Cb Or Not Cb? Centering Theory Applied To NLG

Centering theory (CT) has been mostly discussed from the point of view of interpretation rather than generation, and research has tended to concentrate on problems of anaphora resolution. This paper examines how centering could fit into the generation task, separating out components of the theory which are concerned with planning and lexical choice. We argue that it is a mistake to define a total ordering on the transitions CONTINUE, PJZTAm, SHIFT and that they are in fact epiphenomenal; a partia/ordering emerges from the interaction between cohesion (maintaining the same center) and salience (re-all.qing the center as Subject). CT has generally been neglected by NLG practitioners, possibly because it appears to assume that the center is determined according to feedbsch from the surface grammar, to text planning, but we argue that this is an artefactual problem which can be eliminated on an appropriate interpretation of the CT rules. 1 What is Centering? Centering theory (C~) is a theory of discourse structure which models the interaction of cohesion and salience in the internal organlsation of a text. The main assumptions of the theory as presented by (Gross et a11995 (GJW), Brennan et al 1987) rare: 1. For each utterance in a discourse there is precisely one entity which is the centre of attention or center. 2. There is a preference for consecutive utterances within a discourse segment to keep the same entity as the center, and for the most salient entity ~realised n in an utterance to be interpreted as the center of the following utterance. 3. The center is the entity which is most likely to be pronominalised. These principles will be more precisely expli-cated in Sect. 2. CT has proved attractive to NLP researchers because of the elegance and simplicity of the core proposals; it provides a framework for analysing text without having to make tough decisions about what a partfcular utterance is "about", since all notions are defined in purely structural terms. Much research in CT has concentrated on interpretation , particularly reference resolution, developing algorithms to resolve anaph0ric expressions based on the assumption that the text is constructed according to Rules 1 and 2. So researchers have focussed on filling in de~ tails of the theory which were left unspecified: what counts as an utterance, and how should transitions be handled in complex sentences 1996)? what counts as ~r~Migstion ~-does this include bridging references (Strube and Hahn op …