Ganong's Review of Medical Physiology, 23rd Edition

212 SECTION III Central & Peripheral Neurophysiology

lowest intensity evoke a response varies from unit to unit; with in-
creased sound intensities, the band of frequencies to which a re-
sponse occurs becomes wider. The major difference between the
responses of the first- and second-order neurons is the presence
of a sharper “cutoff” on the low-frequency side in the medullary
neurons. This greater specificity of the second-order neurons is
probably due to an inhibitory process in the brain stem.

OTHER CORTICAL AREAS

CONCERNED WITH AUDITION

The increasing availability of positron emission tomography
(PET) scanning and functional magnetic resonance imaging
(fMRI) has led to rapid increases in knowledge about auditory
association areas in humans. The auditory pathways in the cor-
tex resemble the visual pathways in that increasingly complex
processing of auditory information takes place along them. An
interesting observation is that although the auditory areas look
very much the same on the two sides of the brain, there is
marked hemispheric specialization. For example, Brodmann’s
area 22 is concerned with the processing of auditory signals re-
lated to speech. During language processing, it is much more
active on the left side than on the right side. Area 22 on the right

side is more concerned with melody, pitch, and sound intensity.

The auditory pathways are also very plastic, and, like the visual

and somasthetic pathways, they are modified by experience. Ex-

amples of auditory plasticity in humans include the observation

that in individuals who become deaf before language skills are

fully developed, viewing sign language activates auditory asso-

ciation areas. Conversely, individuals who become blind early

in life are demonstrably better at localizing sound than individ-

uals with normal eyesight.

Musicians provide additional examples of cortical plasticity.

In these individuals, the size of the auditory areas activated by

musical tones is increased. In addition, violinists have altered

somatosensory representation of the areas to which the fin-

gers they use in playing their instruments project. Musicians

also have larger cerebellums than nonmusicians, presumably

because of learned precise finger movements.

A portion of the posterior superior temporal gyrus known

as the planum temporale (Figure 13–13) is regularly larger in

the left than in the right cerebral hemisphere, particularly in

right-handed individuals. This area appears to be involved in

language-related auditory processing. A curious observation,

which is presently unexplained, is that the planum temporale

is even larger than normal on the left side in musicians and

others who have perfect pitch.

FIGURE 13–12 Simplified diagram of main auditory (left) and vestibular (right) pathways superimposed on a dorsal view of the
brain stem. Cerebellum and cerebral cortex have been removed.

Medulla

Thalamus

To cortex (superior

temporal gyrus)

From

cochlea

Spiral

ganglion

Inferior colliculus

Pineal

Reticular

formation

Medial

geniculate

body

Superior

olives

IV ventricle

Dorsal and ventral

cochlear nuclei

From utricle,

semicircular

canals

Anterior vestibulo-

spinal tracts

Lateral

vestibulo-

spinal tract

Vestibular

ganglion

Vestibular nuclei:

superior, lateral

(Deiters’), medial,

spinal

To

cerebellum

To somatosensory

cortex

Medial

longitudinal

fasciculus

Thalamus

III

IV

VI

AUDITORY VESTIBULAR