High-level reasoning, computational challenges for connectionism, and the Conposit solution

Abstract

Sophisticated symbol processing in connectionist systems can be supported by two primitive representational techniques calledRelative-Position Encoding (RPE) andPattern-Similarity Association (PSA), and a selection technique calledTemporal-Winner-Take-All (TWTA). TWTA effects winner-take-all selection on the basis of fine signal-timing differences as opposed to activation-level differences. Both RPE and PSA are for the encoding of highly temporary associations between representations. RPE is based on the way activation patterns are positioned relative to each other within a network. Under PSA, two patterns are temporarily associated if they have (suitable) subpatterns that are (suitably) similar. The article shows how particular versions of the primitives are used to good effect in a system called Conposit/SYLL. This is a connectionist implementation of a slightly simplified version of a complex existing psychological theory, namely Johnson-Laird's account of syllogistic reasoning. The computational processes in this theory present a major implementational challenge to connectionism. The challenge lies in the mutability, multiplicity, and diversity of the working memory structures, and the elaborateness of the processing needed for them. Conposit/SYLL's techniques allow it to meet the challenge. The implementation of symbolic processing in Conposit/SYLL is an interesting application of connectionism partly because it significantly affects the design of the symbolic processing level itself. In particular, it encourages the use of associative as opposed to pointer-based data structures, and the use of random as opposed to ordered iteration over sets of data structures. In addition, the article discusses Conposit/SYLL's somewhat unusual variable-binding approach.