Are both the sensory and the affective dimensions of pain encoded in the face?

The facial expression of pain plays a crucial role in pain communication and pain diagnostics. Despite its importance, it has remained unknown which dimensions of pain (sensory and/or affective) are encoded in the face. To answer this question, we used a well-established cognitive strategy (suggestions) to differentially modulate the sensory and affective dimensions of pain and investigate the effect of this manipulation on facial responses to experimental pain. Twenty-two subjects participated in the study. Their facial expressions, pain intensity, and unpleasantness ratings as well as skin conductance responses to tonic and phasic heat pain were assessed before and after suggestions directed toward increase in affective and sensory qualities of pain, respectively, were provided. Facial expressions were analyzed with the Facial Action Coding system. As expected, suggestions designed to increase the sensory dimension produced a selective increase in pain intensity ratings, whereas suggestions designed to increase pain affect produced increased unpleasantness ratings and elevated skin conductance responses. Furthermore, suggestions for either increased pain affect or pain sensation produced selective modulations in facial response patterns, with facial movements around the eyes mostly encoding sensory aspects, whereas movements of the eyebrows and of the upper lip were closely associated with the affective pain dimension. The facial expression of pain is a multidimensional response system that differentially encodes affective and sensory pain qualities. This differential encoding might have evolved to guarantee that the specific characteristics of one's pain experience are facially communicated, thereby ensuring adequate help and support from others.