Measuring the senses of any animal can be difficult because there is usually no explicit communication (e.g., reading aloud the letters of a Snellen chart) between the subject and the tester.
While a cat's senses of smell and hearing may not be as keen as, say, those of a mouse, they are superior in many ways to those of humans. These along with the cat's highly advanced eyesight, taste, and touch receptors make the cat extremely sensitive among mammals.
Testing indicates that a cat's vision is superior at night in comparison to humans, and inferior in daylight. Cats, like dogs, have a tapetum lucidum that reflects extra light to the retina. While this enhances the ability to see in low light, it appears to reduce net visual acuity, thus detracting when light is abundant. In very bright light, the slit-like iris closes very narrowly over the eye, reducing the amount of light on the sensitive retina, and improving depth of field. The tapetum and other mechanisms give the cat a minimum light detection threshold up to seven times lower than that of humans. Variation in color of cats' eyes in flash photographs is largely due to the interaction of the flash with the tapetum.
Average cats have a visual field of view estimated at 200°, versus 180° in humans, with a binocular field (overlap in the images from each eye) narrower than that of humans. As with most predators, their eyes face forward, affording depth perception at the expense of field of view. Field of view is largely dependent upon the placement of the eyes, but may also be related to the eye's construction. Instead of the fovea which gives humans sharp central vision, cats have a central band known as the visual streak. Cats can apparently differentiate among colors, especially at close range, but without appreciable subtlety.
Cats have a third eyelid, the nictitating membrane, which is a thin cover that closes from the side and appears when the cat's eyelid opens. This membrane partially closes if the cat is sick; although in a sleepy, content cat this membrane is often visible. If a cat chronically shows the third eyelid, it should be taken to a veterinary surgeon.
Cats have a wide variation in eye colour, the most typical colours being golden, green and orange. Blue eyes are usually associated with the Siamese breed, but they are also found in albino cats. If an albino cat has two blue eyes, it is usually deaf; however, orange eyes usually indicate the cat is free of hearing problems. Albino cats having one blue and one orange eye are normally deaf on the same side as the blue eye.
Humans and cats have a similar range of hearing on the low end of the scale, but cats can hear much higher-pitched sounds, even better than dogs. Cats can hear 2 octaves higher than humans, and one-half octave higher than dogs. When listening for something, a cat's ears will swivel in that direction; a cat's ear flaps (pinnae) can independently point backwards as well as forwards and sideways to pinpoint the source of the sound. Cats can judge within three inches (7.5 cm) the location of a sound being made one yard (approximately one meter) away - this can be useful for localizing prey etc.
A domestic cat's sense of smell is about fourteen times as strong as a human's. Cats have twice as many smell-sensitive cells in their noses as people do, which means they can smell things we are not even aware of. Cats also have a scent organ in the roof of their mouths called the vomeronasal, or Jacobson's, organ. When a cat wrinkles its muzzle, lowers its chin, and lets its tongue hang a bit, it is opening the passage to the vomeronasal. This is called gaping, "sneering", or "flehming". Gaping is the equivalent of the Flehmen response in other animals, such as dogs, horses and big cats.
Cats generally have about a dozen whiskers in four rows on each upper lip, a few on each cheek, tufts over the eyes and bristles on the chin. Whiskers may also be found on the cat's inner "wrists", and there are similar hairs which make up the cat's eyebrows. The Sphynx (a nearly hairless breed) may have full length, short, or no whiskers at all.
Whiskers (technically called vibrissae) can aid with navigation and sensation. Whiskers may detect very small shifts in air currents, enabling a cat to know it is near obstructions without actually seeing them. The upper two rows of whiskers can move independently from the lower two rows for even more precise measuring.
It is thought that a cat may choose to rely on the whiskers in dim light where fully dilating the pupils would reduce its ability to focus on close objects. The whiskers also spread out roughly as wide as the cat's body making it able to judge if it can fit through an opening.
Whiskers are also an indication of the cat's attitude. Whiskers point forward when the cat is inquisitive and friendly, and lie flat on the face when the cat is being defensive or aggressive.
According to National Geographic December 8, 2005 cats cannot taste sugary foods due to a faulty sweet receptor gene. Some scientists believe this is related to the cat's diet being naturally high in protein, though it is unclear whether it is the cause or the result of it.