Action Unit (AU)
The basic building block of FACS — a single, anatomically defined facial muscle movement scored independently of any emotional interpretation.
An Action Unit (AU) corresponds to the visible activation of one facial muscle or a small group of muscles working together. FACS defines ~44 numbered AUs covering the upper face, lower face, head position, and eye movement.
AUs are scored on a five-point intensity scale (A=trace, E=maximum) and can co-occur. Combinations of AUs produce the full range of human facial expressions, including the seven universal emotions and the subtler micro-expressions that signal hidden affect.
Modern facial analytics platforms detect AUs using computer vision and landmark tracking, then map AU combinations to emotion vectors, engagement scores, or behavioral signals depending on the use case.
Related terms: facial action coding system, facial landmark detection, au6 cheek raiser, au12 lip corner puller, duchenne smile
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