Typical observers find it easier to describe a person as ‘in pain’ than to describe his behavior in precise physical terms, and do not identify affectively relevant features of pain from austere physical ones. Astute observers often know the conclusions of such alleged inferences without knowing their premises. That indeterminacy is due to complex social and psychological reactions and the complex nature of human beings: the concept of pain expression must be flexible and elastic because human expression, and our reaction to it, is diverse and unpredictable (Wittgenstein, 1958).

This may reflect an indeterminacy which is constitutive of our concept of pain. There is evidence that significant discrepancies exist between self-report, non-verbal expressions of pain, and evidence of tissue damage, reflecting the impact of some of these criteria, in children (Doherty et al., 1993) and in adults (Craig et al., 2001). Prkachin et al. (1994) found that self-report, nonverbal expression and observers’ judgments were in agreement when the pain was severe, but that observers had difficulty judging accurately a sufferer’s inner state when the pain was submaximal, even though evidence was manifest in the face. There is no guarantee that pain expression will be detected by the observer or that the observer will be able to draw accurate conclusions about the state of the sufferer.

Although the indeterminacy of our concept of pain means that the different pain indices are not typically connected in a rigid way, it is possible to increase sensitivity to pain communicated by facial expression by brief periods of training. Solomon et al. (1997) found that exposure to a 30-minute training video increased sensitivity to subtle facial movements associated with low levels of pain. This lends hope to the possibility of developing a clinical tool to detect and correctly interpret pain facial expression.

References
1.  Craig, K.D., Prkachin, K.M., & Grunau, R.V.E. (2001). The facial expression of pain. In: Handbook of pain assessment, ed. D.C. Turk & R. Melzack (2nd Edition). New York: Guilford Press, pp. 257-274.
2. Doherty, E., Yanni, G., Conroy, R.M. and Bresan, B. 1993. A comparison of child and parent ratings of disability and pain in juvenile chronic arthritis. Journal of Rheumatology 20, 1563-1566.
3. Prkachin, K.M., Berzins, S. & Mercer, S.R. (1994). Encoding and decoding of pain expressions: A judgment study. Pain 58, 253-259.
4. Solomon, P.E., Prkachin, K.M., & Farewell, V. (1997). Enhancing sensitivity to facial expression of pain. Pain 71, 279-284.
5. Wittgenstein, L. (1958). Philosophical Investigations. ed. G.E.M. Anscombe and R.Rhees. tr. G.E.M. Anscombe, 2nd edition. Oxford: Blackwell.

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