NBIO 401
Fall 2009
Language and Cortex
Class 11 – Friday, October 23, 2009
Marrs
Objectives:
At the end of this lecture you should be able to:
1) Be able to describe the Wernicke-Geschwind model for language processing.
2) Be able to describe the limitations of the Wernicke-Geschwind model and the more
contemporary view of the neural substrates for language in the human brain.
3) Be able to describe different types of aphasia and the cortical regions associated with
them.
4) Be able to discuss lateralization of the brain.
• Which hemisphere is dominant?
• What tasks are associated with the right hemisphere?
• What tasks are associated with the left hemisphere?
• How have decommisurated patients helped us learn about brain lateralization?
Introduction
Language is a system of communication in which ideas and feelings are encoded into signals
of sounds, gestures, signs, or marks that convey meaning within a group or community.
Language consists of two components – words and grammar. A word is an arbitrary
association between a signal and a meaning. Grammar is a system that specifies how words
can be combined and how the meaning of a combination of words can be determined.
Language Acquisition in Children
Humans are born with the ability to perceive the full range of phonemes (the smallest unit of
speech sound). Babies make language-like sounds at 5-7 months, babble in well-formed
syllables at 7-8 months, and gibber in sentence-like streams by 12 months. They can
discriminate speech sounds, even ones not used in their parents’ language, in their first
months. By 10 months, they discriminate only phonemes used by their parents. By age one,
babies can begin to comprehend words and have a 30-50 word vocabulary. By age three,
children speak in full sentences and have a rich vocabulary, but still have difficulty with
grammar. Older children build up understanding of grammatical structure, rather than simply
imitating the speech of others. By age 6, children comprehend about 13,000 words, and high
school graduates know 60,000 words.
Language development in children led Noam Chomsky to hypothesize in the late 1950’s that
the human brain has evolved an innate neural circuitry dedicated to the acquisition of
language. Some psychologists and linguists disagree, believing that the capacity for
language is one expression of a general cognitive ability to learn patterns, rather than a
specific system for language. Whichever view is correct, it is clear that language is an ability
that is both innate and learned.
There are no homologs to human language in other species of the animal kingdom. The lack
of animal models for language has limited our ability to understand the neural basis of
language. Consequently, most of our knowledge about which brain regions are involved in
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language processing and how they function derives from the description of language
disorders, or aphasias, caused by focal brain lesions, most frequently stroke or head injury.
Lateralization of language processing
Early studies of people with stroke or brain trauma found correlations between patients’
language deficits and the brain areas that had been damaged, determined by an autopsy at
the time of death. These studies led to the discovery that most language processing goes on
in one hemisphere, called the “dominant” hemisphere. In over 90% of people (98% in righthanded people and about 65% of left-handed people) the left hemisphere processes
grammar, lexicon, phonetics, and speech production.
The Wernicke-Geschwind model of language comprehension and speech production
The other major discovery revealed by study of patients with aphasias was that two cerebral
cortical areas – Broca’s area in the lateral frontal lobe and Wernicke’s area in the posterior
superior temporal lobe – are major neural substrates of language.
A useful, if somewhat
simplified, model for
understanding how the
two language areas
interact is the WernickeGeschwind model. This
model has been quite
successful in predicting
the effects of damage in
several brain regions and
how visual information is
processed in naming an
object.
The model posits that
Broca’s area, which is connected to regions of motor cortex that control the face and tongue,
is involved in planning the motor execution of speech, and that Wernicke’s area, which is
connected to auditory cortex, is involved in recognizing and representing the sound pattern of
words. Within this model, the arcuate fasciculus is a unidirectional pathway that brings
information from Wernicke’s area to Broca’s area.
According to the Wernicke-Geschwind model, there
is a clear, linear flow of information through these
two cortical areas for the recognition and
production of speech. Wernicke’s area contains
the auditory codes for words - what they sound like.
Broca’s area contains the articulatory codes for
words - the motor commands that tell the mouth
and larynx how to articulate each word. The
behavior of repeating a spoken word exemplifies
how the Wernicke-Geschwind model understands
language processing. When a spoken word is
Arcuate fasciculus
heard, the sound is processed in auditory cortex and then transmitted to Wernicke's area.
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There the sound is matched to its auditory code, and it’s meaning can be interpreted by other
association areas of the cerebral cortex. The auditory code is transmitted to Broca's area
through the arcuate fasciculus, where the
articulatory code for the word is activated
and sent to motor cortex for speech
production.
