Final answer:
Paul Ekman's research on universal facial expressions underlies theories of emotion such as James-Lange, Cannon-Bard, and Schachter-Singer, which explain the relationship between physiological arousal and emotional experience.
Step-by-step explanation:
Paul Ekman discovered that certain emotions are universal and are associated with unique facial expressions and physiological patterns. Theories of emotion, such as the James-Lange theory, Cannon-Bard theory, and Schachter-Singer two-factor theory, attempt to explain how we experience emotions. The James-Lange theory suggests that physiological arousal precedes and informs emotional experience, the Cannon-Bard theory argues that physiological arousal and emotional experience occur simultaneously but independently, and the Schachter-Singer two-factor theory posits that physiological arousal and cognitive appraisal together result in emotional experiences. Ekman demonstrated the universality of facial expressions in conveying emotions, a concept with foundations in Charles Darwin's work on the expression of emotion.