3.3 Specialisation within language areas: aphasia
Aphasia is caused by localised brain damage, for example due to a stroke or an automobile accident. General intellectual functioning is not necessarily impaired, as the person can still perform non-linguistic tasks. Nor is the understanding and production of language necessarily completely abolished. Instead, there are highly specific patterns of impairment in the way language is processed.
Aphasia is divided into two main types, fluent and non-fluent. For reasons which will become apparent, they are also known as Wernicke's and Broca's aphasia. In non-fluent, or Broca's, aphasia the person has a marked problem with speech production. Speech rate is slow, and the articulation of speech is laboured and distorted. Speech lacks its normal intonation contours, being instead pronounced in rather a monotonic way. The typical feel of non-fluent aphasic speech is given by a sample from a patient in (15). The patient was attempting to explain that he had come to the hospital for dental surgery. The dots indicate long pauses.
(15) Yes … ah … Monday … er … Dad and Peter H … [his own name] and Dad … er … hospital … and ah … Wednesday … Wednesday nine o'clock … and oh … Thursday … ten o'clock, ah doctors … two … an doctors … and er … teeth … yah
Describe as specifically as possible the ways in which this sample of non-fluent aphasic language is similar to and differs from normal language use.
In the sample, all the key referring words (the people involved, places and times, doctors, hospital and teeth) have been appropriately selected. Thus there is no apparent deficit in selecting the correct referring words on the basis of their meaning. These are all nouns, however; there are no verbs. More generally, there is no evidence of words being glued together into higher structures like sentences or phrases.
It was once thought that the comprehension abilities of people with non-fluent aphasia were normal. Thus, non-fluent aphasia was conceptualised as a disorder of language production that did not affect language comprehension. It is indeed the case that people with non-fluent aphasia can sometimes understand even quite complex sentences, in sharp contrast to what they produce. However, some subtle investigations over the last few decades have shown that language comprehension is also affected, and thus that the disorder is one of language in general, rather than just production (Berndt et al., 1997). Consider (16).
(16a) The dog chased the cat.
(16b) The cat chased the dog.
(16c) The girl watered the flowers.
(16d) The flowers were watered by the girl.
(16e) The dog was chased by the cat.
People with non-fluent aphasia can generally pick out the picture corresponding to (16a) or (16b) correctly. It might seem that the difference between (16a) and (16b) is purely syntactic; the words are after all the same in each case. Thus people with non-fluent aphasia seem able to use syntax in understanding if not in production. They can also correctly identify the picture associated with (16c) and (16d). Sentence (16d) is passive; this means that the usual generalisation that the first thing you come across is the subject and the second one the object does not hold in this case. Again, the fact that people with non-fluent aphasia often get the right meaning suggests that they are using syntax. However, in (16a) to (16c), you can get the right meaning simply by assuming the first thing you come across is the agent or doer. In (16d), you can guess the correct meaning simply from the individual meanings of the words present; flowers can't water a girl, so the correct meaning has to be the other way around. Thus, you could get (16a) to (16d) correct without really using any sophisticated syntax, but just by general reasoning from the words present.
Sentence (16e) is different. Using the principle of subject first is a red herring, since it would lead you to assume that the dog was the chaser not the chased; and, unlike (16d), either assignment of roles is plausible. The only way you can tell which meaning is intended in (16e) is by applying a syntactic analysis. People with non-fluent aphasia are very bad at sentences like (16e). They tend to get the roles the wrong way around. In a similar way, non-fluent aphasics are fine with (17a), but bad with (17b).
(17a) The man who pushed the woman is old.
(17b) The man who the woman pushed is old.
Once again, the explanation must be to do with syntax. In (17a), the subject of the verb to push is the man, which comes before the verb, so even though the sentence is moderately complex, the agent is coming first. In (17b), the man is the object of the verb to push; at some more logical level, the sentence means something like The woman pushed the man and that man was old. This logical meaning in (17b) can only be extracted by a syntactic analysis, because the linear order of the words is not the same as their logical order in the meaning.
What does the pattern of non-fluent aphasic comprehension deficit, coupled with the speech production data, suggest is going on in non-fluent aphasia?
It seems that people with non-fluent aphasia do not have access to the syntactic mechanisms. These are the processes which deduce the logical relations between different words in a sentence from the form of a sentence. People with non-fluent aphasia are probably guessing at the meaning of sentences just from the individual words involved, and not from access to the syntactic structure of the sentence. For this reason, non-fluent aphasia has also been called agrammatic aphasia.
The brain damage in non-fluent aphasia was always thought to be in Broca's area in the left frontal lobe (see Figure 15). This generalisation is probably too simplistic; studies of many aphasic and other neurological patients show that damage to Broca's area does not always produce these symptoms, and that damage to other areas does sometimes produce them. Nonetheless, the most frequent source of these symptoms is damage to Broca's area and the most frequent result of damage to Broca's area is these symptoms.
From the aphasia evidence, what is the function of Broca's area?
Broca's area seems especially involved in the syntactic analysis of a sentence. (It may also have other functions to do with the production of speech, which is why articulation in Broca's patients tends to be distorted.)
