CS 4705

CS 4705

Discourse Structure and Text Coherence

What makes a text/dialogue coherent? Incoherent?

“Consider, for example, the difference between passages (18.71) and (18.72). The reason is that these utterances, when juxtaposed, will not exhibit coherence. Almost certainly not. Do you have a discourse? Assume that you have collected an arbitrary set of well-formed and independently interpretable utterances, for instance, by randomly selecting one sentence from each of the previous chapters of this book.”

vs….

“Assume that you have collected an arbitrary set of well-formed and independently interpretable utterances, for instance, by randomly selecting one sentence from each of the previous chapters of this book. Do you have a discourse? Almost certainly not. The reason is that these utterances, when juxtaposed, will not exhibit coherence. Consider, for example, the difference between passages (18.71) and (18.72). (J&M:695)

What makes a text coherent?

• Appropriate use of coherence relations between subparts of the discourse -- rhetorical structure

• Appropriate sequencing of subparts of the discourse -- discourse/topic structure

• Appropriate use of referring expressions

Rhetorical Structure Theory (Mann, Matthiessen, and Thompson ‘89)

• One theory of discourse structure, based on identifying relations between parts of the text– How many rhetorical relations are there?

– MMT say 23

• Nucleus/satellite notion encodes asymmetry

• Some rhetorical relations:– Elaboration (set/member, class/instance/whole/part…)

– Contrast: multinuclear

– Condition: Sat presents precondition for N

– Purpose: Sat presents goal of the activity in N

– Sequence: multinuclear

– Result: N results from something presented in Sat

– Evidence: Sat provides evidence for something claimed in N

• A sample definition:– Relation: evidence

– Constraints on N: H might not believe N as much as S think s/he should

– Constraints on Sat: H already believes or will believe Sat

• An example:George Bush supports Big Business.

He is sure to veto House Bill 1711.

1) Title: Bouquets in a basket – with living flowers

2) There is a gardening revolution going on

3) People are planting flower baskets with living plants

4) Mixing many types in one container for a summer of floral beauty

5) To create your own “Victorian” bouquet of flowers

6) Choose varying shapes, sizes and forms, besides a variety of complementary colors

7) Plants that grow tall should be surrounded by smaller ones and filled with others that tumble over the side of a hanging basket

8) Leaf textures and colors will also be important

9) There is the silver-white foliage of dusty miller, the feathery threads of lotus vine floating down from above, the deep greens, or chartreuse, even the widely varied foliage colors of the coleus.

Some Problems with RST (cf. Moore & Pollack ‘92)

• How many Rhetorical Relations are there?• How can we use RST in dialogue as well as

monologue?• How do we incorporate Intentional Structure into

RST?• RST does not allow for multiple relations holding

between parts of a discourse• RST does not model overall structure of the

discourse

A Simple Example

• System: Hello. How may I help you?• User: I would like to find out why I was charged

for a call?• System: What call would you like to inquire

about?• User: My bill says I made a call to Syncamaloo,

Texas, but I’ve never even heard of this town.• System: May I have the date of the call that

appears on your bill?

A Motivating Example

• A Tutoring System: Welcome to word processing. That’s using a computer to

type letters and reports. Make a typo? No problem. Just back up, type over the mistake, and it’s gone. And, it eliminates retyping.

Structures of Discourse Structure (Grosz & Sidner ‘86)

• Leading alternative theory of discourse structure– Provides for multiple levels of analysis: S’s purpose as

well as content of utterances and S and H’s attentional state

– Identifies only a few, general relations that hold among intentions

• Three components:– Linguistic structure

– Intentional structure

– Attentional structure

Linguistic Structure

• What is actually said/written• How is this represented?

– Assume discourse is segmented into Discourse Segments (DS) -- how?

• what is basic unit of analysis?

• segmentation agreement

• automatic segmentation

– Embedding relations: topic structure

– Cue phrases

• ds1: step 1, enter and get token

first

enter the Harvard Square T stop

and buy a token

• ds2: inbound on red line

then

proceed to get on the

inbound

um

Red Line

uh subway

Boston Directions Corpus: Describe how to get to MIT from Harvard

• ds3: take subway from hs, to cs to ks

and

take the subway

from Harvard Square

to Central Square

and then to Kendall Square

• ds4: describe ks stationyou’ll see a music sculpture there

which will tell you it’s Kendall Square

it’s very nice

• ds5: get off T.

then get off the T

Intentional Structure

• Discourse purpose (DP): basic purpose of the discourse

• Discourse segment purposes (DSPs): how this segment contributes to the overall DP

• Segment relations:– Satisfaction-precedence: DSP1 must be satisfied before

DSP2 (e.g. ds1 satp ds2)

– Dominance: DSP1 dominates DSP2 if fulfilling DSP2 constitutes part of fulfilling DSP1 (e.g. ds3 dom ds4)

Attentional State

• Focus stack:– Stack of focus spaces, each containing objects,

properties and relations salient during each DS, plus the DSP (content plus purpose)

– State changes modeled by transition rules controlling the addition/deletion of focus spaces

• Information at lower levels may or may not be available at higher levels

• Focus spaces are pushed onto the stack when– new DS or embedded DS (e.g. DS that are dominated by

other DS) are begun

– popped when they are completed

How are these structures recognized from a discourse?

• Linguistic markers:– tense and aspect

– cue phrases

– intonational variation

• Inference of S intentions• Inference from task structure

Interruptions, Digressions and Flashbacks

• True interruption: intentional and attentional states unrelatedJohn came by and left the groceries.

Stop that you kids.

And I put them after he left.

• Flashback: intentional and attentional states related -- satisfying DP John came by and left the groceries.

He had promised to help out this week.

I put them away after he left.

• Digressions: intentional separate; attentional relatedJohn came by and left the groceries.

It would be nice if somebody did this every week.

I put them away after he left.

• Semantic return: Remember what I said about John bringing the groceries

yesterday?

He forgot the milk.

Limits of the Theory

• Assumes that discourses are task-oriented• Assumes there is a single, hierarchical structure

shared by S and H• How do we identify entities that are salient (on the

focus stack)?• Do people really build such structures when they

converse?

Next

• Reference resolution• Read Ch. 18:1,4