BROCA’S AREA , WERNICKE’S AREA, AND OTHER LANGUAGE-PROCESSING AREAS IN THE BRAIN

Broca’s area is generally defined as comprising Brodmann areas 44 and 45, which lie anterior to the premotor cortex in the inferior posterior portion of the frontal lobe. Though both area 44 and area 45 contribute to verbal fluency, each seems to have a separate function, so that Broca’s area can be divided into two functional units.

Area 44 (the posterior part of the inferior frontal gyrus) seems to be involved in phonological processing and in language production as such; this role would be facilitated by its position close to the motor centres for the mouth and the tongue. Area 45 (the anterior part of the inferior frontal gyrus) seems more involved in the semantic aspects of language. Though not directly involved in accessing meaning, Broca’s area therefore plays a role in verbal memory (selecting and manipulating semantic elements).

Wernicke’s area lies in the left temporal lobe and, like Broca’s area, is no longer regarded as a single, uniform anatomical/functional region of the brain. By analyzing data from numerous brain-imaging experiments, researchers have now distinguished three sub-areas within Wernicke’s area. The first responds to spoken words (including the individual’s own) and other sounds. The second responds only to words spoken by someone else but is also activated when the individual recalls a list of words. The third sub-area seems more closely associated with producing speech than with perceiving it. All of these findings are still compatible, however, with the general role of Wernicke’s area, which relates to the representation of phonetic sequences, regardless of whether the individual hears them, generates them himself or herself, or recalls them from memory.

Wernicke’s area, of which the temporal planum is a key anatomical component, is located on the superior temporal gyrus, in the superior portion of Brodmann area 22. This is a strategic location, given the language functions that Wernicke’s area performs. It lies between the primary auditory cortex (Brodmann areas 41 and 42) and the inferior parietal lobule.

This lobule is composed mainly of two distinct regions: caudally, the angular gyrus (area 39), which itself is bounded by the visual occipital areas (areas 17, 18, and 19), and dorsally, the supramarginal gyrus (area 40) which arches over the end of the lateral sulcus, adjacent to the inferior portion of the somatosensory cortex.

The supramarginal gyrus seems to be involved in phonological and articulatory processing of words, whereas the angular gyrus (together with the posterior cingulate gyrus) seems more involved in semantic processing. The right angular gyrus appears to be active as well as the left, thus revealing that the right hemisphere also contributes to semantic processing of language.

Together, the angular and supramarginal gyri constitute a multimodal associative area that receives auditory, visual, and somatosensory inputs. The neurons in this area are thus very well positioned to process the phonological and semantic aspect of language that enables us to identify and categorize objects.

The language areas of the brain are distinct from the circuits responsible for auditory perception of the words we hear or visual perception of the words we read. The auditory cortex lets us recognize sounds, an essential prerequisite for understanding language. The visual cortex, which lets us consciously see the outside world, is also crucial for language, because it enables us to read words and to recognize objects as the first step in identifying them by a name.

MODELS OF SPOKEN AND WRITTEN LANGUAGE FUNCTIONS IN THE BRAIN

In the 1980s, American neurologist Marsel Mesulam proposed an alternative to the Wernicke-Geschwind model for understanding the brain’s language circuits. Mesulam’s model posits a hierarchy of networks in which information is processed by levels of complexity.

For example, when you perform simple language processes such as reciting the months of the year in order, the motor and premotor areas for language are activated directly. But when you make a statement that requires a more extensive semantic and phonological analysis, other areas come into play first.

When you hear words spoken, they are perceived by the primary auditory cortex, then processed by unimodal associative areas of the cortex: the superior and anterior temporal lobes and the opercular part of the left inferior frontal gyrus.

Mesulam does, however, still believe that there are two “epicentres”for semantic processing, i.e., Broca’s area and Wernicke’s area. This new conception of these two areas is consistent with the fact that they often work synchronously when the brain is performing a word processing task, which supports the idea that there are very strong connections between them.

Mesulam’s concept of epicentres resembles that of convergence zones as proposed by other authors: zones where information obtained through various sensory modalities can be combined. This combining process is achieved through the forming of cell assemblies: groups of interconnected neurons whose synapses have been strengthened by their simultaneous firing, in accordance with Hebb’s law. This concept of language areas as convergence zones where neuronal assemblies are established thus accords a prominent place to epigenetic influences in the process of learning a language.

