Adult development is often associated with the concept of physical and cognitive decline. However, semantic processes related to the meaning of words are largely preserved: vocabulary increases as we age, and semantic word associations and category membership judgments are preserved, for example. Not only does semantic memory typically remain intact during healthy aging, but the number of associations among lexical items may also increase. In addition, these abilities are maintained despite age-related changes that affect brain structure. Despite the preservation of semantic processes specific deficits in phonological processes have been observed. For example, older adults exhibit decreased speed and accuracy in naming objects and increased slips of the tongue. This pattern of aging effects on semantic and phonological processes suggests a fundamental difference in the cognitive organization of these two abilities. The Transmission Deficit Theory provides one theoretical account of these asymmetric patterns of results by proposing fundamental differences in the cognitive architecture of the semantic and phonological systems. The proposed research tests contrasting predictions of the Transmission Deficit theory and other theories for age differences in semantic and phonological processes. The overarching goal of this proposal is to use behavioral measures, diffusion tensor imaging (DTI), and functional magnetic resonance imaging (fMRI) to investigate the mechanisms underlying the asymmetry in aging effects on phonological and semantic processes. While this asymmetry has been investigated behaviorally, few studies have investigated the age-related neural changes that are related to these cognitive changes. These combined methodologies will relate the underlying structural changes to functional activation and behavioral performance. The proposed experiments will characterize semantic and phonological processes in the aging brain, investigate the role of task-irrelevant information in language production and language comprehension tasks, and investigate age-related changes in transient and sustained semantic and phonological processes. This research will improve the current scientific understanding of age-related changes in language, and describe the neural factors that contribute to cognitive decline. The data will be particularly relevant to the concept of plasticity that underlies the maintenance of behavioral functions in older adults. These results will provide essential information for differentiating normal age-related changes in language from the effects of disease, forge advances in our theoretical conceptualization of age-related change in language, and provide insight into rehabilitative therapies for cognitive decline in both healthy aging and clinical patients.
Public health will benefit from the proposed research by improving our basic knowledge of the neural and behavioral foundations of age-related change in language. This research will facilitate our ability to differentiate healthy aging from clinical conditions, and provide insight into rehabilitative therapies for cognitive decline.
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