Gary Fagan founded the Gage Academy of Fine Arts. He's done quite a bit of work on representing facial expressions. His book is "required" reading for portrait arts. His lectures are fun too.

http://www.garyfaigin.com/faigin-face-blog

http://www.garyfaigin.com/expression-demonstrations.html

The Artist's Complete Guide to Facial Expression https://www.amazon.com/Artists-Complete-Guide-Facial-Express...