Sentence Comprehension

In the 1970s and 1980s, researchers demonstrated that some aphasic patients who understood the content words in a sentence might nonetheless show a failure of comprehension for the sentence as a whole if comprehension depended on the correct analysis of syntactic information. For example, these patients might have difficulty understanding "reversible" sentences—that is, sentences in which either noun could play the thematic roles of agent or theme, as in "The dog was chased by the girl" or "The truck that the car splashed was green." This comprehension difficulty could be demonstrated by asking the patient to choose between a picture depicting the correct thematic role relations and one depicting the reverse role relations (e.g., a girl chasing a dog vs a dog chasing a girl). Although the patients might perform at chance with such picture contrasts, they would do well if one of the pictures substituted an incorrect noun or verb (e.g., a girl chasing a cat or a girl walking a dog). One early hypothesis about the nature of such a comprehension deficit was that it derived from a general failure of syntactic processing because patients who showed this comprehension problem also produced "agrammatic speech." That is, their speech production was marked by simplified syntactic structure and the omission of function words and grammatical markers. However, recent findings have demonstrated that some patients may show only one side of this deficit (i.e., impaired syntactic comprehension but not agrammatic speech, or the reverse). In addition, several studies have shown that patients who show this comprehension problem on sentence-picture matching may do well on judging the grammatical acceptability of sentences. Thus, rather than a global deficit in all aspects of syntactic processing, these patients may have a more restrictive comprehension deficit, such as a deficit in mapping between the grammatical structure of the sentence and the thematic roles that entities play in the sentence. For instance, for the sentence "The truck that the car splashed was green,'' the patient might be able to determine that "car" is the grammatical subject of the verb "splashed" but be unable to determine that the car is doing the splashing.

Other findings indicate that although agrammatic speakers may not provide the clearest evidence of a dissociation between syntactic and semantic knowledge, other patients do provide such evidence. Some patients with Alzheimer's dementia demonstrate very impaired knowledge of word meanings but show preserved grammatical knowledge. For example, they might be unable to realize that a phrase such as "The jeeps walked'' is nonsensical but be able to detect the grammatical error in a phrase such as "The jeeps goes." Other case studies of aphasic patients show the reverse pattern of preserved semantic knowledge but disrupted grammatical knowledge. Thus, the findings indicate some degree of modularity in the sentence comprehension system, with separate modules for semantic and syntactic processing. However, other evidence from patients comports with the findings from normal subjects in showing that these modules do not typically operate in isolation but instead interact. That is, patients may use the grammatical structure of sentences when there are weak semantic constraints (e.g., understanding that "tiger" is the agent of "chased" in "The lion that the tiger chased'') but fail to use the grammatical structure when there are strong semantic constraints (e.g., mistakenly interpreting the "woman" as the agent of "spanked" in "The woman that the child spanked''). These findings support the view that information from both semantic and syntactic sources typically combines during comprehension, but when one of these systems is weakened due to brain damage, the other system may override its influence.

As discussed previously, theories of comprehension often assume a role for a short-term or working memory system that is used to hold partial results of comprehension processes while the rest of a sentence is processed and integrated with earlier parts. Aphasic patients often have very restricted short-term memory spans, being able to recall only one or two words from a list compared to normal subjects' ability to recall five or six words. Many of these patients appear to have a deficit specifically in the ability to retain phonological information. Although it may seem intuitively plausible that restricted short-term memory capacity would impede comprehension, a number of studies have shown that patients with very restricted memory spans may show excellent sentence comprehension even for sentences with complex syntactic structures. Such findings support immediacy of processing theories of comprehension that state that syntactic analysis and semantic interpretation are carried out on a word-by-word basis, to the extent possible. Recently, some patients have been identified whose short-term deficit appears to be due to a difficulty in retaining semantic information rather than phonological. For such patients, their restricted ability to retain semantic information does impede comprehension for certain sentence types—that is, those that put a strain on the capacity to retain individual word meanings. Specifically, these patients have difficulty comprehending sentences in which the structure of the sentences delays the integration of word meanings into larger semantic units. One sentence type causing difficulty includes sentences with several prenominal adjectives, such as ''The drab old red swimsuit was taken to the beach.'' In this example, the meaning of ''drab'' cannot be integrated with the noun it modifies until two intervening words have been processed. These patients do not have difficulty comprehending similar sentences in which word meanings can be integrated immediately. For example, these patients can understand sentences in which several adjectives follow the noun (e.g., ''The swimsuit was old, red, and drab, but she took it along anyway''), because these sentences allow for the immediate integration of each adjective with the preceding noun.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

Get My Free Ebook

Post a comment