| Broca's area | |
|---|---|
Broca's area is made up of Brodmann areas 44 (pars opercularis) and 45 (pars triangularis)
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Broca's area (shown in red)
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| Details | |
| Part of | Frontal lobe |
| Artery | Middle cerebral |
| Vein | Superior sagittal sinus |
| Identifiers | |
| MeSH | D065711 |
| NeuroNames | 2062 |
| FMA | 242176 |
Broca's area, or the Broca area , is a region in the frontal lobe of the dominant hemisphere, usually the left, of the brain with functions linked to speech production.
Language processing has been linked to Broca's area since Pierre Paul Broca reported impairments in two patients. They had lost the ability to speak after injury to the posterior inferior frontal gyrus (pars triangularis) (BA45) of the brain. Since then, the approximate region he identified has become known as Broca's area, and the deficit in language production as Broca's aphasia, also called expressive aphasia. Broca's area is now typically defined in terms of the pars opercularis and pars triangularis of the inferior frontal gyrus, represented in Brodmann's cytoarchitectonic map as Brodmann area 44 and Brodmann area 45 of the dominant hemisphere.
Functional magnetic resonance imaging has shown language processing to also involve the third part of the inferior frontal gyrus the pars orbitalis, as well as the ventral part of BA6 and these are now often included in a larger area called Broca's region.
Studies of chronic aphasia have implicated an essential role of Broca's area in various speech and language functions. Further, fMRI studies have also identified activation patterns in Broca's area associated with various language tasks. However, slow destruction of the Broca's area by brain tumors can leave speech relatively intact, suggesting its functions can shift to nearby areas in the brain.
Structure
Broca's area is often identified by visual inspection of the topography of the brain either by macrostructural landmarks such as sulci or by the specification of coordinates in a particular reference space. The currently used Talairach and Tournoux atlas projects Brodmann's cytoarchitectonic map onto a template brain. Because Brodmann's parcelation
was based on subjective visual inspection of cytoarchitectonic borders
and also Brodmann analyzed only one hemisphere of one brain, the result
is imprecise. Further, because of considerable variability across brains
in terms of shape, size, and position relative to sulcal and gyral
structure, a resulting localization precision is limited.
Nevertheless, Broca's area in the left hemisphere and its homologue in the right hemisphere are designations usually used to refer to the triangular part of inferior frontal gyrus (PTr) and the opercular part of inferior frontal gyrus (POp). The PTr and POp are defined by structural landmarks that only probabilistically divide the inferior frontal gyrus into anterior and posterior cytoarchitectonic areas of 45 and 44, respectively, by Brodmann's classification scheme.
Area 45 receives more afferent connections from the prefrontal cortex, the superior temporal gyrus, and the superior temporal sulcus, compared to area 44, which tends to receive more afferent connections from motor, somatosensory, and inferior parietal regions.
The differences between area 45 and 44 in cytoarchitecture and in
connectivity suggest that these areas might perform different
functions. Indeed, recent neuroimaging
studies have shown that the PTr and Pop, corresponding to areas 45 and
44, respectively, play different functional roles in the human with
respect to language comprehension and action recognition/understanding.
Functions
Language comprehension
For
a long time, it was assumed that the role of Broca's area was more
devoted to language production than language comprehension. However,
there is evidence to demonstrate that Broca's area also plays a
significant role in language comprehension. Patients with lesions
in Broca's area who exhibit agrammatical speech production also show
inability to use syntactic information to determine the meaning of
sentences. Also, a number of neuroimaging studies have implicated an involvement of Broca's area, particularly of the pars opercularis of the left inferior frontal gyrus, during the processing of complex sentences. Further, it has recently been found in functional magnetic resonance imaging (fMRI) experiments involving highly ambiguous sentences result in a more activated inferior frontal gyrus.
Therefore, the activity level in the inferior frontal gyrus and the
level of lexical ambiguity are directly proportional to each other,
because of the increased retrieval demands associated with highly
ambiguous content.
There is also specialisation for particular aspects of comprehension within Broca's area. Work by Devlin et al. (2003) showed in a repetitive transcranial magnetic stimulation (rTMS) study that there was an increase in reaction times when performing a semantic task under rTMS aimed at the pars triangularis
(situated in the anterior part of Broca's area). The increase in
reaction times is indicative that that particular area is responsible
for processing that cognitive function. Disrupting these areas via TMS
disrupts computations performed in the areas leading to an increase in
time needed to perform the computations (reflected in reaction times).
