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Vision Medical terms: visual acuity, Snellen test, Snellen eye test
| What Can Pets See Better Than People Can? |
Objects in twilight |
Cat vision: Cats can see with about 85% less light than humans need for vision.
Dog vision: Dogs also need significantly less light than humans need for vision. |
Moving objects |
Pets see movement so well that they may see flickering images on television rather than the continuous image that humans see. Television images appear at the rate of 60 images/second. |
Shades of gray |
Pets see many more shades of gray than humans can distinguish. |
Field of view |
Pets’ eyes are set wide on their heads so their field of view is 60-70° larger than the human field. Humans see 180°, and pets closer to 240°. |
| What Can’t Pets See As Well As People Can? |
Colors |
Humans are trichromatic and see 3 colors: red, blue and yellow.
Trichromatic vision allows humans to see all visible wavelengths and thousands of color variations.
Are dogs color blind? Pets see yellows and blues, but not reds. They are dichromatic. |
Objects that are close |
Healthy young humans can see objects 3 inches from their noses.
This ability decreases with age as the lens becomes less flexible.
Pets can’t see objects any closer than 1 or 2 feet from their noses.
Pets use their noses and whiskers to identify objects that are close. |
Binocular vision for depth perception |
Humans see 140° of binocular overlap—great depth perception.
Pets see about 40° of binocular overlap—very limited depth perception. |
Visual acuity
(sharpness or detail) |
Humans have about 1.2 million optic nerve fibers.
Dogs and cats have about 150,000 optic nerve fibers—quite a bit fewer than humans.
Humans average one photoreceptor cell to each nerve cell, which gives excellent acuity. Pets have multiple photoreceptor cells feeding into a single nerve cell, so their ability to see detail (acuity) is not equal to that of humans. Pets, however, have better dim-light vision.
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Your pet’s size affects its vision
- A pet with eyes 8 inches from the ground does not see what humans see. It sees grass and human feet.
- A tall dog has eyes 34 inches from the ground, but a human’s eyes are 66 inches from the ground, giving humans greater visibility.
How does color vision affect training?
It’s difficult for dogs to distinguish orange, yellow, or red objects based on color.
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Nearsightedness is genetic in some dogs
About 50% of German shepherds and 65% of Rottweilers are myopic or nearsighted. Among the German shepherds selectively bred to be guide dogs, the number with myopia is about 15%. Thus, it appears that poor vision is partly genetic. Some dogs are also genetically farsighted.
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Do wolves have better vision than ordinary dogs?
Yes. Wolves see better than most dogs because their optic nerve cells are more densely packed.
How is visual acuity measured?
The Snellen test measures visual acuity. Usually the subject is 20 feet from the target.
- 20/20 vision sees what an average person sees standing 20 feet from the target.
- 20/100 sees what an average person sees standing 100 feet from the target—very poor vision.
- 20/400 legally blind.
What is a dog’s visual acuity?
We’re not sure, but tests suggest that dogs’ visual acuity may be 20/60 – 20/80.
Can pets distinguish horizontal and vertical lines?
Definitely. A Nobel prize was won by two scientists who determined that kittens develop the ability to see horizontal and vertical lines during a period of a few weeks. If not exposed to lines during this time—for example, if the kittens are blindfolded—the kittens never develop the ability to see lines.
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Dogs are also able to distinguish horizontal and vertical lines. It is more difficult for dogs to distinguish upright and inverted triangles, than to distinguish horizontal and vertical lines, but once learned, dogs recognize the different triangles easily—regardless of whether the triangles are outlines or filled in.
Nutrition to nourish the eye
Kittens that don’t get the amino acid arginine develop cataracts. Cats without sufficient taurine daily (110 mg/kg) may become blind and develop heart problems. Deeply colored orange, red, and green vegetables provide the antioxidants and nutrients that sensitive eye tissues need to stay healthy. We recommend that pets receive a small amount of fresh or cooked vegetables that have been pureed or put into a blender to make them easily absorbed: sweet potatoes, carrots, spinach, broccoli.
For supplements to nourish the eyes we recommend Missing Link, 1-800-PetMeds Super VitaChews for Dogs, and 1-800-PetMeds VitaChews for Cats.
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The articles here were answered by a variety of pharmacists and veterinarians
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