An understanding of the response of cardiac tissue to an electrical stimulus is crucial for proper treatment and prevention of cardiac arrhythmias, for example in the application of a strong defibrillating shock to halt fibrillation. This project will apply novel techniques for data visualization and non-linear analysis to explore the dynamics of spiral wave formation, interaction, and termination, for electrical activity evident at the epicardial surface and also within the underlying myocardial substrate. Fibrillatory waves rotate about phase singularities (2-D) or filaments (3-D) which define many of the characteristics of the wave itself. Literature addressing a comparison between computational predictions and cardiac experimental observations of the interaction behavior between multiple spiral waves is virtually non-existent. Recent work has demonstrated that such non-linear dynamics of cardiac activation can be quantified using phase portrait analysis. The specific aims are to (1) localize and characterize phase singularity and filament behavior, (2) quantify the shifts in the phase portrait during multiple stimuli to an experimental preparation, and (3) further substantiate the above results by using a previously validated whole-heart panoramic imaging algorithm. This research will rely heavily on the development of new techniques to examine phase resetting during stimulation and defibrillation, and the creation and subsequent interaction of singularities and filaments, as well as whole-heart electrophysiological measurements with the ability of overcoming the difficulty of having phase singularities and filaments, as well as whole-heart electrophysiological with the ability of overcoming the difficulty of having phase singularities move out of the field-of-view. The research will clarify phase singularity and filament behavior as well as the dynamics of phase resetting, on both a regional and a global epicardial basis.