There's a nice little article on the topic from Reuters:
CHICAGO - The arch of an eyebrow or the curve of a lip tells chimps a lot about each other, a finding that may give scientists new understanding about the evolution of human communication, researchers reported Friday.
Human faces can be easy to read, but sometimes people must look in different places on the face to get an accurate picture.
"What we know from humans is that even a single movement added to an expression can change the entire meaning," said Lisa Parr, director of the Yerkes National Primate Research Center at Emory University in Atlanta. "It can significantly affect the outcome of interactions."
The article describes Parr's work classifying facial expressions with the help of a computer program, and getting chimpanzees to try to identify expressions from cartoon images. It alludes to the difficulty of scoring what is essentially a continuous range of variation in expressions -- one of the reasons why the "analog" system of facial expressions poses interpretive difficulties.
There's no link to the original research, though -- if anybody knows where it is appearing, please let me know!