Reading comprehension also requires
Wernicke’s area in the WernickeGeschwind model. Visual information is
sent to the angular gyrus which translates
the visual code to a form accessible by
Wernicke’s area. Wernicke’s area then
matches the word to its auditory code, as
for spoken speech. Thus, according to
this model, reading requires the
phonological recoding of words in
Wernicke’s area.
The Wernicke-Geschwind model provided
a framework for understanding the neural
mechanisms of the aphasias. Patients with damage to Broca’s areas cannot produce
speech, but they can still comprehend speech because Wernicke's area is intact. Damage to
Wernicke's area, on the other hand, produces no problems in speech production because
Broca’a area is unaffected, but the meaning of the speech of others is improperly understood,
and the patient’s own speech has virtually no meaning.
Other brain areas and pathways involved in language
Despite its success predicting clinical outcomes, the Wernicke-Geschwind model has
significant limitations. New lesion studies, research in neuropsychology and linguistics,
functional imaging (PET, fMRI), and direct neural recording techniques (event-related
electrical potentials, direct intraoperative recordings from human cerebral cortex) have better
defined the brain areas and pathways
involved in language processing.
It is now clear that there is a direct
connection between the parieto-occipitotemporal association cortices and Broca’s
area. These pathways are involved in the
understanding and repetition of written
language (i.e., visual recognition). Thus,
read words do not need to be transformed
into auditory representations. Instead,
Broca’s area and other higher order
association areas can handle this
information directly.
In addition, while Broca’s and Wernicke’s
areas are important structures for
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language, they do not operate independently for language production and comprehension to
the extent hypothesized in the Wernicke-Geschwind model. The arcuate fasciculus is in fact
a bidirectional pathway that connects Broca’s and Wernicke’s areas to many other regions of
cortex in the dominant hemisphere. The prefrontal, premotor, and supplementary motor
cortices in the frontal lobes are necessary for higher order aspects of speech planning and
production, and for proper syntax of language comprehension and production. Some areas
of insular cortex are also related to speech articulation. Wernicke’s area has reciprocal
connections with the supramarginal and angular gyri of the parietal lobe as well as with areas
in the temporal lobe in
the dominant
hemisphere. These
regions are important
for language
comprehension, and
also participate in the
mapping of the sounds
of words to their
meanings. The angular
gyrus in the language
dominant hemisphere
contributes to the
understanding of written
language. Finally,
several sub-cortical
structures, such as the
thalamus and basal
ganglia of the dominant
hemisphere, are also
critical for language
processing.
The non-dominant language hemisphere is also involved in many aspects of language.
Lesion studies and studies in split-brain patients reveal that the non-dominant hemisphere
can understand many words and can take on responsibility for many aspects of language in
children with damage to the dominant hemisphere. In normal function, the non-dominant
language hemisphere participates in the emotional aspects of comprehension and speech
production (e.g., inflections, tone of voice, timing) and in the pragmatics of language (e.g.,
contextually appropriate speech). It has been suggested that the non-dominant hemisphere
codes information in a more general way, representing the overall structure of a stimulus.
For instance, the right hemisphere is involved in determining if a statement is funny, which
involves analyzing the content of that statement as a whole rather than the individual words.
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Aphasias and other language disorders
Broca’s aphasia is usually the product of damage to Broca’s area and adjacent structures in
the dominant frontal lobe. Damage to this area results in an impairment of language
production. Patients can be completely mute or else show slow halting speech that utilizes
only key words. The ability to read out loud and write is also impaired. People with Broca’s
aphasia are generally aware of their condition.
Wernicke’s aphasia results from lesions to Wernicke’s area and surrounding regions of the
temporal lobe. Damage to this area results in an impairment of language comprehension.
Patients with severe Wernicke’s aphasia do not respond
appropriately to questions and follow almost no commands.
Patients have fluent speech, but they may use the wrong words
or make up new ones; language tends to be excessive and
devoid of meaning. Patients with Wernicke’s aphasia are
unaware of their condition, behaving as if carrying on a normal
conversation despite their markedly abnormal speech.
Conduction aphasia occurs when a language pathway (usually
the arcuate fasciculus) is disrupted, although damage is not necessarily limited to white
matter. Patients with conduction aphasia show fluent speech, though the number of errors
and word substitutions they make are higher than average. They also have difficulty
repeating words and reading out loud, but can read silently. Overall, their ability to
comprehend language is left intact.
