Why Dogs and People Don’t See Eye to Eye

On a recent hike at dusk, my two dog companions ran ahead while I found my feet slamming the ground and my eyes near-skewered by face-high branches because I couldn’t see well in the fading light. The dogs seemed to see every last twig and leaf and clearly didn’t get what my problem was.

Dog and human eyes are more similar than different: They’re round, filled with gelatin-like liquid and kitted out with retina, lens and the rest. And dogs get glaucoma and cataracts as humans do, says Paul E. Miller, a veterinarian and dog eye expert at the University of Wisconsin School of Medicine. Some dogs are even shortsighted. (And some get fitted with contact lenses, such as a circus dog Miller knew that needed the lenses to continue to do its tricks after cataract surgery.)

But dog and human eyes are distinct in several key ways, Miller says:

* Dogs have only two kinds of color-sensing cells (called cones) in their eyes: one that’s most sensitive for yellow-ish hues and another for blues. This was figured out, in part, by tests in which dogs had to distinguish between different-colored discs to get yummy treats. “The best analogy is that they’re similar to a person who has red-green color blindness,” Miller says. They can detect red and green, but can’t distinguish between them.

Humans, normally, have three cone types, so our world has more hues than a dog’s world does. (People used to think that dogs saw only in shades of gray.)

* Dogs have fewer cones than people and a lot more rods, which merely distinguish light from dark. Rods function better than cones when it’s dark, which explains why when dusk rolls down, the world looks more gray. Dogs’ extra dose of rods allow them to see four to six times better than people in dim light. This helps dogs and their wolf relatives hunt at night, dawn and dusk.

* Dogs, unlike us, have a layer under their retina called the “tapetum,” a reflective coating that further increases the animals’ visual acuity at night. (Reflection from the tapetum is why the eyes of dogs, cats and a range of night critters gleam back at you in the dark.) Should a ray of light shoot through their retina without getting sensed by the rods and cones, it bounces back up and gets another chance to be absorbed.

But there’s a downside to the tapetum. The bouncing-back trick makes the exact source of the light ray more ambiguous--and as a consequence dogs only have about 20:75 vision. In other words, “a sign that a human could read from 75 feet a dog could read from 20 feet,” Miller says. If, of course, a dog could read.

“People ask, ‘Well, how come when I’m across the street my dog barks, but when I walk across the street he starts wagging his tail and feels all sheepish that he barked at me?’” Miller says. “Well, it relates to this 20:75 vision.” Your dog may simply not recognize you that far away.

Dogs, by the way, do get eye tests. But this doesn’t involve growled responses to pictures of cats or postal carriers or anything like that. There are physical ways to detect shortsightedness, such as by measuring the eyes’ ability to focus (the same methods used in children who can’t talk).

* Dogs detect moving images better than still ones, but not as well as we do.

And they can’t focus on objects that are too close. Although a normal child can see--pretty sharply--an image as close as four inches, for a dog crispness starts to decline at about 20 inches. “That’s why I think that when dogs really want to investigate objects near them, they smell them,” Miller says.

These days, he says, dogs with vision problems can get treated much as humans can--glaucoma drugs, cataract surgery and more--in fact, he works closely with his colleagues in medical ophthalmology. Certain breeds are particularly prone to vision problems because of inbreeding.

American cocker spaniels are prone to glaucoma, for instance, and German shepherds have a tendency to nearsightedness. German shepherd guide dogs, meanwhile, have a much lower nearsighted rate-probably because the ones with vision problems were kicked out of guide dog school because they couldn’t perform the tasks well enough.

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