Facial Action Coding System


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 FACS in 2002. Movements of individual facial muscles are encoded by FACS from slight different instant changes in facial appearance. It is a common standard to systematically categorize the physical expression of emotions, and it has proven useful to psychologists and to animators. Due to subjectivity and time consumption issues, FACS has been established as a computed automated system that detects faces in videos, extracts the geometrical features of the faces, and then produces temporal profiles of each facial movement.

Uses

Using FACS 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 AUs 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 FACS Manual is over 500 pages in length and provides the AUs, as well as Ekman's interpretation of their meaning.
FACS defines AUs, which are a contraction or relaxation of one or more muscles. It also defines a number of Action Descriptors, which differ from AUs in that the authors of FACS have not specified the muscular basis for the action and have not distinguished specific behaviors as precisely as they have for the AUs.
For example, FACS can be used to distinguish two types of smiles as follows:
Although the labeling of expressions currently requires trained experts, researchers have had some success in using computers to automatically identify FACS codes.
Computer graphical face models, such as or , allow expressions to be artificially posed by setting the desired action units.
The use of FACS has been proposed for use in the analysis of depression, and the measurement of pain in patients unable to express themselves verbally.
FACS is designed to be self-instructional. People can learn the technique from a number of sources including manuals and workshops, and obtain certification through testing. The original FACS 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 domestic species, including the dog, the horse and the cat. Similarly to the human FACS, the animal FACS have manuals available online for each species with the respective certification tests.
Thus, FACS can be used to compare facial repertoires across species due to its anatomical basis. A study conducted by Vick and others suggests that FACS 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 FACS tools for different species allows the objective and anatomical study of facial expressions in communicative and emotional contexts. Furthermore, a cross-species analysis of facial expressions can help to answer interesting questions, such as which emotions are uniquely human.
EMFACS and FACSAID consider only emotion-related facial actions. Examples of these are:
EmotionAction units
Happiness6+12
Sadness1+4+15
Surprise1+2+5B+26
Fear1+2+4+5+7+20+26
Anger4+5+7+23
Disgust9+15+16
ContemptR12A+R14A

Codes for action units

For clarification, FACS is an index of facial expressions, but does not actually provide any bio-mechanical information about the degree of muscle activation. Though muscle activation is not part of FACS, the main muscles involved in the facial expression have been added here for the benefit of the reader.
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 hasn't been specified and specific behaviors haven't been distinguished as precisely as for the AUs.
For most accurate annotation, FACS suggests agreement from at least two independent certified FACS encoders.

Intensity scoring

Intensities of FACS are annotated by appending letters A–E to the action unit number.
There are other modifiers present in FACS 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 unilateral but has a stronger side is indicated with an "A."

List of action units and action descriptors (with underlying facial muscles)

Main codes

Head movement codes

Eye movement codes

Visibility codes

AU numberFACS name
70Brows and forehead not visible
71Eyes not visible
72Lower face not visible
73Entire face not visible
74Unscorable

Gross behavior codes

These codes are reserved for recording information about gross behaviors that may be relevant to the facial actions that are scored.
AU numberFACS nameMuscular basis
29Jaw thrust
30Jaw sideways
31Jaw clenchermasseter
32 bite
33 blow
34 puff
35 suck
36 bulge
37Lip wipe
38Nostril dilatornasalis
39Nostril compressornasalis and depressor septi nasi
40Sniff-
41Lid droopLevator palpebrae superioris
42SlitOrbicularis oculi muscle
43Eyes closedRelaxation of Levator palpebrae superioris
44SquintCorrugator supercilii and orbicularis oculi muscle
45BlinkRelaxation of Levator palpebrae superioris; contraction of orbicularis oculi
46Winkorbicularis oculi
50Speech-
80Swallow-
81Chewing-
82Shoulder shrug-
84Head shake back and forth-
85Head nod up and down-
91Flash-
92Partial flash-
97*Shiver/tremble-
98*Fast up-down look-