AU4 — Brow Lowerer
The Action Unit produced by the corrugator supercilii and related muscles that pull the brows down and together — central to concentration, anger, and distress.
AU4 is one of the most informative upper-face Action Units. It appears in expressions of anger, sadness, fear, and concentrated cognitive effort. Because it is largely involuntary, AU4 is a reliable signal even when subjects are trained to control the lower face.
AU4 carries particular significance in environments where the lower face is occluded — for example, athletes in helmets, healthcare workers in masks, or operators wearing visors — making upper-face AUs the dominant source of behavioral signal.
Related terms: action unit, facial action coding system, micro expression
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