[2] Ekman, P., and Freisen, W. V.: Unmasking the face: A guide to recognizing emotions from facial clues, Prentice-Hall, Englewood Cliffs, NJ, 1975.
[3] Ekman, P.: Strong evidence for universals in facial expressions: A reply to Russell's mistaken critique, Psychological Bulletin, 115, pp.268-287, 1994.
[4] Lewis, M., Sullivan, M. W., and Vasen, A.: Making faces: Age and emotion differences in the posing of emotional expressions, Developmental Psychology, 23, pp.690-697, 1987.