Zygomaticus activation through facial neuromuscular electric stimulation (fNMES) induces happiness perception in ambiguous facial expressions and affects neural correlates of face processing

The recognition of emotional facial expressions is crucial for social communication, but different views exist on whether facial feedback is a significant factor in this decoding process. Previous studies have manipulated the degree of activation of facial muscles through various techniques, such as voluntary posing of facial expressions, holding a pen in the mouth, or botulinum toxin injections. These methods, however, are limited in their control over which muscles are (de)activated when and to what degree. To overcome these limitations, we used computer-controlled facial neuromuscular electrical stimulation (fNMES) to generate 500ms of weak contractions of bilateral Zygomaticus Major muscle (ZM) at the onset of faces shown on the screen. Ambiguous facial expressions were categorised as happy or sad by 47 participants. In half of the trials, weak smiling was induced through fNMES, as verified with automatic FACS coding. EEG was recorded throughout. The likelihood of categorising ambiguous facial expressions as happy was significantly increased with fNMES, as shown with frequentist and Bayesian linear mixed models. Further, fNMES resulted in a reduction of P1, N170 and LPP amplitudes. These findings suggest that fNMES-induced facial feedback can bias facial emotion recognition and modulate the neural correlates of face processing. We conclude that fNMES has potential as a tool for studying the effects of facial feedback.