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288 pp. per issue, 6 x 9,
illustrated
Founded: 1989
ISSN 0899-7667
E-ISSN 1530-888X
2008 ISI Impact Factor: 2.378

Neural Computation

October 2006, Vol. 18, No. 10, Pages 2529-2567
Posted Online August 14, 2006.
(doi:10.1162/neco.2006.18.10.2529)
© 2006 Massachusetts Institute of Technology
Dynamics and Topographic Organization of Recursive Self-Organizing Maps

Peter Tiňo

School of Computer Science, University of Birmingham, Birmingham B15 2TT, U.K.

Igor Farkaš

Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, and Institute of Measurement Science, Slovak Academy of Sciences, Bratislava, Slovak Republic

Jort van Mourik

Neural Computing Research Group, Aston University, Aston Triangle, Birmingham B4 7ET, U.K.

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Recently there has been an outburst of interest in extending topographic maps of vectorial data to more general data structures, such as sequences or trees. However, there is no general consensus as to how best to process sequences using topographic maps, and this topic remains an active focus of neurocomputational research. The representational capabilities and internal representations of the models are not well understood. Here, we rigorously analyze a generalization of the self-organizing map (SOM) for processing sequential data, recursive SOM(RecSOM) (Voegtlin, 2002), as a nonautonomous dynamical system consisting of a set of fixed input maps. We argue that contractive fixed-input maps are likely to produce Markovian organizations of receptive fields on the RecSOM map. We derive bounds on parameter β (weighting the importance of importing past information when processing sequences) under which contractiveness of the fixed-input maps is guaranteed. Some generalizations of SOM contain a dynamic module responsible for processing temporal contexts as an integral part of the model. We show that Markovian topographic maps of sequential data can be produced using a simple fixed (nonadaptable) dynamic module externally feeding a standard topographic model designed to process static vectorial data of fixed dimensionality (e.g., SOM). However, by allowing trainable feedback connections, one can obtain Markovian maps with superior memory depth and topography preservation. We elaborate on the importance of non-Markovian organizations in topographic maps of sequential data.

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