Why are sign vehicles relevant to studying the face?
To summarize the relevance of facial sign vehicles, if you are interested in measuring identity, you would regard the permanent sign vehicles as relevant, with the other categories as noise. If you are interested in measuring emotion, rapid sign vehicles would be relevant, with other signals noise.
Take a careful look at these two facial expressions. What emotions do these two faces express - Happy, Surprise, Disgust, Fear, Sadness, or Anger?
Images Copyright 1997 by P. Ekman; used with permission.
Now, when people are asked this question and forced to choose from a list of emotions, they usually say that face A looks angry and face B looks happy. However, it is a trick question because as far as the researchers were able to ascertain, these people had relaxed muscles when these pictures were taken and were not "emotional." Therefore, there is no valid information about emotion in these faces. All the messages you may have picked up about emotion are due to static or slow sign vehicles, in other words, noise. If you think these faces show emotion, you have confused signs originating in static sign vehicles for signs of emotion that can originate only in rapid sign vehicles. A measurement system designed to detect rapid sign vehicles must filter other sign vehicles that are noise in communications about emotion.