Neuromuscular electric stimulation and social cognition

How do we perceive and recognize other peoples facial expressions? An influential scientific theory proposes that changes in our own body contribute to inferring other peoples emotions. One particularly relevant bodily signal for recognizing facial emotions is somatosensory and proprioceptive facial feedback. In line with this, a vast number of studies has reported the occurrence of facial mimicry, i.e. the spontaneous imitation of a perceived facial expression, and revealed that blocking or interfering with facial mimicry (and thus with the accompanying facial feedback) can impair emotion recognition. However, conflicting findings also exist, and extant research suffers from several limitations, which this interdisciplinary project aims to overcome. An international team of two experimental psychologists, and one artist/electrophysiologist, aim to investigate the role of facial proprioception in emotion recognition with the help of neuromuscular electric stimulation (NMES) a technique that consists of attaching electrodes to the participants face and activating their facial muscles with weak electric impulses. Facial NMES was first developed in the 19th century, and is currently employed as a form of medical treatment (e.g. to improve cheek and lip strength after a stroke), but was never used in this context. In this project, NMES will be used for the first time to investigate multisensory integration in the domain of social perception, allowing to mimic proprioceptive feedback from specific muscle groups with short duration and accurate timing. Using well-prepared stimuli and especially designed computer tasks, the effects of NMES on emotion recognition will be analyzed at both the perceptual and the cognitive level. Moreover, brain activity will be monitored with electroencephalography (EEG). The results will provide new insights into the psychological mechanisms and the neural processes underlying visual emotion processing, and may help to design better strategies for improving emotion recognition skills in individuals with social deficits.

In this project we developed of a paradigm allowing to stimulate facial muscles in humans using electrical impulses, and combined it with brain recordings, to study how facial actions influence one's own feelings and perception of others' emotional facial expressions. The technique of facial neuromuscular electrical stimulation (fNMES) consists of placing electrodes on the skin and delivering short trains of weak electrical pulses to stimulate the underlying muscles. Despite dating back to the 19th-century neurologist Duchenne de Boulogne, fNMES remained largely unknown to modern-day psychologists, and has never been combined with brain activity recordings with electroencephalography (EEG). To change this, we have published a set of guidelines on how to conduct fNMES in psychophysiological research and conducted a series of experiments to test its effects related to emotion. In one experiment, we showed faces displaying faint happiness or sadness to 47 participants, who decided which emotion was expressed. In half of the trials, fNMES was simultaneously applied for half a second to the main smiling muscles. Results showed that fNMES-induced smiling increases the likelihood that faces are perceived as happy, compared to trials without stimulation. This finding that our own smiling can bias our perception of others' facial expressions, was replicated in a second experiment. In another experiment, we asked if fNMES can change how people feel themselves. A group of 58 participants was asked to report how positively or negatively they felt after receiving five seconds of fNMES to either their smiling or frowning muscles. As expected, participants felt more positive after smiling than frowning, even controlling for the level of discomfort that can sometimes be experienced with fNMES. This result shows that expressing facial emotions can modulate our own feelings. These results were obtained even though only low electrical currents were delivered, which resulted in weak activations of individual muscles. Thus, smiling and frowning were only partially re-created-and at weak intensities. Moreover, we found these results despite the fact that fNMES was only delivered for very short durations-as little as half a second in the first of the experiments described above. We can thus theorize that even stronger effects might be obtained if fNMES were applied with stronger currents and for longer durations-although that comes with the greater risk of inducing discomfort. In summary, we established that electrical stimulation of facial muscles can modulate how people feel and perceive emotions. By allowing to finely control which muscles are activated when and to what degree, fNMES is a powerful tool for science, and may have potential for future clinical applications. It may for example be beneficial to improve mood and emotion recognition in individuals with Parkinson's disease or autism spectrum disorder.