Influencing non-verbal expressions of pain: signal detection analyses.

Non-verbal expressive behaviour may provide important information about pain not available through verbal report. Nevertheless, it has received little attention in pain research. In the present study, changes in expressive behaviour resulting from electric shock were related to shock intensity, self-report of discomfort, observers' judgements of subjects' distress and social modelling influences. Subjects were videotaped while they rated low-, medium- and high-intensity shocks. Simultaneously, they were exposed to a tolerant social model or an inactive companion. Observers then viewed videotapes of subjects and judged the level of shock being delivered on the basis of non-verbal behaviour. Observers' judgements were analysed by signal detection methods to quantify expressive behaviour. Change in expressive reactions to the shocks was directly related to stimulus intensity, self-report of pain, and observers' judgements of subjects' distress. Expressive behaviour resulting from high shocks was diminished by exposure to a tolerant model. These findings establish the feasibility of pain measurement based on expressive behaviour, suggest that expressive behaviour provides a sensitive and valid index of pain, and indicate that tolerant modelling reduces evidence of pain across multiple measures.