Later work by Nixon et al. (2004)
showed that when the pars opercularis (situated in the posterior part
of Broca's area) was stimulated under rTMS there was an increase in
reaction times in a phonological task. Gough et al. (2005)
performed an experiment combining elements of these previous works in
which both phonological and semantic tasks were performed with rTMS
stimulation directed at either the anterior or the posterior part of
Broca's area. The results from this experiment conclusively
distinguished anatomical specialisation within Broca's area for
different components of language comprehension. Here the results showed
that under rTMS stimulation:
- Semantic tasks only showed a decrease in reaction times when stimulation was aimed at the anterior part of Broca's area (where a decrease of 10% (50ms) was seen compared to a no-TMS control group)
- Phonological tasks showed a decrease in reaction times when stimulation was aimed at the posterior part of Broca's area (where a decrease of 6% (30ms) was seen compared to control)
To summarise, the work above shows anatomical specialisation in
Broca's area for language comprehension, with the anterior part of
Broca's area responsible for understanding the meaning of words
(semantics) and the posterior part of Broca's area responsible for
understanding how words sound (phonology).
Action recognition and production
Recent
experiments have indicated that Broca's area is involved in various
cognitive and perceptual tasks. One important contribution of Brodmann's
area 44 is also found in the motor-related processes. Observation of
meaningful hand shadows resembling moving animals activates frontal
language area, demonstrating that Broca's area indeed plays a role in
interpreting action of others. An activation of BA 44 was also reported during execution of grasping and manipulation.
Speech-associated gestures
It
has been speculated that because speech-associated gestures could
possibly reduce lexical or sentential ambiguity, comprehension should
improve in the presence of speech-associated gestures. As a result of
improved comprehension, the involvement of Broca's area should be
reduced.
Many neuroimaging studies have also shown activation of Broca's
area when representing meaningful arm gestures. A recent study has shown
evidence that word and gesture are related at the level of translation
of particular gesture aspects such as its motor goal and intention. This finding helps explain why, when this area is defective, those who use sign language also suffer from language deficits.
This finding that aspects of gestures are translated in words within
Broca's area also explains language development in terms of evolution.
Indeed, many authors have proposed that speech evolved from a primitive
communication that arose from gestures.
Speaking without Broca's area
Damage
to Broca's area is commonly associated with telegraphic speech made up
of content vocabulary. For example, a person with Broca's aphasia may
say something like, "Drive, store. Mom." meaning to say, "My mom drove
me to the store today." Therefore, the content of the information is
correct, but the grammar and fluidity of the sentence is missing.
The essential role of the Broca's area in speech production has
been questioned since it can be destroyed while leaving language nearly
intact. In one case of a computer engineer, a slow-growing glioma tumor was removed. The tumor and the surgery destroyed the left inferior and middle frontal gyrus, the head of the caudate nucleus, the anterior limb of the internal capsule, and the anterior insula.
However, there were minimal language problems three months after
removal and the individual returned to his professional work. These
minor problems include the inability to create syntactically complex
sentences including more than two subjects, multiple causal conjunctions, or reported speech. These were explained by researchers as due to working memory problems. They also attributed his lack of problems to extensive compensatory mechanisms enabled by neural plasticity in the nearby cerebral cortex and a shift of some functions to the homologous area in the right hemisphere.
Clinical significance
Stuttering
A speech disorder known as stuttering is seen to be associated with underactivity in Broca's area.
Aphasia
Aphasia
is an acquired language disorder affecting all modalities such as
writing, reading, speaking, and listening and results from brain damage.
It is often a chronic condition that creates changes in all areas of
one's life.
Expressive aphasia vs. other aphasias
Patients with expressive aphasia, also known as Broca's aphasia, are individuals who know "what they want to say, they just cannot get it out".
They are typically able to comprehend words, and sentences with a
simple syntactic structure (see above), but are more or less unable to
generate fluent speech. Other symptoms that may be present include
problems with fluency, articulation, word-finding, word repetition, and producing and comprehending complex grammatical sentences, both orally and in writing.
This specific group of symptoms distinguishes those who have
expressive aphasia from individuals with other types of aphasia. There
are several distinct "types" of aphasia, and each type is characterized
by a different set of language deficits. Although those who have
expressive aphasia tend to retain good spoken language comprehension,
other types of aphasia can render patients completely unable to
understand any language at all, unable to understand any spoken language
(auditory verbal agnosia),
whereas still other types preserve language comprehension, but with
deficits. People with expressive aphasia may struggle less with reading
and writing than those with other types of aphasia.
Although individuals with expressive aphasia tend to have a good
ability to self-monitor their language output (they "hear what they say"
and make corrections), other types of aphasics can seem entirely
unaware of their language deficits.
In the classical sense, expressive aphasia is the result of
injury to Broca's area; it is often the case that lesions in specific
brain areas cause specific, dissociable symptoms, although case studies show there is not always a one-to-one mapping between lesion location and aphasic symptoms.
