I googled this because: I have a very expressive face and started noticing (perhaps with age, I'm 36), that one side of my face appears more droopy, or sulky, than the other. This lead me to wonder if it's because I raise my eyebrow on the left side of my face quite often and generally have more expression on that side of my face. More expression, more flexion, this more muscle-work. I tried raising just my right eyebrow and realized I cannot, or at least not without raising both. This bothered me and I wanted to know why, hence my Google search. I chose not to believe the answer to this query and started training myself to make little independent movement of my right eyebrow, watching myself on my phone. Within a few minutes I realized I can close my left eye and raise my right eyebrow, seemingly independently. That become a vision that I could see. I opened my left eye and repeated the vision, but in reality. I can now move my right and left up and down eyebrows independently.