Reference-based Discourse Structure for Reference Resolution

Reference-based Discourse Structure for Reference Resolution Helen Seville and Allan Ramsay Centre for Computational Linguistics UMIST, PO Box 88, Manchester M60 IQD, England ' heleng/allan@ccl, umist, ac. uk Abstract The connection between discourse struc- ture and reference is well recognmed. In the system of language understanding de- scribed, discourse structure is exploited for the purposes of reference resolution and then continu/ty of reference is exploited to build the discomle ~etructure for the pur- poses of subsequent reference resohtion. The role of discourse structure in derefer- encing, pronouns, other NPs, and temporal references is explored, as is the role of these in constraining discourse structure. I Introduction, The role of discourse structure in reference resolu- tion, in particular pronoun and temporal reference resolution, is well recognised. However both Rhetor- ical Structure Theory (Mann and Thompson, 1988) and the intention-based approach to diacourse strut- ture associated with C~ntering Theory (GROSS and Sidner, 1986) involve high-level reamning. Such tea- sorting itself presupposes the availability of interpre- tations in which references have already been re- solved. This rakes the question of the legitimacy of assuming such a discourse segmentation for the purposes of reference resolution. From a practical point of view a number of questions arise. Firstly, to what extent can We obtain the discourse structure we need for reference resolution without recourse to such higher-level reasoning? Secondly, having done our reference resolution, what further higher-level reasoning needs to be done if we are to adequately capture the meaning of the discourse, v Being able to label our discourse structure with discourse relations will only be useful insofar as these relations have im- plications for language understanding. Recognising an e.~hnafion can be seen to contribute to our un- derstanding in concrete ways. It allows us to infer, for instance, the fact that one event caused another together with the fact that the first event preceded the second. Given that this information is implicit in the discourse, it is clear that our understanding would be impaired without it. With other discourse relations, however, the contribution to inference and so discourse understanding is le~ dear. We have implemented s unified approach to ref- erence resolution and discourse structure in the sys- tern of language understanding described in (Ram- say, 1999). Having used the existing discourse struc- ture to remlve references in the utterance and an- chor it in the discourse, we then use information about continuity of reference to attach the new dis- course state generated by the utterance to the dis- course structure, or discourse tree. Attachment is by means of a number of interchangeable attach- ment rules which we have used to explore how differ- ent referential cues contribute to discourse structure (Seville, 1999). The kinds of r~er~tial information we use include: ~eme I (as this term is used by (Hal- l/day, 1985)), since this is what the dismurse at any one point is about; reference ~/me ~, since sometimes discourse structure reflects the temporal structure of the events described; and pronommali~ation in general, …

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