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 describe visible facial movement at different durations and intensities without establishing why the movement occurred.
Modern facial analytics platforms estimate AUs using computer vision and landmark tracking. Some systems map AU combinations to model-generated categories, but those outputs are not verified emotions, engagement, intent, or mental state.
Related terms: facial action coding system, facial landmark detection, au6 cheek raiser, au12 lip corner puller, duchenne smile
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