Semantic-Head Based Resolution of Scopal Ambiguities

Semantic-Head Based Resolution of Scopal Ambiguities* BjSrn Gamb/ick Information and Computational Linguistics Language Engineering University of Helsinki SICS, Box 1263 P.O. Box 4 S-164 29 Kista, Sweden SF-00014 Helsinki, Finland gamback@sics, se Johan Bos Computational Linguistics University of the Saarland Postfach 15 11 50 D-66041 Saarbriicken, Germany bos©coli, uni- sb. de Abstract We introduce an algorithm for scope resolution in underspecified semantic representations. Scope pref- erences are suggested on the basis of semantic argu- ment structure. The major novelty of this approach is that, while maintaining an (scopally) underspec- ified semantic representation, we at the same time suggest a resolution possibility. The algorithm has been implemented and tested in a large-scale system and fared quite well: 28% of the utterances were ambiguous, 80% of these were correctly interpreted, leaving errors in only 5.7% of the utterance set. 1 Introduction Scopal ambiguities are problematic for language processing systems; resolving them might lead to combinatorial explosion. In applications like transfer-based machine translation, resolution can be avoided if transfer takes place at a rep- resentational level encoding scopal ambiguities. The key idea is to have a common representa- tion for all the possible interpretations of an am- biguous expression, as in Alshawi et al. (1991). Scopal ambiguities in the source language can then carry over to the target language. Recent research has termed this underspecification (see e.g., KSnig and Reyle (1997), Pinkal (1996)). A problem with underspecification is, how- ever, that structural restrictions are not en- coded. Clear scope configurations (preferences) in the source language are easily lost: (1) das paflt auch nicht that fits also not 'that does not fit either' (2) ich kanni sie nicht verstehen ~i I can you not understand 'I cannot understand you' * This work was funded by BMBF (German Federal Ministry of Education, Science, Research, and Technol- ogy) grant 01 IV 101 R. Thanks to Christian Lieske, Scott McGlashan, Yoshiki Mori, Manfred Pinkal, CJ Rupp, and Karsten Worm for many useful discussions. 433 In (1) the focus particle 'auch' outscopes the negation 'nicht'. The preferred reading in (2) is the one where 'nicht' has scope over the modal 'kann'. In both cases, the syntactic configu- rational information for German supports the preferred scoping: the operator with the widest scope is c-commanding the operator with nar- row scope. Preserving the suggested scope res- olution restrictions from the source language would be necessary for a correct interpretation. However, the configurational restrictions do not easily carry over to English; there is no verb movement in the English sentence of (2), so 'not' does not c-command 'can' in this case. In this paper we focus on the underspecifi- cation of scope introduced by quantifying noun phrases, adverbs, and particles. The representa- tions we will use resembles Underspecified Dis- course Representation Structures (Reyle, 1993) and Hole Semantics (Bos, 1996). Our Underspecified Semantic Representation, USR, is introduced in Section 2. Section 3 shows how USRs are built up in a compositional se- mantics. Section 4 is the main part of the paper. …

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