The visual streak is a horizontal band in the retina right above the optic nerve; this area has the highest concentration of rods and cones and vision is sharpest here. The visual streak varies greatly among breeds, and studies suggest that different breeds see the world differently.* In wolves and in dogs with long heads like wolves, the streak is wide, with the nerves evenly distributed. The shorter a breed’s head, the narrower (more circular) the streak tends to be. Pugs, for example, have a small spot of sharp vision—an “area centralis”—as humans do. Even within breeds, the visual streak can vary from type to type.
These features and others equip the dog to be a good hunter under various light conditions. The tapetum lucidum improves vision in poor light, as does the high proportion of rods to cones. A rod-dense retina also makes dogs excellent at detecting motion and shapes. Because most dogs’ eyes angle slightly to the side, they have a wider field of view than humans. When a wide field of vision combines with a wide visual streak, as in a German Shepherd, the dog can see the whole horizon at once (instead of having to scan the eyes back and forth as humans do).
As hunters of large prey with keen senses of smell and hearing, dogs don’t need to see well close up, and near vision is blurry in long-nosed dogs. (Short-nosed dogs, with their humanlike area centralis, do appear to see well close up. Though the area centralis may lessen their ability as hunters, it may make them better lapdogs, more able to “read” their owners’ faces.) Overall vision is also less sharp.
Common Eye Disorders
Some eye disorders occur more often than others. “As a general practitioner, I was often presented with problems such as conjunctivitis, dry eye and corneal ulcer,” says Christine Lim, DVM, Resident III in Comparative Ophthalmology at the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California–Davis. “Now that I specialize in ophthalmology, I am more often presented with cataracts, glaucoma and retinal disorders.”
Following are a few of the more common canine eye problems. (Note: Some breeds are more prone to eye problems than others, and a mixed-breed with one of those types in the mix could also be affected.)
Conjunctivitis. A condition in which the lining of the eyelids and the front of the sclera (the white of the eye) become inflamed. It can be caused by infection, an object in the eye, an allergic reaction, dry eye, a scratch, or even smoke or dust, and can also be a symptom of other diseases. Treatment depends on the cause.
Dry Eye. A condition in which not enough tears are produced to keep the eyes properly lubricated. Dogs may inherit this condition; among the breeds at higher risk are the American Cocker Spaniel, English Bulldog, Pug, Lhasa Apso, Pekinese, Shih Tzu and West Highland White Terrier. Small, flat-faced dogs sometimes have eyes that bulge so much that their eyelids cannot close, which allows the surface of the eyes to dry out.
Dry eye may also result from an immune system reaction, an injury or a drug side effect. Dryness can be a serious problem because dry eyes are easily irritated and may develop conjunctivitis or corneal ulcers. Artificial tears, good eye hygiene, anti-inflammatory drugs and/or cyclosporine ointment (Optimmune) may help. If the cause is known, the veterinarian treats that as well.
Corneal Ulcer. A slow-healing sore on or in the cornea, accompanied by inflammation. Most ulcers are caused by injuries, and treatment often involves antibiotics. According to Samuel J. Vainisi, DVM, Diplomate, ACVO, of the Animal Eye Clinic in Denmark, Wisc., small dog breeds with very short noses and big eyeballs are more prone to eye injuries. “Because of that, we see a lot of ulcers on the eyes” of breeds such as the Boston Terrier, the Pekinese, and the Shih Tzu.