Corrugator muscle responses are associated with individual differences in positivity-negativity bias.

Corrugator supercilii muscle activity is considered an objective measure of valence because it increases in response to negatively valenced facial expressions (angry) and decreases to positive expressions (happy). The authors sought to determine if corrugator activity could be used as an objective measure of positivity-negativity bias. The authors recorded corrugator responses as participants rated angry, happy, and surprised faces as "positive" or "negative." The critical measure of bias was the percentage of positive versus negative ratings assigned to surprised faces by each participant. Reaction times for surprise expressions were longer than for happy and angry expressions, consistent with their ambiguous valence. Participants who tended to rate surprised faces as negative showed increased corrugator activity to surprised faces, whereas those who tended to rate surprise as positive showed decreased activity. Critically, corrugator responses reflected the participants' bias (i.e., their tendency to rate surprise as positive or negative). These data show that surprised faces constitute a useful tool for assessing individual differences in positivity-negativity bias, and that corrugator activity can objectively reflect this bias.