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Paul Ekman, Robert W Levenson, and Wallace Friesen, «Autonomic Nervous System Activity Distinguishes Between Emotions», Science, 221 (1983): 1208-10; Robert W. Levenson, Paul Ekman, and Wallace Friesen, «Voluntary Facial Action Generates Emotion-specific Autonomic Nervous System Activity», Psychophysiology, 27 (1990): 363-84; Paul Ekman and Richard J. Davidson, «Voluntary Smiling Changes Regional Brain Activity», Psychological Science, 4 (1993): 342-45.