Reference-Based Discourse Structure For Reference Resolution

The connection between discourse structure and reference is well recognmed. In the system of language understanding described, 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 purposes of subsequent reference resohtion. The role of discourse structure in dereferencing, pronouns, other NPs, and temporal references is explored, as is the role of these in constraining discourse structure. I I n t r o d u c t i o n , The role of discourse structure in reference resolution, in particular pronoun and temporal reference resolution, is well recognised. However both Rhetorical Structure Theory (Mann and Thompson, 1988) and the intention-based approach to diacourse strutture associated with C~ntering Theory (GROSS and Sidner, 1986) involve high-level reamning. Such teasorting itself presupposes the availability of interpretations in which references have already been resolved. 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 implications for language understanding. Recognising an e.~hnafion can be seen to contribute to our understanding 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 t h e 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 reference resolution and discourse structure in the systern of language understanding described in (Ramsay, 1999). Having used the existing discourse structure to remlve references in the utterance and anchor it in the discourse, we then use information about continuity of reference to attach the new discourse state generated by the utterance to the discourse structure, or discourse tree. Attachment is by means of a number of interchangeable attachment rules which we have used to explore how different 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 (Hall/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, since this recognisably contributes to the overall coherence of the discourse. There are further referential cues we have yet to explore, including adverbiais like "Then" and "Before that" and bridging descriptions like those in the following examples: (1) John I / ~ Shirley. (2) H~ mo~er doesn't. (I) The house was grand. (2) The door was carved with gilded cherubs. 2 Tim is ~ailar in many ways to the center. 2Asain, this is ,;,,,;lax to the notion of tmpomi center (Kameyama, Pa~onean, and Poeslo, 1993), but with important differences; for instance, re]crones time is an instant, while the tempond ~ is an interval.