Curs 6: Teorii ale discursului: AST şi RST

Dan Cristea Selecţie de slide-uri prezentate în tutoriale (RANLP-03, Borovits, Sept. 2003;

ICON-04, Hyderabad, Dec. 2004) şi conferinţe

ContentI. Introduction

– What is discourse? Text versus discourse. Coherence and cohesion.

II. Theories– attentional state theory– rhetorical structure theory

What is discourse?Longman: 1. a serious speech or piece or

writing on a particular subject: Professor Grant delivered a long discourse on aspects of moral theology. 2. serious conversation between people: You can’t expect meaningful discourse when you two disagree so violently. 3. the language used in particular kinds of speech or writing: scientific discourse.

What is discourse?Longman: 1. a serious speech or piece or

writing on a particular subject: Professor Grant delivered a long discourse on aspects of moral theology. 2. serious conversation between people: You can’t expect meaningful discourse when you two disagree so violently. 3. the language used in particular kinds of speech or writing: scientific discourse.

Text versus discourseSyntactically – a discourse is more than a single

sentence.

From Garcia Marquez

A text is not a discourse!

But it becomes a discourse the very moment it is read or heard by a human... or a machine.

Text versus discourse

Time and discourse

Discourse has a dynamic nature

Time axesreal time

discourse time

story time

1 2

2 11000 1030800 920

1 2

Cohesion and coherence

A text manifests cohesion when its parts closely correlate.

A text is coherent when it makes sense, with respect to an accepted setting, real or virtual.

Interpretation of discourse

discourse interpretation

text setting

knowledge about the language

knowledge about the world

knowledge about the situation

knowledge about the author

Discourse phenomena: interruptions and flash-backs

E: Now attach the pull rope to the top of the engine.By the way, did you buy gasoline today?

A: Yes. I got some when I bought the new lawnmower wheel.I forgot to take the gas with me, so I bought a new one.

E: Did it cost much? A: No, and we could use another anyway to keep with the tractor.E: OK, how far have you got? Did you get it attached?

from (Allen, 1987)

Discourse phenomena: pop-overs

E: Now attach the pull rope to the top of the engine.By the way, did you buy gasoline today?

A: Yes. I got some when I bought the new lawnmower wheel.I forgot to take the gas with me, so I bought a new one.

E: Did it cost much? A: No, and we could use another anyway to keep with the tractor.E: OK, how far have you got? Did you get it attached?

from (Allen, 1987)

Discourse phenomena: pop-overs

E: Now attach the pull rope to the top of the engine.By the way, did you buy gasoline today?

A: Yes. I got some when I bought the new lawnmower wheel.I forgot to take the gas with me, so I bought a new one.

E: Did it cost much? A: No, and we could use another anyway to keep with the tractor.E: OK, how far have you got? Did you get it attached?

from (Allen, 1987)

Discourse phenomena: inference load and pronoun use

Why is it that some discourses seem more difficult to understand than others?

Why do we use the pronouns and other anaphoric means the way we do?

localising the settingcohesion is lowercoherence is lower

Discourse theories?

Sub-domain of Computational Linguistics: searching for the intrinsic laws of the discourse and for models making possible an automated analysis, representation and generation of the discourse.

II. Discourse theories

• atentional state theory• rhetorical structure theory• centering theory• veins theory

Attentional state theory (AST)(Barbara Grosz & Candence Sidner, 1987)

Models the linguistic structure of the discourse Gives an account on intentions and how they are combinedExplains the shift of attention during discourse interpretationExplains interruptions and flash-backsPuts in evidence a dynamic domain of referentiality

3 components

AST: 1st component

• a linguistic structure: – more sentences are aggregated in the same

segment– segments display a recursive structure

AST: 2nd component• an intentional structure:

– a segment communicates an intention, it has a goal to accomplish in the reader;

– the goals of the component segments contribute to the realisation of the goal of the overall segment;

– two type of relations between segment goals: dominance and satisfaction-precedence

AA

AST: 2nd componentRelations: dominance

DSP A dominates DSP AA: the intention associated with DSP AA contributes to the satisfaction of the intention associated with DSP A

