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Pragmatics II: Discourse structure

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Title: 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 Bills car keys.
  • (2) He was drunk.
  • (1) John hid Bills car keys.
  • (2) He likes junk food.
  • (1) George Bush supports big business.
  • (2) Hes 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 Bills 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 Bills car dealership.
  • S2 He needed to buy a car.
  • S3 The company he works for now isnt near any
    public transportation.
  • S4He also wanted to talk to Bill about their
    softball leagues.


Explanation
6
Discourse structure
  • S1 John took a train to Bills car dealership.
  • S2 He needed to buy a car.
  • S3 The company he works for now isnt near any
    public transportation.
  • S4He also wanted to talk to Bill about their
    softball leagues.



Explanation
Parallel
7
Discourse structure
  • S1 John took a train to Bills car dealership.
  • S2 He needed to buy a car.
  • S3 The company he works for now isnt near any
    public transportation.
  • S4He also wanted to talk to Bill about their
    softball leagues.



Explanation

Explanation
Parallel
8
Discourse parsing
Explanation (e1)
S1 (e1)
Parallel (e2e4)
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 Thompsons Rhetorical structure theory
    (1988)
  • Grosz and Sidners 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) Hes 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 NuclSat Hs comprehending Sat in
    Sat increases Hs belief of Nucl.
  • Effect Hs belief of Nucl is increased.

18
A relation Volitional-Cause
  • (a) George Bush supports big business.
  • (b) Hes sure to veto House Bill 1711.
  • Relation Name Volitional-Cause
  • Constraints on Nucl presents a volitional action
  • Constraints on Sat none.
  • Constraints on NuclSat 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 500. (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 dont 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
DS0
DS1
DS3
DS4
DS2
DS5
A1-C2
A2-C3
C1
A3
C4-C7
  • 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

28
Problems with GS 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 Bills car keys. He was drunk.
  • X is drunk ? people do not want X to drive
  • People dont want X to drive ? people hide Xs
    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)
    Rec71, Prec84
  • Id nucleus/satellite labels Rec58, Prec69
  • Id rhetorical relation Rec38, Prec45
  • ?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.
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