The correlation between damage to certain specific brain areas (usually
in the left hemisphere) and the development of specific types of
aphasia makes it possible to deduce (albeit very roughly) the location
of a suspected brain lesion based only on the presence (and severity) of
a certain type of aphasia, though this is complicated by the
possibility that a patient may have damage to a number of brain areas
and may exhibit symptoms of more than one type of aphasia. The
examination of lesion data in order to deduce which brain areas are
essential in the normal functioning of certain aspects of cognition is
called the deficit-lesion method; this method is especially important in
the branch of neuroscience known as aphasiology. Cognitive science - to be specific, cognitive neuropsychology - are branches of neuroscience that also make extensive use of the deficit-lesion method.
| Type of aphasia | Speech repetition | Naming | Auditory comprehension | Fluency |
|---|---|---|---|---|
| Expressive aphasia | Moderate–severe | Moderate–severe | Mild difficulty | Non-fluent, effortful, slow |
| Receptive aphasia | Mild–severe | Mild–severe | Defective | Fluent paraphasic |
| Conduction aphasia | Poor | Poor | Relatively good | Fluent |
| Mixed transcortical aphasia | Moderate | Poor | Poor | Non-fluent |
| Transcortical motor aphasia | Good | Mild–severe | Mild | Non-fluent |
| Transcortical sensory aphasia | Good | Moderate–severe | Poor | Fluent |
| Global aphasia | Poor | Poor | Poor | Non-fluent |
| Anomic aphasia | Mild | Moderate–severe | Mild | Fluent |
Since studies carried out in the late 1970's it has been understood that the relationship between Broca's area and Broca's aphasia is not as consistent as once thought.
Lesions to Broca's area alone don't result in a Broca's aphasia, nor do
Broca's aphasic patients necessarily have lesions in Broca's area.
Lesions to Broca's area alone are known to produce just a transient
mutism that resolves inside 3–6 weeks. This discovery suggests that
Broca's area may be included in some aspect of verbalization or
articulation; however, it does not address its part in sentence
comprehension. Still, Broca's area frequently emerges in functional
imaging studies of sentence processing. However, it also becomes activated in word-level tasks. This suggests that Broca’s area is not dedicated to sentence processing
but supports a function common to both. In fact, Broca's area can show
activation in such non-linguistic tasks as imagery of motion.
Considering the hypothesis that Broca's area may be most involved
in articulation, its activation in all of these tasks may be due to
subjects' covert articulation while formulating a response. Despite this
caveat, a consensus seems to be forming that whatever role Broca's area
may play, it may relate to known working memory functions of the
frontal areas. (There is a wide distribution of Talairach coordinates
reported in the functional imaging literature that are referred to as
part of Broca's area.) The processing of a passive voice sentence, for
example, may require working memory to assist in the temporary retention
of information while other relevant parts of the sentence are being
manipulated (i.e. to resolve the assignment of thematic roles to
arguments). Miyake, Carpenter, and Just have proposed that sentence
processing relies on such general verbal working memory mechanisms while
Caplan and Waters consider Broca’s area to be involved in working
memory specifically for syntactic processing. Friederici (2002) breaks
Broca's area into its component regions and suggests that Brodmann's
area 44 is involved in working memory for both phonological
and syntactic structure. This area becomes active first for phonology
and later for syntax as the time course for the comprehension process
unfolds. Brodmann's area 45 together with Brodmann's area 47 is viewed
as being specifically involved in working memory for semantic features
and thematic structure where processes of syntactic reanalysis and
repair are required. These areas come online after Brodmann's area 44
has finished its processing role and where comprehension of complex
sentences must rely on general memory resources. All of these theories
indicate a move towards a view that syntactic comprehension problems
arise from a computational rather than a conceptual deficit. Newer
theories are taking a more dynamic view of how the brain integrates
different linguistic and cognitive components and are examining the time
course of these operations.
Neurocognitive studies have already implicated frontal areas
adjacent to Broca's area as important for working memory in
non-linguistic as well as linguistic tasks.
Cabeza and Nyberg's analysis of imaging studies of working memory
supports the view that BA45/47 is recruited for selecting or comparing
information, while BA9/46 might be more involved in the manipulation of
information in working memory. Since large lesions are typically
required to produce a Broca's aphasia, it is likely that these regions
may also become compromised in some patients and may contribute to their
comprehension deficits for complex morphosyntactic structures.
Broca's area: A key center in the linking phonemic sequences
Broca's
area has been previously associated with a variety of processes,
including phonological segmentation, syntactic processing, and
unification, all of which involve segmenting and linking different types
of linguistic information.
