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Table 2 Selected single action units from the Facial Action Coding System (FACS) [41];

From: Imitation and recognition of facial emotions in autism: a computer vision approach

AU number FACS name Muscles
AU01 Inner Brow Raiser Frontalis, Pars Medialis
AU02 Outer Brow Raiser Frontalis, Pars Lateralis
AU04 Brow Lowerer Depressor Glabellae; Depressor Supercilli, Corrugator
AU05 Upper Lid Raiser Levator Palpebrae Superioris
AU06 Cheek Raiser Orbicularis Oculi, Pars Orbitalis
AU07 Lid Tightener Orbicularis Oculi, Pars Palebralis
AU09 Nose Wrinkler Levator Labii Superioris, Alaeque Nasi
AU10 Upper Lip Raiser Levator Labii Superioris, Caput Infraorbitalis
AU11* Nasolabial Furrow Deepener Zygomanic Minor
AU12 Lip Corner Puller Zygomatic Major
AU14** Dimpler Buccinator
AU15 Lip Corner Depressor Triangularis
AU16* Lower Lip Depressor Depressor Labii
AU17 Chin Raiser Mentalis
AU20 Lip Stretcher Risorius
AU22* Lip Funneler Orbicularis Oris
AU23 Lip Tightener Orbicularis Oris
AU24* Lip Pressor Orbicularis Oris
AU25 Lips Part Depressor Labii, or Relaxation of Mentalis or Orbicularis Oris
AU26 Jaw Drop Masetter; Temporal and Internal Pterygoid Relaxed
AU27* Mouth Stretch Pterygoids; Digastric
AU28** Lip Suck Orbicularis Oris
  1. *An AU cannot be detected by Open Face [45]
  2. **An AU can be tracked by OpenFace but is not considered relevant for basic emotions in FACS [41], as cited by Gosselin, Kirouac and Doré [74]