The Facial Action Coding System Explained

Facial Action Coding SystemThe Facial Action Coding System (FACS) is a scientific system designed to measure human facial movements. 1 FACS is an anatomically-based comprehensive system. This means FACS can be used to describe any possible set of facial movements. Originally developed by Dr. Paul Ekman, Dr. Wallace Friesen, and Dr. Joseph Hager, the latest version of the FACS manual can be purchased online at face and emotion.

Facial Action Coding System Fundamentals

The basic unit of measurement in FACS is the action unit. (AU) An AU represents the muscular activities that produce momentary changes in facial appearances. 2 Stated another way, an action unit is a numeric code to describe the movements of facial muscles. The mapping between AUs and facial muscles is not necessarily 1-to-1; some AUs are composed of more than one muscle, and other AUs describe separate movements of the same muscle.

The act of describing facial movements using the Facial Action Coding System is called FACS coding (or scoring). Multiple FACS codes are strung together using plus (‘+’) signs. For example 1+2 represents raising the inner and outer portions of the eyebrow (i.e. raising the entire eyebrow.)

FACS also includes a system to describe the intensity of each AU by affixing a letter A through E after the numeric code but before the plus sign. The intensity range for each letter is:

  • Trace (A)
  • Slight (B)
  • Marked / Pronounced (C)
  • Severe / Extreme / Maximum (D)
  • Extreme / Maximum (E)

To continue the previous example, 1E+2E represents extreme (or maximal) raising of the inner and outer portions of the eyebrows.

The Importance of Appearance Changes

Besides representing facial movements, action units will also create changes in facial appearances. For example raising the inner portion of the eyebrows can produce horizontal wrinkles which are usually limited to the center of the forehead.

Many people underestimate the importance of the appearance changes associated with each action unit. Since facial muscles are covered by skin, the muscles themselves can’t be seen directly. Therefore a FACS coder has to infer muscular activity by examining appearance changes. The tricky part comes from the fact that many AUs can produce similar appearance changes. For instance AUs 6, 7, and 12 can all cause crow’s feet wrinkles at the outer corners of the eyes. 3

Further complicating matters is that the appearance changes of one AU can affect another. To illustrate this consider that furrowing the eyebrows (AU 4) can reduce the amount of visible sclera (the white area of the eyeballs.) On the other hand raising the upper eyelid (AU 5) widens the eye opening and therefore increases the amount of visible sclera. When performed together 4 these two AUs work against each other. This means that when the eyebrows are furrowed it can reduce the amount of sclera exposed by raising the upper eyelids, making it look like the upper eyelid wasn’t raised when it in fact was.

Thus a FACS coder has to learn not only the AUs associated with each muscular movement, but also how each AU affects facial appearance, and how the appearance changes of each AU can affect each other.

Training and Certification in the Facial Action Coding System

The FACS Manual is designed to be both a self-study guide for learning how to FACS code, and as a reference guide when FACS coding. This dual-use (learning and reference) can make learning FACS difficult. It is very useful to learn FACS from someone who is already trained and certified. Currently Dr. Erika Rosenberg offers FACS training to the general public.

In order to be certified in the use of FACS a coder has to take and pass the FACS Final Test (FFT). The FFT consists of 34 video clips of people in a natural environment. The instructions for the FFT describe time codes when the test taker should FACS code the faces in each video clip. Once the exam is completed the FACS codes are submitted via email. A minimum score of 70 is need to pass the FFT and become certified in the use of FACS.

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  3. These are not the only AUs that can cause crow’s feet wrinkles at the outer corners of the eyes.
  4. Expressions of anger commonly include AUs 4 and 5 (furrowing the eyebrows while raising the upper eyelids)


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  1. […] EngineeringEmotionNonverbalsDeceptionFramesFacial Action Coding System FAQsBy Mike MurrThe Facial Action Coding System¬†(FACS) is an incredibly useful tool when it comes to dealing with the face. ¬†Despite its utility, […]