AA

AA

ABAB AC

ACAAA

AAA

AAB

AAB

ABA

ABA

ABBABB

AST: 2nd componentRelations: satisfaction-precedenceDSP AA satisfaction-precedes DSP AB: DSP AA must be satisfied before

DSP AB

A

AA AB AC

AAA AAB ABA ABB

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AA AB AC

AAA AAB ABA ABB

A

SA

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

AAA AAB ABA ABB

A

SA

AASAA

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

AAB ABA ABB

A

SA

AASAA

AAA

SAAA

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

ABA ABB

A

SA

AASAA

AAA AAB

SAAB

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

ABA ABB

A

SA

AASAB

AAA AAB

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

ABA ABB

A

SA

AASAB

AAA AAB

SABA

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

ABA ABB

A

SA

AASAB

AAA AAB

SABB

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

ABA ABB

A

SA

AASAC

AAA AAB

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

ABA ABB

A

SA

AA

AAA AAB

AST: 3rd component• an attentional state

– to each segment corresponds a space of entities under focus

– these spaces have the dynamics of a stack

A

AB AC

ABA ABB

A

AA

AAA AAB

AST: 3rd component• an attentional state

– accessibility modeled by the top-down access in the stack

A

AA AB AC

AAA AAB ABA ABB SA

SAB

SABB

AST explains interruptionsE: Now attach the pull rope to the top of the engine.

By the way, did you buy gasoline today?A: Yes. I got some when I bought the new lawnmower wheel.

I forgot to take the gas with me, so I bought a new one.E: Did it cost much? A: No, and we could use another anyway to keep with the tractor.E: OK, how far have you got? Did you get it attached?

from (Allen, 1987)

An interruption is a discourse segment whose DSP is not dominated nor satisfaction-preceded by the DSP of the immediately proceeding segment.

AST: interruptionsE: Now attach the pull rope to the top of the engine.

…

…

…

…

By the way, did you buy gasoline today?A: Yes. I got some when I bought the new lawnmower wheel.

I forgot to take the gas with me, so I bought a new one.E: Did it cost much? A: No, and we could use another anyway to keep with the tractor.E: OK, how far have you got? Did you get it attached?

AST explains flashbacksSinit …SABC …SBill OK. Now how do I say that Bill is...SFB

SBill Now back to Bill.How do I say that Bill is an employee of ABC?

From (Grosz and Sidner, 1987)

Whoops I forgot about ABC.I need an individual concept for the company ABC. …

A flashback is a particular kind of interruption whose DSP satisfaction-precedes the interrupted segment or a segment that dominates the interrupted segment.

AST: flashbacksSinit …SABC …SBill OK. Now how do I say that Bill is...SFB Whoops I forgot about ABC.I need an individual concept for the company ABC. …SBill Now back to Bill.How do I say that Bill is an employee of ABC?

From (Grosz and Sidner, 1987)

A flashback is a particular kind of interruption whose DSP satisfaction-precedes the interrupted segment or a segment that dominates the interrupted segment.

SBill

Sinit

SFB

Sinit

SABC

SBill

SFB

SBill

SFB

Sinit

SABC SBill

Sinit

SFB

flashback starts

flashback ends

AST doesn‘t accommodate left satellites

a. Jack and Sue went to buy a new lawn mowerb. since their old one was stolen.c. Sue had seen the men who took it andd. had chased them down the street,e. but they'd driven away in a truck. f. After looking in the store g. they realized they couldn't afford a new one.h. By the way, Jack lost his job last monthi. so he's been short of cash recently.j. He has been looking for a new one,k. but so far hasn't had any luck. l. Anyway, they finally found a used one at a garage sale.

From (Allen, 1993)

AST doesn‘t accommodate left satellites

a. Jack and Sue went to buy a new lawn mower b. since their old one was stolen.

f. After looking in the store g. they realized they couldn't afford a new one.

l. Anyway, they finally found a used one at a garage sale.

c. Sue had seen the men who took it and d. had chased them down the street, e. but they'd driven away in a truck.

h. By the way, Jack lost his job last month i. so he's been short of cash recently. j. He has been looking for a new one, k. but so far hasn't had any luck.