Although repeating and reading single words do not engage semantic and
syntactic processing, they do require an operation linking phonemic
sequences with motor gestures. Findings indicate that this linkage is
coordinated by Broca's area through reciprocal interactions with
temporal and frontal cortices responsible for phonemic and articulatory
representations, respectively, including interactions with motor cortex
before the actual act of speech. Based on these unique findings, it has
been proposed
that Broca's area is not the seat of articulation per se, but rather is
a key node in manipulating and forwarding neural information across
large-scale cortical networks responsible for key components of speech
production.
History
In a
study published in 2007, the preserved brains of both Leborgne and
Lelong (patients of Broca) were reinspected using high-resolution
volumetric MRI.
The purpose of this study was to scan the brains in three dimensions
and to identify the extent of both cortical and subcortical lesions in
more detail. The study also sought to locate the exact site of the
lesion in the frontal lobe in relation to what is now called Broca's
area with the extent of subcortical involvement.
Broca's patients
Louis Victor Leborgne (Tan)
Leborgne was a patient of Broca's. At 30 years old, he was almost completely unable to produce any words or phrases. He was able to repetitively produce only the word tan. After his death, a neurosyphilitic lesion was discovered on the surface of his left frontal lobe.
Lelong
Lelong
was another patient of Broca's. He also exhibited reduced productive
speech. He could only say five words, 'yes', 'no', 'three', 'always',
and 'lelo' (a mispronunciation of his own name). A lesion within the
lateral frontal lobe was discovered during Lelong's autopsy. Broca's
previous patient, Leborgne, had this lesion in the same area of his
frontal lobe. These two cases led Broca to believe that speech was
localized to this particular area.
MRI findings
Examination of the brains of Broca's two historic patients with high-resolution MRI
has produced several interesting findings. First, the MRI findings
suggest that other areas besides Broca's area may also have contributed
to the patients' reduced productive speech. This finding is significant
because it has been found that, though lesions
to Broca's area alone can possibly cause temporary speech disruption,
they do not result in severe speech arrest. Therefore, there is a
possibility that the aphasia denoted by Broca as an absence of productive speech also could have been influenced by the lesions in the other region.
Another finding is that the region, which was once considered to be
critical for speech by Broca, is not precisely the same region as what
is now known as Broca's area. This study provides further evidence to
support the claim that language and cognition are far more complicated
than once thought and involve various networks of brain regions.
Evolution of language
The pursuit of a satisfying theory that addresses the origin of language
in humans has led to the consideration of a number of evolutionary
"models". These models attempt to show how modern language might have
evolved, and a common feature of many of these theories is the idea that
vocal communication was initially used to complement a far more dominant mode of communication through gesture. Human language might have evolved
as the "evolutionary refinement of an implicit communication system
already present in lower primates, based on a set of hand/mouth
goal-directed action representations."
"Hand/mouth goal-directed action representations" is another way
of saying "gestural communication", "gestural language", or
"communication through body language". The recent finding that Broca's
area is active when people are observing others engaged in meaningful
action is evidence in support of this idea. It was hypothesized that a
precursor to the modern Broca's area was involved in translating
gestures into abstract ideas by interpreting the movements of others as
meaningful action with an intelligent purpose. It is argued that over
time the ability to predict the intended outcome and purpose of a set of
movements eventually gave this area the capability to deal with truly
abstract ideas, and therefore (eventually) became capable of associating
sounds (words) with abstract meanings. The observation that frontal
language areas are activated when people observe Hand Shadows
is further evidence that human language may have evolved from existing
neural substrates that evolved for the purpose of gesture recognition.
The study, therefore, claims that Broca's area is the "motor center for
speech", which assembles and decodes speech sounds in the same way it
interprets body language and gestures. Consistent with this idea is that
the neural substrate that regulated motor control in the common
ancestor of apes and humans was most likely modified to enhance
cognitive and linguistic ability. Studies of speakers of American Sign Language
and English suggest that the human brain recruited systems that had
evolved to perform more basic functions much earlier; these various
brain circuits, according to the authors, were tapped to work together
in creating language.
Another recent finding has showed significant areas of activation
in subcortical and neocortical areas during the production of
communicative manual gestures and vocal signals in chimpanzees. Further, the data indicating that chimpanzees
intentionally produce manual gestures as well as vocal signals to
communicate with humans suggests that the precursors to human language
are present at both the behavioral and neuronanatomical levels. More
recently, the neocortical distribution of activity-dependent gene
expression in marmosets provided direct evidence that the ventrolateral
prefrontal cortex, which comprises Broca's area in humans and has been
associated with auditory processing of species-specific vocalizations
and orofacial control in macaques, is engaged during vocal output in a New World monkey.
These findings putatively set the origin of vocalization-related
neocortical circuits to at least 35 million years ago, when the Old and
New World monkey lineages split.
