Centering: A Framework for Modeling the Local Coherence of Discourse

6 Constraints on center movement and realization

1. State centering Rule 1.
   If any element of \(C_f(U_n)\) is realized by a pronoun in \(U_{n+1}\), then \(C_b(U_{n+1})\) must be realized by a pronoun also. A pronoun for the \(C_b\) signals that the speaker is continuing to talk about the same thing; other pronouns are allowed as long as the \(C_b\) is pronominalized.
   • Grosz et al., 1995, Centering (Computational Linguistics), §6
2. State centering Rule 2.
   Sequences of continuation are preferred over sequences of retaining, which are preferred over sequences of shifting. Frequent shifting lacks local coherence; a coherent segment has shifts followed by runs of continuations.
   • Grosz et al., 1995, Centering (Computational Linguistics), §6
3. What does Rule 2 imply for generation systems versus interpretation systems?
   Speakers and generators should plan ahead to minimize shifts; for interpretation it doesn’t give a direct procedure but predicts that some sequences impose higher inference load (it was not yet validated psycholinguistically).
   • Grosz et al., 1995, Centering (Computational Linguistics), §6
4. How is the \(C_b\) usually realized in English and other languages?
   Preferentially by a pronoun in English, and by equivalent forms such as zero pronouns in languages like Japanese and Italian.
   • Grosz et al., 1995, Centering (Computational Linguistics), §6