Broca's area is a region in the frontal lobe that is thought to play an important role in language production, although its precise linguistic functions are still a bit unclear. In this video, I discuss Broca's area and some hypotheses regarding its function.
Where is Wernicke's area?
Although the location of Wernicke's area is often presented in images and texts as definitive, there is some controversy about the exact location of the region. Typically, however, Wernicke's area is considered to reside in the cortex of the left cerebral hemisphere, surrounding a large groove called the lateral sulcus or Sylvian fissure, near the junction between the parietal and temporal lobes.
What is Wernicke's area and what does it do?
In the second half of the 19th century, neuroscientists were trying to come to grips with a new perspective on the brain that suggested that the two cerebral hemispheres were not completely equivalent in terms of function. The most convincing evidence to support this perspective at that point had been offered up by the famous physician Paul Broca, who had identified a number of cases where damage to the left hemisphere produced deficits in language---whereas damage to the right hemisphere was much less likely to do so. These observations coincided with Broca's identification of what would come to be known as Broca's area---a brain region typically found in the left hemisphere that is thought to be important to the production of speech (see this article for more about Broca's area).
This idea that one hemisphere could be more responsible for a behavior than the other---and in this case that the left hemisphere was dominant when it came to language---was mostly foreign to neuroscientists before Broca (although not completely unheard of as it had previously been proposed by the physician Marc Dax). Many were hesitant to accept it.
Left hemispheric dominance for language got some additional support, however, from the German physician Carl Wernicke in 1874. Wernicke reported that damage to a certain region in the left hemisphere often resulted in a speech deficit where patients were able to produce speech sounds that resembled fluent language, but actually were meaningless. These patients would string together incongruous syllables, neologisms, similar-sounding words substituted for one another, and so on, to produce speech that made little sense. Patients with this disorder, which would come to be known as Wernicke's aphasia, usually also suffer from a deficiency in their ability to understand language. You can see an example of Wernicke's aphasia in the video above.
Wernicke's aphasia contrasted with the syndrome Broca had observed after damage to Broca's area (that syndrome is known as Broca's aphasia). Patients with Broca's aphasia generally have difficulty producing the sounds necessary for speech. Often a patient with Broca's aphasia knows what he or she wants to say, but can't get the words out. Comprehension of language generally remains intact. You can see an example of Broca's aphasia in the video to the right.
Because Wernicke's area seemed to play an important role in language comprehension and the production of language that was intelligible, Wernicke proposed a model for language that involved both his area and Broca's area. Wernicke's area, according to this model, generates plans for meaningful speech. Broca's area, on the other hand, is responsible for taking these plans and generating the movements (e.g. of the tongue, mouth, etc.) required to turn them into vocalizations. To do so, Broca's area sends information about intended speech to the motor cortex, which then signals the muscles involved in speech production to create the vocalizations. Thus, according to this view, Wernicke's area makes sure that language makes sense, while Broca's area helps bring about the muscle movements necessary to actually produce the sounds.This model was later expanded upon by neurologist Norman Geschwind, and it eventually became known as the Wernicke-Geschwind model.
It is now thought, however, that this model is too simplistic. Language is a complex behavior made possible by a list of individual functions---ranging from the retrieval of particular phonemes to the adding of intonation and rhythm---that each likely involves widespread networks; it cannot simply be boiled down to a connection between two brain regions. Additionally, later studies have found that the functions of Broca's and Wernicke's areas are not as circumscribed as once thought. For example, Wernicke's area seems to play a role in speech production and Broca's area contributes to language comprehension. And damage to what is considered Wernicke's area does not always disrupt comprehension, which suggests Wernicke's area is just one component in a larger network involved in understanding language.
Wernicke's area is thus not as anatomically well defined nor functionally well understood as many textbooks would lead you to think. It is thought to be important to language, but researchers are still trying to work out exactly what its role is. It's likely that it functions as part of a larger network, which---when fully understood---might allow us to appreciate the network as the important functional unit for language, rather than focusing so much on the individual brain regions that make up the network.
References (in addition to linked text above):
Binder, JR. The Wernicke area: Modern evidence and a reinterpretation. Neurology. 2015; 85(24): 2170-2175.
Breedlove SM, Watson NV. Biological Psychology. 7th ed. Sunderland, MA: Sinauer Associates, Inc.; 2013.
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
In April of 1861, a 51-year old man was transferred to Paul Broca's surgical ward in a hospital in France. The man, whose name was Leborgne, had epilepsy but was near death due to an uncontrolled infection and the resultant gangrene. There was something curious about Leborgne, however: he had extreme difficulty speaking voluntarily. In fact, one of the only sounds he was able to make--unless antagonized, which could prompt him to curse--was an utterance that resembled the word "tan." This led to "Tan" becoming the patient's nickname, and the way students of psychology and neuroscience would be introduced to him in textbooks for the next 150+ years.
