How Cats' Eyes Work - Nocturnal Adaptations

A cat's daytime vision is somewhat inferior to that of humans, but their nocturnal vision is far superior. The special feline features of cat's eyes have most likely developed for survival reasons, as wild cats are nocturnal and do most of their hunting at night. Thus, the special traits that make a wild cat's eyes work in a way best suited to their nocturnal lifestyle are also present in the domestic cat breeds that make up many of today's most popular pets.

Some primary differences between cat eyes and human eyes are that a cat's pupils are elliptical rather than round and they have more rods than cones in their retinas. The muscles surrounding cat's irises are also constructed differently from those in human eyes, in such a way that their eyes can narrow to a vertical slit in bright light and fully open in dim light. Due to such construction, cats can see clearly with only 1/6^th of the illumination needed by humans. Also, If normal human vision is 20/20, then normal cat vision is more like 20/100, meaning that their vision of objects far away from them is much clearer, whereas their perception of nearby objects is much more blurry. Cats cannot see directly beneath their noses.

Another interesting feature present in cat's eyes is called the tapetum. This is a special reflective layer behind the retina, which reflects incoming light and then bounces it back off the cones, thus making more use of the light. The word tapetum literally translates to 'bright carpet'. Tapetums are present in the eyes of other animals, too, including dogs and sea creatures, but not in the eyes of humans. This is the feature that sometimes makes animal eyes appear to be glowing in the dark; it's because the available light is reflecting off the tapetum, in order to make more use of that light. Cats also have a third, inner eyelid called a nictating membrane. This feature does not play a significant role in their vision itself, but does serve to protect the cat's eyes from dryness or other damage.

It is a misconception that cats are color blind. Although their ability to perceive various colors is not as enhanced as that of humans, they do possess this ability. It is also a misconception that all white, blue-eyed cats are deaf. There is a higher incidence of deafness in white, blue-eyed cats, due to genetic predisposition, but this does not apply to all of them. Also, white cats having one blue and one other-colored eye are called "odd-eyed" and may be deaf on the same side as the blue eye. For the most part, though, cats' eyes of any color have adapted to equip themselves for average daytime vision, but exceptional nighttime vision. I wonder how this might play into the old wives' tale of cats stealing babys' breath in their cribs at night, but I suppose that is an article for another time...

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http://cats.about.com/cs/eyesvision/a/cats_eyes_2.htm

http://chetday.com/catseyes.htm

http://en.wikipedia.org/wiki/Cat_senses#Sight