I. Introduction
How do humans come to understand, recognize, and replicate complex actions? Even if we have witnessed several occurrences of the same activity, each will be unique in terms of the order actions are performed, the explicit motion of the limbs involved, and the appearance of the objects involved. Somehow, the sensory data must be stored in a greatly compressed representation that captures relevant information while discarding what is irrelevant. This representation must be capable of handling actions of any complexity, where activities are composed of previously known actions and subactions.