Owners who want to better understand their canine companions must recognize 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.