Our conceptual knowledge includes the long-term representation and processing of word meanings and basic information about objects and actions. The investigators propose behavioral and functional neuroimaging studies of conceptual representations and processing in neurologically intact subjects and patients with Alzheimer's disease (AD) who have impaired semantic memory. Their cognitive model of such processing assumes that there are two modes of categorization: similarity-based categorization, in which one assesses the similarity of a test item to prototypes or exemplars of various categories; and rule-based categorization, in which one assesses if a test item meets the critical conditions specified in a rule that determines category membership. Their neural model of concepts focuses on support for rule-based processing in left dorsolateral frontal cortex (DLFC), support for similarity-based processing in left posterior heteromodal association cortex (PHAC), and the representation of information associated with a concept in modality-specific association cortex such as the ventral temporal lobe.
Specific Aim 1 investigates how similarity-based and rule-based categorization are affected in AD with behavioral studies, and examines the neural basis for these processes with fMRI.
Specific Aim 2 focuses on the use of categories to guide inductive inference, and investigates different types of category-based induction in AD. They hypothesize that AD patients are impaired at using similarity-based and rule-based processes for categorization and inductive reasoning, and that fMRI studies will associate these limitations with left PHAC and left DLFC, respectively.
Specific Aim 3 is concerned not with the processes that operate on conceptual representations, but with the representations themselves. Here they ask whether such representations distinguish between appearance and action features for natural kind and manufactured artifact categories using tasks that require categorization or feature verification, and they investigate the neural basis for these decisions. They hypothesize that AD patients have a deficit with specific features contributing to a category, and this will be associated with interruption of a network involving visual association cortex in the left temporal lobe and left DLFC.
Specific Aim 4 involves a comparison of object and action conceptual representations in AD, and investigates the neural basis for this knowledge with fMRI. They hypothesize that the mental organization of action concepts is more susceptible to impairment in AD, and this will be associated with left DLFC. This research program will expand our understanding of impaired semantic memory in AD, and provide important information for cognitive neuroscience concerning the cognitive and neural basis for normal concept processing and representation.
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