Movement Differences between Deliberate and Spontaneous Facial Expressions: Zygomaticus Major Action in Smiling

Abstract

Previous research suggests differences in lip movement between deliberate and spontaneous facial expressions. We investigated within participant differences between deliberately posed and spontaneously occurring smiles during a directed facial action task. Using automated facial image analysis, we quantified lip corner movement during periods of visible Zygomaticus major activity. Onset and offset speed, amplitude of movement, and offset duration were greater in deliberate smiles. In contrast to previous results, however, lip corner movement asymmetry was not greater in deliberate smiles. Observed characteristics of deliberate and spontaneous smiling may be related to differences in the typical context and purpose of the facial signal. Keywords emotion expression; nonverbal communication; voluntary movementThe smile is one of the most frequent facial expressions and serves to communicate positive emotional states, as well as to serve social functions including greeting and appeasement (Keltner, 1995;Preuschoft, 1992;Schmidt & Cohn, 2001). Like many facial expressions, the smile can be produced either deliberately by voluntary movement of the Zygomaticus major muscles or spontaneously in response to social or emotional stimuli. The primary objective of this study was to determine whether deliberate and spontaneous smiles observed in the same individuals were significantly different in lip corner movement and in asymmetry of movement. The role of an additional facial movement, specifically Orbicularis oculi activity in these smiles was also addressed. Uncovering systematic differences in movement between deliberate and spontaneous expression has important implications for understanding pathological disruptions in facial movement, as well for understanding the biological basis underlying facial communication.Spontaneous smiles of enjoyment have been found to have a number of distinctive timing and movement characteristics in general; as compared to deliberate smiles, spontaneous smiles tend to be shorter in total duration (from 4 to 6 seconds), slower in offset time (Hess & Kleck, 1997), and less asymmetric in lip corner movements (Hager & Ekman, 1997;Skinner & Mullen, 1991). The findings, obtained with perceptually based measures, have been supported in part by studies using more objective quantitative methods. Cohn and Schmidt (2004) Kleck, & Lanzetta, 1989). This study combines the measurement of temporal differences in smiling movements with the measurement of specific visible characteristics of intra-individual variation in smiling movements to determine which aspects of movement timing differentiate deliberate and spontaneous smiles.Timing differences between deliberate and spontaneous smiles are important because they are likely to significantly affect social judgments of the person smiling. For example, asymmetry in smiling has been shown to relate to negative social judgments and lower ratings of trustworthiness in observers (Brown & Moore, 2002). In addition, Bugental (1986) found that women whose...

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