The expression of emotion is achieved through intricate combinations of verbal and nonverbal information. Nonverbal information encompasses any message that is not expressed in words. Among the communicative channels that express emotions, the face is often considered to be the richest source of non verbal information. Most of the research on facial expression has been based on Ekman’s proposed method of measurement by dividing expressions into categories of predefined emotion labels.
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https://vhil.stanford.edu/mm/2010/ahn-hemm-facial-expression.pdf
https://www.paulekman.com/about/paul-ekman/