by Kellie B. Gormly
When we turn out the lights and go to bed at night, the glow from the moonlight or bedside clock lets us make out dim images, like the outline of our dogs.
But can your dog see you better than can you see him in the dark? Or can he not see you much at all when it’s dark?
Many dog owners ask this question, wondering how their furry buddy’s eyes work. Dr. Eric J. Miller, assistant professor of clinical comparative ophthalmology at Ohio State University’s Veterinary Medical Center, can explain a lot of the mechanics. But fundamentally, he says, a dog’s vision will always retain an element of mystery. After all, we are not dogs, and they can’t describe things to us.
“We have to be careful when assuming what animals actually ‘see’ because we know not what their brain interprets from the information it receives,” Miller says. “We understand fairly well what their eyes are capable of, and it’s likely that their brains interpret something similar to ours, but we really don't know that.”
This is what veterinarians do know: Anatomically and functionally, a dog’s eye is very similar to a human eye and can see in the dark similar to how we can. Your dog’s eye has a cornea, pupil, lens, retina, and rods and cones. Because of the eyes’ position on the front of the head—a sign of a predator rather than a prey animal, which has eyes farther apart—dogs have limited peripheral vision like humans do, and good depth perception, Miller says.
Chances are, he says, dogs rely on other senses—particularly smell—to perceive their environment better than we do, in both the dark and light, Miller says.
Like with human eyes, light enters through the cornea and then the pupil, which expands and contracts to control the amount of light entering, he says. Light then passes through the lens and hits the retina, where light is processed.
Miller says the key difference between dog and human eyes, and night-vision capabilities, is found in the retina, which is composed of rod cells and cone cells that interpret light. Rods deal with low-light vision while cones process bright light and color vision. Dogs have better vision in the dark because their retinas are rod-dominant, while ours are cone-dominant, Miller says.
In addition to lots of dim-lighting rods, dogs have a reflective tissue beneath their retina called the tapetum lucidum. This tissue helps them to use less light more efficiently than we do, he says.
“So basically, they don't see in pitch black either, but can see much better in low lighting or dim light than we can because of those differences,” Miller says.
However, since dogs have more rods and fewer cones in their retinas, they have limited color vision, Miller says. Human eyes are trichromatic, meaning they have three different types of cones that absorb different wavelengths of light. That lets most humans see colors from the red to violet spectrum. Dogs, conversely, are dichromatic, with two types of cones. Dogs then probably see blue and violet colors, but in-between colors—like green, yellow, and red—might blend together and appear to be the same color, Miller says.
“So they do have color vision and may be like some people who are color blind and basically lack the ability to differentiate some colors such as green and red,” Miller explains.
A 2013 study, published in the Proceedings of the Royal Society B, suggests that dogs might have better color vision than we think.
According to the study, Russian researchers printed out four pieces of paper, in shades of dark and light blue, and dark and light yellow. Researchers paired the shades with a piece of raw meat in a feedbox, but only one box was unlocked. The dogs learned to associate a color with the meat; then, researchers switched colors. If the first color had been dark yellow, now the meat color would be dark blue or light yellow. Then, it was assumed, if the dog went after the dark blue paper, he had memorized the brightness; if he went to the light yellow, the dog had memorized the color associated with the meat.
After 10 tests, the eight test dogs went for the color-based choice 70 percent of the time, and six of the dogs went for it 90 to 100 percent of the time, according to the study published in the British journal.