On the relationship between self-report and facial expression of pain

Several investigators have reported weak or no associations between self-report and facial expression of pain, concluding that both parameters appear to be unrelated. However, studies so far have only focused on an overall association, not considering psychophysical relationships between stimulus intensities and pain responses while computing correlations. In the present study these psychophysical relationships, between stimulus intensity on the one hand and response magnitudes (of self-report and facial expression) on the other hand, were described in terms of intercept and slope. Correlation analyses were conducted between intercept and slope parameters of self-report and facial expression of pain. Forty young, pain-free individuals were investigated for their responses to mechanically and electrically induced pain. Self-report was assessed by Visual Analog Scales. Facial expression was examined by using the Facial Action Coding System. There were significant correlations between the linear slopes of the psychophysical functions of self-report and facial expression in pressure pain. Neither the intercepts nor overall mean responses in the 2 pain-signaling systems were significantly correlated. These findings suggest that the facial expression of pain appears to mirror self-report ratings, when their increases over a range of increasing stimulus intensities are considered in parallel.

In future studies, our psycho-physically derived observation that incremental changes in facial expression during developing pain are more characteristic for individuals than static levels needs further corroboration.