The elements of language include sounds, words, and sentences. These are integrated by grammar, which governs the forms that words take on depending on their function in a sentence, as well as the order of words in a sentence. Some aspects of grammar are quite simple. For example, the plural of almost all nouns in English is formed simply by adding –s to the word, and the form of almost all verbs in the present tense varies only in the presence or absence of a final –s. Other aspects of English grammar are complicated and difficult, as any non-native learner of the language is bound to agree!
A neurodegenerative condition such as Frontotemporal degeneration (FTD) may result in a form of primary progressive aphasia (PPA), a specific disorder of declining language. One such syndrome prominently affects grammar. We refer to this as the non-fluent/agrammatic variant of primary progressive aphasia (naPPA). An equivalent term for this condition is progressive non-fluent aphasia (PNFA).
Individuals with naPPA have difficulty producing grammatical sentences when they speak. One kind of grammatical impairment that may occur is the omission of what are called function words. These are the words that do not carry concrete meaning but are required to complete the phrases of a sentence and to relate parts of a sentence to each other. The most frequently omitted words are the articles the, a, and an. Sometimes auxiliary verbs are omitted, as in "Boy climbing on a log," in place of "The boy is climbing on a log." Prepositions are sometimes omitted, but it is more noticeable that they may be misused. For example, a patient may say "The dog is barking on the truck" instead of "… at the truck." Patients may also make errors of noun-verb agreement, as in "The bees is swarming" instead of "The bees are swarming."
Another prominent feature of the speech of individuals with naPPA is that their sentences tend to be shorter than those of healthy speakers. In part, this is due to difficulty with the grammar that is necessary for constructing longer sentences, which requires more complex structure than short sentences. A healthy speaker can easily produce and understand a sentence such as the following: "A man I met at the gas station last Tuesday said he was a friend of the girl I told you about who was supposed to work on the library committee with me.” Although this sentence contains several clauses, all the parts are grammatically interdependent. Patients tend not to produce such long and complex sentences.
The Penn FTD Center studies grammatical expression by conducting quantitative analyses of subjects’ recorded speech. Subjects are recorded while describing a complex picture, “reading” a story from a picture book, or talking about events in their lives, among other tasks. Features of the recorded speech samples are correlated with data from numerous other studies, including both behavioral measures of cognitive functioning and structural and functional neuroimaging studies.
Individuals with naPPA also have difficulty interpreting and understanding sentences. Some of these difficulties are related to an impairment in understanding the grammatical aspects of sentences. For example, the grammar of a sentence indicates who did what to whom, and this may be difficult to understand for patients with naPPA. These deficits may also affect a patient’s ability to read and write grammatical sentences.
The Penn FTD Center investigates grammatical comprehension in FTD in a number of studies. Subjects are asked to read or listen to a sentence or a short passage and make a judgment about the meaning of the sentences. As in studies of sentence production, the results are interpreted in light of performance on behavioral and neuroimaging measures.
Grammatical deficits frequently co-occur with other impairments of language in naPPA. One associated problem is that speech is very effortful. While healthy adults on average speak at a rate of about 140 words per minute, these patients speak at an average rate of 45 words per minute– less than one-third the rate of healthy adults. Another feature seen in naPPA is the production of speech sound errors. While healthy adults occasionally produce speech errors such as the substitution of one sound for another (e.g. “tog” instead of “dog”) or the omission of speech sounds in a word, these kinds of errors occur much more frequently in individuals with FTD.
Grammatically complex sentences are often longer. To keep track of all of the components of a sentence while speaking or listening to another person, a speaker/listener uses working memory. This is a form of very short-term memory with limited capacity and limited duration that keeps information in a temporarily active state so that it can be considered during the processing of the entire sentence. A short-term memory deficit can sometimes mimic a grammatical deficit because an individual cannot process the full sentence all at once.
During the course of understanding or speaking a grammatically complex sentence, it may be useful to attend to the specific bits of information that are crucial to the meaning of a sentence. This is referred to as selective attention. It can ease the processing burden by focusing on the parts of a sentence that contribute most importantly to its meaning.
Up to one-third of sentences are ambiguous in some way, and it may be necessary to disambiguate a sentence. Disambiguation often depends on processes involving strategic judgment and probability assessment. Strategic judgment relates specific aspects of a sentence to particular material (sometimes part of the sentence, sometimes external to the sentence) that helps the speaker/listener establish the meaning of the sentence. Probability assessment weighs the likelihood that a sentence has one meaning rather than another. This may depend on assessments of word frequency or frequency of a grammatical form.
The syndrome of naPPA is frequently associated with disease in the frontal lobe of the left hemisphere and may extend into the adjacent part of the left temporal lobe. This is the part of the brain that is behind the forehead and above the eye. MRI scans of the brain can show atrophy (shrinkage) of this part of the brain that is progressive in nature, and declining grammar and increasing effortfulness are associated with the left frontal lobe becoming progressively smaller as a result of FTD. The grammar of a sentence must be integrated with other aspects of the sentence, such as the meaning of individual words. Connections between the left frontal lobe and other parts of the brain are important for other aspects of language, and these connections appear to suffer changes as well in FTD. The syndrome of naPPA also may be a marker for a specific form of neuropathology.