Hold the drawing of a face upside-down and you can’t tell the expression of the face. Perhaps you can see that it is smiling, but you won’t be able to say what sort of a smile it is. You wouldn’t be able to imitate the smile or describe its character more exactly.

And yet the upside-down picture may represent the object extremely accurately. (Wittgenstein, 1980a, §991) 

In one study of facial expression recognition by three people with congenital facial paralysis (Calder, A. J. et al., 2000), participants were asked to identify the emotion displayed in 10 examples of facial expressions associated with each of 6 basic emotions selected from the ‘Pictures of Facial Affect’ (Ekman and Friesen, 1976). In a second test computer-morphed facial expressions were used. Even when perceptual impairments were found, people with facial paralysis still perceived many of the facial expressions shown to them. This may suggest that the ability to produce facial expressions is not a necessary prerequisite of their recognition.

A difficulty with this study is the employment of prototypical, high intensity, still photographs as stimulus material. As noted above, such stimuli may in fact activate an automatic reaction due to their extremity that is not found for weaker non-prototypical expressions. And there is no evidence showing that this primitive response is inhibited in people with congenital facial paralysis. Using expressions that are not chosen according to their correspondence to an emotion stereotype is therefore an important consideration. To enhance ecological validity in studies of facial expression recognition in people with facial paralysis, dynamic facial expressions should be chosen. Like the drawing of an upside-down face, a static face may make it difficult to see clearly what expression the face expresses. Wittgenstein writes: 

A facial expression that was completely fixed couldn’t be a friendly one. Variability and irregularity are essential to a friendly expression. Irregularity is part of its physiognomy.

The importance we attach to the subtle shades of behavior. (Wittgenstein, 1980b, §§615-616)

Calder, A.J., Keane, J., Cole, J., Campbell, R., and Young, A.W. 2000. Facial expression recognition by people with mobius syndrome. Cognitive Neurospychology 17(1-3), 73-87.