MATHEMATICAL MODELING OF LATERALIZATIONAND ASYMMETRIES IN CORTICAL MAPSbySvetlana

Title of Dissertation: MATHEMATICAL MODELING OF LATERALIZATION AND ASYMMETRIES IN CORTICAL MAPS Svetlana Levitan, Doctor of Philosophy, 1999 Dissertation directed by: Professor James A. Reggia Applied Mathematics Program Recent experimental work in neurobiology has de ned asymmetries and lateralization in the topographic maps found in mirror-image regions of the sensorimotor cerebral cortex. However, the mechanisms underlying these asymmetries are currently not established, and in some cases are quite controversial. In order to explore some possible causes of map asymmetry and lateralization, several neural network models of cortical map lateralization and asymmetries based on self-organizing maps are created and studied both computationally and theoretically. Activation levels of the elements in the models are governed by large systems of highly nonlinear ordinary di erential equations (ODEs), where coe cients change with time and their changes depend on the activation levels. Special metrics for objective evaluation of simulation results (represented as paired receptive eld maps) are introduced and analysed. The behavior of the models is studied when their parameters are varied systematically and also when simulated lesions are introduced into one of the hemispheric regions. Some very sharp transitions and other interesting phenomena have been found computationally. Many of these computationally observed phenomena are explained by theoretical analysis of total hemispheric activation in a simpli ed model. The connection between a bifurcation point of the system of ODEs and the sharp transition in the model's computational behavior is established. More general understanding of topographic map formation and changes under various conditions is achieved by analysis of activation patterns (i.e., !-limit sets of the above system of ODEs). This is the rst mathematical model to demonstrate spontaneous map lateralization and asymmetries, and it suggests that such models may be generally useful in better understanding the mechanisms of cerebral lateralization. The mathematical analysis of the models leads to a better understanding of the mechanisms of self-organization in the topographic maps based on competitive distribution of activation and competitive learning. MATHEMATICAL MODELING OF LATERALIZATION AND ASYMMETRIES IN CORTICAL MAPS by Svetlana Levitan Dissertation submitted to the Faculty of the Graduate School of the University of Maryland at College Park in partial ful llment of the requirements for the degree of Doctor of Philosophy 1999 Advisory Committee: Professor James A. Reggia, Chairman/Advisor Professor Je ery M. Cooper Professor Royal B. Kellogg Professor V.S. Subrahmanian Professor Peter Wolfe TABLE OF CONTENTS List of Tables v List of Figures vi