From the feature “The Uncommon Sense” in the AKC Gazette—
Essentially, dogs see the same way we do. They have pupils, retinas, corneas, and irises just like us. The difference is a matter of adaptation. The canine eye, thanks to evolution and selective breeding, has been adapted to help dogs in their specialized work. Canines evolved as nocturnal hunters and scavengers, so they have large pupils that take in as much available light as possible. This helps account for their keen night vision and diminished “depth of field” vision—that is, the distance over which objects can be put into clear focus.
A human being with perfect eyesight has 20/20 vision, while most dogs see at something closer to 20/75: They must be 20 feet from an object to see it as clearly as a sharp-eyed person standing 75 feet away. The retina houses nerve cells called rods and cones. Rods are light sensitive, and dogs have a greater number of these than we do, so dogs see well in sparse light. Dogs have fewer of the color-sensitive cones, however, leading researchers to believe that dogs see color in the jumbled way as does a person who is red-green colorblind. The retina transforms light into an impulse and kicks it back to the optic nerve, which transmits the impulse to the dog’s brain. There, it becomes an image:
Pizza delivery guy approaching—bark now!
The eyes of some dogs are known to have a “visual streak,” a line of vision cells stretching across the retina’s surface. Until fairly recently it was assumed that this feature was true of all canine eyes. Paul McGreevey, an Australian professor of veterinary science, working with neuroscientist Alison Harman, has challenged this belief. Studying the eyes of several breeds, the team discovered that some dogs don’t have a visual streak. Instead, their vision cells are densely packed in one spot, called an “area centralis.”
The visual-streak breeds are dogkind’s hunters. They have long muzzles, and their eye placement gives them peripheral vision to detect and follow movement over wide vistas at great distance. The coursing breeds—Greyhounds, Salukis, and other hounds developed to scan vast expanses of desert—are the prototypical visual-streak breeds. Not for nothing are they called sighthounds. In retrievers the elongation of the skull is less exaggerated and the vision not quite as panoramic, but the principle is the same. A good working retriever uses widescreen vision to scan the horizon for ducks flying hundreds of yards offshore.
Area-centralis breeds tend to be short muzzled—think Pekingese, Pugs, and other lapdogs famous for being able to read their owner’s moods. These breeds, say researchers, have three times as many nerve endings in the retina as do visual-streak breeds. They see more like human beings: not much peripheral vision, but up close they see in high definition. “So when they’re looking at the owner’s face and different nuances of the owner’s expression, maybe they’re getting a bit more information than a long-nose dog,” McGreevey theorized when he published his findings. “This is perhaps a way of explaining how attentive and charming short-nosed dogs are.”
Photo by Brian Patrick Duggan