Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 13Hearing & Equilibrium 213

SOUND LOCALIZATION

Determination of the direction from which a sound emanates
in the horizontal plane depends on detecting the difference in
time between the arrival of the stimulus in the two ears and the
consequent difference in phase of the sound waves on the two
sides; it also depends on the fact that the sound is louder on the
side closest to the source. The detectable time difference,
which can be as little as 20 μs, is said to be the most important
factor at frequencies below 3000 Hz and the loudness differ-
ence the most important at frequencies above 3000 Hz. Neu-
rons in the auditory cortex that receive input from both ears
respond maximally or minimally when the time of arrival of a
stimulus at one ear is delayed by a fixed period relative to the
time of arrival at the other ear. This fixed period varies from
neuron to neuron.
Sounds coming from directly in front of the individual dif-
fer in quality from those coming from behind because each
pinna (the visible portion of the exterior ear) is turned slightly
forward. In addition, reflections of the sound waves from the
pinnal surface change as sounds move up or down, and the
change in the sound waves is the primary factor in locating
sounds in the vertical plane. Sound localization is markedly
disrupted by lesions of the auditory cortex.

AUDIOMETRY

Auditory acuity is commonly measured with an audiometer.

This device presents the subject with pure tones of various fre-

quencies through earphones. At each frequency, the threshold

intensity is determined and plotted on a graph as a percentage of

normal hearing. This provides an objective measurement of the

degree of deafness and a picture of the tonal range most affected.

DEAFNESS

Hearing loss is the most common sensory defect in humans.

According to the World Health Organization, over 270 mil-

lion people worldwide have moderate to profound hearing

loss, with one fourth of these cases beginning in childhood.

Presbycusis, the gradual hearing loss associated with aging,

affects more than one-third of those over 75 and is probably

due to gradual cumulative loss of hair cells and neurons. In

most cases, hearing loss is a multifactorial disorder caused by

both genetic and environmental factors. Genetic factors con-

tributing to deafness are described in Clinical Box 13–1.

Deafness can be divided into two major categories: conduc-

tive (or conduction) and sensorineural hearing loss. Conduc-

tive deafness refers to impaired sound transmission in the

external or middle ear and impacts all sound frequencies.

Among the causes of conduction deafness are plugging of the

external auditory canals with wax (cerumen) or foreign bod-

ies, otitis externa (inflammation of the outer ear, “swimmer’s

ear”) and otitis media (inflammation of the middle ear) caus-

ing fluid accumulation, perforation of the eardrum, and

osteosclerosis in which bone is resorbed and replaced with

sclerotic bone that grows over the oval window.

Sensorineural deafness is most commonly the result of

loss of cochlear hair cells but can also be due to problems with

the eighth cranial nerve or within central auditory pathways.

It often impairs the ability to hear certain pitches while others

are unaffected. Aminoglycoside antibiotics such as streptomy-

cin and gentamicin obstruct the mechanosensitive channels

in the stereocilia of hair cells and can cause the cells to degen-

erate, producing sensorineural hearing loss and abnormal

vestibular function. Damage to the outer hair cells by pro-

longed exposure to noise is associated with hearing loss.

Other causes include tumors of the eighth cranial nerve and

cerebellopontine angle and vascular damage in the medulla.

Conduction and sensorineural deafness can be differenti-

ated by simple tests with a tuning fork. Three of these tests,

named for the individuals who developed them, are outlined

in Table 13–1. The Weber and Schwabach tests demonstrate

the important masking effect of environmental noise on the

auditory threshold.

VESTIBULAR SYSTEM

The vestibular system can be divided into the vestibular appa-

ratus and central vestibular nuclei. The vestibular apparatus

FIGURE 13–13 Left and right planum temporale in a brain
sectioned horizontally along the plane of the sylvian fissure.
Plane of section shown in the insert at the bottom. (Reproduced with
permission from Kandel ER, Schwartz JH, Jessel TM [editors]: Principles of Neural
Science, 3rd ed. McGraw-Hill, 1991.)

Left planum

temporale

Right planum

temporale

Occipital

pole

Frontal

Heschl’s pole

sulcus