Humor is a subject of debate in affect theory. In studies of humor's physiological manifestations, humor provokes highly characteristic facial expressions. Some research has shown evidence that humor may be a response to a conflict between negative and positive affects, such as fear and enjoyment, which results in spasmodic contractions of parts of the body, mainly in the stomach and diaphragm area, as well as contractions in the upper cheek muscles. Further affects that seem to be missing for Tomkins's taxonomy include relief, resignation, and confusion, among many others.