It is now known that patients with dementia of the Alzheimer's type often show impaired language abilities. Most researchers agree that these patients have difficulty naming objects and comprehending words. However, there is disagreement about whether their sentence comprehension abilities are impaired. The results of our preliminary studies support the view that DAT patients have only minimal difficulty in understanding the literal meaning of a sentence but a substantial impairment in situations requiring them to use the meaning derived from a sentence to perform tasks such as verifying whether a sentence is plausible. In addition, preliminary data from studies in which we have asked patients to understand sentences while performing another task (such as following a sequence of dots or digits on a computer screen) supports the hypothesis that these sentence comprehension impairments result from a reduction in the processing resources available to DAT patients to accomplish these tasks. The proposed research will extend these preliminary studies in three ways. First, we will use a wide range of sentence types and sentence comprehension tasks, to see if the results generalize to other sentences and tasks. Second, we will use a wider range of concurrent tasks, to allow us to study the effects on sentence comprehension of tasks that vary in their processing resource requirements and in the degree to which they overlap cognitively with the sentence comprehension task. Third, we will investigate possible trade-offs in subjects' allocating processing resources to only one task in dual-task situations, by generating Performance Operating Curves in dual-task situations. This will allow us to interpret the effect of dual-task conditions on sentence comprehension more clearly. The proposed studies will provide data that test two hypotheses: (a) the patients with DAT have little difficulty with tasks simply requiring them to obtain the literal meaning of sentences but greater difficulty with tasks that required them to use meaning obtained for other purposes, and (b) that the impairments these patients show on these tasks result from a reduction in the processing resources they can devote to sentence comprehension.
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