I posted this article yesterday which reports on the role of motion in the perception of facial expressions (the paper is called ‘Deciphering the Enigmatic Face: The Importance of Facial Dynamics’ in the May 2005 issue of Psychological Science). The study found that facial motion – seeing the range of movement in the arching of an eyebrow or the curve of a smile – improves perception (judgement) of facial expressions depicting sadness, anger, disgust, fear, happiness, and surprise. “The participants were more accurate and more confident judging facial expressions in the conditions in which they perceived motion (dynamic) than in conditions in which motion perception was prevented.”

The importance of motion in human facial expression was also understood by Wittgenstein. Facial motion is an important feature of what makes subtle facial expressions meaningful:

‘It is as if one were trying to imagine a facial expression not susceptible of gradual and subtle alterations; but which had, say, just five positions; when it changed it would snap straight from one to another. Would this fixed smile really be a smile? And why not? – I might not be able to react as I do to a smile. Maybe it would not make me smile myself’ (RPP II 614).

Most studies of facial perception have used static models of intense expressions. But, facial expressions do not ‘snap straight from one to another’. A different concept from ours would be at play in a community where regular and fixed facial expressions were the norm. But, to us, a facial expression that was completely fixed couldn’t (e.g) be a friendly or sincere one. ‘Variability and irregulaity are essential to a friendly expression’ (RPP II 615). Irregulaity resonates with our reactions: we do not explain the perception of different facial expressions by reference to exact measurement. We do not measure the difference between expressions as though to each expression there corresponds a precise static image (PI 285). For, we react differently to such different facial expressions. If variability (subtle and gradual alterations) is absent from human facial expression, then facial behavior would be to us something completely different (our reactions would be thrown out). Wittgenstein is saying that the facial features of emotion are not more important – meaningful – than their mobility (RPP II 627).