The cat's eye has all the structural components as a human's, with the addition of a light-reflecting membrane called the tapetum lucidum, which is located behind the retina. The retina in the cat also contains the two photoreceptor cells found in the human retina – rods and cones. Rods are responsible for seeing in low light levels whereas the cones are responsible for colour sensitivity. The tapetum lucidum improves light sensitivity and contributes to the ability of cats to see well in low light situations. When light enters the eye, the tapetum reflects the light passing through the rods not once, but twice so that there is a double exposure to the light. This is mostly why cats see so well in near darkness, however there still needs to be some light (i.e. from the stars) for them to see. In complete darkness, cats are unable to see as there must be some light available, even if just a little. Another contributing factor to the cat's vision is that they have many rods; in fact, they have six times more rods than humans, which is the reason for their superb vision in environments with low levels of lighting.The pupil in the center of the iris is responsible for regulating the amount of light that enters the eye. In situations of bright light, the pupil involuntarily constricts to reduce the intensity of light entering the eye and in low light, the pupil dilates to maximize the amount of light entering. Humans have round pupils whereas cats have elliptical, or slit-shaped pupils. The slit-shaped pupils of cats are able to open and close faster than human eyes. Furthermore, the cat's pupil can dilate to a significantly larger size than the round pupils of humans, which allows for more light to enter the eye. Therefore, this is another adaptation of the eye that enables cats to have superior vision at night and thus, are known as successful hunters.
Although cats can detect motion and have excellent vision in low light levels, their colour perception is not as well developed. As you may recall, the other photoreceptor cells, the cones, are responsible for colour vision. In this case, the numbers are reversed in that humans have about six times more cone cells than cats. As a result, cats are limited in the colours they can detect. Studies have shown that cats are able to respond to colours in the purple, blue, green and yellow ranges. However colours in the orange, red and brown ranges are unable to be detected and most likely cats see these colours as shades of gray or purple. Since it is more important for cats to detect motion, for example to catch small prey, than it makes sense that colour perception is not as well developed because detecting colours does not really help in hunting.
Binocular vision (using 2 eyes at the same time to see a single fused image)allows for better depth perception and a wider field of view. Like humans, cats have binocular vision, which helps them to judge distances. This is especially important in activities such as hunting prey or jumping up onto fences to avoid danger. This combined with their excellent balance allows cats to jump as high as 6 feet or even higher as some owners will attest to. Cats are also somewhat nearsighted, which implies that their vision is more suited for detecting closer objects, like prey.
So the next time you discover a little present your cat left for you in the night, you can now better appreciate the exceptional hunting ability and the structural differences in your cat's eye that makes it a hunter.