Attentional state stack

a

a. Jack and Sue went to buy a new lawn mower

a,b

a. Jack and Sue went to buy a new lawn mower b. since their old one was stolen.

c,d,e

a. Jack and Sue went to buy a new lawn mower b. since their old one was stolen.

c. Sue had seen the men who took it and d. had chased them down the street, e. but they'd driven away in a truck.

a,b

Attentional state stack

a. Jack and Sue went to buy a new lawn mower b. since their old one was stolen.

f. After looking in the storeg. they realized they couldn't afford a new one.

c. Sue had seen the men who took it and d. had chased them down the street, e. but they'd driven away in a truck.

a,b,f,g

Attentional state stack

a. Jack and Sue went to buy a new lawn mower b. since their old one was stolen.

f. After looking in the storeg. they realized they couldn't afford a new one.

c. Sue had seen the men who took it and d. had chased them down the street, e. but they'd driven away in a truck.

h. By the way, Jack lost his job last month i. so he's been short of cash recently. j. He has been looking for a new one, k. but so far hasn't had any luck.

h,i,j,k

a,b,f,g

Attentional state stack

a,b,f,g,l

a. Jack and Sue went to buy a new lawn mower b. since their old one was stolen.

f. After looking in the storeg. they realized they couldn't afford a new one.

l. Anyway, they finally found a used one at a garage sale.

c. Sue had seen the men who took it and d. had chased them down the street, e. but they'd driven away in a truck.

h. By the way, Jack lost his job last month i. so he's been short of cash recently. j. He has been looking for a new one, k. but so far hasn't had any luck.

Attentional state stack

Problem:a finer granularity

f. After looking in the store

a. Jack and Sue went to buy a new lawn mower

g. they realized they couldn't afford a new one.

l. Anyway, they finally found a used one at a garage sale.

h, i, j, k

b. since their old one was stolen.c, d, e

Problem

a a a

b

a

b

c,d,e

a, g

f

AST: pluses• Discourse structure:

– a proposal for discourse structure– stack behavior models hierarchical relationships

among text segments• Reference: accounted for by accessibility in

the stack• Explains interruptions• Explains flash-backs

AST: minusesStack mechanism fails for certain

dominant/dominated segment configurations when granularity is sufficiently fine

Does not accommodate left satellitesThe stack model is impurified with an artificial

border (in treating interruptions)Do we have an additional memory from where

states have to be restored (in treating flashbacks)?

Rhetorical structure theory

Basics• text span: un uninterrupted linear interval of text• relation: holds between two or more non-overlapping spans• arguments of relations are of a nuclear type and a satellite type

– a nucleus is more important than a satellite (deletion and substitution tests)

– relations: hypotactic (one nucleus + satellites) and paratactic (all nuclear)• scheme: integrates by a relation two or more text spans (like grammar

rules)• RST analysis are trees• they reflect a judge interpretation (therefore could be subjective)

(William Mann and Sandra Thompson, 1987)

RST schemes

relation

text span: nucleus

text span: satellite

relation

text span: nucleus

text span: nucleus

RST schemes: equivalences

relation1 relation2

relation1

relation2

relation1

relation2

RST schemes: equivalences

relation relation

relation

relation

relation

relation

RST: a relation definition

EVIDENCEconstraint on N: R might not believe N to a

degree satisfactory to Wconstraint on S: R believes S or finds it

credibleeffect: R’s belief of N is increased

EVIDENCE relation

1. The program as published for calendar year 1980 really works.

2. In only a few minutes, I entered all the figures from my 1980 tax return

3. and got a result which agreed with my hand calculations to the penny.

EVIDENCE

1-3

2-31

EVIDENCEconstraint on N: R might not believe N to a

degree satisfactory to Wconstraint on S: R believes S or finds it

credibleeffect: R’s belief of N is increased

CONCESSION relationCONCESSION

constraint on N: W has positive regard to the situation presented in N

constraint on S: W is not claiming that the situation presented in S doesn’t hold

constraint on the combination N+S: W acknowledges a potential incompatibility between the situations presented in N and S; W regards the situation presented in N and S as compatible

effect: R’s positive regard for the situation presented in N is increased

CONCESSION relationCONCESSION

constraint on N: W has positive regard to the situation presented in Nconstraint on S: W is not claiming that the situation presented in S doesn’t holdconstraint on the combination N+S: W acknowledges a potential incompatibility between

the situations presented in N and S; W regards the situation presented in N and S as compatible

effect: R’s positive regard for the situation presented in N is increased

1. Although Dioxin is toxic to certain animals,2. evidence is lacking that it has any serious

long-term effects on human beings.

