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).

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

The role of facial feedback in facial emotion recognition remains controversial, partly due to limitations of the existing methods to manipulate the activation of facial muscles, such as voluntary posing of facial expressions or holding a pen in the mouth. These procedures are indeed limited in their control over which muscles are (de)activated when and to what degree. To overcome these limitations and investigate in a more controlled way if facial emotion recognition is modulated by one's facial muscle activity, we used computer-controlled facial neuromuscular electrical stimulation (fNMES). In a pre-registered EEG experiment, ambiguous facial expressions were categorised as happy or sad by 47 participants. In half of the trials, weak smiling was induced through fNMES delivered to the bilateral Zygomaticus Major muscle for 500 ms. 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.

Application of facial neuromuscular electrical stimulation (fNMES) in psychophysiological research: Practical recommendations based on a systematic review of the literature.

Facial neuromuscular electrical stimulation (fNMES), which allows for the non-invasive and physiologically sound activation of facial muscles, has great potential for investigating fundamental questions in psychology and neuroscience, such as the role of proprioceptive facial feedback in emotion induction and emotion recognition, and may serve for clinical applications, such as alleviating symptoms of depression. However, despite illustrious origins in the 19th-century work of Duchenne de Boulogne, the practical application of fNMES remains largely unknown to today's researchers in psychology. In addition, published studies vary dramatically in the stimulation parameters used, such as stimulation frequency, amplitude, duration, and electrode size, and in the way they reported them. Because fNMES parameters impact the comfort and safety of volunteers, as well as its physiological (and psychological) effects, it is of paramount importance to establish recommendations of good practice and to ensure studies can be better compared and integrated. Here, we provide an introduction to fNMES, systematically review the existing literature focusing on the stimulation parameters used, and offer recommendations on how to safely and reliably deliver fNMES and on how to report the fNMES parameters to allow better cross-study comparison. In addition, we provide a free webpage, to easily visualise fNMES parameters and verify their safety based on current density. As an example of a potential application, we focus on the use of fNMES for the investigation of the facial feedback hypothesis.