Discussion
anger, reduction of smile (
CONTRACTION
In order to shed light on the nature of facial mimicry, we tested the effects of amisulpride, a D2/D3 dopamine receptor antagonist, and naltrexone, a non-selective opioid receptor antagonist, on facial mimicry of happiness and anger. We found clear facial mimicry in all three participant groups, using a tried and tested task and a dynamic stimulus set. Thus, videos starting with decreasing anger and gradually changing into increasing happiness elicited greater contraction of the ZM muscle and relaxation of the CS muscle. In contrast, videos featuring the opposite change in expression, i.e. starting with decreasing happiness and gradually containing more anger, elicited ZM relaxation and CS activation. These changes in facial muscle activity, reflecting mimicry of the second emotion depicted in each video, were expected based on previous publications that utilised the same stimuli and paradigm (Korb et al. Several non-pharmacological studies have indeed shown that facial mimicry is modulated by the reward value of the face stimuli encountered. For example, conditioning participants to associate particular face identities with winning money, compared to losing money, resulted in greater smile mimicry when these same faces were later shown dynamically changing from a neutral to a happy expression (Korb et al. Dopamine and endogenous opioids have been established as the lead chemicals involved in the anticipation of and response to both social and non-social rewards (Berridge and Kringelbach Instead, we were unable to find a significant reduction of smile (as well as anger) mimicry after inhibition of D2/D3 dopamine and opioid receptors after administration of, respectively, 400 mg of amisulpride and 50 mg of naltrexone. This was the first measurement of facial mimicry after a targeted pharmacological challenge of the dopamine system in healthy participants, and the first direct comparison of the effects of dopaminergic and opioidergic antagonists on facial mimicry. Our null finding speaks against the (‘contextual’, or ‘mimicry as social regulator’) view that facial mimicry reflects a response that follows emotion understanding (Hess and Fischer In conclusion, facial mimicry amplitude of happy and angry faces was not significantly modulated, neither by 400 mg of the D2/D3 dopamine receptor antagonist amisulpride, nor by 50 mg of the non-selective opioid receptor antagonist naltrexone. This null finding suggests that spontaneous facial mimicry is not primarily conducible to a reward/punishment response and does not support the claim that facial mimicry is an affiliative or strategic response to an already-perceived and recognised emotional expression. It is, however, compatible with the idea, advanced by theories of embodied cognition, that facial mimicry is an automatic motor copy, which can be, but is not necessarily, modulated by context and other factors. More research is however needed to conclusively establish the nature of facial mimicry, as it should be acknowledged that it is notoriously challenging to interpret null findings for novel effects.