| |
Through the eyes of your dog...
Understanding what your dogs
'sees' could mean the difference between a clear round and a dropped jump
bar or missed contact. If you want to better understand canine vision,
therefore, first you need to recognise that dogs see like a colour-blind
human. This does not mean that dogs can't see green or red objects! It only
means that they can't distinguish green, yellow or red objects based on
their colour so you had better work on your obstacle discrimination as well
as contacts.
 Dogs
are red-green colour blind. They see a brighter and less detailed world when compared to
humans. Peripheral vision is better than humans (dogs see more of the world), but distance is
not judged quite as well. Dogs excel at night vision and the detection of moving objects.
These
differences in visual ability make sense in light of evolutionary theory. Good depth
perception and visual acuity are necessary for a primate (from which humans evolved) jumping
from tree limb to tree limb. Good colour vision enabled this primate to choose the ripest and
most nutritious fruit. The canine, on the other hand, is well adapted as a nocturnal hunter
of camouflaged prey.
Particulars
- Colour
Dogs see something like a human deuteranope, that
is, they are red-green color blind (occurs in 4% of male humans). Simply put, this is due
to having only two cone types rather than three (light sensitive cells include
cones and rods).
- Detail or Acuity
Since dogs have no fovea (or area with 100% cones),
their estimated eye for detail is (roughly) six times poorer than in an average human.
- Night Vision
Dogs have much better night vision for two reasons:
- The have more rods (which enable night
vision).
- They have a structure called the Tapetum
Lucidum. This is a reflective surface behind the retina (area including the light sensitive
cells) that reflects light back through it (gives the eerie shine at night).
-
- Sensitivity to Movement
Dogs are better able to detect movement.
- Depth & Field
Due to the placement of the eyes, humans have an overlap of the field of
each eye of 140. In dogs, it is about 100. This results in the dog having limited
ability to accommodate (focus on items at different distances), but a wider overall field
allowing them to see more of the world.
Created 10/17/98. Text and images
copyright © 1998 by Mark Plonsky, Ph.D.
http://www.uwsp.edu/psych/dog/DVision.htm
Other Relevant links include:
Through the Eyes of
Your Canine
Owners who want to better understand their canine
companions must recognise that dogs see the world from a different visual perspective. The
differences begin with the structure of the eye. 'We have a good idea what canines see
because we know the make-up of the retina of a dog's eye,' says Dr. Ralph Hamor, a
veterinarian and specialist in ophthalmology at the University of Illinois College of
Veterinary Medicine Teaching Hospital.
The retina, which covers the back of the inside of
the eyeball, contains cones and rods-two types of light-sensitive cells. Cones provide
color perception and detailed sight, while rods detect motion and vision in dim light.
Dogs, which have rod-dominated retinas, see better in the dark than humans do and have
motion-oriented vision. However, because they have only about one-tenth the concentration
of cones that humans have, dogs do not see colors as humans do.
'I generally explain that dogs see like a color-blind
human,' says Dr. Hamor. 'Many people think that a person who is red/green color blind
cannot see any color, but there are variations of being color blind. Most people have
vision that is trichromatic (three color variations). People who are red/green color blind
are dichromatic (two color variations).
Dogs can pick out two colors-blue-violet and
yellow-and they can differentiate among shades of gray.' Dogs are unable to distinguish
among green, yellow, orange, and red. They also have difficulty differentiating greens and
grays.
Dogs use other cues (such as smell, texture,
brightness, and position) rather than rely on color. Seeing-eye dogs, for example, may not
distinguish whether a stoplight is green or red; they look at the brightness and position
of the light. This and the flow and noise of traffic will tell the dog that it is the right
time to cross the street.
The set of dog's eyes determines the amount of field
of view and depth perception. Prey species tend to have eyes set on the sides of their head
because the increased field of view allows them to see approaching predators. Predator
species, like humans and dogs, have eyes set closer together. 'Human eyes are set straight
forward while dog eyes, depending on the breed, are usually set at a 20 degree angle. This
angle increases the field of view and therefore the peripheral vision of the dog.'
However, this increased peripheral vision compromises
the amount of binocular vision. Where the field of view of each eye overlaps, we have
binocular vision, which gives us depth perception. The wider-set eyes of dogs have less
overlap and less binocular vision.
Dogs' depth perception is best when they look
straight ahead, but is blocked by their noses at certain angles. 'Predators need binocular
vision as a survival tool,' Dr. Hamor says. Binocular vision aids in jumping, leaping,
catching, and many other activities fundamental to predators.
In addition to having less binocular vision than
humans, dogs also have less visual acuity. Humans with perfect eyesight are said to have
20/20 vision-we can distinguish letters or objects at a distance of 20 feet. Dogs typically
have 20/75 vision-they must be 20 feet from an object to see it as well as a human standing
75 feet away. Certain breeds have better acuity. Labradors, commonly used as seeing-eye
dogs, have been bred for better eyesight and may have closer to 20/20 vision.
Don't expect your dog to recognize you across the
field by sight. He'll recognize you when you do some sort of motion particular to yourself
or by smell or hearing. Because of the number of rods in the retina, dogs see moving
objects much better than they do stationary objects. Motion sensitivity has been noted as
the critical aspect of canine vision. 'So much of dog behaviour deals with posture and
appropriateness. Small changes in your body posture mean a lot to your dog,' Dr. Hamor
adds. Dog owners need to modify training based on this fact. If you want your dog to
perform an action based on a silent cue from you, Dr. Hamor suggests using a wide sweeping
motion to cue your dog.
