Chapter 12
The Central Nervous System
437
12
that is
dominant for language
. In most people (about 90%), the lef
hemisphere has greater control over language abilities, math, and
logic. Tis so-called dominant hemisphere is working when we
compose a sentence, balance a checkbook, and memorize a list. Te
other hemisphere (usually the right) is more Free-spirited, more
involved in visual-spatial skills, intuition, emotion, and artistic and
musical skills. It is the poetic, creative, and the “Ah-ha!” (insightFul)
side oF our nature, and in males it is better at recognizing Faces.
Most individuals with lef cerebral dominance are right-
handed. In the remaining 10% oF people, the roles oF the hemi-
spheres are reversed or the hemispheres share their Functions
equally. ±ypically, right-cerebral-dominant people are lef-
handed and male. Some “lefies” who have a cerebral cortex that
Functions bilaterally are ambidextrous.
Te two cerebral hemispheres have almost instantaneous com-
munication with each other via connecting fiber tracts, as well as
complete Functional integration. ²urthermore, although laterali-
zation means that each hemisphere is better than the other at cer-
tain Functions, neither hemisphere is better at everything.
Cerebral White Matter
Te second oF the three basic regions oF each cerebral hemi-
sphere is the internal
cerebral white matter
. ²rom what we
have already described, you know that communication within
the brain is extensive. Te white matter deep to the cortical gray
matter is responsible For communication between cerebral areas
and between the cerebral cortex and lower CNS centers.
White matter consists largely oF myelinated fibers bundled
into large tracts. Tese fibers and tracts are classified according
to the direction in which they run as
association
,
commissural,
or
projection
(Figure 12.8)
.
Association fibers
connect different parts oF the same hemi-
sphere. Short association fibers connect adjacent gyri. Long
association fibers are bundled into tracts and connect differ-
ent cortical lobes.
Commissural fibers
connect corresponding gray areas oF
the two hemispheres. Tese
commissures
(kom
9
ĭ-shūrz) al-
low the two hemispheres to Function as a coordinated whole.
Te largest commissure is the
corpus callosum
(kah-lo
9
sum;
“thickened body”), which lies superior to the lateral ventri-
cles, deep within the longitudinal fissure. Less prominent
examples are the
anterior
and
posterior commissures
(see
²igure 12.10, p. 440).
Projection fibers
either enter the cerebral cortex From lower brain
or cord centers or descend From the cortex to lower areas. Sensory
inFormation reaches the cerebral cortex and motor output leaves
it through these projection fibers. Tey tie the cortex to the rest oF
the nervous system and to the body’s receptors and effectors. In
contrast to commissural and association fibers, which run hori-
zontally, projection fibers run vertically (²igure 12.8a).
At the top oF the brain stem, the projection fibers on each
side Form a compact band, the
internal capsule
, that passes
between the thalamus and some oF the basal nuclei. Beyond
that point, the fibers radiate Fanlike through the cerebral
white matter to the cortex. Tis distinctive arrangement is
called the
corona radiata
(“radiating crown”).
perceptions come together in the multimodal association areas.
Along with Feelings oF panic, these perceptions are woven into a
seamless whole, which (hopeFully) recalls instructions about what
to do in this situation. As a result your premotor and primary mo-
tor cortices direct your legs to propel you to the saFety shower.
Te multimodal association areas can be broadly divided
into three parts: the anterior association, posterior association,
and limbic association areas.
1.
Anterior association area.
Te
anterior association area
in the Frontal lobe, also called the
prefrontal cortex
, is the
most complicated cortical region oF all (²igure 12.6). It is
involved with intellect, complex learning abilities (called
cognition), recall, and personality. It contains working
memory, which is necessary For abstract ideas, judgment,
reasoning, persistence, and planning. Tese abilities de-
velop slowly in children, which implies that the preFrontal
cortex matures slowly and depends heavily on positive and
negative Feedback From our social environment.
2.
Posterior association area.
Te
posterior association area
is a large region encompassing parts oF the temporal, pari-
etal, and occipital lobes. Tis area plays a role in recognizing
patterns and Faces, localizing us and our surroundings in
space, and binding different sensory inputs into a coherent
whole. In the spilled acid example above, your awareness oF
the entire scene originates From this area. Attention to an
area oF space or an area oF one’s own body is also a Function
oF this part oF the brain. Many parts oF the posterior associa-
tion area (including Wernicke’s area, ²igure 12.6a) are also
involved in understanding written and spoken language.
3.
Limbic association area.
Te
limbic association area
in-
cludes the cingulate gyrus, parahippocampal gyrus, and
hippocampus (see ²igures 12.6b and 12.16). Part oF the
limbic system (which we describe later), the limbic asso-
ciation area provides the emotional impact that makes a
scene important to us. In our example above, it provides
the sense oF “danger” when the acid splashes on our legs.
Te hippocampus establishes memories that allow us to
remember this incident. More on this later.
Homeostatic Imbalance
12.3
±umors or other lesions oF the
anterior association area
may cause
mental and personality disorders including loss oF judgment, atten-
tiveness, and inhibitions. Te affected individual may be oblivious to
social restraints, perhaps becoming careless about personal appear-
ance, or rashly attacking a 7-Foot opponent rather than running.
Different problems arise For individuals with lesions in the
part oF the
posterior association area
that provides awareness oF
selF in space. Tey may reFuse to wash or dress the side oF their
body opposite to the lesion because “that doesn’t belong to me.”
Lateralization of Cortical Functioning
We use both cerebral
hemispheres For almost every activity, and the hemispheres ap-
pear nearly identical. Nonetheless, there is a division oF labor,
and each hemisphere has abilities not completely shared by its
partner. Tis phenomenon is called
lateralization
.
Although one cerebral hemisphere or the other “dominates”
each task, the term
cerebral dominance
designates the hemisphere
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