The World According to Cat - Part III


Cats also show little interest in color; among mammals, color seems a unique primate, especially human, obsession like dogs, cats have only two types of cones and see only two colors, blue and yellow; in humans, we call this red-green color blindness.

To cats, both red and green probably look grayish. Moreover, even colors they can distinguish seem to be of little relevance to them.

 Their brains contain only a few nerves dedicated to color comparisons, and it is difficult to train cats to distinguish between blue and yellow objects. Any other difference between objects brightness, pattern, shape, or size seems to matter more to cats than does color.

Another drawback of having such large eyes is that they are not easy to focus. We have muscles in our eyes that distort the shape of the lens to allow close vision; cats seem to have to move their whole lens back and forth, as happens in a camera, a much more cumbersome process.

Perhaps because it is just too much effort, they often do not bother to focus at all, unless something exciting, such as a bird flying past, catches their attention. Close focus, anything nearer than about a foot away, is also out of the question with such large eyes.

Furthermore, the muscles that focus the lens seem to set themselves according to the environment the cat grows up in: outdoor cats are slightly longsighted, whereas all indoor cats tend to be shortsighted.

Despite the largeness of their eyes, cats can swivel them quickly to keep track of rapidly moving prey. To avoid image blurring, the eyes do not move smoothly but in a series of jerks, known as saccades, about a quarter of a second apart, so that the cat’s brain can process each separate image clearly.

Like humans, cats have binocular vision. The signals from each of their forward-facing eyes are matched up in their brains and are converted there into three-dimensional pictures.

Most mammalian carnivores have eyes that point forward to provide them with binocular vision so that they can judge precisely how far away potential prey is, and judge their pounce accordingly. Presumably, because their eyes do not focus any closer than about a foot away from their noses, cats do not bother to converge their eyes on objects any closer than this.

 To compensate, cats can swing their whiskers forward to provide a 3D tactile “picture” of objects that are right in front of their noses. Binocular vision is the best way of judging how far away something is, but it is not the only method available.

Cats that lose an eye because of disease or injury can compensate by making exaggerated bobbing movements of their heads, monitoring how the images of the various objects they can see move relative to one another.

Prey animals such as rabbits commonly do this: because their eyes are on the sides of their heads to maximize surveillance, they have little or no binocular vision and have to rely on other, slightly cruder ways of judging distance.

The cat’s ability to detect tiny movements is another legacy of its predatory past. The visual cortex, the part of the brain that receives signals from the eyes, does not simply construct pictures as if the eyes were two still cameras; it also analyzes what has changed between one picture and the next.

The cat’s visual cortex compares these “pictures” sixty times each second slightly more frequently than our visual cortex does, meaning that cats see fluorescent lights and older TV screens as flickery.

Dedicated brain cells analyze movements in various directions up and down, left to right, and along both diagonals—and even local brightening or dimming of specific parts of the image.

Thus, the most important features of the image the parts that are changing rapidly are instantly singled out for attention. Cats learn how to integrate all this information when they are kittens, unlike amphibians, for example, which already have specialized prey-detector circuits formed in their brains when they metamorphose from tadpole to adult.


Cats use their movement detectors to behave flexibly when they are hunting, paying equal attention to a mouse, it spots attempting to make an escape, or to a movement of the grass that betrays the mouse’s position. Both help the hunting cat to find a meal.

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