Broca's area

Broca's area is a region of the hominid brain with functions linked to speech production.

The production of language has been linked to the 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 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. 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 areas 44 and 45. Studies of chronic aphasia have implicated an essential role of Broca’s area in various speech and language functions. Further, functional MRI 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.

Anatomy and connectivity
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 pars triangularis (PTr) and pars opercularis (POp) of the inferior frontal gyrus. 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 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.

Broca's area revisited
In a recent study, the preserved brains of both Leborgne and Lelong (patients of Paul Pierre 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.

Leborgne
Leborgne was a patient of Paul Pierre Broca. He was unable to produce any words or phrases. The only word he could repetitively produce was 'tan'. After his death, a lesion was discovered on the surface of the left frontal lobe.

Lelong
Lelong was another patient of Paul Pierre Broca. He also exhibited reduced productive speech. He could only say five words, 'yes,' 'no,' 'three,' 'always,' and 'lelo' (a mispronunciation of his own name). At autopsy, a lesion was also found in the same region of lateral frontal lobe as in Leborgne. These two cases led Paul Pierre Broca to believe that speech was localized to this particular area.

MRI findings
Examination of the brains of Paul Pierre 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 interesting 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 that language and cognition are far more complicated than once thought and involve various networks of brain regions.

Speaking without Broca’s area
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 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.

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, recent evidence demonstrates 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.

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 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. (See below.)

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. ''

Broca's aphasia vs. other aphasias
Patients with Broca's aphasia are individuals who know "what they want to say, they just cannot get it out." They are typically able to understand what is being said to them, but unable to fluently speak. This is also known as non-fluent aphasia. 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 Broca's aphasics 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. While Broca's aphasics 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 (pure word deafness),  while still other types preserve language comprehension, but with deficits. People with Broca's aphasia may struggle less with reading and writing (see alexia) than those with other types of aphasia. While individuals with Broca’s 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 oblivious to their language deficits.

Broca's aphasia classically 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 - specifically cognitive neuropsychology - are branches of neuroscience that also make extensive use of the deficit-lesion method.

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 deaf "speakers" of American Sign Language and English speakers 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.