AU1 — Inner Brow Raiser
The Action Unit produced by the medial frontalis, which lifts the inner corners of the brows — a key signal of sadness, distress, and concern.
AU1 raises the inner portions of the eyebrows, producing the characteristic oblique forehead lines associated with sadness, worry, and empathy. When combined with AU4 (brow lowerer), it forms the AU1+4 configuration that is a strong marker of distress.
AU1 is one of the most commonly studied AUs in clinical and therapeutic contexts and is part of every modern facial analytics platform's core feature set.
Related terms: action unit, au4 brow lowerer, facial action coding system
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