AU4 — Brow Lowerer
The Action Unit produced by the corrugator supercilii and related muscles that pull the brows down and together. It does not establish concentration, anger, or distress.
AU4 appears in many facial configurations and can accompany concentration, physical effort, discomfort, or other contexts. The movement does not establish anger, sadness, fear, or cognitive state.
When the lower face is occluded, upper-face movements may remain observable, but coverage and model confidence depend on image quality and the extent of occlusion.
Related terms: action unit, facial action coding system, micro expression
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