Where is Broca's area?
Although the anatomical definitions of Broca's area are not completely consistent, it is generally considered to make up some part of a region called the inferior frontal gyrus, which is found in the frontal lobe. Some researchers ascribe Broca's area to the entire inferior frontal gyrus, while others consider it to only make up a portion of the inferior frontal gyrus. Still others consider the boundary of Broca's area to expand slightly outside of the inferior frontal gyrus.
In the vast majority of individuals, Broca's area is considered to reside in the left cerebral hemisphere. This is due to the role of Broca's area in language and the typical left hemisphere dominance of language function; there is, however, a corresponding region in the right hemisphere---it is just not thought to play as significant a role in language production.
What is Broca's area and what does it do?
In April of 1861, a 51-year old man named Louis Victor Leborgne was admitted to the surgical unit of young physician named Paul Pierre Broca. Leborgne had a severe leg infection that had become gangrenous, and Broca did not think it likely he would survive. Broca took much more interest in Leborgne than he would have in just another patient with cellulitis, however, as Leborgne also had a more unique disorder. The disorder, which Broca would come to all aphemia and which would later be named aphasia (aphasia is the name that would stick), caused Leborgne to have an extremely difficult time producing language. In fact the only word he could consistently generate was the word "tan," which he would often utter in two-word refrains of "tan, tan." Leborgne had thoughts he wanted to communicate, but he was unable to. He used gestures to interact with Broca, but sometimes became frustrated at his inability to express himself---causing him to utter the only other words Broca reported hearing him say: "sacre nom de Dieu," or God damn.
Broca saw an opportunity in Leborgne. At the time there was a debate occurring in some circles of the scientific community; it was centered around the question of whether certain areas of the brain were specialized for certain functions, or if the entire brain was utilized in the performance of every function. The former view, sometimes referred to as localization of function, was the perspective Broca was leaning toward.
One function that advocates of localization (sometimes called localizationists) had argued strongly in favor of being localized was speech. Previous evidence had suggested that the faculty for speech might be centered in the frontal lobes. Thus, when Broca encountered Leborgne he saw an opportunity to test this hypothesis. After Leborgne died, Broca quickly performed an autopsy. Upon examining the brain, Broca found a crater in the left frontal lobe that he described as being as large as a "chicken's egg."
The combination of a left frontal lobe lesion with a deficit in the production of speech caused Broca to recognize this case as a seminal one in the localization argument. He presented the case before groups of intrigued scientists in Paris, and for some it was the evidence that swayed them to favor a more localizationist approach to the brain. Broca was considered a respectable and cautious scientist---not one who jumped to conclusions without an adequate amount of evidence. Thus, the fact that he had come to believe that speech might be localized to the frontal lobes was influential.
Not being completely convinced by only one case, however, Broca continued to look for other cases involving frontal lobe damage and speech deficits after Leborgne. Within just a couple of years, he had identified eight cases. What was perhaps most shocking to Broca was that---in every case---the damage was not only in a similar location in the frontal lobe, but it was also always on the left side. The idea that the two cerebral hemispheres were different in some way was relatively unheard of at this point in time, but the clinical evidence would soon have Broca arguing for that hypothesis along with the localization of speech.
The region Broca had discovered would first be known as Broca's convolution, then Broca's centre, and then---by the early 1900s---Broca's area. In addition to becoming recognized as an important part of the brain for language production, Broca's area would be a critical piece of evidence in the debate over localization of function. Although it would not on its own end the localization debate, it helped to convince many that at least some functions are assigned to relatively circumscribed areas of the brain.
Leborgne's condition became known as Broca's aphasia (also known as expressive aphasia). Its main symptom is a deficit in the ability to produce language (often any type of language, including both spoken and written). Thus, the primary function most often attributed to Broca's area involves language production. Not long after Broca, however, investigators realized that a behavior as complex as speech is not likely to involve only one small region of the brain. Thus, it is now believed that Broca's area plays an important role in language production through communication with several other brain regions.
The precise role of Broca's area in language production is still debated. In other words, evidence suggests that damage to Broca's area can disrupt language production, but nobody is quite sure exactly what specific language-related function is lost to cause that disruption. Some have asserted Broca's area is involved with producing motor movements that allow speech to be produced. Others have argued that it is involved with verbal working memory, syntax, grammar, or all of the above.
Broca's area is thought to also have a variety of other linguistic and non-linguistic functions. In addition to language production, it is now recognized that Broca's area plays an important role in language comprehension. Broca's area is also believed to be involved in movement and action, and has been found to be active during planning movement, imitating movement, and understanding another's movement. Additionally, it has been hypothesized that Broca's area contains mirror neurons that are activated during hand and lip movements and when observing others make similar movements.
Although some of these additional functions linked to Broca's area may be associated with the region's role in language, they also make it clear that the function of Broca's area is much more complex than originally thought. Thus, the role of Broca's area in linguistic and non-linguistic functions is still being elucidated, and will likely be modified and expanded upon many times in the future.
Reference (in addition to linked text above):
Schiller F. 1979. Paul Broca: Founder of French Anthropology, Explorer of the Brain. New York: Oxford University Press.
Read more - History of neuroscience: Paul Broca