The retina uses special cells called rods and cones to process light, and cones sense color, and they need more light than rods, while rods work better in dim light. People with typical color vision have three types of cones, and each type is most sensitive to a slightly different wavelength of light, and the three types of cone opsins respond to different wavelengths of light and give us color vision. In opponent-process theory, color is coded in pairs: black-white, yellow-blue and green-red, and because of opponent processing, we do not see greenish-reds or yellowish-blues as colors. An afterimage is a visual sensation that continues after the stimulus is removed, and rhodopsin, the photopigment in rods, is most sensitive to light of about 498 nanometers wavelength.

Rods and cones are photoreceptors that change their membrane potential when light hits them. Rods are so sensitive that a single photon of light can make a rod's connected retinal ganglion cell send an action potential, while cones do not work in dim light, so you cannot see colors in dim light. Cones are packed most densely in the center of the retina and give sharp detail and color. Axons from the retinal ganglion cells gather and leave through the back of the eye to form the optic nerve, and the optic nerves from the two eyes meet at a point just below the brain called the optic chiasm. Each eye has a blind spot, an area of the retina with no light-detecting receptors, but an octopus does not have a blind spot.

Sound waves go into your ears and make your eardrums vibrate, and the cochlea is a snail-shaped part of the inner ear that turns vibrations into nerve signals. Hair cells in the cochlea sit on the basilar membrane and ride the wave made by sound vibrations. The place theory of pitch says that different parts of the basilar membrane react to different pitches, and hair cells near the wide end of the cochlea detect higher-pitched sounds. In the temporal theory of pitch, frequency is coded by how active a sensory neuron is. Low-frequency sounds are lower pitched, and high-frequency sounds are higher pitched. Hearing humans can pick up sound frequencies from about 20 to 20,000 hertz, and hair cells in the cochlea make neural impulses that travel along the auditory nerve to the brain.

Three tiny bones in the middle ear, called the malleus, incus and stapes, pass on the eardrum's vibrations, and the auditory nerve carries signals from the ear to the brain. Hearing and balance are also sensed by mechanoreceptors. In the hair cells, bending opens pore-like channels at the tips of the stereocilia, and when the channels open, chemicals rush into the hair cell and create an electrical signal. You may have heard that you have only five senses. In fact, you also have a sense of balance, because the vestibular sense helps you keep your balance and body posture. The vestibular organs are fluid-filled and have hair cells similar to the ones in the auditory system, and when your head moves, fluid in the semicircular canals moves tiny hair cells, which send a signal to your brain.

Food has to dissolve in saliva before you can taste it, and once food chemicals have dissolved, receptors on the taste buds can sense them. Taste buds can pick up sweet, sour, bitter, salty and umami (savory) flavors. Salty taste is the perception of sodium ions in the saliva, and sour taste is the perception of hydrogen ion concentration. You may have heard that there is a map on your tongue, with sweet at the tip and bitter at the back. In fact, in humans the different types of taste cells are scattered throughout the tongue. Taste buds live for about ten days to two weeks, and then new ones grow in. Special smell receptors high in the nose pick up odor molecules that travel through the air, and the nose has about 10 million smell receptors. The brain can recognize about 10,000 different smells from the combination of smell receptors that fire, and smell receptors in the nose send messages to the brain, which identifies the smell. Flavor comes from taste and smell working together, and smell helps you taste, because without smell, foods taste bland.

Touch starts with touch receptors in your skin, which send nerve impulses to the brain. Fingers and lips have more touch receptors than your back, so they feel more. Meissner's corpuscles react to pressure and lower-frequency vibrations, and Pacinian corpuscles detect brief pressure and higher-frequency vibrations. Stretch in the skin is sensed by receptors called bulbous corpuscles, and how sensitive a patch of skin is depends on the number of receptors per unit area and the distance between them. Braille is a way of reading with the fingers, using raised dots, and a full braille cell has six raised dots arranged in two rows of three. A single braille cell can stand for a letter, a number, a punctuation mark, or even a whole word, and sixty-four combinations are possible using one or more of the six dots. Louis Braille was born in Coupvray, France, on January 4, 1809.

American Sign Language (ASL) is the primary language of many North Americans who are deaf or hard of hearing, and it is expressed by movements of the hands and face. You may have heard that it is English on the hands. In fact, ASL is a language completely separate and distinct from English. Fingerspelling is part of ASL and is used to spell out English words, and there is no universal sign language. Sensory receptors are specialized neurons that respond to specific types of stimuli, and transduction is the conversion of sensory stimulus energy into an action potential. Sensation is a physical process, whereas perception is psychological, and it is the way sensory information is organized, interpreted and consciously experienced. Your skin has nerve endings that let you feel hot, cold and touch, and your senses send messages to your brain.

Words to know

• opsins: light-sensing proteins
• afterimage: lingering sight
• basilar: ear membrane
• vestibular: balance-related
• fingerspelling: spelling words