What and how do we know about how Broca's and Wernicke's areas contribute to language functions?

The highly developed verbal communication skills found in humans is one of the main factors that sets us apart from the rest of the animal kingdom. It has allowed us to create the multi-cultural society in which we live. Our ability to share information, factual down to gossip and deception via various methods; talking, listening, reading and writing is unique to humans but how much is known about it? As the complex nature of language is a uniquely human ability it is mainly studied through stroke victims that is those people who have suffered a cerebrovascular accident. Other causes of brain lesions have also been studied however, for example, brain surgery, head injuries and infections or tumors. A further way of studying brain function in language is through speech disorders called aphasias. Aphasias involved deficits in comprehension or production of speech due to localised brain damage. Not all speech deficits are categorised as aphasias however, deafness or paralysis of speech muscles for example aren't aphasias.

In 1861, Paul Broca described the case of a patient who stopped speaking following a stroke. The patient could comprehend speech but could only utter one syllable 'tan' which is what he became known as. Despite his lack of speech, his motor control of his facial muscles was unimpaired prompting Broca to consider that an area of the brain had been damaged by the stroke. This deficit became known as Broca's aphasia, which may or may not also involve grammatical misunderstanding. Broca's aphasia is characterised by good comprehension but a deficit in production. The range of impairments goes from complete mutism to slow, laboured speech using very simple word forms. Patients with this aphasia rarely use plurals and often leave out articles, adjectives and adverbs altogether. The ability to repeat words, read aloud and write is also impaired. Broca's aphasiacs are aware of their errors and try to compensate for them. In fact they are usually aware of the important features of their native language and try not to overlook them for example Chinese patients will rarely omit noun classifiers. It was suggested by Broca that this deficit in language production is produced by a lesion to the inferior portion of premotor and posterior prefrontal cortex (frontal operculum). Further research supported this idea and the region became known as Broca's area. Wernicke suggested that this area is the location of motor memories- the sequences of muscular movement involved in articulation.

Broca's area is involved in the action of words; it is located in the premotor cortex. It has been found that the equivalent area in the monkey brain contains mirror neurons. These are specialised cells for motor actions that fire if a certain movement is performed or witnessed. The basis of understanding language is after all, the movements of the mouth. The perception of an action may be the basis of comprehension and the evolution of communication. It has been thought, for example that a baby imitating the facial expressions of an adult may be a function of Broca's area.

Carl Wernicke reported the cases of six patients in 1874 that had lost the ability to comprehend speech and produce meaningful speech despite normal articulation, grammar and prosody (rhythm and intonation). This deficit is known as Wernicke's aphasia and is characterised by severely impaired comprehension and some production difficulties, for example they struggle to find the right word. Unlike Broca's aphasiacs, Wernicke's aphasiacs are unaware of their speech failures and do not have impaired repetition. The excessive speech of Wernicke's aphasiacs sounds, from a distance to be normal and grammatically correct but in fact despite an adequate use of function words for example, the and but and complex tenses, they use few content words (words that convey meaning) and word strings often make no sense. Lesions in the posterior superior temporal cortex, known as Wernicke's area, cause this deficit; it can also be affected by damage to the surrounding areas such as the auditory association cortex. This aphasia has been classified as a receptive aphasia; Wernicke suggested that Wernicke's area is involved in remembering sound sequences that make up words. Wernicke's area does appear to be involved in learning. Jacobs et al. (93) found a positive correlation between the length of apical dendrites of pyramidal cells in the area and educational level of a person. Wernicke's aphasia doesn't directly disrupt people's ability to speak; rather it disrupts recognition of spoken words, comprehension of meaning and the ability to convert thoughts into words.

Wernicke predicted a neuronal circuit for critical language functions involving Wernicke's area in a perceptual role and Broca's area in a motor or action role. This was later confirmed with the discovery of the arcuate fasciculus that connects the two areas. Lesions to this neuronal connection results in a conduction aphasia, characterised by less fluent speech than with Wernicke's aphasia, impaired repetition but normal comprehension. For example patient L.B. made very few errors in normal speech but was unsuccessful when it came to repeating words, nonsense words such as 'blaynge' were not even attempted Other symptoms are consistent with a functional separation of the two areas, impaired naming and reading aloud although silent reading is unaffected and patients can comprehend what they read. The arcuate fasciculus therefore seems to be involved in conveying information about the sounds but not the meanings of words. The symptoms of this aphasia suggest that the arcuate fasciculus plays an important role in short-term memory of new words and speech sounds they've just heard.

