Human Physiology, 14th edition (2016)

(Tina Sui) #1
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
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