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The Visual System in Humans and Canines

A detailed follow-up to Thursday's canine vision article, including a discussion of visual accuity 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 different forms of colorblindness in humans, including just what colors can be seen, but we do not have a good understanding of just what those colors are for dogs. The
published literature does not include a detailed spectral sensitivity chart. The description of color vision in dogs is further complicated by the structure of the dog's eye to improve low light vision.

The tapetum lucidum is a highly reflective layer of cells in the top half of the dog's retina that reflects light back through the retina, giving the rods and cones a second chance to detect the light. This layer, which is responsible for the shine of the dog's eye in the dark in response to light, may shift the color of the light to be more aptly noticed by the rods. This would effectively enhance contrast, but at the cost of "true" color detection. In addition, the tapetum varies in color, which would further alter the color of the light perceived.

So what is our best guess as to what colors they see? Based on research with mixed breeds, Beagles, Cocker Spaniels, Italian Greyhounds, a Toy Poodle and other canid species, we can say that dogs can see shades of yellow, grey, and blue. All of the colors from red through yellow down to yellowish-green appear the same shade of yellow to a dog. Greenish-blue is the neutralpoint, the color value which can be distinguished from other colors but easily confused by the
dog with white or grey. Blue through violet are perceived as different shades of blue.

What dog toys would be easiest for the animal to see? Colors that come from different sides of the neutral point would be most easiest to distinguish, a blue and yellow ball, for example. We have a teal-colored flying disc that my Border Collie loves, and she can find it in the grass. What about a red flying disc? According to the above analysis, it should blend right in with the grass.

Brightness is still a major visual cue for a dog and may serve to make the disc stand out from the grass background. The dog's visual system is very sensitive to movement (a rod dependent skill), so they can follow the movement of the toy even though they may not be able to discern it due to
poor acuity or color confusion. Other cues, like the dog's own scent on a toy, will make it "visible" in other ways. The visual system of the dog has evolved to suit their needs. While
different from the human visual system, it is every bit as sophisticated. But the world they see is very different from the one we see!

So what is the verdict regarding the dog agility errors? In the case of the wall jump, there was very little to visually discriminate between the obstacles and the wall, especially given the distances and angle of viewing of dogs running the course. With respect to the tire, the striping would not be particularly "colorful" for the dog, and set against the green outdoor background, it would be difficult for the animal to see the jump. Knowing a little more about how dogs view the
world can help us create obstacles as colorful and visible for dogs as for us!

Resources:

Canine Vision by Mark Plonsky
http://www4.uwsp.edu/psych/dog/la/drp4.htm
[originally http://www.uwsp.edu/psych/dog/dvision.htm]
(this site contains matching color spectrums, for dog and human color vision)

Color Vision and Color Deficiency
http://www.firelily.com/opinions/color.html
(a site that addresses color vision and deficiencies in some technical detail, but at a level
accessible to non-scientists)

Color Vision in the Dog
Jay Neitz, Timothy Geist, and Gerald H. Jacobs, Visual Neuroscience, 1989, Vol 3, pp.
119-125.

Experiments in Colour Discrimination in Dogs
A. Rosengren, Acta Zoologica Fennica, 1969, Vol 121, pp. 1-19.

Vision in Dogs
Paul E. Miller and Christopher J. Murphy, Journal of the American Veterinary Medical
Association, Vol 207, No 12, December 15, 1995, pp. 1623-1634

This article first appeared in In Good Health, December, 2001, Issue 24, p. 15 and was reprinted in Clean Run, May, 2002, 7(5), p. 12. Posted with permission.

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