Studying aphasias in these areas has led to an elaboration of the Wernicke-Geschwind model of language. When you hear a word, the information is transferred from the ear to the auditory nerve and the medial geniculate nucleus. Following this, the information moves to the primary auditory cortex and on to the higher order auditory cortex and a specific region of the parietal-temporal-occipital association cortex called the angular gyrus that processes incoming auditory, visual and tactile information. From the angular gyrus, the information is transferred to Wernicke's area, which is concerned with the comprehension of the word. The information travels to Broca's area via the arcuate fasciculus where the auditory representation is transformed into the grammatical structure of the phrase and the memory of how the word is articulated is stored here. The information about the pattern of sounds is then received by the motor cortex.

This model made several true predictions such as if Wernicke's area is damaged, spoken words reach the auditory cortex but fail to activate Wernicke's area resulting in a lack of comprehension. A lesion in Broca's area results in no comprehension disruption but there are problems with speech production, as the patterns of sounds and structure aren't passed on to the motor cortex. Lastly, a lesion to the arcuate fasciculus disconnects the two areas and disrupts normal speech due to the disruption of the normal feedback from language production to language comprehension.

Despite these successful predictions, there are problems with the model mainly due to the over simplification of it. The importance of Wernicke's and Broca's areas are overemphasised, the patients first studied in relation to these areas had lesions that extended to surrounding areas as well. If lesions are localised to the areas only then the full symptoms of the two aphasias aren't usually observed. The model also emphasised the importance of cortical regions and interconnecting pathways running through subcortical white matter. Evidence now exists that suggests subcortical structures such as the left thalamus and the left caudate nucleus are also important for language. The left caudate nucleus for example is needed for auditory-motor integration. Lesions in this structure lead to deficits in auditory comprehension. Further, visual information doesn't actually go to Wernicke's area; it travels directly from the visual association cortex to Broca's area. Lastly, cognitive studies disagree with the model on more than just the pathways involved. There is now good evidence that not all auditory inputs are processed in the same way. Nonsense sounds are processed separately from meaningful words. This evidence suggests that language is a complex parallel process not a serial pathway between Wernicke's and Broca's areas.

Damage restricted to the cortex of Broca's area doesn't appear to lead to Broca's aphasia. The damage must extend to surrounding regions of the frontal lobe and underlying subcortical white matter. There is also evidence that suggests the importance of the basal ganglia. Lesions in this area, especially the top of the caudate nucleus can cause a Broca-like aphasia. Different symptoms of Broca's aphasia seem to involve different brain regions. Dronkers (1996) suggests that the left precentral gyrus of the insula is the critical location for speech articulation. He discovered this by plotting the lesions of people with and without apraxia (impaired ability to program movement of lips, tongue and throat required to produce a sequence of sounds) of speech following strokes. A region of 100% overlap was found in the patients with apraxia falling on the left precentral gyrus of the insula. Conversely none of the 19 patients without apraxia had damage in this area. Functional imaging studies have supported Dronkers conclusion that this area is involved in speech production.

We know about Broca's and Wernicke's areas through the study of specific speech deficits, i.e. aphasias. The two areas are directly involved in comprehension and production of words. Wernicke's area seems to have a perceptual role, recognising speech sounds whilst Broca's area has a more active role, being near the motor cortex it is involved in producing speech sounds and storing memories of how to articulate words. Despite the importance of these areas they are not the be all and end all of language. Communication is a more complicated process than simply a link between the two areas.

Language Short Notes

Aphasia: Aphasia is the name given to speech disorders whose basis lies in the inability to comprehend or produce speech. However, aphasia does not refer to disorders of speech caused by motor or sensory problems (e.g. deafness). One of the characteristics of aphasia is that the sufferer seems to understand that someone is trying to communicate with them. There are several types of aphasia with distinct symptoms, for example Broca's aphasia, Wernicke's aphasia or conduction aphasia.

Broca's area: This is a region of the brain in the inferior left frontal lobe. It is named after a French surgeon, Paul Broca (1861) who first proposed that this area was responsible for some aspect of speech (now thought to be the motor control of speech). This followed his study of a patient (later called Tan as this was the only syllable he could say) who had damage to this specific area of his brain following a stroke and so could not speak properly. This disorder is now known as Broca's aphasia.

