Nicole Roybal, DVM, DACVO.
Although our pets can’t tell us what they’re seeing, there are actually many clues that can help us piece together how dogs and cats view the world. The most significant difference between our eyes and those of our pets is in the structure of the retina. The retina is a layer of tissue at the back of the eye that is responsible for transforming light entering the eye into electric signals that are then sent to the brain for vision processing. The cells that make up the retina have been carefully examined and results have given us some interesting insight into the lives of the animals that share our world.
The human retina is structured to pick up a rainbow of colors and allows us to appreciate intricate detail. Dog and cat retinas are structured more for detecting fine movement and operating in low light conditions, great adaptations to have while hunting small prey.
Dogs do have some degree of color vision. However, they do not experience the wide range of colors appreciated by humans, other primates and birds. Based on the composition of their retinal cells, it is thought that dogs most likely see ‘colors’ with a palette made up of different shades of blue and yellow and varying shades of gray. Based on the types of cone cells in their retinas, cat color vision may actually be a bit better compared to dogs. Despite this lack of a complete color palette, pets are able to detect subtle difference in shades of gray that would be indistinguishable to their owners. They also rely on their highly developed senses of hearing and smell to navigate and differentiate between objects. In fact, a study has shown that working dogs trained to detect explosive devices are equally effective at finding bombs whether in the light or in complete darkness.
Enhanced night vision is due to special adaptive features of our pets’ eyes. One of these is a structure called the tapetum lucidum. This is a layer of reflective tissue behind the retina that results in the ‘eye shine’ you may have observed in a photograph or in the beam of a flashlight at night. The tapetum causes light to be reflected internally within the eye, allowing more efficient use of small amounts of light.
Another feature that allows for enhanced night vision is the distribution of photoreceptors in the retinas. Photoreceptors are specialized cells that absorb light and convert it to an electrical signal that the brain can read. The two types of photoreceptors are called rods and cones. Dogs and cats have a much higher percentage of rods than humans. Rods are more sensitive to lower light levels. Humans have a greater amount of cones in the center of the retina, which are better for visualizing color and function better in bright light.
Also, neurologic connections in their retinas are arranged so that very little light is required to form an image. However, there is a trade-off. These same features that allow for better night vision result in worsened visual acuity, producing a more ‘grainy’ image with poorer detail compared to humans.
Nicole Roybal, DVM, DACVO
Veterinary Specialty Hospital