Background: Individuals with intense frowning are commonly perceived as expressing negative emotions. Anger, fear, and sadness are associated with corrugator ("frown") muscle activity.
Objective: We sought to study how faces were perceived by others after denervation of frown muscles with localized botulinum toxin injections for treatment of facial frown lines.
Methods: Facial photographs were taken from volunteers before and after botulinum toxin injection. These photographs were shown to viewers who were naive to the procedure and asked to rate the expressed intensity of anger, sadness, fear, and happiness. As reference for this task we used a standard set of pictures of facial affect displaying different intensity levels for each emotion tested.
Results: Of 40 viewers, 39 were able to discriminate different intensity levels (0%, 25%, 50%, 75%, and 100%) of emotional affect in the control task. According to their ratings faces with denervated frown muscle activity expressed relatively less anger (-40%), fear (-49%), sadness (-10%), and more happiness (+71%).
Conclusion: Frown muscle activity is essential for both negative and positive emotional expressions. Temporary denervation using botulinum toxin enhances the facial expression of positive emotion resulting in a shift rather than a loss of facial affect.