Our fascination with cats is largely based on their facial expression, with their vertical pupils and eyesight that does not seem to diminish at night. Indeed, the cat's eyes are well-adapted to nocturnal predation. Like humans, cats have fully frontal eyes directed forward. This makes the cat one of the mammals with the best binocular vision (approximately 120¡), allowing it to better perceive outlines and distances. On each side, this binocular vision is complemented by monocular vision of approximately 80¡. The cat's lens (the eye's "autofocusing" device) is highly mobile and enables rapid focusing on prey.
The structure of the eye is the same in all mammals. Light penetrates the cornea, then the anterior chamber, which is filled with a liquid called the aqueous humor. After passing through the pupil, light is then diverted by the lens, which works like a regular convex lens. Light then passes through the posterior chamber to the retina.
The eye is equipped with groups of muscles that enable it to move in all directions. It is protected by the eyelids. Cats have a third eyelid that is highly mobile and can sweep over the entire surface of the cornea, cleaning it and covering it with a protective liquid film.
Color perception requires that the retina contain photoreceptors called cones. But the sensitivity to many shades of color is based on families of cones that are sensitive to different wavelengths of light. Primarily nocturnal, or monochromatic, species such as rats have one type of cone. These animals distinguish objects based only on their light intensity. Cats are dichromatic, meaning they have two sorts of cones. We may thus assume that they can perceive two colors and the shades that result from mixing these two colors, from red to green. Humans are trichromatic, meaning we have three families of cones whose associated stimulation allows us to detect shades of color in the entire visible spectrum.
The cat's nocturnal vision, however, is much keener than ours. Other cells, the rods, are involved in this function. Cats have 200 million rods, versus only 120 in humans. In addition, the tapetum in the lower part of the back of the cat's eye serves as a mirror, reflecting light that is ÒunusedÓ by the retina and enabling this light to be saved and returned to the retina. The cat's tapetum is easy to see when your car headlights shine into a cat's eyes.
The perception of form, motion, and probably outlines is linked to a certain specialization of the retinal cells: some cells respond only to the perception of lines arranged in a specific direction. In short, some cells detect vertical elements, others detect horizontal elements, and still other cells detect each angulation (in increments of 10 to 30 degrees). The displacement of these lines during motion enables more effective perception. In other words, the more an object or animal moves, the better the cat perceives it.
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