Their primary goal in developing the Facial Action Coding System was to develop a comprehensive system which could distinguish all possible visually distinguishable facial movements.
They chose to derive FACS from an analysis of the anatomical basis of facial movement. Since every facial movement is the result of muscular action, a comprehensive system could be obtained by discovering how each muscle of the face acts to change visible appearance. With that knowledge it would be possible to analyze any facial movement into anatomically based minimal action units.
Constraints
A constraint in the development of FACS was that it deals with what is clearly visible in the face, ignoring invisible changes (e.g. certain changes in muscle tonus), and discarding visible changes too subtle for reliable distinction.FACS excludes visible changes in muscle tonus which do not entail movement; changes in skin coloration are usually not visible on black and white records. Also excluded from FACS are: facial sweating, tears, rashes, pimples and permanent facial characteristics.
The user of FACS must learn the mechanics -- the muscular basis -- of facial movement, not just the consequence of movement or a description of a static landmark. FACS emphasizes patterns of movement, the changing nature of facial appearance. Distinctive actions are described: the movements of the skin, the temporary changes in shape and location of the features, and the gathering, pouching, bulging and wrinkling of the skin.
Development
The first step in developing FACS was to study various anatomical texts to discover the minimal units. The authors expected to find a listing of the muscles which can fire separately, and how each muscle changes facial appearance.Next, they examine the photographs taken of each of their faces, scrambling the pictures so they would not know what muscle had been fired. Their purpose was to determine if all the separate muscular actions could be distinguished accurately from appearance alone. Note that they call the measurements action not muscle units; this is because they have combined more than one muscle in their unitization of appearance changes. Another reason for using the term action unit is because they also have separated more than one action from what most anatomists described as one muscle.
For example, the frontalis muscle which raises the brow was separated into two action units, depending upon whether the inner or outer portion of this muscle lifts the inner or outer portions of the eyebrow.
Table: Single Action Units
The table indicates where they have collapsed more than one muscle into a single Action Unit from a single muscle.
AU No. FACS Name Muscular Basis 1 Inner Brow Raiser Frontalis, Pars Medialis 2 Outer Brow Raiser Frontalis, Pars Lateralis 4 Brow Lowerer Depressor Glabellae; Depressor Supercilli; Corrugator 5 Upper Lid Raiser Levator Palebrae Superioris 6 Cheek Raiser Orbicularis Oculi, Pars Orbitalis 7 Lid Tightener Orbicularis Oculi, Pars Palebralis 8 Lips Toward Orbicularis Oris Each Other 9 Nose Wrinkler Levator Labii Superioris, Alaeque Nasi 10 Upper Lip Raiser Levator Labii Superioris, Caput Infraorbitalis 11 Nasolabial Furrow Zygomatic Minor Deepener 12 Lip Corner Puller Zygomatic Major 13 Cheek puffer Caninus 14 Dimpler Buccinnator 15 Lip Corner Depressor Triangularis 16 Lower Lip Depressor Depressor Labii 17 Chin Raiser Mentalis 18 Lip Puckerer Incisivii Labii Superioris; Incisivii Labii Inferioris 20 Lip Stretcher Risorius 22 Lip Funneler Orbicularis Oris 23 Lip Tightner Orbicularis Oris 24 Lip Pressor Orbicularis Oris 25 Lips Part Depressor Labii, or Relaxation of Mentalis or Orbicularis Oris 26 Jaw Drop Masetter; Temporal and Internal Pterygoid 27 Mouth Stretch Ptergoids; Digastric 28 Lip suck Orbicularis Oris 38 Nostril Dilator Nasalis, Pars Alaris 39 Nostril Compressor Nasalis, Pars Transversa and Depressor Septi Nasi 41 Lid Droop Relaxation of Levator Palpebrae Superioris 42 Slit Orbicularis Oculi 43 Eyes Closed Relaxation of Levator Palpebrae Superioris 44 Squint Orbicularis Oculi, Pars Palpebralis 45 Blink Relaxation of Levator Palpebrae and Contraction o Orbicularis oculi, Pars Palpebralis 46 Wink Orbicularis Oculi