Broca's aphasia: This disorder is sometimes called anterior aphasia and is usually caused by damage to Broca's area but may also arise when the primary motor cortex, or the sub cortical white matter of the frontal lobe is damaged. It is characterised by the inability to speak although almost normal comprehension and motor skills remain intact. Three main disorders are associated with Broca's aphasia, agrammatism, anomia and articulation difficulties. Agrammatism is the inability to understand or use grammar effectively, often grammatical words such as 'it', 'for' or 'have' are omitted from speech. Anomia is characteristic of all types of aphasia and refers to the inability to find and use correct or appropriate words. The final characteristic of Broca's aphasia is a difficulty with articulation; patients may mispronounce words or use them in the wrong order.

Wernicke's area: Carl Wernicke (1874) studies on three patients suffering from a type of aphasia (but not Broca's aphasia) lead him to locate another area of the brain involved in language in the posterior and superior part of the temporal cortex. He proposed that this part of the brain was involved in the perception of language and that there was a pathway linking this area to Broca's area.

Wernicke's aphasia: The aphasia that Wernicke described is sometimes called posterior aphasia as Wernicke's area is in the posterior part of the temporal cortex. The characteristics of Wernicke's aphasia are that the sufferer is unable to comprehend speech or speak normally. Unlike sufferers of Broca's aphasia people suffering from Wernicke's aphasia seem unaware of their disorder. They talk fluently, use grammar and inclinations of tone but their speech is nonsense. They will often also make up words and use these in the place of the proper word.

Arcuate fasciculus: The arcuate fasciculus translates as 'arch shaped bundle' and is the name given to the fibres that connect Wernicke's area to Broca's area. It is thought that this pathway conveys information about the sounds of words (but not their meanings). The best way to study the arcuate fasciculus is to look at the effects of damage to it; this produces the syndrome conduction aphasia.

Conduction aphasia: Damasio and Damasio (1980) stated that damage to the inferior parietal lobe that extends into the sub cortical white matter or any damage to the arcuate fasciculus results in conduction aphasia. People suffering from conduction aphasia can repeat any word that has a meaning but cannot repeat words that are nonsense, (e.g. L.B. observed by Margolin and Walker, 1981). It is thought that the arcuate fasciculus conveys information about the sounds of words to the frontal lobes; hence any damage to this pathway causes an inability to learn or say new words. However, there is a second indirect pathway that remains intact in people with conduction aphasia. This pathway conveys information about the meanings of words, so if a sentence is read to someone with conduction aphasia, they can repeat a sentence with the same meaning, even if they do not exactly repeat the original sentence.

Hemispheric Specialisation (of language): It is thought that the left hemisphere is responsible for language (in the majority of people). Evidence for this comes from the study of people with lesions to the left hemisphere who have marked language difficulties. TMS studies have also located language in the left hemisphere. A test called the Wada test involves injecting an anaesthetic into each hemisphere alternatively, when tests like this are carried out it can be seen that language does in fact seem to be located in most people in the left hemisphere. This may explain the hemisphere asymmetry, which is biased towards the left hemisphere. However, there is some language specialisation in the right hemisphere, specifically it seems that prosody (tone and inclination in speech) is organised in the right hemisphere. Damage to the right frontal cortex leaves a person with a monotonous voice; damage to the posterior (right) cortex leaves a person unable to discern prosody in other peoples speech.

Visual recognition of words: Other than the areas previously mentioned some other brain areas involved in language recognition have been discovered. These areas have been found along the ventral stream and seem to be involved in the visual and semantic recognition of words. Intracranial and non-invasive techniques have identified a part of the posterior fusiform gyrus which seems to respond to the shapes of words. (This comes from work by Nobre and Price).

Meaning representations of words: The second brain area identified that is thought to be involved in the semantic (meaning) recognition is located in the ventral anterior temporal cortex. Brain imaging studies have also shown that when different words are said to people different parts of their brains are activated. These areas seem to be related to the word itself. For example, colours seem to be identified in visual areas of the brain. In this area of study lesion studies again play a significant role. There are a number of case studies that describe the inability of people to name certain types of things, for example one man could not access the information needed to describe a rhinoceros, but when a visual cue was presented to him he was able to perform this task. It seems therefore that the semantic representation of words is distributed over the brain.