How does Ekman and Friesen's FACS measure facial expression?
Facial Action Coding System measures facial activity with Action Units or AUs. With minor exceptions, each AU represents one visibly distinguishable muscular action.
Three Muscles and Their Actions
The image to the right shows three muscles that act straight up and down in the lower face. The left of the image shows roughly where the muscle fibers are (under the skin, of course), and the right of the image shows how these muscles pull the skin. The muscle pulls towards the number in the schematic representation of the facial action. The AUs of focus now are:
You can see how these Action Units look on the face in the videos above.
Although FACS can be used to score still photos, it is typically used with video or film records that preserve the motion of facial action. Any FACS scorer can tell you that the motion is an extremely helpful aid in identifying what action has occurred, especially when the intensity of the action is low, much lower than the contractions seen in these examples.
When technicians score actual records of behavior, they look for signs of each muscle, sometimes acting in combinations with each other, to determine what is happening. Look at the video here and try to determine a score using the information about AUs 10, 15, and 17. Then, click on the link to see the answer.