Now here is an example of how Broca’s aphasia affects the sufferers’ ability to understand those grammatical components of language. Sentence (2) can be harder for Broca’s aphasics to understand than sentence (3):
The cat was chased by the dog.
The car was chased by the dog.
Sentence (2) requires knowledge of syntax in order to be interpreted, because the sentence is in the passive voice, whose word order is not the more common English order. (By the way, the “more common” English order, which is “subject + verb + object,” like “The cheetah ate the cake,” is called “canonical.”) So, imagine a sufferer who is lacking the ability to process those functional language pieces, such as the “-ed” past tense verb ending (that’s called a bound morpheme—read more about those here), and the preposition “by.” Even the verb “was” is a function word, because it is an auxiliary verb, not a main verb.
So, when aphasics can only process content words, like “cat,” “chase,” and “dog,” they are less likely to correctly process the doer and the receiver of the chasing action in sentence (2). That’s because if you strip the sentence down to content words, it might be “cat chase dog,” so the person with Broca’s aphasia may be misled, and say that the cat must be doing the chasing, because canonical word order in English is the most common and straightforward.
However, sentence (3) has stronger semantic information (again, that means “meaning”): Cars are not animate, and cannot really chase things, so these aphasics may more aptly ascertain/guess that the car is the recipient of the chasing action, even with the tricky word order.
Whether something is animate or not is one of the many features that are encoded within every word that we learn in the languages we know. In other words, in sentence (2), semantics does not provide enough information, because both cats and dogs can be chasers, and this would be difficult for a speaker who is dependent on semantic knowledge, due to having suffered grammar loss.
A final example of this function/content distinction, which is both fascinating and sad, is the nature of the dyslexia that aphasics often acquire along with the spoken-language loss. In reading tests, because of their difficulty with function words, Broca’s patients are unable to read function words out loud from test cards, yet they can read homophones—those are words that sound the same but mean something else—if the homophone is a content word! For example, a patient may read the word witch without a problem, because witch is a content word. It is a noun that you can define, and picture in your mind. However, this same patient may struggle immensely to read the homophone which, because which is a relative pronoun: a grammatical, closed-class word. (2)
Now, let’s talk about damage to Wernicke’s area of the brain. The resulting Wernicke’s aphasia is a type of the fluent aphasia, which means that the speech is not labored, and the sufferers are still able to use function words like articles and prepositions. Their words most often occur in a grammatically correct order (remember, this type is not “agrammatic,” like Broca’s aphasia). Instead, they have trouble with picking the right content words. So, the Wernicke’s sufferers may use one content word when they need another, but they speak at a normal pace. They tend to produce meaningless utterances and they also, unfortunately, tend not to be able to understand what is spoken to them, so meaning is impaired for them in a very general sense. (4) Noam Chomsky, the “grandfather” of linguistics, created the following sentence to show that in human language, meaning is separate from syntax/word order:
“Colorless green ideas sleep furiously.”
This sentence sounds a bit like something that someone with Wernicke’s aphasia could say. It is grammatically correct, in that the word order follows syntactic rules, and, for example, the subject-verb agreement is correct for most English dialects, but it makes no logical sense at all.