Hearing begins with pressure waves hitting the auditory canal and ends when the brain perceives sounds.
Sound reception occurs at the ears, where the pinna collects, reflects, attenuates, or amplifies sound waves. These waves travel along the auditory canal until they reach the ear drum, which vibrates in response to the change in pressure caused by the waves. The vibrations of the ear drum cause oscillations in the three bones in the middle ear, the last of which sets the fluid in the cochlea in motion. The cochlea separates sounds according to their place on the frequency spectrum.
Sound waves travel along the auditory canal and strike the tympanic membrane, causing it to vibrate. This vibration results in movement of the three ossicles. As the ossicles move, the stapes presses into a thin membrane of the cochlea known as the oval window. As the stapes presses into the oval window, the fluid inside the cochlea begins to move, which in turn stimulates hair cells, which are auditory receptor cells of the inner ear embedded in the basilar membrane. The basilar membrane is a thin strip of tissue within the cochlea.
The activation of hair cells is a mechanical process: the stimulation of the hair cell ultimately leads to activation of the cell. As hair cells become activated, they generate neural impulses that travel along the auditory nerve to the brain. Auditory information is shuttled to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe of the brain for processing. Like the visual system, there is also evidence suggesting that information about auditory recognition and localization is processed in parallel streams.
The ability to locate sound in our environments is an important part of hearing. Localizing sound could be considered similar to the way that we perceive depth in our visual fields. Like the monocular and binocular cues that provided information about depth, the auditory system uses both monaural (one-eared) and binaural (two-eared) cues to localize sound.
Each pinna interacts with incoming sound waves differently, depending on the sound’s source relative to our bodies. This interaction provides a monaural cue that is helpful in locating sounds that occur above or below and in front or behind us. The sound waves received by your two ears from sounds that come from directly above, below, in front, or behind you would be identical; therefore, monaural cues are essential.
Binaural cues, on the other hand, provide information on the location of a sound along a horizontal axis by relying on differences in patterns of vibration of the eardrum between our two ears. If a sound comes from an off-center location, it creates two types of binaural cues: interaural level differences and interaural timing differences. Interaural level difference refers to the fact that a sound coming from the right side of your body is more intense at your right ear than at your left ear because of the attenuation of the sound wave as it passes through your head. Interaural timing difference refers to the small difference in the time at which a given sound wave arrives at each ear. Certain brain areas monitor these differences to construct where along a horizontal axis a sound originates.
Different frequencies of sound waves are associated with differences in our perception of the pitch of those sounds. Low-frequency sounds are lower pitched, and high-frequency sounds are higher pitched.
Several theories have been proposed to account for pitch perception, they are temporal theory and place theory. The temporal theory of pitch perception asserts that frequency is coded by the activity level of a sensory neuron. This would mean that a given hair cell would fire action potentials related to the frequency of the sound wave. The place theory of pitch perception suggests that different portions of the basilar membrane are sensitive to sounds of different frequencies. More specifically, the base of the basilar membrane responds best to high frequencies and the tip of the basilar membrane responds best to low frequencies. Therefore, hair cells that are in the base portion would be labeled as high-pitch receptors, while those in the tip of basilar membrane would be labeled as low-pitch receptors.
Practice Questions
Khan Academy
MCAT Official Prep (AAMC)
Section Bank C/P Section Question 16
Section Bank P/S Section Passage 8 Question 60
Section Bank P/S Section Passage 1 Question 2
Key Points
• The human sense of hearing is attributed to the auditory system, which uses the ear to collect, amplify, and transduce sound waves into electrical impulses that allow the brain to perceive and localize sounds.
• The ear can be divided into the outer ear, middle ear, and inner ear, each of which has a specific function in the process of hearing.
• The fluid-filled inner ear transduces sound vibrations into neural signals that are sent to the brain for processing.
• The cochlea is the major sensory organ of hearing within the inner ear. Hair cells within the cochlea perform the transduction of sound waves.
• As hair cells become activated, they generate neural impulses that travel along the auditory nerve to the brain. Auditory information is shuttled to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe of the brain for processing.
• Humans are capable of estimating a sound’s origin through a process called sound localization, which relies on timing and intensity differences in sound waves collected by each of our two ears.
• Place theory of pitch perception states that different portions of the basilar membrane are sensitive to sounds of different frequencies.
• Temporal theory of pitch perception states that a sound’s frequency is coded by the activity level of a sensory neuron.
Key Terms
Afferent: Leading to the brain.
Interaural: Describing the differences between the reception of sound (especially timing and intensity) by each ear.
