Veterinary ophthalmologists are often asked, “How well do animals see?” Visual function involves a combination of many factors, including: the field of view, depth perception (ability to judge distances), acuity (focusing ability), perception of motion, and color differentiation. All of these functions must then be integrated by the brain to produce useful vision. Although we are unable to ask our pets to read an eye chart, through comparative studies, it is possible to make some educated assumptions about their vision.
The position of the eyes within the head determines the degree of peripheral vision as well as the amount of the visual field that is seen simultaneously with both eyes. This binocular vision is necessary for judgment of distances. Dogs have eyes which are placed on the sides of the head, resulting in a visual field of 240 degrees compared with the human field of 200 degrees. The central, binocular field of vision in dogs and cats is approximately half that possessed by humans.
The eyes of dogs and cats have many of the modifications typically seen in animals which evolved as nighttime hunters. The pupil functions much as the aperture for a camera and can dilate for maximal light capturing ability in dogs and cats. In addition, there is a reflective layer under the retina called the tapetum which serves to intensify vision in dim light. The “mirror” effect of the tapetum results in the “eye shine” observed when an animal looks into a car’s headlights. While dim light vision is enhanced by the tapetum, scattering of the reflected light may result in reduced acuity.
Although it is commonly believed that dogs and cats see only in black and white, recent evidence suggests that animals may have some degree of useful color vision. The perception of color is determined by the presence of cone photoreceptors within the retina. These cone cells function in bright light conditions and comprise approximately 20% of the photoreceptors in the central retina of the dog. In humans, the central retina (macula) is 100% cones. Behavioral tests in dogs suggest that they can distinguish red and blue colors but often confuse green and red.
Acuity is the ability to focus so that two objects appear as distinct entities. This is the value that is measured in people using an eye chart. In animals (and in infants) this can be measured using retinoscopy. Visual acuity in normal dogs has been estimated to be 20-40% that of humans. This means that, at 20 feet, a dog can distinguish an object that a human could see at 90 feet. Acuity is a function of the clarity of the structures of the eye (cornea, aqueous humor, lens, and vitreous) as well as the combined refractive powers to focus the image clearly on the retina. One of the most common diseases which can affect the clarity of the eye is formation of a cataract within the lens. Cataracts in dogs are most often inherited and may affect dogs at any age. As the lens becomes progressively opaque, an animal’s vision deteriorates so that only light and dark perception exists. Surprisingly, animals function relatively well in familiar surroundings, even with severe vision impairment. This illustrates the ability of dogs and cats to depend heavily on their other senses, namely smell and hearing.