Processing Local Coherence of Discourse in Centering Theory

In this paper we will investigate the centering theory proposed by Grosz, Joshi, and Weinstein (1995) (henceforth GJW) and revised in Walker, Joshi, and Prince (1998) (henceforth WJP), and argue that their theory needs to be further extended and revised. We show that the centering theory proposed by GJW and WJP would make wrong predictions about the preferred transition states in discourse processing since their backward-looking center (Cb) is not primitive and thus cannot be used to adequately model the local coherence of discourse. We propose that Cb should be distinguished from the discourse segment topic (DST), and provide a more restrictive definition of Cb, which restricts Cb(U) to be the element in Cf(U 11 ) that is realized by the subject pronoun or the pronoun contained in the subject in U. 1 Centering Theory in GJW and WJP The term centers of an utterance is used to refer to the entities that regulate the local information flow in a discourse. GJW define a set of forward-looking centers (CO, which are assigned to each utterance in a discourse segment, and a single Cb, which is assigned to each utterance other than the segment initial utterance. The members in the set of Cf are ranked according to discourse salience, and the highest-ranked member is referred to as the Preferred Center, Cp. GJW use the following definition to define the centers of utterance in a sequence: (1) The Definition of Centers (GJW 1995) For Un : Cb(Un) = a, Cf(Un) = (el, e2,... ep), a = ek, for some k. For U, 1 : Cb(11, 1 ) realizes em and, for all j, j < m, ej is not realized in Un+ 1 ; i.e., em is realized in U,1, and no higher ranked ei is realized in Un+1. Centering Theory also specifies a set of constraints and rules. (2) Constraints (WJP 1998) For each utterance U i in a discourse segment D consisting of utterances U 1 , ..., Um: a. There is precisely one backward-looking center Cb(U 1, D). b. Every element of the forward centers list, Cf(U 1 , D), must be realized in Ui.