Special Electives of Comp Linguistics Processing Anaphoric Expressions

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

Main points from Lecture 2 • Pronouns – Bound variables • • The man who 1 Mary likes t 1 The man who 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 2

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

CT: Basic intuition Perceived difference in coherence due to attention structure: 1. 2. 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. 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. 4

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 6 am. 5. He was sick and furious at being woken up so early 5

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 6 am. 5. Tony was sick and furious at being woken up so early. 6

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 7

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

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

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 10

Centering transitions 11

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 12

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 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 1. He is getting his math degree this year. 1. 2. 3. 4. Cb= JOHN Cfs= JOHN, MATH, DEGREE, MATH DEGREE, YEAR Cp=JOHN Transition: Smooth-Shift 14

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 15

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 16

Outline of the Centering model • • • Discourse segments utterances Set of forward-looking centers, {Cf 1, Cf 2…} Preferred center, Cp Backward-looking center, Cb Cf ranking (Sub>Obj>Other) Ordering transitions 17

Applying the Centering model Mary likes Jane. Mary > Jane Cb=Mary Cp=Mary She invited her for dinner. She > her Cb=Mary Cp=Mary Tr=CONTINUE Jane accepted the invitation. Cb=Jane > Invitation Cp=Jane invitation Tr=SMOOTH-SHIFT Cb(Ui)=Cb(Ui-1) Cb(Ui)=Cp(Ui) Continue Cb(Ui)≠Cb(Ui-1) Smooth-Shift Cb(Ui)≠Cp(Ui) Retain Rough-Shift

Historical development CT 1977 -1981 • Complexity of inferencing • Almost Monadic PC Joshi&Kuhn 1979 Joshi&Weinstein 1981 CT 1986 -1995 • Attention • Reference • Coherence G&S 1986 GJ&W 1986 G, J& W 1995 Alternatives 1996 -2000 • Functional • Linear • Semantic Strube&Kuhn 1996 Walker 1996 Stevenson et al 2000