Human Physiology, 14th edition (2016)

218 Chapter 8

hemisphere (in most people). This results in speech that is rapid

and fluid but without meaning. People with Wernicke’s apha-

sia produce speech that has been described as a “word salad.”

The words used may be real words that are chaotically mixed

together, or they may be made-up words. Language comprehen-

sion is destroyed; people with Wernicke’s aphasia cannot under-

stand either spoken or written language.

It appears that the concept of words originates in Wer-

nicke’s area. Thus, in order to understand words that are read,

information from the visual cortex (in the occipital lobe) must

project to Wernicke’s area. Similarly, in order to understand

spoken words, the auditory cortex (in the temporal lobe) must

send information to Wernicke’s area.

To speak intelligibly, the concept of words originating in

Wernicke’s area must be communicated to Broca’s area; this

is accomplished by a fiber tract called the arcuate fasciculus.

Broca’s area, in turn, sends fibers to the motor cortex (precen-

tral gyrus), which directly controls the musculature of speech.

Damage to the arcuate fasciculus produces conduction aphasia,

which is fluent but nonsensical speech as in Wernicke’s aphasia,

even though both Broca’s and Wernicke’s areas are intact.

The angular gyrus, located at the junction of the parietal,

temporal, and occipital lobes, is believed to be a center for the

integration of auditory, visual, and somatesthetic information.

Damage to the angular gyrus produces aphasias, which sug-

gests that this area projects to Wernicke’s area. Some patients

with damage to the left angular gyrus can speak and under-

stand spoken language but cannot read or write. Other patients

can write a sentence but cannot read it, presumably because of

damage to the projections from the occipital lobe (involved in

vision) to the angular gyrus.

Language

Knowledge of the brain regions involved in language has been
gained primarily by the study of aphasias —speech and language
disorders caused by damage to the brain through head injury or
stroke. In most people, the language areas of the brain are pri-
marily located in the left hemisphere of the cerebral cortex, as
previously described. Even in the nineteenth century, two areas
of the cortex—Broca’s area and Wernicke’s area ( fig.  8.14 )—
were recognized as areas of particular importance in the produc-
tion of aphasias.
Broca’s aphasia is the result of damage to Broca’s area,
located in the left inferior frontal gyrus and surrounding areas.
Common symptoms include weakness in the right arm and the
right side of the face. People with Broca’s aphasia are reluctant to
speak, and when they try, their speech is slow and poorly articu-
lated. Their comprehension of speech is unimpaired, however.
People with this aphasia can understand a sentence but have dif-
ficulty repeating it. It should be noted that this is not simply due
to a problem in motor control, because the neural control over the
musculature of the tongue, lips, larynx, and so on is unaffected.
Wernicke’s aphasia is caused by damage to Wernicke’s
area, located in the superior temporal gyrus of the left

Figure 8.14 Brain areas involved in the control of
speech. Damage to these areas produces speech deficits,
known as aphasias. Wernicke’s area, required for language
comprehension, receives information from many areas of the
brain, including the auditory cortex (for heard words), the visual
cortex (for read words), and other brain areas. In order for a
person to be able to speak intelligibly, Wernicke’s area must send
messages to Broca’s area, which controls the motor aspects of
speech by way of its input to the motor cortex.

Motor cortex

(precentral gyrus)

Vision

Hearing

Motor speech area

(Broca’s area)

Wernicke’s

area

CLINICAL APPLICATIONS

A spontaneous full recovery of language ability often occurs

within a few days of a transient ischemic attack ( TIA ),

but there is usually only partial recovery over the longer

term after a stroke. Recovery is faster in children, possibly

because of transfer of language ability from the left to the

right hemisphere, but is reduced after adolescence. Recov-

ery is aided by speech therapy and other techniques. Some

recovery usually occurs after damage to Broca’s area, but

damage to Wernicke’s area produces more severe and per-

manent aphasias.

Clinical Investigation CLUES

Kevin’s speech became slow and difficult after the acci-

dent, but it was understandable.

• Which language area of the brain was likely affected
  in the accident?


• Which brain area would have been involved if his
  speech were fluent but nonsensical?