Hierarchical hidden Markov models with general state hierarchy

  • Authors:
  • Hung H. Bui;Dinh Q. Phung;Svetha Venkatesh

  • Affiliations:
  • Artificial Intelligence Center, SRI International, Menlo Park, CA;Department of Computing, Curtin University of Technology, Perth, Western Australia;Department of Computing, Curtin University of Technology, Perth, Western Australia

  • Venue:
  • AAAI'04 Proceedings of the 19th national conference on Artifical intelligence
  • Year:
  • 2004

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Abstract

The hierarchical hidden Markov model (HHMM) is an extension of the hidden Markov model to include a hierarchy of the hidden states. This form of hierarchical modeling has been found useful in applications such as handwritten character recognition, behavior recognition, video indexing, and text retrieval. Nevertheless, the state hierarchy in the original HHMM is restricted to a tree structure. This prohibits two different states from having the same child, and thus does not allow for sharing of common substructures in the model. In this paper, we present a general HHMM in which the state hierarchy can be a lattice allowing arbitrary sharing of substructures. Furthermore, we provide a method for numerical scaling to avoid underflow, an important issue in dealing with long observation sequences. We demonstrate the working of our method in a simulated environment where a hierarchical behavioral model is automatically learned and later used for recognition.