When dogs go blind, owners often wonder if the dogs'
quality of life has diminished to the point where they are no longer happy. 'We know that
humans deal well with being blind, and humans are much more dependent on their eyes than
are dogs,' Dr. Hamor says. 'Blind dogs lead happy lives if they are comfortable.' The owner
may need to make some adjustments in the pet's environment, such as having a fenced yard,
taking leashed walks, and not leaving unusual objects in normal pathways. 'When blind dogs
are in their normal environment, most people don't know they are blind.' When clients visit
Dr. Hamor asking about quality of life for their newly blind dog, Dr. Hamor suggests that
they take a month to see if they and their dog are happy. In the majority of cases, the
owners never come back.
For further information on dog
vision and problems with your dog's eyes, contact your local veterinarian.
Source: Pet Column
for the week of June 15, 1998
CEPS/Veterinary Extension
2938 Vet. Med. Basic Sciences Bldg.
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Revised July 10, 2001 by Matt Drake
Your comments about the site are welcome, but we cannot dispense medical advice via the
Internet. |
From Mona Gitter
I found this useful since my dog has trouble seeing yellow tunnels in bright
sunlight.
Estimates of Visual Acuity
The Snellen fraction is a common method of describing
visual acuity in humans, with the normal person having a visual acuity of 20/20. This ratio
means that the test subject can discern the details of an image (letters on a chart) from 20
feet away that a normal person could differentiate from 20 feet away. When this scheme is
applied to animals, the visual acuity of the typical dog is about 20/75, and the average cat is
between 20/100 and 20/200.1, 17, 22 This means that from 20 feet away, normal dogs could
distinguish the details of an object that a person with normal vision could differentiate from
75 feet away.
The most common methods of evaluating vision in animals
(a menace response or following a cotton ball) only crudely estimate visual acuity because they
test the motion sensitivity of virtually the entire retina. Positive responses with these
techniques may still be present even if visual acuity is less than 20/800 and a person with
such vision would be legally blind. It also must be remembered that visually distinguishing the
details of an object is less important for a dog or cat's lifestyle than it is for people, and
that improved vision in dim light allows the exploitation of ecological niches inaccessible to
us.
Color Vision
Recent studies suggest that dogs, and to a lesser extent
cats, possess and use color vision, although they have many fewer color sensitive cone
photoreceptors than do humans.23-25 Dogs appear to be similar to humans who lack green cones
and are “red-green color-blind”, whereas cats have a limited, but detectable capacity for color
vision if the stimuli are large and differ greatly in spectral content (color).
Dogs have 2 main types of cone photoreceptors, one which
is maximally sensitive to violet wavelengths (429 to 435 nm), and the other which is maximally
sensitive yellow-green light (about 555 nm).23-25 Although it is not known whether dogs
perceive these 2 colors in the same way as people do, the canine visible spectrum may be
divided into 2 hues: one in the violet to blue-violet range (430 to 475 nm), which is probably
seen as blue by dogs, and a second in the range seen by people as greenish-yellow, yellow, and
red (500 to 620 nm wavelengths), which is probably seen as yellow by dogs.23 Dogs also appear
to have a narrow band in the blue-green range (475-485 nm) that is without color and seen
simply as shades of white or gray (a spectral neutral point).23 Wavelengths at the two ends of
the spectrum (blue at one end and yellow at the other) probably provide the most saturated
colors.
Intermediate wavelengths are less intensely colored,
appearing as if they were blends with white or gray. Dogs differ from a “red-green color blind”
human, however, in that the fewer numbers of cones provide less color saturation and the canine
spectral neutral point is shifted towards the blue end of the spectrum (480 nm), whereas, in
people the spectral neutral point is in a greener (505 nm) region of the spectrum.
Limitations in color vision are probably of little
consequence to dogs and cats in dim light, however, as insufficient light is available to
stimulate cone photoreceptors.1-3 It may be problematic, however, to teach dogs to distinguish
among red, orange, yellow, and green objects solely on the basis of color. Additionally, a
guide dog would be unable to differentiate among the signals at a stop light on the basis of
color alone. In these cases, other clues such as position, relative brightness, or smell,
taste, and texture, must be used differentiate between similarly colored objects. On the other
hand, dogs have been reported to be able to differentiate perfectly among closely related
shades of gray that are indistinguishable to the human eye.3 This ability would be a greater
aid in visual discrimination in low light levels than would enhanced color vision which
requires bright light.
C.J. Murphy, School of Veterinary Medicine, University of
Wisconsin, Madison, Wisconsin: Unpublished data.
(16/02/02)
From Rose O'down (USA)...
Just a thought... my eldest son has been told by competent testers that his vision
is virtually identical to a dog's. Yes, he can see at night well enough to read a book in the
dark outdoors or indoors. He is considered color blind.
I asked him if he had any difficulty seeing a
red dog walk as opposed to, say,
a blue one.
He replied that while it wasn't red as apparently I visually experience the color, he had no
trouble picking it out from it's background and could run over it as well as if it were
blue
- and he proved it by doing it.
He also noted that he could pick out the
yellow contacts vs. the
red
board just fine, too. It is interesting to note that this person who has dog vision' is
a graphic artist and is employed as such. Though color blind, he managed to pass the Color
Theory course in art school with a B. The course instructor had announced to the class that
anyone who was color blind might as well withdraw from the class the first day, since they
would be unable to pass it - and it was required for graduation on that degree program.
While some dogs may not like red equipment, we can't ignore the fact that
there may be other factors rather than color that contribute to that apparent problem, as
another poster pointed out.
| |
|
