The retina uses special cells called rods and cones to process light. Rods and cones are photoreceptors that change their membrane potential when light hits them, and special cells in the retina turn light into electrical signals. Cones sense color and need more light than rods, while rods work better in dim light. Rods are so sensitive that a single photon of light can make a rod's connected retinal ganglion cell send an action potential. Cones do not work in dim light, so you cannot see colors in dim light. In dim light the rods do most of the work, but they give no information about color.

Axons from the retinal ganglion cells gather and leave through the back of the eye to form the optic nerve. The optic nerve carries signals from the eye to the brain, 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. Light entering the eye forms an upside-down image on the retina, and the brain turns it right-side up. An octopus does not have a blind spot. Two eyes also help you judge depth, how far away things are.

Sound waves go into your ears and make your eardrums vibrate, and three tiny bones in the middle ear pass on the eardrum's vibrations. 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 respond to different pitches, and hair cells near the wide end of the cochlea detect higher-pitched sounds. Hair cells in the cochlea make neural impulses that travel along the auditory nerve to the brain. Hearing humans can detect sound frequencies from about 20 to 20,000 hertz.

Special smell receptors high in the nose detect odor molecules that travel through the air. The nose has about 10 million smell receptors, and the brain can recognize about 10,000 different smells from the combination of smell receptors that fire. Food has to dissolve in saliva before you can taste it, and once food chemicals have dissolved, receptors on the taste buds can detect them. 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.

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. Braille is a way of reading with the fingers, using raised dots. 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 in a braille cell. Sensory receptors are specialized neurons that respond to specific types of stimuli. Transduction is the conversion of sensory stimulus energy into an action potential.

American Sign Language 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. There is no universal sign language.

Sensation is a physical process, whereas perception is psychological. Perception is the way sensory information is organized, interpreted and consciously experienced. You may have heard that you have only five senses. In fact, you also have a sense of balance. When your head moves, fluid in the semicircular canals moves tiny hair cells, which send a signal to your brain. Your senses send messages to your brain.

Words to know

• photoreceptors: light-sensing cells
• chiasm: meeting point
• cochlea: snail-shaped part
• perception: interpreted sensing
• transduction: energy conversion