Date of Degree

6-2016

Document Type

Dissertation

Degree Name

Ph.D.

Program

Speech-Language-Hearing Sciences

Advisor

Loraine Obler

Committee Members

Loraine Obler

James Babb

Susan DeSanti

Subject Categories

Cognitive Psychology | Medical Neurobiology | Neurosciences

Keywords

Lexicon, Brain, Gray Matter Volumes, Language

Abstract

This study investigated the longitudinal relationship among aging, performance on lexical tasks, and regional gray matter volumes over 2-7 years. A total of 137 older participants who remained cognitively normal were administered four lexical tasks at each time point: the Boston Naming Test (BNT), Vocabulary Test, Semantic- and Phonemic-Fluency task. In addition, they underwent repeated magnetic resonance imaging (MRI) scanning acquired within two months of the lexical tasks. The average interval between time points was 2.36 years (range 1.50-7.64) and the average number of time points was 2.65 times (range 2-5).

Results indicated that age differentially affects lexical task performance in two ways. First, baseline age was significantly negatively related to the scores on the BNT and Semantic Fluency task but not related to the Vocabulary test and Phonemic Fluency task. Second, the interval between the baseline and follow-ups was significantly related to the performance on the BNT, Vocabulary test and Semantic Fluency task. The longer the interval between the observations, the lower the scores on these tasks. This shows that as people get older, general lexical production ability within participants declines. Moreover, the rate of change in performance over time varies across tasks.

Older participants tended to score lower at each successive time point and the rate of decline was greater on the tasks that impose more constraints on semantic and phonemic specificity than the tasks that demand less specificity in lexical selection. Thus, the BNT and Semantic Fluency tasks are more sensitive to age-related lexical performance changes in cognitively normal older adults than are Phonemic Fluency tasks or Vocabulary Test. Further, the interval effect clearly shows that as time progresses older adults’ lexical production abilities decline on all tasks except in the Phonemic Fluency.

Regarding the relationship between regional brain volumes and lexical performance over time, common and specific association patterns were found. Individual brain regions whose volume reduction predicted lexical performance decline were found in the bilateral temporal, parietal and frontal cortices. More specifically, the volumes of the medial temporal lobes (MTL) were significantly related to performance on all four lexical task, while other brain regions showed distinctive association patterns with individual tasks; the frontal pole was related to accuracy on the BNT; the temporal pole, supramarginal gyrus, superior frontal and superior parietal cortices to the scores on the Vocabulary Test; the dorsolateral prefrontal and precuneus to the scores on the Semantic Fluency task; the fusiform gyrus, inferior frontal, superior parietal, superior and inferior temporal cortices to the scores on the Phonemic Fluency task. When these significant regions were jointly analyzed, significant task-related clusters emerged for the BNT and Vocabulary test. For the BNT performance, the bilateral MTL and left frontal pole were crucial; for the Vocabulary test, while the left superior temporal cortex and bilateral MTL were important.

These results provide evidence that both common (shared across lexical tasks) and distinct regional volume reduction in the left and right hemispheres are associated with performance changes over time on different lexical tasks in cognitively healthy elderly individuals. Beyond the classic language areas determined from aphasiology, far more distributed regions, such as MTL, should be included as part of the lexical network for people experiencing a gradual aging process without prominent brain damage. A combination of common and specific brain regions contribute to maintenance and decline of lexical retrieval and semantic/phonemic processing.

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