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, 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.

Keywords: Electrical stimulation; Emotion; Facial feedback; Facial muscles; NMES.

Fig. 1

A mid-eighteenth-century photograph depicting Duchenne de Boulogne applying fNMES to his patient, from Duchenne (1862)

Fig. 2

Ideal electrode positions for bipolar fNMES are similar to those for facial EMG (see guidelines by Fridlund & Cacioppo, 1986). For a monopolar configuration, the active (cathode) electrode should be placed in the centre of these ideal locations (on the motor point), while the reference (anode) is placed distally

Fig. 3

An example of a square wave in three different phases: A monophasic, B biphasic, C polyphasic

Fig. 4

A screenshot of the Shiny app allowing us to compute current density and to visualize stimulation waveforms

Fig. 5

PRISMA flow diagram depicting the information at the different phases of the systematic review. The parameter table and R code to compute current density are available on OSF (bit.ly/3faUYkP)

Fig. 6

Classical proprioceptive manipulations, e.g., preventing smiling by holding a pen between the lips or inducing a smile by holding a pen between the teeth, are in place before the onset of the visual stimulus. This is not fully in line with theories of embodied cognition, which conceive facial mimicry and its accompanying change in facial feedback as a reaction to the visual stimulus. fNMES, on the other hand, allows us to provide physiologically sound proprioceptive inputs that can be targeted in both time (before, during, and after the visual stimulus) and space (congruent or incongruent muscles)