Sound waves are bands of compressed and expanded air. Our ears detect these changes in air pressure and transform them into neural impulses, which the brain decodes as sound. Sound waves vary in amplitude, which we perceive as differing loudness, and in frequency, which we experience as differing pitch.

The outer ear is the visible portion of the ear. The middle ear is the chamber between the eardrum and cochlea. The inner ear consists of the cochlea, semicircular canals, and vestibular sacs. Through a mechanical chain of events, sound waves traveling through the auditory canal cause tiny vibrations in the eardrum. The bones of the middle ear amplify the vibrations and relay them to the fluid-filled cochlea. Rippling of the basilar membrane, caused by pressure changes in the cochlear fluid, causes movement of the tiny hair cells, triggering neural messages to be sent (via the thalamus) to the auditory cortex in the brain.
     Sensorineural hearing loss (or nerve deafness) results from damage to the cochlea’s hair cells or their associated nerves. Conduction hearing loss results from damage to the mechanical system that transmits sound waves to the cochlea. Cochlear implants can restore hearing for some people.

Loudness is not related to the intensity of a hair cell’s response. The brain interprets loudness from the number of activated hair cells.
     Place theory explains how we hear high-pitched sounds, and frequency theory explains how we hear low-pitched sounds. (A combination of the two theories explains how we hear pitches in the middle range.) Place theory proposes that our brain interprets a particular pitch by decoding the place where a sound wave stimulates the cochlea’s basilar membrane. Frequency theory proposes that the brain deciphers the frequency of the neural impulses traveling up the auditory nerve to the brain.
     Sound waves strike one ear sooner and more intensely than the other. To locate sounds, the brain analyzes the minute differences in the sounds received by the two ears and computes the sound’s source.

Our sense of touch is actually several senses—pressure, warmth, cold, and pain—that combine to produce other sensations, such as “hot.”

Pain reflects bottom-up sensations (such as input from nociceptors, the sensory receptors that detect hurtful temperatures, pressure, or chemicals) and top-down processes (such as experience, attention, and culture). One theory of pain is that a “gate” in the spinal cord either opens to permit pain signals traveling up small nerve fibers to reach the brain, or closes to prevent their passage. The biopsychosocial perspective views our perception of pain as the sum of biological, psychological, and social-cultural influences. For example, our experience of pain is influenced by activity in the spinal cord’s large and small fibers (a biological influence), attention to pain (a psychological influence), and cultural expectations (a social-cultural influence).
     Pain treatments often combine physical and psychological elements. Placebos can help by dampening the central nervous system’s attention and response to painful experiences. Distractions draw people’s attention away from painful stimulation. Hypnosis, which increases our response to suggestions, can also help relieve pain. Posthypnotic suggestion is used by some clinicians to control undesired symptoms.

Taste and smell are both chemical senses. Taste is a composite of five basic sensations—sweet, sour, salty, bitter, and umami—and of the aromas that interact with information from the taste receptor cells of the taste buds.
     There are no basic sensations for smell. We smell something when molecules of a substance carried in the air reach a tiny cluster of 20 million receptor cells at the top of each nasal cavity. Odor molecules trigger combinations of receptors, in patterns that the olfactory cortex interprets. The receptor cells send messages to the brain’s olfactory bulb, then to the temporal lobe, and to parts of the limbic system.

Through kinesthesia, we sense the position and movement of our body parts. We monitor our head’s (and thus our body’s) position and movement, and maintain our balance, with our vestibular sense.

Our senses can influence one another. This sensory interaction occurs, for example, when the smell of a favorite food amplifies its taste. Embodied cognition is the influence of bodily sensations, gestures, and other states on cognitive preferences and judgments.

Parapsychology is the study of paranormal phenomena, including extrasensory perception (ESP) and psychokinesis. The three most testable forms of ESP are telepathy (mind-to-mind communication), clairvoyance (perceiving remote events), and precognition (perceiving future events).
     Skeptics argue that (1) to believe in ESP, you must believe the brain is capable of perceiving without sensory input, and (2) researchers have been unable to replicate ESP phenomena under controlled conditions.