1.2 Functional Neuroanatomy
11
injury. These data indicate that certain regions of cortex may be nec-
essary for a specific function, but they do not define the complete set
of structures that suffices for a complex task. Anecdotal evidence from
surface electrocorticography for the study of epilepsy, for example, sug-
gests that a right parietal seizure impulse may shoot immediately to the
left frontal lobe and then to the right temporal lobe before spreading
locally to the remainder of the parietal lobe. This evidence illustrates
the limitations of naively assigning a mental function to a single brain
region. Functional neuroimaging studies frequently reveal simultaneous
activation of disparate brain regions during the performance of even
a simple cognitive task. Nevertheless, particularly in the processing of
vision and language, fairly well-defined lobar syndromes have been
confirmed.
Language
The clearest known example of hemispheric lateralization is
the localization of language functions to the left hemisphere.
Starting with the work of Pierre Broca and Karl Wernicke in the
19
th
century, researchers have drawn a detailed map of language
comprehension and expression.
At least eight types of aphasias in which one or more com-
ponents of the language pathway are inured have been defined.
Prosody,
the emotional and affective components of language,
or “body language,” appears to be localized in a mirror set of
brain units in the right hemisphere.
Because of the major role of verbal and written language in
human communication, the neuroanatomical basis of language
is the most completely understood association function. Lan-
guage disorders, also called
aphasias,
are readily diagnosed in
routine conversation, whereas perceptual disorders may escape
notice, except during detailed neuropsychological testing,
although these disorders may be caused by injury of an equal
volume of cortex. Among the earliest models of cortical local-
ization of function were Broca’s 1865 description of a loss of
fluent speech caused by a lesion in the left inferior frontal lobe
and Wernicke’s 1874 localization of language comprehension to
the left superior temporal lobe. Subsequent analyses of patients
rendered aphasic by strokes, trauma, or tumors have led to the
definition of the entire language association pathway from sen-
sory input through the motor output.
Language most clearly demonstrates hemispheric localiza-
tion of function. In most persons, the hemisphere dominant for
language also directs the dominant hand. Ninety percent of the
population is right-handed, and 99 percent of right-handers have
left hemispheric dominance for language. Of the 10 percent who
are left-handers, 67 percent also have left hemispheric language
dominance; the other 33 percent have either mixed or right hemi-
spheric language dominance. This innate tendency to lateraliza-
tion of language in the left hemisphere is highly associated with
an asymmetry of the planum temporale, a triangular cortical patch
on the superior surface of the temporal lobe that appears to harbor
Wernicke’s area. Patients with mixed hemispheric dominance for
language lack the expected asymmetry of the planum temporale.
That asymmetry has been observed in prenatal brains suggests a
genetic determinant. Indeed, the absence of asymmetry runs in
families, although both genetic and intrauterine influences prob-
ably contribute to the final pattern.
Language comprehension is processed at three levels. First,
in
phonological processing,
individual sounds, such as vow-
els or consonants, are recognized in the inferior gyrus of the
frontal lobes. Phonological processing improves if lip reading is
allowed, if speech is slowed, or if contextual clues are provided.
Second,
lexical processing
matches the phonological input with
recognized words or sounds in the individual’s memory. Lexical
processing determines whether a sound is a word. Recent evi-
dence has localized lexical processing to the left temporal lobe,
where the representations of lexical data are organized accord-
ing to semantic category. Third,
semantic processing
connects
the words to their meaning. Persons with an isolated defect in
semantic processing may retain the ability to repeat words in the
absence of an ability to understand or spontaneously generate
speech. Semantic processing activates the middle and superior
gyri of the left temporal lobe, whereas the representation of the
conceptual content of words is widely distributed in the cortex.
Language production proceeds in the opposite direction, from the
cortical semantic representations through the left temporal lexical
nodes to either the oromotor phonological processing area (for
speech) or the graphomotor system (for writing). Each of these
areas can be independently or simultaneously damaged by stroke,
trauma, infection, or tumor, resulting in a specific type of aphasia.
The garbled word salad or illogical utterances of an aphasic
patient leave little uncertainty about the diagnosis of left-sided
cortical injury, but the right hemisphere contributes a somewhat
more subtle, but equally important, affective quality to lan-
guage. For example, the phrase “I feel good” may be spoken
with an infinite variety of shadings, each of which is understood
differently. The perception of prosody and the appreciation of
the associated gestures, or “body language,” appear to require an
intact right hemisphere. Behavioral neurologists have mapped
an entire pathway for prosody association in the right hemi-
sphere that mirrors the language pathway of the left hemisphere.
Patients with right hemisphere lesions, who have impaired com-
prehension or expression of prosody, may find it difficult to
function in society despite their intact language skills.
Developmental dyslexia is defined as an unexpected diffi-
culty with learning in the context of adequate intelligence, moti-
vation, and education. Whereas speech consists of the logical
combination of 44 basic phonemes of sounds, reading requires
a broader set of brain functions and, thus, is more susceptible
to disruption. The awareness of specific phonemes develops at
about the age of 4 to 6 years and appears to be prerequisite to
acquisition of reading skills. Inability to recognize distinct pho-
nemes is the best predictor of a reading disability. Functional
neuroimaging studies have localized the identification of let-
ters to the occipital lobe adjacent to the primary visual cortex.
Phonological processing occurs in the inferior frontal lobe, and
semantic processing requires the superior and middle gyri of the
left temporal lobe. A recent finding of uncertain significance is
that phonological processing in men activates only the left infe-
rior frontal gyrus, whereas phonological processing in women
activates the inferior frontal gyrus bilaterally. Careful analysis
of an individual’s particular reading deficits can guide remedial
tutoring efforts that can focus on weaknesses and thus attempt
to bring the reading skills up to the general level of intelligence
and verbal skills.
In children, developmental nonverbal learning disorder is pos-
tulated to result from right hemisphere dysfunction. Nonverbal
learning disorder is characterized by poor fine-motor control in
the left hand, deficits in visuoperceptual organization, problems
with mathematics, and incomplete or disturbed socialization.
