Application of facial Neuromuscular Electrical Stimulation (fNMES) in psychophysiological research – practical recommendations based on a systematic review of the literature

Abstract

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, as well as for clinical applications, such as alleviating depression symptoms. However, despite illustrious origins in 19th century work of Duchenne de Boulogne, the practical application of fNMES remains largely unknown to researchers in psychology and human physiology. In addition, published studies vary dramatically in use and reporting of parameters, such as stimulation frequency, amplitude, duration, and electrode size. 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. Here, we provide an introduction to fNMES, a systematic review of the existing literature focusing on stimulation parameters used, and we offer recommendations on how to safely and reliably deliver fNMES. In addition, we provide a free webpage, allowing to easily verify and compare the safety of fNMES parameters 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.

Measurement of the N170 during facial neuromuscular electrical stimulation (fNMES)

“…These stimulation parameters were chosen based on our own pilot data that aimed to minimise discomfort whilst engaging facial muscles, and on previous studies using fNMES (e.g. Kapadia et al, 2019; for a review of the literature see Efthimiou, Perusquía-Hernández, et al, 2022). The 70 Hz frequency is also convenient for other reasons, as it facilitates estimation and removal of the stimulation artefacts, which will mostly lie outside the frequency range that is typically analysed for ERPs (1-30 Hz), and will also differ from line noise (50/60 Hz).…”

“…Prior to beginning of the task, participants were introduced to the fNMES procedure so as to gain initial experience with the electrical stimulation of facial muscles, and to allow the experimenter to configure the appropriate fNMES amplitudes to use during the task. An alcohol wipe (70% isopropyl) was used to clean the skin, before the disposable surface electrodes were placed above the approximate location of the DAO muscle on each side of the face (guided by EMG guidelines, for a step-by-step guide see Efthimiou, Perusquía-Hernández, et al, 2022). The experimenter systematically applied fNMES to each side of the face independently, starting with a low current.…”

“…before, during, or after the onset of a visually presented face). As such, fNMES bears enormous potential to study the effects of proprioceptive facial inputs in healthy participants, but remains largely unknown to both researchers and volunteers (Efthimiou, Hanel, et al, 2022;Efthimiou, Perusquía-Hernández, et al, 2022).…”

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

“…fNMES has already shown promise as a treatment for depression (Demchenko et al, 2023;Kapadia et al, 2019), and we have recently emphasised its potential for testing aspects of the FFH (Efthimiou, Hernandez, et al, 2023). For a recent review of the literature on human surface fNMES, and recommendations on how to implement it in the psychology laboratory and combine it with EEG (see, Baker et al, 2023;Efthimiou, Baker, et al, 2023;Efthimiou, Hernandez, et al, 2023).…”