Your senses send messages to your brain. 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. Perception is the way sensory information is organized, interpreted and consciously experienced, and it can be shaped by our beliefs, expectations and life experiences. Top-down processing is perception driven by knowledge and expectations. We often do not perceive stimuli that stay about the same for a long time, which is called sensory adaptation. The just noticeable difference, or difference threshold, is how much difference between two stimuli a person needs in order to notice that they differ. The optic nerve carries signals from the eye to the brain, and the auditory nerve carries signals from the ear to the brain.

People with typical color vision have three types of cones, each 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. 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. Rods and cones are photoreceptors that change their membrane potential when light hits them. Rhodopsin, the photopigment in rods, is most sensitive to light of about 498 nanometers wavelength, and 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.

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. The lens and the cornea work together to focus light on the retina, and light entering the eye forms an upside-down image on the retina, which the brain turns right-side up. When light levels are high, the pupil constricts to reduce the amount of light that enters the eye, and muscles attached to the iris change its shape to control how much light gets in.

Sound waves go into your ears and make your eardrums vibrate, and three tiny bones in the middle ear, called the malleus, incus and stapes, 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, and hair cells in the cochlea make neural impulses that travel along the auditory nerve to the brain. 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. 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, and hearing humans can detect sound frequencies from about 20 to 20,000 hertz.

Interaural timing difference is the small difference in when a sound wave reaches each ear, a cue for where a sound is. Each pinna changes incoming sound waves in a way that depends on where the sound comes from, which helps us locate sounds above or below us. 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 detect them. Taste buds can detect 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.. Special smell receptors high in the nose detect 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. Our chemical senses of taste and smell strongly interact, and flavor comes from taste and smell working together. Different species vary a lot in how sensitive their sense of smell is, and dogs' extraordinary sense of smell may be due to more functional genes for olfactory receptors.

Touch starts with touch receptors in your skin, which send nerve impulses to the brain. Meissner's corpuscles respond to pressure and lower-frequency vibrations, and Pacinian corpuscles detect brief pressure and higher-frequency vibrations. 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 full braille cell has six raised dots arranged in two rows of three, and a single braille cell can stand for a letter, a number, a punctuation mark, or even a whole word. Sixty-four combinations are possible using one or more of the six dots in a braille cell. 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, and fingerspelling is part of ASL and is used to spell out English words. There is no universal sign language.

Words to know

• transduction: energy conversion
• afterimage: lingering sight
• chiasm: meeting point
• stereocilia: hair tips
• adaptation: sensory fading