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Computational Intelligence and Neuroscience
Volume 2015 (2015), Article ID 986574, 18 pages
http://dx.doi.org/10.1155/2015/986574
Research Article

Encoding Sequential Information in Semantic Space Models: Comparing Holographic Reduced Representation and Random Permutation

1University of Cambridge, Cambridge CB2 1TN, UK
2Swedish Institute of Computer Science, 164 29 Kista, Sweden
3Redwood Center for Theoretical Neuroscience, University of California, Berkeley, Berkeley, CA 94720, USA
4Indiana University, Bloomington, IN 47405, USA

Received 14 December 2014; Accepted 26 February 2015

Academic Editor: Carlos M. Travieso-González

Copyright © 2015 Gabriel Recchia et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Circular convolution and random permutation have each been proposed as neurally plausible binding operators capable of encoding sequential information in semantic memory. We perform several controlled comparisons of circular convolution and random permutation as means of encoding paired associates as well as encoding sequential information. Random permutations outperformed convolution with respect to the number of paired associates that can be reliably stored in a single memory trace. Performance was equal on semantic tasks when using a small corpus, but random permutations were ultimately capable of achieving superior performance due to their higher scalability to large corpora. Finally, “noisy” permutations in which units are mapped to other units arbitrarily (no one-to-one mapping) perform nearly as well as true permutations. These findings increase the neurological plausibility of random permutations and highlight their utility in vector space models of semantics.