How Dogs See...
Dogs
see the world from a different visual perspective than we do. 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 light-sensitive cells called cones and rods. In humans,
cones provide color perception and detail, 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
the way we do.
Most people have tri-chromatic vision (three color
variations). Color blind individuals have di-chromatic (two
color variations) vision.
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 relying 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, or position, of a 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, the amount of binocular vision is affected by the increased
peripheral 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. 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 20/20 vision can
distinguish letters or
objects at a distance of 20 feet. Dogs typically have 20/75 vision,
which means they must be 20 feet from
an object to see it as well as a human who is 75 feet away.
Labradors, commonly used as seeing-eye dogs, have been
bred for better eyesight
and may have something resembling 20/20 vision.
Your dog recognizes you at a distance by the way you move or by
your scent or your voice. Because
of the
number of rods in the retina, dogs see moving objects much better
than they do
objects that are standing still.
As always, for further information regarding your dog's vision
and/or problems with your dog's eyes, contact your local
veterinarian.
This article was adapted from an article by Sarah Probst, Information
Specialist at the UIUC College of Veterinary Medicine.
|
