The Facial Action Coding System (FACS) is the most comprehensive and objective facial coding system available to social scientists. Traditionally a manual coding system, which quantifies all possible movements a person can make with his or her face.

Check out what Facial Action Units look like!

Recent advances in computer vision have allowed for reliable automated facial action coding. Below you can see the 20 Action Units offered in the most recent version of FaceReader as well as some frequently occurring or difficult action unit combinations.  Some images have been zoomed in on the area of interest to explicitly show what muscle movement corresponds to the specific Facial Action Unit. 

Traditionally a very time-consuming task, reliable action unit coding is automated using FaceReader.

AU 1. Inner Brow Raiser

Contributes to the emotions sadness, surprise, and fear, and to the affective attitude interest. Muscular basis: frontalis (pars medialis).

AU 2. Outer Brow Raiser

Contributes to the emotions surprise and fear, and to the affective attitude interest. Frontalis (pars lateralis) is the underlying facial muscle.

AU 4. Brow Lowerer

Contributes to sadness, fear, and anger, and to confusion. Muscles: depressor glabellae, depressor supercilii, and corrugator supercilii.

AU 5. Upper Lid Raiser

Contributes to surprise, fear, and anger, and to interest. Muscular basis: levator palpebrae superioris, and superior tarsal muscle.

AU 6. Cheek Raiser

Contributes to the emotion happiness. Orbicularis oculi (pars orbitalis) is the underlying facial muscle.

AU 7. Lid Tightener

Contributes to the emotions fear and anger, and to confusion. Orbicularis oculi (pars palpebralis) is the underlying facial muscle.

AU 9. Nose Wrinkler

Contributes to the emotion disgust. Levator labii superioris alaeque nasi are the underlying facial muscles.

AU 10. Upper Lip Raiser

Levator labii superioris, caput infraorbitalis are the underlying facial muscles.

AU 12. Lip Corner Puller

Contributes to the emotion happiness and contempt when the action appears unilateraly. Muscular basis: zygomaticus major.

AU 14. Dimpler

Contributes to the emotion contempt when the action appears unilateraly, and to boredom. Buccinator is the underlying muscle.

AU 15. Lip Corner Depressor

Contributes to the emotions sadness and disgust, and to confusion. Depressor anguli oris is the underlying muscle.

AU 17. Chin Raiser

This Action Unit contributes to the affective attitudes interest and confusion. The underlying facial muscle is mentalis.

AU 18. Lip Pucker

The underlying facial muscles are incisivii labii superioris and incisivii labii inferioris.

AU 20. Lip Stretcher

Contributes to the emotion fear. The underlying facial muscle is risorius w/ platysma.

AU 23. Lip Tightener

Contributes to the emotion anger, and to the affective attitudes confusion and boredom. Muscular basis: orbicularis oris.

AU 24. Lip Pressor

This Action Unit contributes to the affective attitude boredom. The underlying facial muscle is orbicularis oris.

AU 25. Lips Part

The muscular basis consists of depressor labii inferioris, or relaxation of mentalis or orbicularis oris.

AU 26. Jaw drop

Contributes to the emotions surprise and fear. Muscular basis: masseter; relaxed temporalis and internal pterygoid.

AU 27. Mouth Stretch

The underlying facial muscle are pterygoids and digastric.

AU 43. Eyes Closed

Contributes to the affective attitude boredom. The muscular basis consists of relaxation of Levator palpebrae superioris.

Combinations of action units:

AU 1 - 2 - 4

Contributes to the emotions fear and can be recognized by the wavy pattern of the wrinkles across the forehead.

AU 1 - 2

Contributes to the emotion surprise and can be recognized by a smooth line formed by the wrinkles across the forehead.

AU 1 - 4

Contributes to sadness. Recognizable by a wavy pattern of the wrinkles in the center of the forehead. Eye-brows come together and up.

AU 4 - 5

Contribute to the emotion anger.

AU 6 - 12

Contributes to happiness. Notice the wrinkles around the eyes caused by cheek raising, also know as the "Duchenne Marker".

AU 10 - 25

Contributes to the emotion disgust. When AU10 is activated intensily, it causes the lips to part as the upper lip raises.

AU 18 - 23

Often confused as solely AU18. Notice the lips almost appear to be pulled by a single string outward (AU18) and then tightened (AU23).

AU 23 - 24

The AUs marking lip movements are often the hardest to code. The lips are being pushed together (AU24) and tightened (AU23)

Reference

Ekman, P.; Friesen, W. V.; Hager, J. C. (2002). Facial action coding system: The manual on CD-ROM. Instructor’s Guide. Salt Lake City: Network Information Research Co.