian and specialist in ophthalmology at the University of Illinois College of Veterinary Medicine Teaching Hospital.

The retina, which covers the back of the inside of the eyeball, contains cones and rods-two types of light-sensitive cells. Cones provide color perception and detailed sight, while rods detect motion and vision in dim light. Dogs, which have rod-dominated retinas, see better in the dark than humans do and have motion-oriented vision. However, because they have only about one-tenth the concentration of cones that humans have, dogs do not see colors as humans do.

"I generally explain that dogs see like a color-blind human," says Dr. Hamor. "Many people think that a person who is red/green color blind cannot see any color, but there are variations of being color blind. Most people have vision that is trichromatic (three color variations). People who are red/green color blind are dichromatic (two color variations). Dogs can pick out two colors-blue-violet and yellow-and they can differentiate among shades of gray." Dogs are unable to distinguish among green, yellow, orange, and red. They also have difficulty differentiating greens and grays.

Dogs use other cues (such as smell, texture, brightness, and position) rather than rely on color. Seeing-eye dogs, for example, may not distinguish whether a stoplight is green or red; they look at the brightness and position of the light. This and the flow and noise of traffic will tell the dog that it is the right time to cross the street.

The set of dog's eyes determines the amount of field of view and depth perception. Prey species tend to have eyes set on the sides of their head because the increased field of view allows them to see approaching predators.

Predator species, like humans and dogs, have eyes set closer together. "Human eyes are set straight forward while dog eyes, depending on the breed, are usually set at a 20 degree angle. This angle increases the field of view and therefore the peripheral vision of the dog."

However, this increased peripheral vision compromises the amount of binocular vision. Where the field of view of each eye overlaps, we have binocular vision, which gives us depth perception. The wider-set eyes of dogs have less overlap and less binocular vision. Dogs' depth perception is best when they look straight ahead, but is blocked by their noses at certain angles. "Predators need binocular vision as a survival tool," Dr. Hamor says. Binocular vision aids in jumping, leaping, catching, and many other activities fundamental to predators.

In addition to having less binocular vision than humans, dogs also have less visual acuity. Humans with perfect eyesight are said to have 20/20 vision-we can distinguish letters or objects at a distance of 20 feet. Dogs typically have 20/75 vision-they must be 20 feet from an object to see it as well as a human standing 75 feet away. Certain breeds have better acuity. Labradors, commonly used as seeing-eye dogs, have been bred for better eyesight and may have closer to 20/20 vision.

Don't expect your dog to recognize you across the field by sight. He'll recognize you when you do some sort of motion particular to yourself or by smell or hearing. Because of the number of rods in the retina, dogs see moving objects much better than they do stationary objects. Motion sensitivity has been noted as the critical aspect of canine vision. "So much of dog behavior deals with posture and appropriateness. Small changes in your body posture mean a lot to your dog," Dr. Hamor adds. Dog owners need to modify training based on this fact. If you want your dog to perform an action based on a silent cue from you, Dr. Hamor suggests using a wide sweeping motion to cue your dog.

When dogs go blind, owners often wonder if the dogs' quality of life has diminished to the point where they are no longer happy. "We know that humans deal well with being blind, and humans are much more dependent on their eyes than are dogs," Dr. Hamor says. "Blind dogs lead happy lives if they are comfortable." The owner may need to make some adjustments in the pet's environment, such as having a fenced yard, taking leashed walks, and not leaving unusual objects in normal pathways. "When blind dogs are in their normal environment, most people don't know they are blind." When clients visit Dr. Hamor asking about quality of life for their newly blind dog, Dr. Hamor suggests that they take a month to see if they and their dog are happy. In the majority of cases, the owners never come back.

For further information on dog vision and problems with your dog's eyes, contact your local veterinarian.