I have usually described A.s.a. H. as learning layers of categories/concepts. (In the form of vectors.) After learning a set of first-order features A.s.a. H. May then learn the ANDing together of some of these on the next level in its hierarchy. Are these concepts or second-order features? Second-order features are thought to be important for doing things like facial recognition. Interactions among features may be represented by such higher-order features prior to their use to define some concept.