Anomic aphasia occurs when the posterior aspect of the left inferior temporal lobe (near the
tempero-occipital border) is damaged. This is the rarest of aphasias. Patients with this form
of aphasia have difficulty finding the correct words when speaking or writing. Otherwise, their
comprehension, production, and repetition abilities are intact.
Global aphasia is the most debilitating form of language impairment. Patients show impaired
comprehension, production, and repetition. Gobal aphasia is caused by rather widespread
damage of the entire perisylvian region, including Broca’s area, Wernicke’s area, and the
arcuate fasciculus.
Transcortical motor aphasia is characterized by the inability to produce creative speech.
Patients attempt to initiate speech, but produce only a few words. However, they can repeat
words and phrases. Comprehension is less disturbed, but writing ability is impaired. This
form of aphasia is due to lesions that disconnect Broca’s area from the supplementary motor
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cortex.
Transcortical sensory aphasia consists of an inability to read or write, but the patient can
repeat spoken language easily and fluently. Comprehension is defective and meanings of
words are lost. This results from lesions in the parieto-occipitotemporal junction.
Naming deficits occur after damage to the anterior temporal and inferotemporal cortices in
the dominant hemisphere. Patients have trouble retrieving words, but do not have
grammatical, phonemic, or speech problems. Focal lesions in these regions of temporal
cortex can separately affect the recall of names for unique place or person, common names,
or the names of specific types of items (e.g., tools) without affecting the recall of words for
actions or relationships between things.
Insular lesions result in difficulty pronouncing phonemes in their proper order. People with
insular damage usually produce combinations of sounds that are not quite correct, even
though they have no trouble selecting words and can recognize their mistakes. There is no
difficulty comprehending speech sounds.
Non-dominant language hemisphere damage disrupts the ability to interpret the emotional
content of spoken language and to produce contextually appropriate speech. People with
such lesions may produce speech with inappropriate intonation and may have difficulty using
socially appropriate language. They may have deficits in interpreting the tone of others’
speech, understanding jokes, and incorporating sentences into written or spoken narratives.
Lateralization of the brain:
The functional lateralization of the cerebral cortex was first suggested by Geshwind and
Levitsky (1968), who discovered that about 60% of human brains display anatomical
differences between the two hemispheres in the posterior temporal lobe (the region which
encompasses Wernicke’s area). In line with these anatomical observations, language was
the first function that was demonstrated to be lateralized in the human cerebral cortex. More
recently, various other anatomical and functional differences have been documented
between the two cerebral hemispheres.
The study of “split-brained” subjects in the late
20th century provided a great deal of information
on the specific roles of the two hemispheres. This
condition results from complete lesion of the
corpus collosum, known as commissurotomy.
Because the callosum is severed, the two
hemispheres can no longer communicate with
each other and there is no transfer of auditory,
visual, or sensory information from one
hemisphere to the other. Commissurotomy is
sometimes the only treatment available for
patients with intractable epilepsy. The procedure
can diminish the number and severity of the
patients’ seizures.
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The corpus collosum is essential for melding the actions of each hemisphere into a unitary
whole. Once the callosum is bisected, the hemispheres appear to act independently. The
study of this phenomenon has led to some surprising findings in the field of perception and in
trying to understand human consciousness. Split-brain experiments often employ a
tachistoscope, an instrument that presents visual stimuli only to the patient’s left or right
visual field (LVF or RVF, respectively). Information about an image presented to the LVF of a
split-brain subject only reaches the RIGHT cerebral hemisphere, and vice versa. When the
image of an object is projected onto the RVF, the subject can easily name the object because
the information is accessible to the left, dominant, language hemisphere. When the image is
projected onto the LVF, the subject cannot verbally identify it and may even deny ever seeing
it - the language areas of the left hemisphere are unaware of the image. However, the
patient can readily identify the object nonverbally, such as pointing to it with the left hand,
or using tactile cues to distinguish it from several
other unseen objects. This suggests that while
the right hemisphere cannot talk, it is able to
perceive, learn, remember, and issue commands
for motor tasks, even when the subject is not
consciously aware of these processes.
In another set of experiments, the split-brained
patients’ ability to identify written words was
examined. Words presented to the RVF were
easily read out loud. On the other hand, the
patients were unable to pronounce the word if it
was presented to the LVF. Interestingly, some
patients could write words projected onto the
LVF, but not the RVF. This finding demonstrates
that some people possess language skills in the
right hemisphere, and that writing may require
non-dominant hemisphere skills such as creativity
or artistic ability.
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