Unquestionably, one of these convergence zones is the left inferior parietal lobule, which comprises the angular gyrus and the supramarginal gyrus. In addition to receiving information from the right hemisphere, the left inferior parietal lobule also integrates emotional associations from the amygdala and the cingulate gyrus.

Some scientists believe that over the course of evolution, language remained under limbic control until the inferior parietal lobule evolved and became a convergence zone that provides a wealth of inputs to Broca’s area. Some scientists also think that it was the emergence of the inferior parietal lobule that gave humans the ability to break down the sounds that they heard so as to make sense of them and, conversely, to express sounds in a sequential manner so as to convey meaning. In this way, primitive emotional and social vocalizations would have eventually come to be governed by grammatical rules of organization to create what we know as modern language.

Lastly, a number of researchers now reject classic locationist models of language such as Geschwind’s and Mesulam’s. Instead, they conceptualize language, and cognitive functions in general, as being distributed across anatomically separate areas that process information in parallel (rather than serially, from one “language area” to another). Even those researchers who embrace this view that linguistic information is processed in parallel still accept that the primary language functions, both auditory and articulatory, are localized to some extent. This concept of a parallel, distributed processing network for linguistic information constitutes a distinctive epistemological paradigm that is leading to the reassessment of certain functional brain imaging studies. The proponents of this paradigm believe that the extensive activation of various areas in the left hemisphere and the large number of psychological processes involved make it impossible to associate specific language functions with specific anatomical areas of the brain. For example, the single act of recalling words involves a highly distributed network that is located primarily in the left brain and that includes the inferolateral temporal lobe, the inferior posterior parietal lobule, the premotor areas of the frontal lobe, the anterior cingulate gyrus, and the supplementary motor area. According to this paradigm, with such a widely distributed, parallel processing network, there is no way to ascribe specific functions to each of these structures that contribute to the performance of this task.

The brain does seem to access meanings by way of categories that it stores in different physical locations. For example, if the temporal pole (the anterior end of the temporal lobe) is damaged, the category “famous people” is lost; if a lesion occurs in the intermediate and inferior parts of the temporal lobe, the category “animals” disappears. It also seems that the networks involved in encoding words activate areas in the motor and visual systems. The task of naming tools activates the frontal premotor areas, while that of naming animals activates the visual areas. But in both cases, Broca’s area and Wernicke’s area are not even activated.

HANDEDNESS, LANGUAGE, AND BRAIN LATERALIZATION

The brain’s anatomical asymmetry, its lateralization for language, and the phenomenon of handedness are all clearly interrelated, but their influences on one another are complex. Though about 90% of people are right-handed, and about 95% of right-handers have their language areas on the left side of their brains, that still leaves 5% of right-handers who are either right-lateralized for language or have their language areas distributed between their two hemispheres. And then there are the left-handers, among whom all of these patterns can be found, including left-lateralization.

Some scientists suggest that the left hemisphere’s dominance for language evolved from this hemisphere’s better control over the right hand. The circuits controlling this “skilful hand” may have evolved so as to take control over the motor circuits involved in language. Broca’s area, in particular, is basically a premotor module of the neocortex and co-ordinates muscle contraction patterns that are related to other things besides language.

Brain-imaging studies have shown that several structures involved in language processing are larger in the left hemisphere than in the right. For instance, Broca’s area in the left frontal lobe is larger than the homologous area in the right hemisphere. But the greatest asymmetries are found mainly in the posterior language areas, such as the temporal planum and the angular gyrus.

Two other notable asymmetries are the larger protrusions of the frontal lobe on the right side and the occipital lobe on the left. These protrusions might, however, be due to a slight rotation of the hemispheres (counterclockwise, as seen from above) rather than to a difference in the volume of these areas. These protrusions are known as the right-frontal and left-occipital petalias (“petalias” originally referred to the indentations that these protrusions make on the inside of of the skull).

The structures involved in producing and understanding language seem to be laid down in accordance with genetic instructions that come into play as neuronal migration proceeds in the human embryo. Nevertheless, the two hemispheres can remain just about equipotent until language acquisition occurs. Normally, the language specialization develops in the left hemisphere, which matures slightly earlier. The earlier, more intense activity of the neurons in the left hemisphere would then lead both to right-handedness and to the control of language functions by this hemisphere.

