Facial Action Coding System, usually the abbreviation FACS is used. It is a methodology developed for the identification and classification of human facial expressions. It was developed by American psychologists Paul Ekman and Wallace V. Friesen in 1978.
FACS contains all possible combinations of activities of mimic muscles in the face, and each of them is assigned a specific emotion. Altogether there are hundreds of combinations of facial muscle activities, however just few of them are meaningful.
One of the key elements of FACS are so-called microexpressions.
A microexpression is involuntary (independent of the will of the subject) expression of specific emotions in a specific situation within a fraction of a second. Microexpression is entirely spontaneous. This means that you can’t control the will and in spite of any attempt of the subject to suppress the expression of actual current emotion. It is fully automatic, and corresponds thus at the moment with really felt emotion.
Use of FACS methodology in practice: After training, the trained subject is able to distinguish microexpressions of other subjects and “read” when communicating real emotions of his counterpart or counterparts (communication partners).