Contextual Filtering of Rhetorical Relations for Discourse Structuring in Dialogue Vladimir Popescu * , Jean Caelen Laboratoire d’Informatique de Grenoble, Grenoble Institute of Technology, France {vladimir.popescu, jean.caelen}@imag.fr Abstract In this paper we propose an alternative to the Segmented Discourse Representation Theory (SDRT) mechanism for dealing with ambiguous rhetorical connections between utterances. Thus, starting from the observation that the SDRT mechanism for ordering discourse structures (the Maximize Discourse Coherence – MDC principle) relies on a fragile scalar interpretation of rhetorical relations in terms of their quality, we propose a reduction of the discourse structures to abstract attribute grammars (AAG) that will allow us to check the constraints they impose on the paths to the utterances, in the discourse structure. Thus, the non-unifiability of the restriction sets determined by the AAGs for each particular utterance represent a criterion for ruling out the rhetorical relations that contributed the AAGs with non- unifiable restriction sets. This “contextual filtering” approach is first presented, theoretically grounded and discussed with respect to several potential weak points. Then, an extended example illustrates the feasibility of the proposed method for dialogue situations. 1 Introduction Determining the discourse structure of texts is a thoroughly-studied, yet not completely solved, issue, either from a theoretical standpoint, or guided by practical, computational goals. On the theoretical side, first discourse coherence was hinted at, abstracting away from the types of rhetorical connections between text spans; Discourse Representation Theory (DRT) is a typical illustration of this strand (Kamp and Reyle, 1993). However, although DRT could predict some phenomena, such as certain anaphoric chains, other aspects such as ellipsis (Asher et al, 2001) or illocutionary aspects (Vanderveken, 1990–1991) were left untackled. Hence, one has moved to a more fine-grained analysis of the nature of rhetorical connections between text spans. One of the first attempts in this respect is Rhetorical Structure Theory (RST) (Mann and Thompson, 1988), where an open set of several tens of rhetorical relation types are proposed, such as ELABORATION, CONCESSION, etc. However, RST misses the depth of the (utterance-level) analysis performed in DRT, thus failing to predict sub-sentential linguistic phenomena (Asher, 1993). This is why, the idea of combining insights from both DRT and RST was quite natural. One of the first and most representative attempts in this respect was Asher’s SDRT (Asher, 1993), where the fine-grained predictive power of DRT was extended to multi-utterance discourses, by specifying the types of the rhetorical relations that connect spans of text. This early work of Asher (1993) was mainly concerned only with monologue, written, texts. An extension of this approach to dialogue situations was started with Asher and Lascarides (2003), and henceforth continued in Lascarides and Asher (2009), for dealing with more dialogue-specific issues, such as commitments modeling (Asher and Lascarides, 2008), or common ground (Traum, 1994). However, it was evident even from early work of Asher (1993) and Asher and Lascarides (2003) that a thorny issue had not received enough attention an adequate account: the underspecification of the discourse relations (Reyle, 1993). This underspecification is due either to the lack of enough contextual elements for computing a unique appropriate rhetorical relation between two utterances or text spans, or to the inherent * Now at Laboratoire Informatique d’Avignon, University of Avignon, France; e-mail: vladimir.popescu@univ-avignon.fr. 1