(A) Mean corrugator supercilii activity (i.e., brow knit) following odor onset (in seconds). (B) Mean medial frontalis activity (i.e., brow lift) following odor onset (in seconds). Facial muscle activity displayed here was measured before the start of the facial expression classification task, to isolate the effect of odor. Error bars reflect 68% within-subjects CI of the interaction between odor and time.

https://doi.org/10.1371/journal.pone.0128717.g001

Facial muscle activity displayed here was measured before the start of the facial expression classification task, to isolate the effect of odor. Above each bar, the time after odor onset (in seconds) is depicted (see Y-axis). The more each bar is located toward the upper-right end point (vs. bottom-left starting point) of the dashed diagonal, the more the medial frontalis and corrugator supercilii muscles co-activated (μV), resembling a fearful facial expression [cf. 11, 12].

https://doi.org/10.1371/journal.pone.0128717.g002

Odor condition: Baseline, fast stress, slow stress. Facial expressions that had to be classified: Neutral, happy, fear, disgust. For clarification purposes, the display of mean facial muscle activity on the emotional facial expression classification task was collapsed over the variable noise level (20%, 40%, 60%, 80%, 100%). (A) Mean corrugator supercilii activity, averaged over 1 second following the onset of the presented expression. (B) Mean medial frontalis activity, averaged over 1 second following the onset of the presented expression. Error bars reflect 68% within-subjects CI of the main effect of odor.

https://doi.org/10.1371/journal.pone.0128717.g003