1. Ekman, P., Friesen, W.V.: Facial Action Coding System: A Technique for the Measurement of Facial Movement. Consulting Psychologists Press, Palo Alto (1978)
2. Ekman, P.: An argument for basic emotions. Cogn. Emot. 6(3–4), 169–200 (1992)
3. Russell, J.A., Bullock, M.: Multidimensional scaling of emotional facial expressions: similarity from preschoolers to adults. J. Pers. Soc. Psychol. 48(5), 1290–1298 (1985)
4. Sumiya, R., Lenz, R., Chao, J.: Measurement of discrimination thresholds in facial expression recognition and investigation on expression space. In: HCG symposium 2017, pp. 25–28 (2017)
5. Chojnacki, W., Brooks, M.J., van den Hengel, A., Gawley, D.: On the fitting of surfaces to data with covariances. IEEE Trans. Patt. Anal. Mach. Intell. 22(11), 1294–1303 (2000)