The scientific interest in the investigation and measurement of facial movement and its meaning dates back to the works of John Bulwer in the 17th century. He dealt with the study of expression of affect in the face (Bulwer, 1649) and with lipreading in hearing impaired persons (Bulwer, 1648). In the 19th century Duchenne de Boulogne (1862) with his experimental work on muscular movement in the face set the foundations for an anatomically based measurement of facial expressions. Furthermore Darwin (1872) with his book "The expression of the emotions in man and animals" opened new horizons for the investigation of expressivity in the human face and its relation to affect. In a review article on assessing facial activity Ekman (1982) describes different coding methods that have been developed and applied on scientific research. These diverse systems have been developed for various research purposes and for the investigation of different populations (e.g. for infants or adults, for normal or clinical samples or disabled individuals) in various situations (e.g. spontaneous or posed expressions, in interpersonal interaction or in experimental situations). Table 1 presents the different measurement systems of facial activity with respect to their theoretical background and area of implementation.
Table 1: Assessment instruments of facial activity according to
theoretical background and major area.
The most frequently employed coding systems in psychological research are the FACS (Ekman & Friesen, 1978) and the MAX (Izard, 1979). The major difference of these instruments lies in the focus of their measurement. The MAX concentrates only on facial expressions that corresponds to the basic emotions and ignores other movements not relevant to affect. On the contrary FACS describes all possible movements of the facial musculature that produce a visible change in the face. Each discrete movement is called action unit. Activation of different action units results in a combination that make up a facial expression - not only of affective meaning. A shorter version of FACS that focuses selectively only on facial activity related to indicators of affect was also developed (EM-FACS). Furthermore the Baby-FACS version (Oster, 1993) was adapted in order to take into account particularities of the facial musculature of infants.
In the present work different facial expressions of emotion were defined by taking reference to the Facial Action Coding System (Ekman & Friesen, 1978). FACS is based on an anatomy notation system that describes the muscular basis of facial expression and classifies the appearance changes on the face mediated by muscular activity. In this way it can represent all possible appearance changes in the face. Table 2 indicates the correspondence of different facial muscles to discrete Action Units representing visible changes in the face.
| AU-Nr. | 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 Papebrae Superioris | ||
| 6 | Cheek Raiser | Orbicularis Oculi, Pars Orbitalis | ||
| 7 | Lid Tightener | Orbicularis Oculi, Pars Palebralis | ||
| 9 | Nose Wrinkler | Levator Labii Superioris, Alaeque Nasi | ||
| 10 | Upper Lip Raiser | Levator Labii Superioris, Caput Infraorbitalis | ||
| 11 | Nasolabial Deepener | Zygomaticus Minor | ||
| 12 | Lip Corner Puller | Zygomaticus 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 Tightener | Orbicularis Oris | ||
| 24 | Lip Pressor | Orbicularis Oris | ||
| 25 | Lips Part | Depressor Labii o. Entspannung des Mentalis | ||
| o. des Orbicularis Oris | ||||
| 26 | Jaw Drop | Masetter, temporale u. internale Entspannung | ||
| des Pterygoid | ||||
| 27 | Mouth Stretch | Pterygoids, Digastric | ||
| 28 | Lip Suck | Orbicularis Oris |
Trained coders can note the single elements that make up complicated facial expressions without providing an interpretation of facial activity. That is, a face will not be described as happy, sad, or aggressive but rather will be coded according to specified Action Units (AUs) such as 4 (= frown) + 15 (= lip corner depressor) + 17 (= chin lift). The interpretation of the combination of these appearance changes in the face as aggressive, happy, etc. remains open. The Facial Action Coding System provides high accuracy in detecting and describing produced changes in the facial musculature from either still photos or video records.