The Facial Action Coding System (FACS) refers to a set of facial muscle movements that correspond to a displayed emotion. Originally created by Carl-Herman Hjortsjö with 23 facial motion units in 1970, it was subsequently developed further by Paul Ekman, and Wallace Friesen. The FACS as we know it today was first published in 1978, but was substantially updated in 2002.

Using FACS, we are able to determine the displayed emotion of a participant. This analysis of facial expressions is one of very few techniques available for assessing emotions in real-time (fEMG is another option). Other measures, such as interviews and psychometric tests, must be completed after a stimulus has been presented. This delay ultimately adds another barrier to measuring how a participant truly feels in direct response to a stimulus.

Researchers have for a long time been limited to manually coding video recordings of participants according to the action units described by the FACS. This process is now possible to complete with automatic facial expression analysis, using tools such as Affectiva. This saves vast amounts of time and money, as scoring no longer requires analysis of each frame by a trained researcher – the software simply does the work for you.

Below we have listed the major action units that are used to determine emotions. Roll your mouse over the image to start the movement! Below the action units we have also listed the combinations of action units that are determined as a specific emotion.

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Main Action Units

Action UnitDescriptionFacial MuscleExample (Hover to Play)
 1Inner Brow RaiserFrontalis, pars medialisAU1 FACS
 2Outer Brow Raiser (unilateral, right side)Frontalis, pars lateralisAU2 right only FACS
 4Brow LowererDepressor Glabellae, Depressor Supercilli, Currugatorau4 brow lowerer
 5Upper Lid RaiserLevator palpebrae superiorisAU5 FACS
 6Cheek RaiserOrbicularis oculi, pars orbitalisAU6 cheek raiser
 7Lid TightenerOrbicularis oculi, pars palpebralisAU7 lid tightener
 9 (also shows slight AU4 and AU10)Nose Wrinkler Levator labii superioris alaquae nasiAU9 with 4+10
 10 (also shows slight AU25)Upper Lip Raiser Levator Labii Superioris, Caput infraorbitalis AU10 with 25
 11Nasolabial DeepenerZygomatic Minor AU11 - nasolabial deepener
 12Lip Corner PullerZygomatic MajorAU12
 13Cheek PufferLevator anguli oris (Caninus)AU13 cheek puffer
 14DimplerBuccinatorAU14 dimpler
 15Lip Corner DepressorDepressor anguli oris (Triangularis)AU15 FACS
 16 (with AU25)Lower Lip DepressorDepressor labii inferiorisAU16 with 25
 17Chin RaiserMentalisAU17 FACS guide
 18 (with slight AU22 and AU25)Lip PuckererIncisivii labii superioris and Incisivii labii inferiorisAU18 with 22A and 25A
 20Lip stretcherRisorius AU20 lip stretcher
 22 (with AU25)Lip FunnelerOrbicularis orisAU22 with 25 FACS
 23Lip TightenerOrbicularis orisAU23 lip tightener
 24Lip PressorOrbicularis orisAU24 image FACS guide
 25Lips partDepressor Labii, Relaxation of Mentalis (AU17), Orbicularis OrisAU25 lips part
 26 (with AU25)Jaw DropMasetter; Temporal and Internal Pterygoid relaxedAU26 with 25 FACS affectiva
 27Mouth StretchPterygoids, DigastricAU27 mouth stretcher
 28 (with AU26)Lip SuckOrbicularis orisAU28 with 26 FACS affectiva
 41Lid droopRelaxation of Levator Palpebrae SuperiorisAU41 lid droop
42SlitOrbicularis oculiAU42 slit
 43Eyes ClosedRelaxation of Levator Palpebrae SuperiorisAU43 eyes closed
 44SquintOrbicularis oculi, pars palpebralis AU44 squint
 45BlinkRelaxation of Levator Palpebrae and Contraction of Orbicularis Oculi, Pars Palpebralis. AU45 blink
 46WinkLevator palpebrae superioris; Orbicularis oculi, pars palpebralisAU46 wink

