Pragmatics II: Discourse structure

1. Pragmatics II: Discourse structure Ling 571 Fei Xia Week 7: 11/10/05

2. Pragmatics outline • Reference resolution • Discourse structure • What makes a text coherent? • What are discourse structures? • Two theories on discourse structures • Approaches to build discourse structures

3. Text Coherence • Example: • (1) John hid Bill’s car keys. • (2) He was drunk. • (1) John hid Bill’s car keys. • (2) He likes junk food. • (1) George Bush supports big business. • (2) He’s sure to veto House Bill 1711. • Hearers try to find connections between utterances in a discourse. • The possible connections between utterances can be specified as a set of coherence relations.

4. Coherence relations (Hobbs,1979) • Result: S0 causes S1 • John bought an Acura. His father went ballistic. • Explanation: S1 causes S0. • John hid Bill’s car keys. He was drunk. • Parallel: S0 and S1 are parallel. • John bought an Acura. Bill bought a BMW. • Elaboration: S1 is an elaboration of S0. • John bought an Acura this weekend. He purchased it for $40 thousand dollars. • …

5. Discourse structure S1: John took a train to Bill’s car dealership. S2: He needed to buy a car. S3: The company he works for now isn’t near any public transportation. S4:He also wanted to talk to Bill about their softball leagues. ] Explanation

6. Discourse structure S1: John took a train to Bill’s car dealership. S2: He needed to buy a car. S3: The company he works for now isn’t near any public transportation. S4:He also wanted to talk to Bill about their softball leagues. ] ] Explanation Parallel

7. Discourse structure S1: John took a train to Bill’s car dealership. S2: He needed to buy a car. S3: The company he works for now isn’t near any public transportation. S4:He also wanted to talk to Bill about their softball leagues. ] ] Explanation ] Explanation Parallel

8. Discourse parsing Explanation (e1) S1 (e1) Parallel (e2;e4) Explanation (e2) S4 (e4) S2(e2) S3(e3)

9. Why compute discourse structure? • Natural language understanding • Summarization • Information retrieval • Natural language Generation • Reference resolution

10. Two theories on discourse structure • Mann and Thompson’s Rhetorical structure theory (1988) • Grosz and Sidner’s attention, intention and structure of discourse (1986)

11. Rhetorical structure theory (RST) • Mann and Thompson (1988) • One theory of discourse structure, based on identifying relations between parts of the text: • Defined 20+ rhetorical relations • Presentational relations: intentional • Subject matter relations: informational • Nucleus: central segment of text • Satellite: more peripheral segment • Relation definitions and more.

12. Presentational relations • Those whose intended effect is to increase some inclination in the hearer. • Relations: • Antithesis - Justify • Background - Motivation • Concession - Preparation • Enablement: - Restatement • Evidence - Summary

13. Subject matter relations • Those whose intended effect is that the hearer recognize the relation in question. • Relations • Circumstance - Otherwise • Condition - Purpose • Elaboration - Solutionhood • Evaluation - Unconditional • Interpretation - Unless • Means - Volitional cause • Non-volitional cause - Volitional result • Non-volitional result

14. Multinuclear relations • Contrast • Joint • List • Multinuclear restatement • Sequence

15. Some examples • Explanation: John went to the coffee shop. He was sleepy. • Elaboration: John likes coffee. He drinks it every day. • Contrast: John likes coffee. Mary hates it.

16. Discourse structure John likes coffee They argue a lot contrast cause elaboration Mary hates coffee. He drinks it every day

17. A relation: Evidence • (a) George Bush supports big business. • (b) He’s sure to veto House Bill 1711. • Relation Name: Evidence • Constraints on Nucl: H might not believe Nucl to a degree satisfactory to S. • Constraints on Sat: H believes Sat or will find it credible • Constraints on Nucl+Sat: H’s comprehending Sat in Sat increases H’s belief of Nucl. • Effect: H’s belief of Nucl is increased.

18. A relation: Volitional-Cause • (a) George Bush supports big business. • (b) He’s sure to veto House Bill 1711. • Relation Name: Volitional-Cause • Constraints on Nucl: presents a volitional action • Constraints on Sat: none. • Constraints on Nucl+Sat: Sat presents a situation that could have caused the agent of the volitional action in Nucl to perform the action. • Effect: H recognizes the situation presented in Sat as a cause for the volitional action presented in Nucl.

