Towards Automatic Recognition of Spontaneous Facial Actions
Charles Darwin (1872/1998) was the first to fully recognize that facial expression is one of the most powerful and immediate means for human beings to communicate their emotions, intentions, and opinions to each other. In addition to providing information about affective state, facial expressions also provide information about cognitive state, such as interest, boredom, confusion, and stress, and conversational signals with information about speech emphasis and syntax. Facial expressions also contain information about whether an expression of emotion is posed or felt (Ekman, 2001; Frank, Ekman, & Friesen, 1993). In order to objectively measure the richness and complexity of facial expressions, behavioral scientists have found it necessary to develop objective coding standards. The Facial Action Coding System (FACS) from Ekman and Friesen (1978) is arguably the most comprehensive and influential of such standards. FACS is based on the anatomy of the human face, and codes expressions in terms of component movements, called " action units " (AUs). Ekman and Friesen defined 46 AUs to describe each independent movement of the face. FACS measures all visible facial muscle movements, including head and eye movements, and not just those presumed to be related to emotion. When learning FACS, a coder is trained to identify the characteristic pattern of bulges, wrinkles, and movements for each facial AU. The AUs approximate individual facial muscle movements but there is not always a 1:1 correspondence. FACS has been used to verify the physiological presence of emotion in a number of studies, with high (over 75%) agreement Because it is comprehensive, FACS also allows for the discovery of new patterns related to emotional or situational states. For example, using FACS Ekman et al (1990) and Davidson et al (1990) found that smiles which featured both orbicularis oculi (AU6), as well as zygomatic major action (AU12), were correlated with self-reports of enjoyment, as well as different patterns of brain activity, whereas smiles that featured only zygomatic major (AU12) were not. Subsequent research demonstrated that the presence of smiles that involve the orbicularis oculi (hereafter " enjoyment smiles ") on the part of a person who has survived the death of their romantic partner predicts successful coping with that traumatic loss (Bonnano & Keltner, 1997). Other work has shown a similar pattern. For example, infants show enjoyment smiles to the presence of their mothers, but not to strangers (Fox & Davidson, 1988). Mothers do not show as many enjoyment …