P. Ekman, ′′Basic emotions,′′ in Handbook of Cognition and Emotion, T. Dalgleish and M.J. Power, eds., pp. 45-60, John Wiley & Sons Ltd., New York, 1999.
J.A. Russell, ′′A circumplex model of affect,′′ Journal of Personality and Social Psychology, vol. 39, no. 6, pp. 1161-1178, 1980.