At the time, there was a fairly contentious debate among scientists about the organization of brain function. Some argued that the brain had to be viewed from a holistic perspective. They believed that the activities of the brain were carried out by the brain acting as a whole, and that one couldn't attribute functions to specific regions of the brain, as that approach ignored the necessity of the entire brain being involved. Others contended that the functions of the brain could be localized to certain areas of the organ; thus, from this perspective it was argued that there were centers of the brain specialized for certain tasks.
Speech was an important part of this debate because some believed there was evidence that there were areas of the frontal cortex where the capacity to produce articulate speech was located. Broca, who was already an influential figure in the scientific community, leaned toward the holistic perspective before Tan became his patient.
Tan died six days after he came under Broca's care. Broca performed an autopsy and found that, although Tan's brain was in relatively bad shape overall, there was an especially distinct lesion in his left frontal lobe. This supported the hypothesis that language was localized to the frontal lobes, as it could be interpreted that Tan's brain damage was the root cause of his speech deficit. This finding would eventually lead to Broca putting his full support behind the localization of function argument, and his support would be influential in turning the scientific consensus away from holism.
Over the next several years, Broca discovered a number of other patients who had left frontal lobe damage along with a deficit in producing articulate speech. Over time, the area that Broca repeatedly observed to be lesioned in these cases came to be called Broca's area. The speech disorder that resulted, which involves difficulty primarily with speech production, was called Broca's aphasia.
Although Broca would be best known for his work supporting the importance of the frontal lobe in speech, and the influence this had on the localization of function debate, he also was a pioneering neurosurgeon. He developed several neurosurgical methods that advanced our ability to examine the brain postmortem. However, Broca's findings regarding speech production are what have immortalized him in the history of neuroscience.
If you were listening in on a discussion about the evolutionary origins of language, you might expect to hear theories bandied about concerning evidence for language-like processes in apes. You probably wouldn’t be too shocked to hear someone bring up an example of language in parrots. You might, however, be a little surprised if the conversation turned to the origins of human vocalization in toadfish.
Perhaps this isn’t that surprising, though, when one considers how much of our evolutionary beginnings are shared with fishes. While (of course) fish don’t have language in a human sense, some species do have the ability to make vocalizations in certain situations, like courtship or defense of territory. Although they lack an air tube leading to the mouth, and a larynx to create the vibrational variations more common to land animal utterances, some are able to make noises with an air sac used primarily for buoyancy control and secondary respiration, known as the gas bladder. Fish of the batrachoidid family in particular (i.e. the midshipman and toadfish) have a diverse group of vocalizations. They vary depending on the context, with specific calls for aggression, surprise, or mating (among others).
This leads to a couple of different hypotheses. One is that the ability to vocalize evolved independently a number of times throughout history: in fish, amphibians, reptiles, mammals, and birds. Another is that there is a common origin for the ability to vocalize that can be traced back millions of years to a piscine ancestor. A study published in this week’s Science explores the latter hypothesis by investigating the development of the neural circuitry for vocalization in larval fish.
Studying embryos or larvae is a method used in evolutionary developmental biology. Similarities in the embryonic development of two organisms are considered evidence of a common ancestor. This conclusion is based on the fact that evolution works by the alteration of existing structures. Thus, two related organisms will theoretically have similar embryonic development to a certain point, where it will then diverge in order to form the structures that make the two creatures taxonomically different. A commonly given example of this is the vestigial pharyngeal pouches (gill slits) that human embryos possess early in development.
The authors of the study in Science found that the vocal motor neurons in batrachoidid fish develop in a segmental region that spans the caudal hindbrain and rostral spinal cord. This is similar to the pattern of development found in other vertebrates like frogs and birds. Adult phenotypes seem to indicate a comparable developmental process in reptiles and mammals as well, although embryological studies here are lacking.
The authors conclude that these analogies in the distribution of vocal neurons indicate a conserved developmental pathway that involves Hox gene expression. They suggest this pathway predates the radiation of fish, originating over 400 million years ago. Thus, perhaps the Big Mouth Billy Bass is a more astutely-developed toy than it first appears to be…no, it’s still stupid.
Bass, A.H., Gilland, E.H., Baker, R. (2008). Evolutionary Origins for Social Vocalization in a Vertebrate Hindbrain-Spinal Compartment. Science, 321 (5887), 417-421. DOI:10.1126/science.1157632