Pragmatics II: Discourse structure

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Pragmatics II Discourse structure

• Ling 571
• Fei Xia
• Week 7 11/10/05

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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

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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.

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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.

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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
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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
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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
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Discourse parsing
Explanation (e1)
S1 (e1)
Parallel (e2e4)
Explanation (e2)
S4 (e4)
S2(e2)
S3(e3)
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Why compute discourse structure?

• Natural language understanding
• Summarization
• Information retrieval
• Natural language Generation
• Reference resolution

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Two theories on discourse structure

• Mann and Thompsons Rhetorical structure theory
  (1988)
• Grosz and Sidners attention, intention and
  structure of discourse (1986)

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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.

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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

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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

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Multinuclear relations

• Contrast
• Joint
• List
• Multinuclear restatement
• Sequence

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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.

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

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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.

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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
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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.

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Grosz Sidner (1986)
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Grosz and Sidner (1986)

• A leading theory of discourse structure
• Three components
• A linguistic structure
• An intentional structure
• An attentional state

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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?

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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.

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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.

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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
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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

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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?

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Building discourse structure
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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

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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.
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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, .

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Cues (cont)

• Linguistic structure
• Elaboration for example,
• Concession although
• Condition if
• Sequence and, first, second.
• Contrast and,

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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,

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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.

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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.