 

Head Movement Action Units

Action UnitDescriptionExample (Hover to Play)
 51Head Turn LeftAU51 head turn left
 52Head Turn RightAU52 head turn right
 53Head UpAU53 head up
 54Head DownAU54 head down
 55Head Tilt LeftAU55 head tilt left
 56Head Tilt RightAU56 head tilt right
 57Head ForwardAU57 head forward
 58Head Back AU58 head back

 

Eye Movement Action Units

Action UnitDescriptionExample (Hover to Play)
 51Eyes Turn LeftAU61-eyes-turn-left
 51Eyes Turn RightAU62 eyes right
 51Eyes UpAU63 eyes up
 51Eyes DownAU64 eyes down

Emotions and Action Units

The Action Units described above show the different movements of facial muscles. Certain combined movements of these facial muscles pertain to a displayed emotion. Emotion recognition is completed in iMotions using Affectiva, which uses the collection of certain action units to provide information about which emotion is being displayed. For example, happiness is calculated from the combination of action unit 6 (cheek raiser) and 12 (lip corner puller). A complete list of these combinations and the emotion that they relate to is shown below. The gifs on the right are shown in the same order that the action units listed.

EmotionAction UnitsDescriptionExamples (Hover to Play)
Happiness / Joy6 + 12Cheek Raiser, Lip Corner PullerAU6 cheek raiser

AU12 lip corner puller

Sadness1 + 4 + 15Inner Brow Raiser, Brow Lowerer, Lip Corner DepressorAU1 inner brow raiser

au4 brow lowerer

AU15 lip corner depressor

Surprise1 + 2 + 5 + 26Inner Brow Raiser, Outer Brow Raiser, Upper Lid Raiser, Jaw DropAU1 inner brow raiser

au2 outer brow raiser

au5 upper lid raiser

AU26 jaw drop

Fear1 + 2 + 4 + 5 + 7 + 20 + 26Inner Brow Raiser, Outer Brow Raiser, Brow Lowerer, Upper Lid Raiser, Lid Tightener, Lip Stretcher, Jaw DropAU1 inner brow raiser

au2 outer brow raiser

au4 brow lowerer

au5 upper lid raiser

AU7 lid tightener

AU20 lip stretcher

AU26 jaw drop

Anger4 + 5 + 7 + 23Brow Lowerer, Upper Lid Raiser, Lid Tightener, Lip Tightenerau4 brow lowerer

au5 upper lid raiser

AU7 lid tightener

AU23 lip tightener

Disgust9 + 15 + 16Nose Wrinkler, Lip Corner Depressor, Lower Lip DepressorAU9 nose wrinkler

AU15 lip corner depressor

AU16 lower lip depressor

Contempt12 + 14 (on one side of the face)Lip Corner Puller, DimplerAU12 lip corner puller

AU14 dimpler

 

Putting it all together

When measuring facial expressions within iMotions, the stimuli are paired automatically to the FACS analysis, allowing you to pinpoint the exact moment that the stimulus triggered a certain emotion. The FACS is also graded on a scale of intensity, which gives a measure of how strongly the emotion is displayed. These measurements can also be synchronized with recordings of galvanic skin response, which provides a measure of arousal. With this information combined, it’s possible to start drawing conclusions about how strongly an individual felt, and what those emotions consisted of, in response to a set stimulus.

The screenshot below shows how the facial expression data is displayed while a participant watches an advertisement.

 imotions action units affdex

If we zoom in, we can see the intensity of the displayed emotion. There are five emotions displayed in the image below, however iMotions provides a measure of the seven central emotions (shown in the table above), alongside, and in conjunction with measurements of action units.

 imotions action units facs

I hope this explanation of action units and FACS has been helpful, and informative. If you’d like to learn even more about facial expressions, then we also have a free pocket guide that you can download for free below!

 

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