In stark contrast to non-fluent aphasia is fluent aphasia. In fluent aphasia, the patient has very obvious comprehension problems. He makes semantic errors with words, such as pointing to his ankle when asked to point to his knee. By contrast, the articulation, intonation and fluency of speech sound normal. For this reason, fluent aphasia was initially thought of as a disorder of comprehension rather than of production. However, linguistic output is far from normal, as illustrated by (18), a fluent aphasic's description of a picture.
(18) Well this is … mother is away here working her work o'here to get her better, but when she's looking, the two boys looking in the other part. One their small tile into her time here. She's working another time because she's getting, too.
Compare and contrast the language of (18) to the language of the non-fluent patient in (15).
Unlike (15), the fluent patient's output is in full sentences, with verbs as well as nouns. There is evidence of grammatical relations; for example, ‘is working’ is inflected to agree with ‘mother’. The non-fluent sample was economically worded but informative, since all the key elements were specified. Here the speech is fluent but the information content is very low. This is because the words selected tend to lack concreteness and specificity of meaning (‘working her work’ rather than what she is actually doing; ‘the other part’ rather than the exact place). At other times, it seems that the wrong content words have been selected, and as a consequence the sentence is uninterpretable (‘One their small tile into her time here’).
From the above, what aspects of language processing would you guess are impaired in fluent aphasia?
Syntax is generally unaffected. There seems to be a specific deficit in semantics. That is, there is trouble hooking the concrete meanings of words, especially nouns, to the objects to which they refer.
This view is reinforced by the fact that fluent aphasia can often be linked to another condition known as anomia. Anomia is a deficit in accessing the names of objects. It can take several forms, including the inability to give the name of an object when confronted with a picture of it, or the inability to find a name from a description or definition. One of the most striking discoveries about anomia is that the impairment can be specific to particular categories of things.
There are many patients who have trouble retrieving abstract names, like supplication, pact or culture, whilst still being able to find relatively concrete ones like cheese or thimble. This is perhaps not surprising; the abstract meanings tend to be rarer, more complex, and learned later in life, so they might be expected to be the first ones abolished by brain damage. What is striking is that there are also patients in whom the pattern of preservation is the other way around. Some examples of the definitions that this second group of patients volunteer are given below in Table 1.
Table 1 Definitions of abstract and concrete words produced by patients with a superiority for abstract words
|Supplication||Making a serious request for help|
|Geese||An animal but I've forgotten precisely|
|Malice||To show bad will against somebody|
|Caution||To be careful how you do something|
|Ink||Food – you put on top of food you are eating – a liquid|
|Cabbage||Use for eating, material – it's usually made from an animal|
|Society||A large group of people who live in the same manner and share the same principles|
|Culture||A way to learn life's customs; it varies from country to country|
|Duck||A small animal with four legs|
|Thimble||We often say sewing thimble|
These patients clearly have real trouble with the meanings of things that are based on concrete sensory (visual, touch or taste) properties, whereas they have no trouble with meanings that are expressible entirely in terms of other words. There are patients in whom the impairment is even more specific. There are individuals who have trouble finding the names of living things, but are fine with manufactured objects such as tools, and there are other patients with the opposite pattern.
How are we to interpret the loss of concrete word meaning in fluent aphasia and anomia? The best explanation is that there are neuronal circuits somewhere that carry the meanings of individual words by connecting the cells that respond to the phonological shape of that word to other cells which represent its non-linguistic attributes such as smell, colour, taste and so on. These circuits appear to be disrupted in fluent aphasia. In that disorder, the machinery of language is all working but it is not anchored to the concrete, non-linguistic meaning of nouns. These word-meaning circuits clearly have some functional specialisation within them, so that the area that represents living things is different from that which represents manufactured objects, which is different again from that which represents abstract concepts.
Damage in fluent aphasia is generally located in the vicinity of Wernicke's area, near the junction of the temporal and parietal lobes. Anomias for various types of nouns are generally a consequence of bilateral temporal lobe damage (Figure 16). From this we conclude that the posterior and temporal parts of the language areas are specialised towards the linking of particular words to their concrete meanings. The distinction between living and non-living things probably arises as a consequence of the different modalities we use to identify them. Living things are generally distinguished by their perceptual, particularly visual, properties. A ‘thing’ being an elephant is related to the way it looks. The circuits associated with anomias for living things are down in the temporal lobe, adjacent to circuits from the visual system that we know are involved in identifying objects. Non-living things, like tools, are more defined by what we do with them (a saw is for cutting wood). It is likely that the circuits for the meanings of these words are hooked up to motor circuits for performing the related actions.
Anomia, as described here, is a problem with nouns in particular. There are word-finding difficulties associated particularly with verbs, too. They are more particularly associated with frontal lobe damage and non-fluent aphasia. This is a satisfying finding. Verbs are much more ‘syntactic’ words than nouns. They do not have meaning outside of sentences in quite the way a concrete noun can have, and they are intimately tied up with the creation of sentences, the assigning of subject and object roles and so on. Thus this finding generally supports the view outlined here that sentence processing has a more anterior basis, and individual word meaning has a more posterior one.