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SECTION III
Central & Peripheral Neurophysiology
of artists, musicians, and mathematicians are left-handed. For
unknown reasons, left-handers have slightly but significantly
shorter life spans than right-handers.
Some anatomic differences between the two hemispheres
may correlate with the functional differences. The
planum
temporale,
an area of the superior temporal gyrus that is
involved in language-related auditory processing, is regularly
larger on the left side than the right. It is also larger on the left
in the brain of chimpanzees, even though language is almost
exclusively a human trait. Imaging studies show that other por-
tions of the upper surface of the left temporal lobe are larger in
right-handed individuals, the right frontal lobe is normally
thicker than the left, and the left occipital lobe is wider and pro-
trudes across the midline. Chemical differences also exist
between the two sides of the brain. For example, the concentra-
tion of dopamine is higher in the nigrostriatal pathway on the
left side in right-handed humans but higher on the right in left-
handers. The physiologic significance of these differences is
unknown.
In patients with schizophrenia, MRI studies have demon-
strated reduced volumes of gray matter on the left side in the
anterior hippocampus, amygdala, parahippocampal gyrus,
and posterior superior temporal gyrus. The degree of reduc-
tion in the left superior temporal gyrus correlates with the
degree of disordered thinking in the disease. There are also
apparent abnormalities of dopaminergic systems and cerebral
blood flow in this disease.PHYSIOLOGY OF LANGUAGE
Language is one of the fundamental bases of human intelli-
gence and a key part of human culture. The primary brain ar-
eas concerned with language are arrayed along and near theCLINICAL BOX 19–2
Lesions of Representational
& Categorical Hemispheres
Lesions in the categorical hemisphere produce language
disorders, whereas extensive lesions in the representational
hemisphere do not. Instead, lesions in the representational
hemisphere produce
astereognosis
—the inability to iden-
tify objects by feeling them—and other agnosias.
Agnosia
is the general term used for the inability to recognize ob-
jects by a particular sensory modality even though the sen-
sory modality itself is intact. Lesions producing these de-
fects are generally in the parietal lobe. Especially when they
are in the representational hemisphere, lesions of the infe-
rior parietal lobule, a region in the posterior part of the pa-
rietal lobe that is close to the occipital lobe, cause
unilat-
eral inattention
and
neglect.
Individuals with such lesions
do not have any apparent primary visual, auditory, or so-
matesthetic defects, but they ignore stimuli from the con-
tralateral portion of their bodies or the space around these
portions. This leads to failure to care for half their bodies
and, in extreme cases, to situations in which individuals
shave half their faces, dress half their bodies, or read half of
each page. This inability to put together a picture of visual
space on one side is due to a shift in visual attention to the
side of the brain lesion and can be improved, if not totally
corrected, by wearing eyeglasses that contain prisms.
Hemispheric specialization extends to other parts of the
cortex as well. Patients with lesions in the categorical hemi-
sphere are disturbed about their disability and often de-
pressed, whereas patients with lesions in the representa-
tional hemisphere are sometimes unconcerned and even
euphoric. Lesions of different parts of the categorical hemi-
sphere produce
fluent, nonfluent,
and
anomic aphasias
(see text for more details). Although aphasias are produced
by lesions of the categorical hemisphere, lesions in the rep-
resentational hemisphere also have effects. For example,
they may impair the ability to tell a story or make a joke.
They may also impair a subject’s ability to get the point of a
joke and, more broadly, to comprehend the meaning of dif-
ferences in inflection and the “color” of speech. This is one
more example of the way the hemispheres are specialized
rather than simply being dominant and nondominant.CLINICAL BOX 19–3
Dyslexia
Dyslexia,
which is a broad term applied to impaired ability to
read,
is characterized by difficulties with learning how to de-
code at the word level, to spell, and to read accurately and
fluently. It is frequently due to an inherited abnormality that
affects 5% of the population. Many individuals with dyslexic
symptoms also have problems with short-term memory skills
and problems processing spoken language. Although its pre-
cise cause is unknown, there is evidence that dyslexia is of
neurological origin. Acquired dyslexias occur due to brain
damage in the left hemisphere’s key language areas. Also, in
many cases, there is a decreased blood flow in the angular
gyrus in the categorical hemisphere. There are numerous the-
ories to explain the causes of dyslexia. The
phonological hy-
pothesis
is that dyslexics have a specific impairment in the
representation, storage, and/or retrieval of speech sounds.
The
rapid auditory processing theory
proposes that the pri-
mary deficit is the perception of short or rapidly varying
sounds. The
visual theory
is that a defect in the magnocellu-
lar portion of the visual system slows processing and also
leads to phonemic deficit. More selective speech defects have
also been described. For example, lesions limited to the left
temporal pole (area 38) cause inability to retrieve names of
places and persons but preserves the ability to retrieve com-
mon nouns, that is, the names of nonunique objects. The abil-
ity to retrieve verbs and adjectives is also intact.