The long term goal of the proposed research is to increase our understanding of the neurobiology of language, particularly the neural mechanisms that underlie the representation and access of words. The more immediate objective of the research is to understand the cortical organization of noun and verb processing. We will achieve this through a two-pronged experimental approach using event-related functional magnetic resonance imaging (fMRI) and repeated transcranial magnetic stimulation (rTMS) in order to infer patterns of brain activation during processing of words of different grammatical categories. The first specific aim is to identify cortical regions that are differentially implicated in processing nouns and verbs. One set of experiments uses event-related fMRI to chart the areas of cortical activation differentially associated with noun and verb processing. Other experiments use rTMS to establish whether the areas of cortical activation play a central or a secondary role in this processing. The second specific aim is to distinguish those cortical regions that are primarily involved in processing the semantics of nouns and verbs from those that are implicated specifically in their grammatical processing. To this end, event-related fMRI experiments explore the effects of grammatical versus semantic factors on cortical activation patterns, and rTMS experiments determine whether the identified regions are essential for semantic and/or grammatical processing of words. The third specific aim is to determine the extent to which it is specifically grammatical category membership and not the different morphological processes undertaken with nouns and verbs that are represented in the regions of maximal cortical activation for nouns and verbs. The fMRI and rTMS experiments designed to achieve this goal involve comparison of performance on tasks that require regular morphological changes (jump/jumps) versus irregular (take/took) and/or no morphological changes. The fourth specific aim is to clarify the role that noun- and verb-specific processing areas play in the lexical retrieval tasks used to investigate the noun/verb distinction. This is addressed through the comparison of performance across the various task types used in the fMRI and rTMS experiments. The proposed research should increase our understanding of the cortical organization of noun and verb processing and provide the basis for explaining the causes of grammatical category deficits in various types of aphasia.
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