Within research on memory, a broad distinction is made between semantic and episodic memory (Tulving, 1972). Typically, episodic memory is defined as memory for events that are part of our personal histories. In contrast, semantic memory is memory for general factors about the world that are embodied in our knowledge of the meanings of words, and concepts, that do not have an autobiographical component. This distinction is particularly relevant in distinguishing Alzheimer's disease (AD) from the effects of normal aging. Memory impairments in the normal elderly appear to be most strongly linked to a decline in episodic memory. In contrast, AD patients exhibit severe impairments both in episodic and semantic memory (Nebes, 1989). For example, AD patients have word finding difficulties (Nicholas, Obler, Albert & Helm- Estabrooks, 1985), a reduced ability to name pictured objects (Bayles & Tomoeda, 1983), and in difficulty in remembering details from recently presented text (Haut, Demarest, Keefover, & Rankin, 1994).
The specific aim of this application is to test two explanations of the semantic memory deficits that occur in AD. Each hypothesis proposes a specific mechanism to explain these deficits. According to the first hypothesis, AD patients have more trouble identifying semantic relations than the normal old. This difficulty is attributed to a decreased ability to evaluated semantic relations rather than to a change in the basic organization of semantic memory. The second hypothesis claims that a narrower set of semantic features is typically accessed by AD patients when a concept is retrieved from semantic memory. Previous explanations of memory deficits in AD have ranged from accounts that assume that a common mechanism is responsible for these deficits (Nebes & Brady, 1992) to accounts that have advanced a mechanism whose effect is limited to a particular deficit (Martin & Fedio, 1983). The broader objective of this application is to develop a coherent account of a range of memory deficits by examining the current hypotheses within the context of several previous explanations that have advanced a common mechanism to explain these deficits. The rationale for this strategy is that a variety of factors are probably responsible for memory deficits in AD and performance within a task may be better understood in terms of the relative impact of each factor rather than in terms of a single factor that is responsible for most deficits. This application claims that specific changes in semantic memory processing occurs early in AD. If this content is correct, this knowledge could be used to develop behavioral measures that facilitate an early diagnosis of AD. In addition to facilitating early diagnosis, the results of these studies may facilitate the development of instruments that can be used to document the impact of drug treatments on the course of the disease.
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