This part of our project explores the declarative knowledge and the deterioration of the declarative knowledge system as a function of organic brain disease. We seek to test the hypothesis that declarative memory can be seen as arising from the behavior of adaptive, distributed, interactive network models, rather than a conventional propositional network. We will test this hypothesis by assessing whether simulations of such models can capture the patterns of spared and impaired performance that are seen in patients suffering from the progressive syndrome of semantic dementia. The project will employ both computational modeling and experimental investigations with semantic dementia patients. We will a) develop a model that learns internal representations of concepts from exposure to a corpus of representative propositional statements; b) test the model~s ability to account for seven key characteristics of the deficits shown by semantic dementia patients, which include progressive deterioration of conceptual knowledge, partial residual knowledge even of severely affected concepts, sparing of aspects of concepts shared with other related concepts, possible effects of category coherence on spared knowledge, frequency sensitivity, occurrence of high-frequency category coordinates as errors in picture naming, and the high degree of consistency in the pattern of loss within and between tasks; c) Test the model~s ability to address phenomena exhibited by normal adults, such as effects of prototypicality, basic level, and expertise; order of acquisition; and sensitivity to domain relevant underlying relational information as opposed to surface similarity. We will then develop a second version of the model to increase the flexibility of the propositional content that can be represented, so that specific fact and event information can be stored along with general conceptual knowledge. We will test the new model's ability to account for pattern of deterioration of fact, and episodic information in semantic dementia patients. The experimental work will generate materials to be used in training the network, and to test use in testing the model~s adequacy in accounting for patterns of semantic deterioration. Additional aims of the experimental work are to a)Examine the pattern of deficits within individual patients with respect to verbally reportable knowledge and knowledge accessible without requiring verbalization. b)Assess the breakdown of knowledge of specific instances (people, places), with respect to frequency, consistency, and category coherence; and explore the relationships between semantic and episodic memory and between semantic memory and speech production. These tests will assess the degree of integration of interdependency among the processes in question.
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