But if the left hemisphere is damaged or defective, language can be acquired by the right hemisphere. An excess of testosterone in newborns due to stress at the time of birth might well be one of the most common causes of slower development in the left hemisphere resulting in greater participation by the right.

This hypothesis of a central role for testosterone is supported by experiments which showed that in rats, cortical asymmetry is altered if the rodents are injected with testosterone at birth. This hormonal hypothesis would also explain why two-thirds of all left-handed persons are males.

Interindividual variations, which are essential for natural selection, are expressed in various ways in the human brain. Volume and weight can vary by a factor of two or even more. The brain’s vascular structures are extremely variable; the deficit caused by an obstruction at a given point in the vascular system can vary greatly from one individual to another. At the macroscopic anatomical level, the folds and grooves in the brain also vary tremendously from individual to individual, especially in the areas associated with language. Variability in the language areas can also be observed at the microscopic level, for example, in the synaptic structure of the neurons in Wernicke’s area. Interindividual variability is also expressed in the brain’s functional organization, and particularly in the phenomenon of hemispheric asymmetry. For instance, some data indicate that language functions may be more bilateral in women than in men. The percentage of atypical lateralization for language also varies with handedness: it is considerably higher among left-handers than among right-handers. Lastly, as if all this were not enough, there is also such a thing as intraindividual variability. In the same individual, a given mental task can sometimes activate different neuronal assemblies in different circumstances—for instance, when the individual is performing this task for the first time, as opposed to when he or she has already performed it many times before.

THE RIGHT HEMISPHERE’S CONTRIBUTION TO LANGUAGE

To follow a conversation, a written document, or an exchange of witticisms, you must be able not only to understand the syntax of sentences and the meanings of words, but also to interrelate multiple elements and interpret them with respect to a given context. While various types of damage to the left hemisphere produce the many documented forms of aphasia, right hemisphere damage (RHD) causes a variety of communication deficits involving the interpretation of context. These deficits can be divided into two main categories.

The first category of RHD-induced deficits affect communication indirectly, by disrupting people’s ability to interact effectively with their environment.

The individual may also suffer from anosognosia: unawareness of such deficits. For instance, some people who have damage just posterior to the central sulcus in their right hemispheres cannot even recognize certain parts of their own bodies as being their own. Thus this type of RHD produces a kind of indifference that is the opposite of the minimum emotional investment required to establish harmonious communication.

The other major family of RHD-induced deficits affect communication and cognition directly. These deficits can be grouped under the heading of pragmatic communication disorders, pragmatics being the discipline that studies the relationships between language and the way that people use it in context. Pragmatic disorders can be subdivided into disorders in prosody, discourse organization, and understanding of non-literal language.

Last but not least among the types of pragmatic communication disorders caused by RHD are disorders in the understanding of non-literal language. It is estimated that fewer than half of the sentences that we speak express our meaning literally, or at least they do not do so entirely. For instance, whenever we use irony, or metaphors, or other forms of indirect language, people’s ability to understand our actual meaning depends on their ability to interpret our intentions.

To understand irony, for example, people must apply two levels of awareness, just as they must do to understand jokes. First, they must understand the speaker’s state of mind, and second, they must understand the speaker’s intentions as to how his or her words should be construed. Someone who is telling a joke wants these words not to be taken seriously, while someone who is speaking ironically wants the listener to perceive their actual meaning as the opposite of their literal one.

Metaphors too express an intention that belies a literal interpretation of the words concerned. If a student turns to a classmate and says “This prof is a real sleeping pill”, the classmate will understand the implicit analogy between the pill and the prof and realize that the other student finds this prof boring. But someone with RHD that affects their understanding of non-literal language might not get this message.

Lastly, the various indirect ways that we commonly use language in everyday life can cause problems for people with RHD. In such cases, the speaker’s actual intention underlies their oral statement as such. For example, someone who says “I wonder what the time is now ” is indirectly asking for someone to tell them the time, but a person with RHD may not understand that.

Though the left hemisphere is still regarded as the dominant hemisphere for language, the role of the right hemisphere in understanding the context in which language is used is now well established. We know that in the absence of the left hemisphere (for example, when Wada’s test is performed), the right hemisphere can produce some rudimentary language. But lesion studies have shown that the right hemisphere’s role in language appears to be far wider—so much so that it is now more accurate to think of the two hemispheres’ language specializations not as separate functions, but rather as a variety of abilities that operate in parallel and whose interaction makes human language in all its complexity possible.