Vision is based on the spatial pattern of light waves on the retina. The sense of hearing, by contrast, is all about sound waves: changes in air pressure unfolding over time. Plenty of things produce sound waves: the collision of a tree hitting the forest floor, the vibration of vocal cords during a stirring speech, the resonance of a bass guitar string during a thrash metal concert. Understanding auditory experience requires understanding how we transform changes in air pressure into perceived sounds.

Striking a tuning fork produces a pure tone, a simple sound wave that first increases air pressure and then creates a relative vacuum. This cycle repeats hundreds or thousands of times per second as sound waves travel outward in all directions from the source. Just as there are three physical dimensions of light waves corresponding to three dimensions of visual perception, so, too, there are three physical dimensions of a sound wave that determine what we hear (see TABLE 4.4).

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Most sounds—such as voices, music, the sound of wind in trees, the screech of brakes, the purring of a cat—are composed of not one, but many different frequency components. Although you perceive the mixture (the cat’s purr, for example, not the 206 Hz component in the purr), the first thing the ear does to a sound is to break it down—to analyze it—into its separate component frequencies. The psychological attributes of pitch, loudness, and timbre are then “built up” by the brain from the separate frequency components that are represented in the inner ear, just as visual perceptions are “built up” from the spatial pattern of activity on the retina. The focus in our discussion of hearing, then, is on how the auditory system encodes and represents sound frequency (Kubovy, 1981).

How does the auditory system convert sound waves into neural signals? The process is very different from the visual system, which is not surprising, given that light is a form of electromagnetic radiation, whereas sound is a physical change in air pressure over time: Different forms of energy require different processes of transduction. The human ear is divided into three distinct parts, as shown in FIGURE 4.21: the outer ear, the middle ear, and the inner ear.

The outer ear consists of the visible part on the outside of the head (called the pinna); the auditory canal; and the eardrum, an airtight flap of skin that vibrates in response to sound waves gathered by the pinna and channeled through the canal. The middle ear, a tiny, air-filled chamber behind the eardrum, contains the three smallest bones in the body, called ossicles. Named for their appearance as hammer, anvil, and stirrup, the ossicles fit together into a lever that mechanically transmits and intensifies vibrations from the eardrum to the inner ear.

The inner ear contains the spiral-shaped cochlea (Latin for “snail”), a fluid-filled tube that is the organ of auditory transduction. The cochlea is divided along its length by the basilar membrane, a structure in the inner ear that undulates when vibrations from the ossicles reach the cochlear fluid (see FIGURE 4.22). Its wavelike movement stimulates thousands of tiny hair cells, specialized auditory receptor neurons embedded in the basilar membrane. The hair cells then release neurotransmitter molecules, initiating a neural signal in the auditory nerve that travels to the brain.

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From the inner ear, action potentials in the auditory nerve travel to the thalamus and ultimately to an area of the cerebral cortex called area A1, a portion of the temporal lobe that contains the primary auditory cortex (see FIGURE 4.23). Neurons in area A1 respond well to simple tones, and successive auditory areas in the brain process sounds of increasing complexity (see FIGURE 4.23, inset; Rauschecker & Scott, 2009; Schreiner, Read, & Sutter, 2000; Schreiner & Winer, 2007). For most of us, the auditory areas in the left hemisphere analyze sounds related to language and those in the right hemisphere specialize in rhythmic sounds and music. There is also evidence that the auditory cortex is composed of two distinct streams, roughly analogous to the dorsal and ventral streams of the visual system. Spatial (“where”) auditory features, which allow you to locate the source of a sound in space, are handled by areas toward the back (caudal) part of the auditory cortex, whereas nonspatial (“what”) features, which allow you to identify the sound, are handled by areas in the lower (ventral) part of the auditory cortex (Recanzone & Sutter, 2008).

How is the frequency of a sound wave encoded in a neural signal? Our ears have evolved two mechanisms to encode sound-wave frequency, one for high frequencies and one for low frequencies.

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Just as the differing positions of our eyes give us stereoscopic vision, the placement of our ears on opposite sides of the head gives us stereophonic hearing. The sound arriving at the ear closer to the sound source is louder than the sound in the farther ear, mainly because the listener’s head partially blocks sound energy. This loudness difference decreases as the sound source moves from a position directly to one side (maximal difference) to straight ahead (no difference).

Another cue to a sound’s location arises from timing: Sound waves arrive a little sooner at the near ear than at the far ear. The timing difference can be as brief as a few microseconds, but together with the intensity difference, this time difference is sufficient to allow us to perceive the location of a sound. When the sound source is ambiguous, you may find yourself turning your head from side to side to localize the source. By doing this, you are changing the relative intensity and timing of sound waves arriving in your ears and collecting better information about the likely source of the sound. Turning your head also allows you to use your eyes to locate the source of the sound—and your visual system is much better at pinpointing the location of things than your auditory system is.

Broadly speaking, hearing loss has two main causes. Conductive hearing loss arises because the eardrum or ossicles are damaged to the point that they cannot conduct sound waves effectively to the cochlea. In many cases, medication or surgery can correct the problem. Sound amplification from a hearing aid also can improve hearing through conduction to the cochlea via the bones around the ear directly.

Sensorineural hearing loss is caused by damage to the cochlea, the hair cells, or the auditory nerve, and it happens to almost all of us as we age. Sensorineural hearing loss can be heightened in people regularly exposed to high noise levels (such as rock musicians or jet mechanics). Simply amplifying the sound does not help because the hair cells can no longer transduce sound waves. In these cases, a cochlear implant may offer some relief.

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A cochlear implant is an electronic device that replaces the function of the hair cells (Waltzman, 2006). The external parts of the device include a microphone and speech processor, about the size of a USB key, worn behind the ear, and a small, flat, external transmitter that sits on the scalp behind the ear. The implanted parts include a receiver just inside the skull and a thin wire containing electrodes inserted into the cochlea to stimulate the auditory nerve. Sound picked up by the microphone is transformed into electric signals by the speech processor, which is essentially a small computer. The signal is transmitted to the implanted receiver, which activates the electrodes in the cochlea. Cochlear implants are now in routine use and can improve hearing to the point where speech can be understood.

Marked hearing loss is commonly experienced by people as they grow older, but is rare in an infant. However, infants who have not yet learned to speak are especially vulnerable because they may miss the critical period for language learning (see the Learning chapter). Without auditory feedback during this time, normal speech is nearly impossible to achieve, but early use of cochlear implants has been associated with improved speech and language skills for deaf children (Hay-McCutcheon et al., 2008). Efforts are under way to introduce cochlear implants to children as young as 12 months old or younger to maximize their chances of normal language development (DesJardin, Eisenberg, & Hodapp, 2006; Holman et al., 2013). (For research on the importance of music and brain development, see the Hot Science: Music Training: Worth the Time.)

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