CONCESSION

1-2

21

CIRCUMSTANCE relationCIRCUMSTANCE

constraint on N: noneconstraint on S: S presents a situationconstraint on the combination N+S: S sets a framework

(spatial or temporal) within which R is intended to interpret the situation presented in N

effect: R recognizes that the situation presented in S provides the framework for interpreting N

CIRCUMSTANCE relationCIRCUMSTANCE

constraint on N: noneconstraint on S: S presents a situationconstraint on the combination N+S: S sets a framework (spatial or temporal) within

which R is intended to interpret the situation presented in Neffect: R recognizes that the situation presented in S provides the framework for

interpreting N

1. Probably the most extreme case of Visitors Fever I ever witnessed was a few summers ago

2. when I visited relatives in Midwest.

CIRCUMSTANCE

1-2

21

A more complex example1. Farmington Police had to help control traffic recently2. when hundreds of people lined up to be among the first

applying for jobs at the yet-to-open Marriot Hotel.3. The hotel’s help-wanted announcement – for 300 openings –

was a rare opportunity for many unemployed.4. The people waiting in line carried a message of claims that the

jobless could be employed if only they showed enough moxie.5. Every rule has exceptions,6. but the tragic and too-common tableaux of hundreds of people

snake-lining up for any task with a paycheck illustrates a lack of jobs,

7 not laziness.

A more complex example1. Farmington Police had to help control traffic recently2. when hundreds of people lined up to be among the first applying for jobs at the yet-to-open Marriot Hotel.3. The hotel’s help-wanted announcement – for 300 openings – was a rare opportunity for many unemployed.4. The people waiting in line carried a message of claims that the jobless could be employed if only they

showed enough moxie.5. Every rule has exceptions,6. but the tragic and too-common tableaux of hundreds of people snake-lining up for any task with a paycheck

illustrates a lack of jobs,7 not laziness.

circumstance

32

2-3

volitional result

1-3

4

evidence

5

6

antithesis

7

6-7

concession

5-7

4-7

background

1-7

RST relationsSubject matter

(informational)

ElaborationCircumstanceSolutionhoodVolitional CauseVolitional ResultNon-Volitional CauseNon-Volitional ResultPurposeConditionOtherwiseInterpretationEvaluationRestatementSummarySequenceContrast

Presentational (intentional)

MotivationAntithesisBackgroundEnablementEvidenceJustifyConcession

Problem: multiple interpretations(Moore and Polack, 1992)

1

2

motivation

3

motivation

Intentional level

3

1

condition

2

condition

Informational level

1. Come back at 5:00.2. Then we can go to the hardware store before it closes.3. This way we can finish the bookshelves tonight.

How distant are AST & RST?

• Mosser&Moore (1996):– granularity: AST - undefined, RST - fine (clause

level)– structure: trees– internal nodes: relations (AST:2, RST: 28,

Hobbs, Knott: hierarchy of relations)• Marcu (1997)

– uses a logical formalism that allows him to prove the equivalence between the AST and RST

RST bibliography or related readingsMann,W. and Thompson,S. (1987): Rhetorical Structure TheoryMoore and Polack (1992): A problem for RST: The need for multi-

level discourse analysisMosser and Moore (1996): Towards a synthesis of two accounts of

discourse structureHobbs Jerry – a lot of writings on discourse coherence, but see also

the abductive modelKnott Alistair: his PhD thesisMarcu,D. (1997): The rhetorical parsing of natural language textsMarcu,D. (2000): The theory and practice of discourse parsing

and summarization, The MIT PressCarlson, L., Marcu, D. and Okurowski, M.E. (2003): Building a

doscourse-tagged corpus in the framework of Rhetorical Structure Theory.