19. Another example S: (a) Come home by 5:00. (b) Then we can go to the hardware store before it closes. (c) That way we can finish the bookshelves tonight. (a) (a) (b) (c) motivation motivation (b) (c) condition condition

20. Problems with RST (Moore & Pollack, 1992) • How many rhetorical relations are there? • How can we use RST in dialogues? • How do we incorporate speaker intentions into RST? • RST does not allow for multiple relations between parts of a discourse: informational and intentional levels must coexist.

21. Grosz & Sidner (1986)

22. Grosz and Sidner (1986) • A leading theory of discourse structure • Three components: • A linguistic structure • An intentional structure • An attentional state

23. Linguistic structure • The structure of the sequence of utterances that comprises a discourse. • Utterances form Discourse Segment (DS); and a discourse is made up of embedded DSs. • What exactly is a DS? • Any evidence that humans naturally recognize segment boundaries? • Do humans agree on segment boundaries? • How to find the boundaries automatically?

24. Intentional structure • Speakers in a discourse may have many intentions: public or private. • Discourse purpose (DP): the intention that underlies engaging in a discourse. • Discourse segment purpose (DSP): the purpose a DS. How this segment contributes to achieving the overall DP? • Two relations between DSPs: • Dominance: if DSP1 contributes to DSP2, we say DSP2 dominates DSP1. • Satisfaction-precedence: DSP1 must be satisfied before DSP2.

25. Attentional State • The attentional state is an abstraction of the participants’ focus of attention as their discourse unfolds. • The state is a stack of focus spaces. • A focus space (FS) is associated with a DS, and it contains DSP and objects, properties, and relations salient in the DS. • When a DS ends, its FS is popped. • When a DS starts, its FS is pushed onto the stack.

26. An example DS1 C1: I need to travel in May. A1: And, what day in May do you want to travel? C2: I need to be there for a meeting on 15th. A2: And you are flying into what city? C3: Seattle. A3: And what time would you like to leave Pittsburgh? C4: Hmm. I don’t think there are many options for non-stop. A4: There are three non-stops today? C5: What are they? …. DS2 DS0 DS3 DS4 DS5

27. Discourse structure with intention info • I0: C wants A to find a flight for C • I1: C wants A to know that C is traveling in May. • I2: A wants to know the departure data • I3: A wants to know the destination • I4: A wants to know the departure time • I5: C wants A to find a nonstop flight DS0 DS1 DS3 DS4 DS2 DS5 C1 A1-C2 A2-C3 A3 C4-C7

28. Problems with G&S 1986 • Assume that discourses are task-oriented • Assume there is a single, hierarchical structure shared by speaker and hearer • Do people really build such structures when they speak? Do they use them in interpreting what others say?

29. Building discourse structure

30. Tasks • Identify discourse segment boundaries • Determine relations between segments • Determine intentions of the segments • Determine the attentional state • Methods: • Inference-based approach: symbolic • Cue-based approach: statistical

31. Inference-based approach • Ex: John hid Bill’s car keys. He was drunk. • X is drunk  people do not want X to drive • People don’t want X to drive  people hide X’s car key. • Abduction: •  AI-complete: Require and utilize world knowledge.

32. Cue-based approach • Attentional state: • Attentional changes: • (push) now, next, but, …. • (pop) anyway, in any case, now back to, ok, fine,... • True interruption: excuse me, I must interrupt • Flashback: oops, I forgot • Intention: • Satisfaction-precedes: first, second, furthermore, …. • Dominance: for example, first, second, ….

33. Cues (cont) • Linguistic structure • Elaboration: for example, … • Concession: although • Condition: if • Sequence: and, first, second. • Contrast: and, … • …

34. One example • (Marcu 1999): Train a parser on a discourse treebank. • 90 trees, hand-annotated for rhetorical relations (RR) • Learn to identify Elementary discourse units (EDUs) • Learn to identify N, S, and their relation. • Features: WordNet-based similarity, lexical, structural, …

35. Results • Id EDUs: 96%-98% accuracy • Id hierarchical structures (2 EDUs are related): Rec=71%, Prec=84% • Id nucleus/satellite labels: Rec=58%, Prec=69% • Id rhetorical relation: Rec=38%, Prec=45% Hierarchical structure is easier to id than rhetorical relations.

36. Summary of Pragmatics • Reference resolution • Discourse structure • What makes text coherent? • What are discourse structures? • Two discourse theories: • RST (1988) • Grosz & Sidner (1986) • Two approaches to build discourse structures.