The Facial Action Coding System is a system for describing facial expression. It is based on 46 defined ‘Action Units’, which are each the contraction of a facial muscle or group of muscles.
So, the six basic emotional expressions can be expressed in terms of combinations of action units. Disgust is Action Unit 7 + Action Unit 9, for example.
Described in terms of the Action Unit space, each emotion must have an inverse (when all involved action units are inactive, and all action units not involved in the expression of that emotion are active).
Question: What do the Action-Unit Space inverses of the fundamental emotional expressions look like? Are they recognisable in any way as the opposite of the expression in emotional space? Does the action-unit inverse of sadness look like happiness, for example? What is the muscle-opposite of surprise, is it similar to the feeling-opposite (boredom presumably)?