Graded Unification: A Framework for Interactive Processing

GRADED UNIFICATION: A FRAMEWORK FOR INTERACTIVE PROCESSING Albert Kim * Department of Computer and Information Sciences University of Pennsylvania Philadelphia, Pennsylvania, USA email: alkim©unagi, cis. upenn, edu Abstract An extension to classical unification, called graded unifica- tion is presented. It is capable of combining contradictory information. An interactive processing paradigm and parser based on this new operator are also presented. Introduction Improved understanding of the nature of knowledge used in human language processing suggests the fea- sibility of interactive models in computational linguis- tics (CL). Recent psycholinguistic work such as (Stowe, 1989; Trueswell et al., 1994) has documented rapid em- ployment of semantic information to guide human syn- tactic processing. In addition, corpus-based stochas- tic modelling of lexical patterns (see Weischedel et al., 1993) may provide information about word sense fre- quency of the kind advocated since (Ford et al., 1982). Incremental employment of such knowledge to resolve syntactic ambiguity is a natural step towards improved cognitive accuracy and efficiency in CL models. This exercise will, however, pose difficulties for the classical ('hard') constraint-based paradigm. As illus- trated by the Trueswell et al. (1994) results, this view of constraints is too rigid to handle the kinds of effects at hand. These experiments used pairs of locally am- biguous reduced relative clauses such as: 1) the man recognized by the spy took off down the street 2) the van recognized by the spy took off down the street The verb recognized is ambiguously either a past par- ticipial form or a past tense form. Eye tracking showed that subjects resolved the ambiguity rapidly (before reading the by-phrase) in 2) but not in 1) 1. The con- clusion they draw is that subjects use knowledge about thematic roles to guide syntactic decisions. Since van, which is inanimate, makes a good Theme but a poor Agent for recognized, the past participial analysis in 2) is reinforced and the main clause (past tense) sup- pressed. Being animate, man performs either thematic role well, allowing the main clause reading to remain *I thank Christy Doran, Jason Eisner, Jeff Reynar, and John Trueswell for valuable comments. I am grateful to Ewan Klein and the Centre for Cognitive Science, Edin- burgh, where most of this work was conducted, and also ac- knowledge the support of DARPA grant N00014-90-J-1863. 1In fact, ambiguity effects were often completely elimi- nated in examples like 2), with reading times matching those for the unambiguous case: 3) the man/van that was recognized by the spy ... plausible until the disambiguating by-phrase is encoun- tered. At this point, readers of 1) displayed confusion. Semantic constraints do appear to be at work here. However, the effects observed by Trueswell et al. are graded. Verb-complement combinations occupy a con- tinuous spectrum of "thematic fit", which influences reading times. This likely stems from the variance of verbs with respect to the thematic roles they allow (e.g., Agent, Instrument, Patient, etc.) and the syntactic po- sitions of these. The upshot of such observations is that classical uni- fication …

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