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How Do Dogs
See
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Studies on canine
vision were conducted in the 1980s by Jay Neitz in the University of
California, Santa Barbara which dispelled the myth that dogs can only
see in black and white. For many test trials, dogs were shown
three light panels in a row--two of the panels were the same colour,
while the third was different. The dogs' task was to find the one that
was different and to press that panel. If the dog was correct, he was
rewarded with a treat that the computer delivered to the cup below that
panel. This research proved that dogs do in fact see in colour, but
their vision is less detailed than what we humans have.
The eyes in both canines and humans have special cells that catch the light. These cells are called photoreceptor cells and consist of cones and rods and are found in the central retina. Dogs are said to have dichromatic vision -- they can see only part of the range of colours in the visual spectrum of light wavelengths. Humans have trichomatic vision, meaning that they can see the whole spectrum.
Cones are responsible for helping the brain detect colours. There are several different types of cones, each tuned to different wavelengths of light. Human beings have three different kinds of cones and the combined activity of these gives humans a complete range of colour vision whereas dogs only have two types. By disecting the eyeball of a dog, researchers found that only 10% of the dogs photoprecepters are cones. The photoreceptors in our eyes are 100% cones. When a person is colourblind it means that they are missing one of the three kinds of cones. With only two cones, a person can still see colours, but fewer than a person with normal colour vision. This is the situation with dogs who also have only two kinds of cones.
Rods can only detect black and white, but they process movement extremely well and help to see in the dark.
The eyes in both canines and humans have special cells that catch the light. These cells are called photoreceptor cells and consist of cones and rods and are found in the central retina. Dogs are said to have dichromatic vision -- they can see only part of the range of colours in the visual spectrum of light wavelengths. Humans have trichomatic vision, meaning that they can see the whole spectrum.
Cones are responsible for helping the brain detect colours. There are several different types of cones, each tuned to different wavelengths of light. Human beings have three different kinds of cones and the combined activity of these gives humans a complete range of colour vision whereas dogs only have two types. By disecting the eyeball of a dog, researchers found that only 10% of the dogs photoprecepters are cones. The photoreceptors in our eyes are 100% cones. When a person is colourblind it means that they are missing one of the three kinds of cones. With only two cones, a person can still see colours, but fewer than a person with normal colour vision. This is the situation with dogs who also have only two kinds of cones.
Rods can only detect black and white, but they process movement extremely well and help to see in the dark.
| What
Humans See |
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| What
Dogs see |
This means that
we can process all the colours and see them more vividly whereas a dog
can see extremely well in low light conditions and see different shades
of grey to a higher degree than we can. They can also differinciate the
tiniest movements we make.
Dogs also have a highly reflective layer of cells called the tapetum lucidum a structure that is not present in human eye. It is located behind the photoreceptors in the top half of the dog's retina (termed the tapetal area of the retina); the bottom half of the dog's retina is composed of the tapetum nigrum, a layer of darkly pigmented cells that is not reflective. In most instances, the top half of the retina receives light from the darker ground, and the bottom half of the retina receives light form the brighter sky. This is felt to enhance the view of both the darker ground and brighter sky. This mirror-like structure in the back of the eye reflects light, giving the retina a second chance to register light that has entered the eye. This reflective layer is the glow that we see when we shine a light on a dogs eyes in the dark. It is believed that a dog can see in 4 times lower light levels than we can because of this reflective layer.
Dogs also have a highly reflective layer of cells called the tapetum lucidum a structure that is not present in human eye. It is located behind the photoreceptors in the top half of the dog's retina (termed the tapetal area of the retina); the bottom half of the dog's retina is composed of the tapetum nigrum, a layer of darkly pigmented cells that is not reflective. In most instances, the top half of the retina receives light from the darker ground, and the bottom half of the retina receives light form the brighter sky. This is felt to enhance the view of both the darker ground and brighter sky. This mirror-like structure in the back of the eye reflects light, giving the retina a second chance to register light that has entered the eye. This reflective layer is the glow that we see when we shine a light on a dogs eyes in the dark. It is believed that a dog can see in 4 times lower light levels than we can because of this reflective layer.
| “Although
the tapetum improves vision in dim light, it also scatters some light,
degrading the dog’s vision from the 20:20 that you and I normally see
to about 20:80,” Paul E Miller DVM. (Journal of the Veterinary Medical Association (JAVMA, vol. 207, no. 12, pp. 1623-1634, Dec. 15, 1995). |
A dogs field of
vision varies from breed to breed because of the differences in the
shape of the skull, placement of the eyes in the skull, and the shape
and size of the nose. It is this that 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. In the average dog, the eyes are placed such
that they move approximately 20 degrees lateral to the midline ie. dogs
have eyes which are placed on the sides of the head. In humans, the
eyes do not move, but rather look straight ahead. When both eyes are
considered together, two studies have indicated that the total field of
view in dogs is approximately 240 to 250 degrees, which is 60 to 70
degrees greater than the normal human's field of view (180
degrees). Even if you are standing diagonally behind your dog,
chances are he can still see you.
