The following post is about AU1 – inner brow raiser, driven by the medial frontalis muscle. It covers: anatomical variation, common mistakes, and provides clean references. Scroll down to view AU1 references.

 

What is Inner Brow Raiser?

Inner brow raiser, or AU1, is a Facial Action Coding System (FACS) action unit (AU); its function is to raise the medial brow and forehead area. Inner brow raiser is triggered by the contraction of the medial area of the frontalis muscle. It also happens to be one of the most difficult facial actions to find clean references for. When building FACS references, many sources fail to:

1. Find models who can voluntarily trigger an inner brow raiser free from other facial actions like outer brow raiser (from the lateral frontalis) and brow lowerer (from corrugator supercilii). 
2. Properly direct models to perform inner brow raiser.

Though in our everyday lives, most of us can effortlessly raise our inner brows without thinking twice, when commanded to, many people struggle to lift their medial foreheads.

FACS References & Misinformation

I recently critiqued a set of FACS shapes for a client and noticed his inner brow raiser shape had some corrugator (from brow lowerer) mistakenly mixed in.

Client’s Inner Brow Raiser + My Notes:

Adding corrugator to inner brow raiser is a very common mistake. This commonality is partly due to the fact that: Good references are hard to find!

Even my client – an established industry artist with a great portfolio and CV – unknowingly added corrugator features to his model, because he trusted a tainted reference. The reference he used was from a prominent anatomy and facial expression book: Anatomy of Facial Expressions. Many art students and professionals use this textbook. It is a great textbook, but some of its FACS shapes are incorrect.

Inner brow raiser reference used by client:

See the vertical and wavy lines in the center of the forehead in the images below? Those vertical lines are from corrugator. Corrugator should not be activated for the inner brow raiser base shape. Corrugator should only be present when one is intentionally combining inner brow raiser with brow lowerer.

To view some clean examples of AU1, observe the GIFs below. Read about the primary and secondary effects of inner brow raiser as well as how its features can vary across individuals.

Clean AU1  – Inner Brow Raiser Examples

AU1 i

AU1 ii 

For more information on inner brow raiser, see: the secret life of inner brow raiser.