Friday, November 23, 2012

Eye see.

Cats’ eyes are beautiful: big round and deep with fascinating pupils that go from round to slit. I always wondered how cats see the world and did some reading up on it. What follows is just a brief summary of what I learned, but it gives a fair idea of how cats’ eyes work.

Most people know that cats are nocturnal hunters. Cats don’t just rely on their eyes to hunt. They have touch-sensitive whiskers, their smell is 14 times better than a human’s and they can hear sounds 1.6 octaves higher than a human. All this helps them to hunt at night. They are, however, active during the day as well. That’s why their eyes have evolved in such a way that they can see very well in the dark and fairly well in daylight.

A cat’s field of view is about 200 degrees, which is only a bit larger than that of a human (180 degrees). Both the pupil and the lens are bigger than the pupil and lens in the human eye, letting in more light. Their pupils contract from round to slit, which is a faster process than contracting from big round to small round as with humans. There is a trade-off for these advantages.  When the pupil is in slit mode, (in bright light) a cat can’t focus as well as when it is round (dim light) and although the cat’s lens is much bigger than a human’s lens, it can’t change size like a human’s lens. Therefore a cat’s depth of field is not as good as ours. We can focus on objects far away and close by. A cat can’t focus on objects close by. You can compare it to the healthy eyes of an older human. Human eyes loose their flexibility with age and you end up needing reading glasses for objects close by.

The lining of the back of the eye is called the retina. It is where the image focuses and where the rods and cones are situated. The rods are highly light-sensitive but not sensitive to colour. They help us see in dim light conditions. The cones are the ones that are colour-sensitive and also deal with detail detection in daylight conditions.
The human eye has an area on the retina called the fovea. This area is very high in cones, and has virtually no rods. That’s why humans have very good colour vision and can also detect a lot of finer detail in good light (try seeing how much detail and colour you can detect in a dark room with only some moonlight coming in through the window).
The retina in a cat’s eye contains more rods than cones and doesn’t have an area with only cones. That’s why cats can see very well in darker conditions, six times better than humans, but don’t see as much detail and colour in daylight conditions. As colour is the reflection of light off surfaces, you can’t really see colour in the dark anyway. Therefore, A nocturnal hunter like a cat doesn’t really need the ability to see colour that well. Instead of a fovea, cats have an area behind the retina called the tapetum. This area contains reflective cells that reflect the light back to the rods and cones, so the slightest bit of light in the dark is put to good use. That’s why cats’ eyes light up in the headlights at night: the cells on the tapetum work as mirrors.

Cats can give a really intense stare because, unlike humans, cats don’t blink to moisten their eyes. Cats do blink slowly as a form of communication. Some people call it cat kisses. Slow blinking makes cats feel at ease and is a show of affection. Try it out some time. If the cat slow blinks back at you, it means that you are communicating successfully, in cat language, with your cat.

Have you ever looked at a sleepy cat that has just opened the eyes slightly and seen a whitish membrane partly over the eyes? Well, you were looking at their third eyelid, called the nictitating membrane. This is a translucent membrane that moves from the side. It protects the eye and keeps it moist while maintaining visibility. This membrane is sometimes slightly visible when a cat is sleepy and content. It is also visible when a cat is sick. If the membrane stays in view while the cat is awake, you should contact your vet and get it checked out.

Some people still believe that cats are completely colour-blind. This is not true. Cats can see colours, but not all colours. They can distinguish between blues and greens but not between shades of red.

So to recap: a bigger lens and pupil, combined with a higher density in rods and the reflective cells on the tapetum, allow a cat to see 6 times better in the dark than humans. Cats’ eyes are highly light sensitive but, as a trade-off, lack the ability to detect a lot of detail and colour sensitivity in daylight. A cat can see certain colours (greens and blues) and is therefore only partly colour-blind. A cat can’t focus on objects close by because of non-flexible lenses. Cats don’t blink to keep their eyes moist. They slow blink to communicate. They have a third eyelid called the nictitating membrane that protects their eyes and keeps it moist.

Right, I will include a few pictures and then I’m done. I hope you liked this little write-up. :)

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