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The Visual System in Humans and Canines

A discussion of visual acuity and depth perception. By David Bozak

Among the several kinds of beauty, the eye takes most delight in colors.
- Joseph Addison, 1672-1719

Watching an indoor dog agility competition a few years ago, my wife and I noticed an unusual number of errors being made by dogs on two obstacles. Both the wall jump and weave poles were white with red stripes and were viewed by the competitors with the white walls of the indoor facility directly behind them. It was hard for us to discriminate these obstacles, and we speculated that the dogs were having the same difficulties. Later that year at another competition, we saw dogs having great difficulties negotiating a particular tire jump. In this case it was easy for us to discriminate the tire against the outdoor background, and we wondered about the purple, lime, white and pink striping on the light aqua framed tire. Perhaps some of the errors dogs were making were due to equipment coloring decisions that resulted in more difficult object discrimination. While trained in human sensation and perception mechanisms, I didn't know what the equivalent mechanisms were in dogs.

Vision is such a dominant perceptual mechanism for us that we forget that our view of the world is just one of many possible perceptions. It is no wonder, then, that when I talk about color vision in dogs, people seem surprised. Often I hear, "Oh, I thought dogs see only in black and white." In fact, dogs do see color, just not as we do. There are four aspects of the visual system that differ from humans to canines. Understanding those differences will give you a better sense of how a dog sees its world.

The first two differences relate to overall sensitivity and visual acuity. The retina of the eye is packed with two types of photosensitive cells, rods (sensitive to overall illumination levels) and cones (sensitive to colors). In humans, the cones are packed into the fovea, that central part of the retina that provides us with the ability to resolve great detail as well as color. In dogs, both the distribution and proportion of rods and cones differs. First, dogs are rod-dominant with about one-tenth the relative number of cones as in the human eye. Second, there is a more uniform distribution throughout the retina, not a concentrated area of color receptivity. Combined, this implies that while dogs do see color, overall illumination levels are much more important. And dogs have less visual acuity than do humans. While we have 20-20 vision, dogs typically have 20-75 vision: they must be 20 feet from an object to see what we can discern from 75 feet away. In short, dogs are near-sighted.

A third difference is based on the set of the eyes in dogs. While the eyes of dogs, like humans, are set relatively close, dogs have their eyes set at an angle, affording them a wider field of vision and greater peripheral vision. They see a great deal more of the world than we do. The tradeoff for this is that they have a smaller region of overlap between what their two eyes see, restricting the size and effectiveness of their binocular vision. Thus their depth perception is limited to the visual area straight ahead of them, excluding that portion blocked by their muzzles! You would expect that dogs would more capably perform tasks requiring good depth perception (like jumping and catching) when the task at hand was relatively head on.

The final major difference has to do with the perception of colors. Human color vision is based on three different kinds of cones, each sensitive to a different range of colors. These cones are maximally sensitive to red, green, and blue colors. Dogs possess a two-color system rather than a three-color system. In this way they are like the approximately 8% of male humans who are colorblind. These people perceive colors in a different and in some sense a more limited way because they lack one of the three types of cones in the normal visual system. We know a great deal about the differen


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