Smiling and frowning induced by facial neuromuscular electrical stimulation (fNMES) modulate felt emotion and physiology.

According to the facial feedback hypothesis, feedback from facial muscles can initiate and modulate a person's emotional state. This assumption is debated, however, and existing research has arguably suffered from a lack of control over which facial muscles are activated, when, to what degree, and for how long. To overcome these limitations, we carried out a preregistered experiment including 58 participants. Facial neuromuscular electrical stimulation (fNMES) was applied to the bilateral zygomaticus major and depressor anguli oris muscles for 5 s at 100% and 50% of the participants' individual motor threshold. After each trial, participants reported their emotional valence and intensity and levels of experienced discomfort. Facial muscle activations were verified with automatic video coding; heart rate and electrodermal activity were recorded throughout. Results showed that muscle activation through fNMES, even when controlling for fNMES-induced discomfort, modulated participants' emotional state as expected, with more positive emotions reported after stronger stimulation of the zygomaticus major than the depressor anguli oris muscle. The addition of expression-congruent emotional images increased the effect. Moreover, fNMES intensity predicted intensity ratings, reduced HR, and skin conductance response. The finding that changes in felt emotion can be induced through brief and controlled activation of specific facial muscles is in line with the facial feedback hypothesis and offers exciting opportunities for translational intervention. (PsycInfo Database Record (c) 2024 APA, all rights reserved).