SENTENCE COMPREHENSION IN ALZHEIMERS-DISEASE

A visual sentence-picture matching task was used to clarify the nature of the comprehension deficit in Alzheimer's Disease (AD). Test sentences varied according to how crucial the processing of syntax was in computing a correct interpretation. Half the sentences could be understood through the comprehension of semantic cues alone; the other half required the processing of syntactic cues. In the first experiment, sentences were removed from view before the test pictures were presented; in the second, sentences and pictures were presented together to decrease demands on working memory. Two factors determined the accuracy of sentence comprehension: whether a syntactic analysis was needed and whether additional working-memory resources were needed for temporary storage. When storage demands were minimized, patients were able to use semantic cues to guide comprehension. When semantic cues were unavailable, comprehension was impaired, regardless of whether or not there were additional storage requirements, providing evidence for a genuine syntactic deficit that is not due to coexisting semantic impairment or working-memory dysfunction.