Brain areas active while learning hierarchical structure of a problem
According to "Frontal Cortex and the Discovery of Abstract Action Rules" by David Badre, Andrew S. Kayser, and MarkD'Esposito:


  ... the rostro-caudal axis of frontal cortex support rule learning at
  higher levels of abstraction. Moreover, these results indicate that
  when humans confront new rule learning problems, this rostro-caudal
  division of labor supports the search for relationships between
  context and action at multiple levels of abstraction simultaneously.


They found this by contrasting fMRI signals collected while the subject performed two tasks, one requiring a hierarchical, abstract rule and one requiring a flat, basic mapping.