This project seeks to characterize and quantify psychological and physiological differences that occur across alter personality states of patients with MPD. Alter personalities are studied using a range of psychological and physiological measures including: EEG, auditory and visual evoked potentials, Xe131 cerebral blood flow, histamine sensitivity, autonomic nervous system measures and psychological testing. Simulating normal controls serve as a comparison group for many of the studies. Important recent findings include the discovery that MPD patients demonstrate an almost total absence of savings for implicit memory tasks across alter personality states at the same time they have essential normal savings on test-retest (approximately 40% improvement in performance) for the same tasks within alter personality state. Measures of explicit memory performance are normal for these patients.