India, Jan. 27 -- A number of factors are involved in vision. These include not only the perception of light and motion, but also visual perspective, visual field of view, depth perception, visual acuity and the perception of colour and form.
It may not be possible to accurately describe canine vision because of fundamental differe-nces in vision between the canine and human species, and because the understanding of some of the factors in canine vision is imperfect. Nonetheless it is important to pursue and disseminate an understanding of canine vision as it aids our overall understanding of vision and visual sciences.
Sensitivity to light
The canine visual system is designed to operate well under low light conditions and is capable of functioning under a wide range of lighting condition. This canine adaptation to low light conditions enhance their ability to function as predators, while the human visual system performs best in bright light.
The retina of the dog's eye is composed primarily of rod photoreceptors while the human retina is composed primarily of cone photoreceptors. Rods function better in dim light, while cones are used for colour vision and require bright light. Human and canine both have rods and cones present in their retinas but the relative amount of each is very different.
The tapetum lucidum is a highly reflective layer of cells located behind the photoreceptors in the canine retina. This reflective layer is responsible for the bright shine of a dog's (and other species) eyes when a bright light is shone at them in the dark. This reflective layer functions to improve low light vision by reflecting light back through the retina.
The tapetum lucidum is located in the top half of the dog's retina; the bottom half is composed of the tapetum nigrum, a layer of darkly pigmented cells that are not reflective. This is believed to enhance the view of both the darker ground and brighter sky.
Sensitivity to motion
Dogs could discern an object in motion at 810 to 900 meters, but were only able to discern the same object when stationary at 585 meters or less.
Sensitivity to light
The frequency at which rapidly flickering light fuses or appears to fuse into a constantly illuminated light is termed 'Flicker fusion'; in most humans flicker fusion occurs at around 50 to 60 hz, while for dogs it may be as high as 70 to 80 hz.
Why dogs don't spend much time watching television? The refresh rate on television is about 60 hz, so images are perceived smoothly, but for a dog it may appear as a rapidly flickering image.
Visual perspective
Height of the eyes above the ground has a major effect on the view by an animal or person.
Visual field of view
This has not been well studied probably because of the differen-ces in shape of the skull, placement of eyes in the skull, shape and size of the nose. Dogs are probably more aware of activity occurring around them than we are due to larger field of view.
Depth perception
This may vary between breeds because of anatomical differences in skull shape of human and dogs. It appears the area of visual overlap is approximately 30 to 60 degrees in most dogs, which is much less than in humans with 140 degrees of binocular overlap.
Acuity depends on three factors: (1) optical properties of the eye, (2) retinal detection and processing of the image, and (3) proper interpretation of the images by higher centers in brain.
Optical factors: Some dogs are myopic. One study demonstrated that 53% of a group of German Shepherds and 64% of a group of Rottweilers were myopic. Optical aberrations may occur within the eye creating vision problems such as astigmatism, spherical aberrations of lens and chromatic aberration.
The canine eye has limited accommodative ability compared to human eye. They are able to view image objects within 50 to 33 cm of their eye. Dogs compensate for this by using other senses such as smell and taste.
Retinal factors: The canine optic nerve contains approximately 167, 000 nerve fibers, as compared to the human optic nerve, which contains 1.2 million nerve fibers. The canine eye lacks a fovea, but has a region termed visual streak. It aids in enhancing vision in dim light. The oval shape of the visual streak probably helps improve a dog's ability to scan the horizon, and nasal location of the visual streak probably enhances a dog's ability to use peripheral vision.
Estimates: A normal dog has a visual acuity of approximately 20/75. This means that a dog must be 20 feet away from an object to clearly visualise its details that a human with normal vision could clearly visualise from a distance of 75 feet. Their life style does not require visual distinction of fine details.
Color vision
Two distinct types of cones appear to be present in the canine retina. One type is maximally sensitive to light in the wavelength that appears violet to people and the other type is maximally sensitive to light in the wavelength that appears yellow-green to people. Dogs are unable to differentiate colours that appear as green, yellow-green, orange or red, and are unable to differentiate greenish-blue from grey. But dogs are better able to differentiate between subtle shades of grey than people.
The canine visual system may be considered inferior to human visual system in such aspects as degree of binocular overlap, colour perception, accommodative range and visual acuity, but the canine visual system is superior to the human one in other aspects such as functional ability in low light conditions, flicker fusion, field of view, ability to differentiate shades of grey and possibly ability to detect motion.
By understanding the stren-gths and weakness of the canine visual system we will be able to understand our hunting companion's capabilities.
Published by HT Syndication with permission from Star of Mysore. For more information on news feed please contact Sarabjit Jagirdar at htsyndication@hindustantimes.com
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