Why do you smile at me while I'm in pain? --Pain selectively modulates voluntary facial muscle responses to happy faces.

It has been well documented that emotional stimuli modulate pain perception, but little is known about the reverse influence pain may have on emotion processing. According to the motivational priming theory, pain should facilitate the processing of unpleasant and hamper the processing of pleasant affective stimuli. To this end, we investigated the influence of tonic pain on the processing of happy and angry faces in 30 healthy participants. As a measure of affect processing, the interference of picture content on participants' voluntary reactions to the pictures either with a compatible or an incompatible facial muscle (M. zygomaticus major ["smile"] or M. corrugator supercilii ["frown"]) was recorded by electromyogram (EMG). Additionally, participants rated valence and arousal of each picture. During both tasks, the participants received either painful or non-painful pressure stimulation. Pain stimulation was generally associated with slower compatible and incompatible muscle responses (M. zygomaticus and M. corrugator) and fewer erroneous incompatible (M. corrugator) responses to happy faces. However, pain did not affect muscle responses to angry faces and explicit valence and arousal ratings. Thus, pain seems to selectively influence the responses to happy faces, which may result from a slowed processing of incongruent information (happy/pain). That happy faces are processed differently during pain may bear important implications for social interactions during acute and possibly even chronic pain states.