1 Special Electives of Comp.Linguistics: Processing Anaphoric Expressions Eleni Miltsakaki AUTH Fall 2005-Lecture 3

2 Main points from Lecture 2 Pronouns –Bound variables The man who 1 Mary likes t 1 The man who 1 t 1 talked to the boy who 2 t 2 visited him 1/*2 –Referring expressions John invited George for a drink. He told him it was his birthday. Background for Centering –Comes from two strands of work Centered logic (Joshi and Kuhn 1979) Discourse structure (Grosz and Sidner 1981) Centered logic –Makes n-ary predicates monadic by singling out one argument of the many that a predicate might have. –Facilitates inferencing Discourse structure –Linguistic structure –Intentional structure –Attentional state

3 What is Centering about? A theory that relates focus of attention, choice of referring expression and perceived coherence of utterances within a discourse segment

4 CT: Basic intuition Perceived difference in coherence due to attention structure: 1. a. John went to his favorite music store to buy a piano b. He had frequented the store for many years. c. He was excited that he could finally buy a piano. d. He arrived just as the store was closing for the day. 2. a. John went to his favorite music store to buy a piano. b. It was a store he had frequented for many years. c. He was excited that he could finally buy a piano. d. It was closing just as John arrived.

5 Garden paths 1.Terry really goofs sometimes. 2.Yesterday was a beautiful day and he was excited about trying out his new sailboat. 3.He wanted Tony to join him on a sailing expedition. 4.He called him at 6am. 5.He was sick and furious at being woken up so early

6 1.Terry really goofs sometimes. 2.Yesterday was a beautiful day and he was excited about trying out his new sailboat. 3.He wanted Tony to join him on a sailing expedition. 4.He called him at 6am. 5.Tony was sick and furious at being woken up so early.

7 Basic definitions: Centers Centers –Forward-looking centers (Cfs) List of entities evoked in the current utterance (ranked) –Preferred center (Cp) The highest ranked entity in the list of forward-looking centers –Backward-looking center (Cb) The highest ranked entity in the previous utterance that is realized in the current utterance

8 Basic rules: Center ranking Ranking rule –Depends on the language –Grammatical hierarchy for English SUBJ>OBJ>OTHER

9 Example Mary loves music. –Cb=none –Cfs= MARY, MUSIC –Cp= MARY She plays the guitar –Cb= MARY –Cfs= MARY, GUITAR –Cp= MARY

10 Basic definitions: Center transitions Continue: the Cb in the current utterance is the same as in the previous utterance and it is also the Cp of the current utterance Retain: the Cb of the current utterance is the same as in the previous utterance but it is not realized as the Cp of the current utterance Smooth-Shift: the Cb of the current utterance is not the same as in the previous utterance but it is realized as the Cp of the current utterance Rough-Shift: the Cb of the current utterance is not the same as in the previous utterance and it is not realized as the Cp of the current utterance

11 Centering transitions

12 Example Assume the previous discourse was about Mary Mary loves music. –Cb=MARY –Cfs= MARY, MUSIC –Cp= MARY –Transition=none She plays the guitar –Cb= MARY –Cfs= MARY, GUITAR –Cp= MARY –Transition=Continue

13 Assume the previous discourse was about Mary Mary loves John. –Cb=MARY –Cfs= MARY, JOHN –Cp= MARY –Transition=none John loves her too. –Cb= MARY –Cfs= JOHN, MARY –Cp= JOHN –Transition=Retain

14 Assume the previous discourse was about Mary Mary loves John. –Cb=MARY –Cfs= MARY, JOHN –Cp= MARY –Transition=none John loves her too. –Cb= MARY –Cfs= JOHN, MARY –Cp= JOHN –Transition=Retain He is getting his math degree this year. –Cb= JOHN –Cfs= JOHN, MATH, DEGREE, MATH DEGREE, YEAR –Cp=JOHN –Transition: Smooth-Shift

15 Basic rules: Transition rule Continues are preferred over Retains Retains are preferred over Smooth-shifts Smooth-shifts are preferred over Rough- Shifts  Continue>Retain>Smooth-Shift>Rough- Shift

16 Basic rules: Pronoun rule Pronoun rule –If any element of the preceding utterance is realized as a pronoun then so is the backward looking center of the current utterance  Intuition: Backward-looking centers are pronominalized first

17 Outline of the Centering model Discourse  segments  utterances Set of forward-looking centers, {Cf1, Cf2…} Preferred center, Cp Backward-looking center, Cb Cf ranking (Sub>Obj>Other) Ordering transitions

Mary likes Jane. She invited her for dinner. Jane accepted the invitation. > > > MaryJane Sheher Janeinvitation MaryJane Cp=Mary Sheher Cp=Mary JaneInvitation Cp=Jane Mary likes Jane. Cb=Mary She invited her for dinner. Cb=Mary Jane accepted the invitation. Cb=Jane Tr= CONTINUE Tr= SMOOTH-SHIFT Cb(Ui)=Cb(Ui-1)Cb(Ui)≠Cb(Ui-1) Cb(Ui)=Cp(Ui)ContinueSmooth-Shift Cb(Ui)≠Cp(Ui)RetainRough-Shift Applying the Centering model

Historical development CT CT Complexity of inferencing Almost Monadic PC Attention Reference Coherence Alternatives Functional Linear Semantic Joshi&Kuhn 1979 Joshi&Weinstein 1981 G&S 1986 GJ&W 1986 G,J& W 1995 Strube&Kuhn 1996 Walker 1996 Stevenson et al 2000