Dogs do not have the
ability to focus as well on the shape of objects (their visual acuity
is lower) . An object a human can see clearly may appear to be blurred
to a dog looking at it from the same distance. An early study indicated
that dogs could discriminate an object in motion at 810 to 900 meters,
but were only able to discriminate the same object when stationary at
585 meters or less.
| 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. http://www.veterinaryvision.com/See.htm |
| Visual
acuity is the ability to see the details of an object separately and
clearly. Visual acuity depends on three factors: a) optical properties
of the eye, b)retinal detection and processing of the image, and c)
proper interpretation of the images by higher centers in the brain.
Postretinal processing has not been extensively studied in dogs, and
was beyond the scope of the article being reviewed. Optical factors in visual acuity: The optical media of the eye consists of the cornea, aqueous humor, lens, and vitreous humor. These structures are responsible for creating a properly focused image on the retina (emmetropia). If the image is focused in front of the retina, myopia, or nearsightedness, results, and if the image is focused behind the retina, hyperopia, or farsightedness, results. Some dogs are myopic, or nearsighted. One study demonstrated that 53% of a group of German Shepherds were myopic, and another study indicated that 64% of a group of Rottweilers were myopic. However, another study of a group of guide dog German Shepherds found only 15% of this group of dogs were myopic, indicating that selection for normal vision may have occurred. Myopic dogs may have been removed from the program due to poorer performance, without an understanding of why they were performing poorly. This finding may have important implications for retriever breeds, indicating that poor marking skills may possibly be related to vision disturbances, and use of animals with poor marking skills in breeding programs may possibly propagate those vision problems. Other optical aberrations may occur within the eye, creating vision problems, such as astigmatism, spherical aberrations of the lens, and chromatic aberrations. The clinical importance of these conditions is unclear in dogs - they are either an uncommon finding or the canine eye may be able to accommodate for these problems. In addition, the normal eye is able to accommodate, or change focus, which allows normal vision of objects at different distances. The canine eye has limited accommodative ability compared to the human eye. They appear to be only able to accurately image objects within 50 to 33 cm of their eye, where as human children are able to accurately image objects as close as 7 cm. Dogs compensate for this limited accommodative ability by using other senses, such as smell or taste, to augment vision of very close objects. As humans age, it is normal to lose some accommodating ability (age-related presbyopia), and it is thought that dogs also undergo similar age related changes in accommodation, but the incidence or significance has not be studied in dogs. Retinal factors in visual acuity: It is felt that the retina is the principle limiting factor of visual acuity in dogs. To improve vision in dim light, a greater number of photoreceptors converge on a single ganglion cell (a nerve cell that gathers input from receptor cells and then transmits the information to higher nervous centers in the brain). The more photoreceptors converging on a single ganglion cell, the less detail is present in the image produced, just as high speed photographic film, designed for low light situations, produces a grainier image than lower speed (brighter light) photographic film. The more ganglion cells present, the more nerve fibers present in the optic nerve that relays visual information to the brain. The canine optic nerve contains approximately 167,000 nerve fibers, compared with the human optic nerve which contains 1.2 million nerve fibers. In the human eye, there is a circular area of the retina that contains the densest concentration of photoreceptors and ganglion cells, called the fovea. It is centrally located, and is the area of the retina that produces the sharpest image. The canine eye lacks a fovea, but rather has a region termed the visual streak. The visual streak is an oval area of the retina located just above the optic nerve, and is positioned with its long axis on a horizontal plane, and is centered on the area of the retina closer to the nose. Like the fovea, it contains the highest concentration of photoreceptors and ganglion cells, and is the area that produces the greatest visual acuity. It is located in the tapetal area of the retina, which again 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 the nasal location of the visual streak probably enhances a dog's ability to use peripheral vision. Individual variations in the density of ganglion cells within the visual streak have been noted. Wolves have a higher density of ganglion cells than dogs (12,000 to 14,000 ganglion cells/mm2 in wolves compared to 6,400 to 14,400 ganglion cells/mm2 in dogs) which suggests that wolves have better visual acuity than dogs. Differences are found between dogs, also, and this may be another area where selection for visual ability may occur. One study indicated that one strain of Beagles had a pronounced visual streak, whereas another strain had only a moderately pronounced streak. It is possible, although difficult, to differentiate between pronounced and moderate visual streaks with ophthalmoscopic evaluation, and this may potentially be a tool that could be used in selection of breeding stock. Careful evaluation of the retina by a competent ophthalmologist, using sophisticated equipment, is required to make this differentiation. Studies to validate the effectiveness of this technique are required before this technique can be recommended in breeding stock selection. Estimates of visual acuity: The most commonly used indicator of visual acuity is the Snellen fraction, which relates the ability to distinguish objects or letters at a fixed distance with the standard response. Snellen fractions of 20/20, 20/40, 20/60 or 20/100 indicates that the test subject needs to be 20 feet away from an image to clearly visualize the details that a normal subject could discern at 20, 40, 60, and 100 feet away, respectively. A variety of studies have been done, using a number of different methodologies, to estimate the visual acuity of dogs. We can assume from these studies that the 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 visualize details of that object that a human with normal vision could clearly visualize from a distance of 75 feet. Again, this less acute vision in dogs is a trade-off for improved vision in dim light, and their life style does not require visual distinction of fine details as does our life style. "Vision in dogs", was written by Paul E. Miller, DVM and Christopher J. Murphy, DVM, and published in the Journal of the Veterinary Medical Association (JAVMA, vol. 207, no. 12, pp. 1623-1634, Dec. 15, 1995). http://psychlops.psy.uconn.edu/eric/class/dogvision.html |
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