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Association Cortex

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Association Cortex
68
Chapter 2 Biology and Behavior
disembodied, like a ghost. On one occasion, for example, she became annoyed at a visitor for tapping her fingers on a tabletop. But it was Christina’s fingers, not the visitor’s,
that were tapping. Her body was acting on its own, doing things she did not know about.
■ What was the researcher’s question?
Christina could not walk or use her hands and arms. Why was a seemingly normal,
healthy young woman falling and dropping things?
■ How did the researcher answer the question?
A psychiatrist at the hospital thought that Christina was suffering from conversion disorder, a condition in which psychological problems cause physical disabilities (see the
chapter on psychological disorders). Unconvinced, Sacks conducted a careful case study
of Christina.
■ What did the researcher find?
It turned out that the psychiatrist was wrong. Sacks’s examinations and tests revealed
that Christina had lost all sensory feedback about her joints, muscle tone, and the position of her limbs. Christina had suffered a breakdown, or degeneration, of the sensory
neurons that normally bring kinesthetic information to her brain. In other words, there
was a biological reason that Christina could not walk or control her hands and arms.
■ What do the results mean?
In his analysis of this case, Sacks noted that the sense we have of our bodies is provided
partly through our experience of seeing but also partly through proprioception (sensing
the self). Christina herself put it this way: “Proprioception is like the eyes of the body,
the way the body sees itself. And if it goes, it’s like the body’s blind.” With great effort and
determination, Christina was eventually able to regain some of her ability to move about.
If she looked intently at her arms and legs, she could coordinate their movement somewhat. She was able to leave the hospital and resume many of her normal activities, but
Christina never recovered her sense of self. She still feels like a stranger in her own body.
■ What do we still need to know?
Notice that Christina’s case study did not confirm any hypotheses about kinesthesia in
the way an experiment might. It did, however, focus attention on what it feels like to
have lost this sense. It also highlighted a rare condition that, though almost unknown
when Sacks reported it, has been observed more often in recent years, especially among
people taking large doses of vitamin B6, also known as pyridoxine (Sacks, 1985). These
large doses—or even smaller doses taken over a long period of time—can damage sensory neurons (Dordain & Deffond, 1994). How and why vitamin B6 does such damage still needs to be determined. Are there other causes of this kinesthetic disorder?
What treatments might best combat it? These questions remain to be answered by the
research of psychologists and other scientists who continue to unravel the mysteries of
behavior and mental processes.
Association Cortex
association cortex The parts of the
cerebral cortex that integrate sensory
and motor information and perform
complex cognitive tasks.
Parts of the cortex that do not directly receive specific sensory information or control
specific movements are referred to as association cortex. The term association
describes these areas well, because they receive input from more than one sense or input
that combines sensory and motor information. For instance, these areas associate words
with images. Association cortex appears in all lobes of the brain and forms a large part
of the cerebral cortex in humans. For this reason, damage to association areas can create serious problems in a wide range of mental abilities.
The Central Nervous System: Making Sense of the World
LINKAGES
Where are the brain’s
language centers? (a link to
Thought, Language, and
Intelligence)
doing
2
learn
by
LANGUAGE AREAS OF THE
BRAIN Have you ever tried
to write notes while you were
talking to someone? Like this teacher,
you can probably write and talk at the
same time, because each of these language functions uses different areas of
association cortex. However, stop reading
for a moment, and try writing one word
with your left hand and a different word
with your right hand. If you had trouble,
it is partly because you asked the same
language area of your brain to do two
things at once.
69
Consider language. Language information comes from the auditory cortex for spoken language or from the visual cortex for written language. Areas of the motor cortex
produce speech. Putting it all together in the complex activity known as language
involves activity in association cortex. In the 1860s, a French surgeon named Paul Broca
described the effects of damage to association cortex in the frontal lobe near motor
areas that control face muscles. This part of the cortex is on the left side of the brain
and is called Broca’s area (see Figure 2.10). Damage to Broca’s area disrupts speech
organization, a condition called Broca’s aphasia. Victims have difficulty speaking, often
making errors in grammar. Each word comes out slowly.
Other language problems result from damage to a portion of association cortex
described in the 1870s by a Polish neurologist named Carl Wernicke (pronounced
“VER-nick-ee”). Figure 2.10 shows that, like Broca’s area, Wernicke’s area is on the left
side of the brain, but it is in the temporal lobe, near the area of the sensory cortex that
receives information from the ears. Wernicke’s area also receives input from the visual
cortex and is involved in the interpretation of both speech and written words. Damage
to Wernicke’s area produces complicated symptoms. It can leave patients with the ability to speak but disrupts the ability to understand the meaning of words or to speak
understandably.
Case studies illustrate the different effects of damage to each area (Lapointe, 1990).
In response to the request “Tell me what you do with a cigarette,” a person with
Wernicke’s aphasia replied, “This is a segment of a pegment. Soap a cigarette.” This
speech was fluent but without meaning. In response to the same request, a person with
Broca’s aphasia replied, “Uh . . . uh . . . cigarette [pause] smoke it.” This speech was
meaningful, but halting and awkwardly phrased. Surprisingly, when a person with
Broca’s aphasia sings, the words come easily and correctly. Apparently, words set to
music are handled by one part of the brain and spoken words by another (Jeffries, Fritz,
& Braun, 2003). Some speech therapists take advantage of this fact through “melodic
intonation” therapy, which helps Broca’s aphasia patients to gain fluency by speaking
in a “singsong” manner (Lapointe, 1990).
It appears that written language and spoken language require the use of different
areas of association cortex. So does language involving specific parts of speech (Shapiro,
Moo, & Caramazza, 2006). For example, two women—H.W. and S.J.D—each had a
stroke in 1985, causing damage to different language-related parts of their association
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