Human Physiology, 14th edition (2016)

Sensory Physiology 289

the distance between the ears. This information is supplemented

by an interaural time difference if sound arrives at one ear

before the other. The time difference is particularly important

for localizing low-frequency sounds (below 2000 Hz). Humans

can detect an interaural intensity difference of as little as

1–2 decibels and an interaural time difference as short as

10 microseconds. Sound localization based on intensity differ-

ences and time differences between the two ears is primarily

a function of the lateral and medial superior olive, respectively.

Hearing Impairments

There are two major categories of deafness: (1) conduction

deafness, in which the transmission of sound waves through

the outer and middle ear to the oval window is impaired, and

(2) sensorineural, or perceptive, deafness, in which the

transmission of nerve impulses anywhere from the cochlea to

the auditory cortex is impaired. Conduction deafness can be

caused by a variety of problems in the ability of sound waves

to move through the external auditory meatus to produce vibra-

tions of the tympanic membrane. This is most commonly due

to the buildup of ear wax (cerumen), and to middle-ear dam-

age from otitis media or otosclerosis (discussed in the previ-

ous Clinical Application box). Sensorineural deafness may

result from a wide variety of pathological processes and from

exposure to extremely loud sounds (as from gunshots or rock

concerts). Unfortunately, the hair cells in the inner ears of

mammals cannot regenerate once they are destroyed. Experi-

ments have shown, however, that the hair cells of reptiles and

birds can regenerate by cell division when they are damaged.

Scientists are currently trying to determine if mammalian sen-

sory hair cells might be made to respond in a similar fashion.

Conduction deafness impairs hearing at all sound frequen-

cies. Sensorineural deafness, by contrast, often impairs the

ability to hear some pitches more than others. This may be due

to pathological processes or to changes that occur during aging.

Age-related hearing impairment—called presbycusis —begins

after age 20, when the ability to hear high frequencies (18,000

to 20,000 Hz) diminishes. Men are affected to a greater degree

than women, and although the progression is variable, the

deficits may gradually extend into the 4,000–8,000-Hz range.

These impairments can be detected by audiometry, a technique

in which the threshold intensity of different pitches is deter-

mined. The ability to hear speech is particularly affected by

hearing loss in the higher frequencies.

People with conduction deafness can be helped by

hearing aids —devices that amplify sounds and conduct the

sound waves through bone to the inner ear. Some people with

sensorineural deafness choose to have cochlear implants.

The cochlear implant consists of electrodes threaded into the

cochlea, a receiver implanted in the temporal bone, and an

external microphone, processor, and transmitter. Although

hair cells and most of the associated sensory dendrites have

degenerated in sensorineural deafness, these devices may be

effective because some dendrites survive and can be stimu-

lated by implanted electrodes. Thus, some neurons of the

that innervate the basilar membrane. This is because hair cells
located in different places along the basilar membrane are
most effectively stimulated by different frequencies of sound.
This is known as the place theory of pitch, and has been previ-
ously described. Sensory neurons stimulated by low-frequency
sounds, and those stimulated by high-frequency sounds, pro-
ject their axons to different regions of the cochlear nucleus.
The cochlear nucleus displays a tonotopic organization, in
that different regions represent different “tones” (pitches). This
separation of neurons by pitch is preserved in the tonotopic
organization of the auditory cortex ( fig.  10.25 ), which allows
us to perceive the different pitches of sounds.
The analysis of pitch can be quite amazing; for example, we
can recognize that a given sound frequency (such as 440 Hz) is
the same regardless of whether it is played by a violin or a piano.
The harmonics (multiples of a common fundamental frequency)
can vary, depending on their amplitudes, and this helps produce
the different characteristics of each instrument. However, if the
fundamental frequency is the same, the pitch is recognized as
being the same on the different instruments.
The loudness (intensity) of sounds, unlike their pitch, is
coded by the frequency of action potentials. Differences in the
intensity of the sounds arriving at each ear can be used to local-
ize a sound. This interaural intensity difference is produced
when one ear is closer to the source of the sound than the other
ear and the sound frequency is above about 2000 Hz. At these
higher frequencies, the wavelengths of sound are shorter than

Figure 10.25 Correlation between pitch location
in the cochlea and auditory cortex. Sounds of different
frequencies (pitches) cause vibration of different parts of the basilar
membrane, exciting different sensory neurons in the cochlea. These,
in turn, send their input to different regions of the auditory cortex.

Correspondence between the

cochlea and the acoustic

area of the cortex:

Blue—low tones

Red—medium tones

Yellow—high tones

Cochlea

Cerebral

cortex