The role of spontaneous facial mimicry in modulating instructed overt facial actions: evidence from facial electromyography

Instructed facial actions are executed more quickly when they match observed expressions (e.g., smiling in response to a smile) and more slowly when they do not (e.g., smiling in response to a frown). The underlying mechanism could be motor matching to positive and negative stimuli on the valence dimension rather than expression-specific facial mimicry (defined as spontaneous and precise motor matching to observed facial expressions). Using facial electromyography, the present study investigated whether the onset speed of instructed overt facial actions associated with emotions is shaped by spontaneous expression-specific motor matching rather than mere valence. To provide evidence against a simple valence effect, facial action onset modulation was expected to be seen beyond smiles and frowns. The relationships between muscles – synergistic (i.e., muscles involved in a specific movement) vs. antagonistic (i.e., muscles opposing the target action) – informed the expected onset times. Participants (N = 80, 50:50 sex ratio, M(age) = 23.4 years, SD = 4.8) were pre-instructed to perform five facial actions (frowning, nose-wrinkling, pouting, eyebrow lifting, smiling) while viewing facial expressions of anger, disgust, fear, sadness, surprise, happiness, and neutral. For smiling, nose-wrinkling, and frowning, onset times were faster when instructed actions and observed expressions were synergistic and slower when they were antagonistic as expected; this pattern was not supported for pouting and eyebrow lifting. Difficulties performing the instructed pout and the weak resemblance of participants’ eyebrow lifting to the observed expressions may explain the unexpected results. The findings imply that spontaneous expression-specific motor-matching modulates onsets of pre-instructed expressions, thereby influencing intentional facial behaviour. Facial actions may need to be well-established in a person’s motor repertoire for facial action facilitation (as reflected in faster onset times) to occur, relying on perception-action coupling through learned associations. Overall, the results are better explained by expression-specific motor matching than valence. Covert facial mimicry may have occurred serving to overtly transmit facial signals.