Introduction
The human face is a rich means of nonverbal communication and can constitute a powerful social reward (Chelnokova et al. A heated debate persists around whether facial mimicry contributes to emotion recognition. Theories of embodied cognition (Niedenthal et al. In contrast, the ‘contextual’ or ‘mimicry as social regulator’ view proposes that facial mimicry results from an affiliative intention and reflects emotion understanding (Hess and Fischer Rewards are powerful emotion elicitors and share many commonalities with emotions (Sander and Nummenmaa The manipulation of the major underlying neurochemical brain systems regulating reward processing (i.e. dopamine and opiates) constitutes a promising tool to further explore the nature of facial mimicry. Pharmacologically challenging either of these neurochemical systems should indeed impact facial mimicry, and an eventual asymmetry in the effects of dopaminergic and opioidergic manipulations would reveal if facial mimicry reflects more wanting or liking — although some changes in affiliative motivation would also be expected following opioid modulation, given that the opioid system is thought to be, together with oxytocin, the main system behind affiliations. On the other hand, if changes to the dopamine and/or opioid systems have no effects on facial mimicry, then the hypothesis that facial mimicry is primarily an anticipation of, or a response to, a reward is not supported, and instead the hypothesis that facial mimicry is the result of an automatic motor copy becomes more likely. The goal of the experiment reported here was to test these two contrasting hypotheses.Interest in the modulation of facial mimicry by changes in the brain’s levels of hormones and neurotransmitters, such as testosterone, oxytocin, vasopressin, and opioids, has recently increased (Hermans et al. In summary, the currently available evidence is scarce and mixed, but suggests that opioids, and potentially also dopamine, may play a role in the facial mimicry response by acting on the motivation to affiliate and/or the hedonic pleasure related to it. However, to date, a direct comparison of the effects of dopaminergic and opioidergic modulations on facial mimicry is lacking. In order to fill this gap, the following experiment investigated the roles of the dopamine and opioid systems in facial mimicry by studying healthy participants in a double-blind placebo-controlled pharmacological intervention. Using a between-subjects design, participants received 400 mg of amisulpride (a dopamine D2/3 receptor antagonist), 50 mg of naltrexone (an opioid receptor antagonist), or a placebo. Four hours after drug/placebo administration, facial mimicry of dynamic happy and angry facial expressions morphing into each other was measured with facial EMG of the corrugator supercilii (CS) and zygomaticus major (ZM) muscles.Based on the understanding that pleasure responses are promoted/inhibited by opioid agonists/antagonists (Nummenmaa and Tuominen 