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Dogs & Myopia (nearsightedness)
from:Optical measurements of dogs' eyes have found a surprising incidence of myopia in some breeds. A study of about two hundred dogs by a veterinarian named Christopher J. Murphy and his colleagues found the average canine refractive error to be pretty close to normal (within a quarter of a diopter of perfect, an amount that would not provoke any person to get glasses). Several breeds of sporting dogs, such as Chesapeake Bay retrievers, golden retrievers, Labrador retrievers, cocker spaniels, and springer spaniels, were on average a bit farsighted. But two-thirds of Rottweiler and half of German shepherds and miniature schnauzers in this study were significantly myopic, by more than 1.5 diopters. The myopic Rottweilers were close to 3 diopters nearsighted on average. Generally, people who have more than about 0.75 diopters of nearsightedness will complain of noticeable impairment and find they need to wear glasses or contact lenses to function in everyday life.
The animals in this study population were all pets. Interestingly, when Murphy and his coworkers looked at a second population of German shepherds - animals kenneled at Guide Dogs for the Blind in San Rafael, California - they found that the guide dogs had average normal vision, with fewer than a third showing even as much as 0.5 diopters of nearsightedness. The guide dog program did not specifically test dogs' vision in selecting animals, but they did flunk out any dogs that failed to perform well in training, which suggests that myopia results in a real impairment in getting the job done. The average farsightedness of sporting dog breeds suggests that there has likewise been selection at work in these breeds - that good distance vision has a demonstrable effect on making a good working dog.
The researchers noted a tendency for severe nearsightedness to run in families, which suggests a strongly inherited component. In breeds that are not expected to perform anything more demanding than lying on the carpet, walking on a leash, and finding their supper bowl, there has no doubt been little selection for good vision, which has allowed myopia to sneak into the gene pool.
There are distinct breed differences in peripheral vision and overall field of view as well. Human eyes look straight ahead, giving us just about a 180-degree field of view, but with a lot of overlap between left and right eyes. Animals can see in true 3-D vision only when they use both eyes together, and the overlap in the human visual field thus maximizes the region in which we can perceive depth by using this binocular vision. The eyes of dogs are turned a bit to the side, which allows them to see a bit to the rear, with a wider overall field of vision.
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