Facial Action Coding System
The Facial Action Coding System is a system to taxonomize human facial movements by their appearance on the face, based on a system originally developed by a Swedish anatomist named Carl-Herman Hjortsjö. It was later adopted by Paul Ekman and Wallace V. Friesen, and published in 1978. Ekman, Friesen, and Joseph C. Hager published a significant update to F.A.C.S. in 2002. Movements of individual facial muscles are encoded by the F.A.C.S. from slight different instant changes in facial appearance. It has proven useful to psychologists and to animators.
Background
In 2009, a study was conducted to study spontaneous facial expressions in sighted and blind judo athletes. They discovered that many facial expressions are innate and not visually learned.Method
Using the F.A.C.S., human coders can manually code nearly any anatomically possible facial expression, deconstructing it into the specific "action units" and their temporal segments that produced the expression. As A.U.s are independent of any interpretation, they can be used for any higher-order decision-making process including recognition of basic emotions, or pre-programmed commands for an ambient intelligent environment. The F.A.C.S. manual is over five hundred pages in length and provides the A.U.s, as well as Ekman's interpretation of their meanings.The F.A.C.S. defines A.U.s as contractions or relaxations of one or more muscles. It also defines a number of "action descriptors", which differ from A.U.s in that the authors of the F.A.C.S. have not specified the muscular basis for the action and have not distinguished specific behaviors as precisely as they have for the A.U.s.
For example, the F.A.C.S. can be used to distinguish two types of smiles as follows:
- the insincere and voluntary Pan-Am smile: contraction of zygomatic major alone
- the sincere and involuntary Duchenne smile: contraction of zygomatic major and inferior part of orbicularis oculi.
Although the labeling of expressions currently requires trained experts, researchers have had some success in using computers to automatically identify the F.A.C.S. codes. One obstacle to automatic FACS code recognition is a shortage of manually coded ground truth data.
Uses
Baby F.A.C.S.
Baby F.A.C.S. is a behavioral coding system that adapts the adult F.A.C.S. to code facial expressions in infants aged 0–2 years. It corresponds to specific underlying facial muscles, tailored to infant facial anatomy and expression patterns.It was created by Dr. Harriet Oster and colleagues to address the limitations of applying adult F.A.C.S. directly to infants, whose facial musculature, proportions and developmental capabilities differ significantly.
Use in medicine
The use of the F.A.C.S. has been proposed for use in the analysis of depression, and the measurement of pain in patients unable to express themselves verbally.Interspecial applications
The original F.A.C.S. has been modified to analyze facial movements in several non-human primates, namely chimpanzees, rhesus macaques, gibbons, and siamangs, and orangutans. More recently, it was developed also for domesticated species, including dogs, horses and cats. Similarly to the human F.A.C.S., the non-human F.A.C.S. has manuals available online for each species with the respective certification tests.Thus the F.A.C.S. can be used to compare facial repertoires across species due to its anatomical basis. A study conducted by Vick and others suggests that the F.A.C.S. can be modified by taking differences in underlying morphology into account. Such considerations enable a comparison of the homologous facial movements present in humans and chimpanzees, to show that the facial expressions of both species result from extremely notable appearance changes. The development of F.A.C.S. tools for different species allows the objective and anatomical study of facial expressions in communicative and emotional contexts. Furthermore, an interspecial analysis of facial expressions can help to answer interesting questions, such as which emotions are uniquely human.
The Emotional Facial Action Coding System and the Facial Action Coding System Affect Interpretation Dictionary consider only emotion-related facial actions. Examples of these are:
| Emotion | Action units |
| Happiness | 6+12 |
| Sadness | 1+4+15 |
| Surprise | 1+2+5B+26 |
| Fear | 1+2+4+5+7+20+26 |
| Anger | 4+5+7+23 |
| Disgust | 9+15+17 |
| Contempt | R12A+R14A |
Computer-generated imagery
F.A.C.S. coding is also used extensively in computer animation, in particular for computer facial animation, with facial expressions being expressed as vector graphics of A.Us. F.A.C.S. vectors are used as weights for blend shapes corresponding to each A.U., with the resulting face mesh then being used to render the finished face. Deep-learning techniques can be used to determine the F.A.C.S. vectors from face images obtained during motion capture acting, facial motion capture or other performances.Codes for action units
For clarification, the F.A.C.S. is an index of facial expressions, but does not actually provide any biomechanical information about the degree of muscle activation. Though muscle activation is not part of the F.A.C.S., the main muscles involved in the facial expression have been added here.Action units are the fundamental actions of individual muscles or groups of muscles.
Action descriptors are unitary movements that may involve the actions of several muscle groups. The muscular basis for these actions has not been specified and specific behaviors have not been distinguished as precisely as for the A.U.s.
For the most accurate annotation, the F.A.C.S. suggests agreement from at least two independent certified F.A.C.S. encoders.
Intensity scoring
Intensities of the F.A.C.S. are annotated by appending letters A–E to the action unit number.- A Trace
- B Slight
- C Marked or pronounced
- D Severe or extreme
- E Maximum
Other letter modifiers
There are other modifiers present in F.A.C.S. codes for emotional expressions, such as "R" which represents an action that occurs on the right side of the face and "L" for actions which occur on the left. An action which is unilateral but has no specific side is indicated with a "U" and an action which is bilateral but has a stronger side is indicated with an "A" for "asymmetric".List of A.U.s and A.D.s (with underlying facial muscles)
Visibility codes
| A.U. number | F.A.C.S. name |
| 70 | Brows and forehead not visible |
| 71 | Eyes not visible |
| 72 | Lower face not visible |
| 73 | Entire face not visible |
| 74 | Unscorable |
Gross behavior codes
These codes are reserved for recording information about gross behaviors that may be relevant to the facial actions that are scored.| A.U. number | F.A.C.S. name | Muscular basis |
| 29 | Jaw thrust | |
| 30 | Jaw sideways | |
| 31 | Jaw clencher | masseter |
| 32 | bite | |
| 33 | blow | |
| 34 | puff | |
| 35 | suck | |
| 36 | bulge | |
| 37 | Lip wipe | |
| 38 | Nostril dilator | nasalis (pars alaris) |
| 39 | Nostril compressor | nasalis and depressor septi nasi |
| 40 | Sniff | - |
| 41 | Lid droop | levator palpebrae superioris |
| 42 | Slit | orbicularis oculi muscle |
| 43 | Eyes closed | relaxation of levator palpebrae superioris |
| 44 | Squint | corrugator supercilii and orbicularis oculi muscle |
| 45 | Blink | relaxation of levator palpebrae superioris; contraction of orbicularis oculi |
| 46 | Wink | orbicularis oculi |
| 50 | Speech | - |
| 80 | Swallow | - |
| 81 | Chewing | - |
| 82 | Shoulder shrug | - |
| 84 | Head shake back and forth | - |
| 85 | Head nod up and down | - |
| 91 | Flash | - |
| 92 | Partial flash | - |
| 97* | Shiver/tremble | - |
| 98* | Fast up-down look | - |