Inner brow raiser, or, AU 1, is the medial portion the frontalis muscle. As its name suggests, inner brow raiser lifts the inner (medial) portion of the brows. It can either move independently from the lateral portion of the frontalis (frontalis, pars laterials aka outer brow raiser, or AU 2) or in combination with the lateral portion. The combination of frontalis, pars medialis and frontalis, pars lateralis creates what we know as a full brow raise.
While people can generally activate inner brow raiser spontaneously, inner brow raiser stakes claim as one of the most notoriously difficult action units to trigger during posed expressions. Its elusive nature plagues various fields – from academic research to machine learning and character art.
So, what gives?
Cross-species comparisons of facial anatomy between humans and chimpanzees have shown the distinct movements of inner brow raiser and outer brow raiser to be unique to humans. Due to morphological differences – i.e. chimpanzees having prominent brow ridges, chimpanzees have been observed to only move their brows in a full raise configuration (inner brow raiser + brow raiser). Researchers speculate that our ability to separate inner and outer brow movements derived from our need for more enriched expression communication.
Because our ancestors from the Middle Pleistocene age – like chimpanzees – possessed pronounced brow ridges, it’s possible our ability to differentiate inner and outer brow raises is too new in our morphology to be fully fleshed out. (Pun intended.)
In any case, though inner brow raiser is the bane of many people’s existence, all hope is not lost. Over the years, I have developed a series of hacks to either trigger, simulate, or enhance the look of inner brow raiser. I explore these hacks in the video below: