Dialogue systems

1
Dialogue systems

• R Mitkov (ed.) The Oxford Handbook of
  Computational Linguistics, Oxford (2004) OUP
  Chapters 6 (Discourse Allan Ramsay), 7
  (Pragmatics and dialogue Geoffrey Leech Martin
  Weisser) and 14 Anaphora resolution (Ruslan
  Mitkov)
• D Jurafsky JH Martin Speech and Language
  Processing, Upper Saddle River NJ (2000)
  Prentice Hall, Chapters 18 19

2
Dialogue systems

• Computer enters a dialogue with user
• Usually specific cooperative task-oriented
  dialogue
• Often over the phone
• Examples?
• Usually speech-driven, but text also appropriate
• Modern application is automatic transaction
  processing
• Limited domain can simplify language aspect
• Domain model will play a big part

3
Dialogue systems

• Apart from speech recognition and synthesis
  issues, NL components include
• Anaphora/Reference resolution
• Use of pronouns, ellipsis
• Understanding speakers intentions
• Reply generation
• Cooperative responses

this class
next class
4
Reference in discourse
John went to Bills car showroom to check out the
new Toyota. He looked at it for about half an
hour.

• What do pronouns he and it refer to? How do you
  know?
• Identify possible referents
• Then decide which ones fit the model of a
  coherent discourse

5
Referring expressions

• Expressions which refer to a specific entity in
  the discourse
• Reference to a previously introduced entity is
  called anaphora (more rarely cataphora, if
  reference syntactically precedes the referent)
• I hate it when you do that.
• Hes big, hes funny, hes Peter Kay.
• Before he bought it, John checked the new Toyota
  very carefully.
• For anaphoric reference, we need to establish the
  antecedent
• Anaphora can be pronouns (it, this), vague
  expressions (the car) or synonymous expressions
  (the President Mr Bush)

6
Reference phenomena

• Indefinite vs definite noun phrases
• Pronouns
• Demonstratives
• One-anaphora

Important both for anaphora resolution, but also
for generating coherent (realistic) discourse
7
Indefinite vs. definite noun phrases

• Indefinite (a, some) introduces a new entity
• Definite (the) refers to an entity identifiable
  to the hearer
• Because already mentioned
• Because it is identifiable from the hearers
  discourse model or beliefs about the world
• Because it is inherently unique

• I met a stupid man at the meeting. The idiot was
  only interested in football.
• I ran a bath. The water was too hot.
• Did you get the cake?
• I went to see a/the doctor yesterday.
• The Queen is coming to visit.

8
Pronouns

• Like definite noun phrases, but more strongly
  constrained must be salient
• John went to Bobs party and parked next to a
  shiny new car. He went inside and talked to Bob
  for half an hour. Bob said that he had recently
  got engaged. He also said that he had bought it
  yesterday.
• Agree with antecedent in gender (sex) and number
• he, she, it, they ils, elles
• Ich bin ein Mädchen, das sagt was es denkt.
• Marie disait que leau était froide, mais elle
  était calme.
• Plural pronouns can refer to
• Discontinuous sets
• John has a Telecaster and Bill has a Strat. They
  play them all the time.
• Generic reference
• There were six Les Pauls in the shop. They are my
  favourite guitar.
• Interaction with quantifiers complicates the
  picture
• Every man loves his mother.
• Every farmer who owns a donkey beats it.

9
Demonstratives

• Demonstrative adjectives / pronouns
• this, that etc.
• Notion of spatial (or temporal) proximity
• I bought a new Telecaster, like the one I bought
  five years ago. That guitar was nice, but this is
  even better.
• Many languages have more complex systems
• This can be used as an indefinite determiner!
• I heard this wicked joke yesterday.

10
One-anaphora

• The cakes looked so nice that I ate one.
• i.e one of them plural referent but the anaphor
  picks out a single member of the set
• Not to be confused with the formal pronoun, or
  the number
• One cant be too careful.
• John has two cars but I only have one.
• John has two cars but Ive only seen one.

11
How to identify referents

• Number and gender agreement
• Usually trivial for humans, but computer has to
  have the relevant knowledge
• John met Mary. She was wearing a new dress.
• The German chancellor is on a state visit. She is
  expected to address the upper house.
• Reflexive pronouns
• Generally corefer with the subject of the most
  immediate clause, while a non-reflexive cannot
  refer to this
• John bought himself/him a new guitar.
• John said that Bill bought himself/him a new
  guitar.

12
How to identify referents

• Selectional restrictions
• John parked his new car on the street. He left
  his hat on the back seat.
• He liked to wear it on rainy days.
• He liked to sit on it when his wife was driving.
• He liked to drive it on rainy days.
• He liked to walk down it on rainy days.
• It was really dirty.
• The monkey ate the banana because it was
  hungry/ripe/teatime
• But beware of metaphor
• John drove his new car down the street.
• She was as smooth as anything.
• It went all the way into town.
• It drank petrol like nobodys business.

13
Preferences in pronoun interpretation

• Recency most recent utterances are most salient,
  so pronouns more likely to refer to them.
• John has a Toyota. Fred has a Mini. He lets me
  drive it.
• Grammatical role
• subject gt object gt prepobj gt
• John met Fred in town. He had some good news.
• John went for a meal with Bill. He ordered
  spaghetti.

14
Preferences in pronoun interpretation

• Repeated mention
• John wanted to buy a new guitar. He thought hed
  get a semi-acoustic. Bill went to the shop with
  him. He tried out a few guitars and then changed
  his mind.
• Parallelism
• John went with Bill to the City match. Fred went
  with him to the Arsenal game.
• John went with Bill to the City match. Fred asked
  him to take some photos.

15
Preferences in pronoun interpretation

• Verb semantics (how to explain the following?)
• John phoned Bill. He had forgotten what time the
  meeting was.
• John criticized Bill. He had forgotten what time
  the meeting was.
• John grabbed the book from Bill. He was an avid
  reader.
• John passed the book to Bill. He was an avid
  reader.
• The car dealer admired John. He knew cars inside
  out.
• The car dealer impressed John. He knew cars
  inside out.
• Intonation can play a role
• John punched Bill and then he kicked him.
• John punched Bill and then he kicked him.
  (contrastive stress)

16
How to identify referents

• Linguistic model
• Rules of grammar
• Selectional restrictions
• Preference rules
• World-knowledge
• Possible implications and inferences
• John phoned Bill because he wanted some
  information
• John phoned Bill because he withheld some
  information
• Specific facts and relationships may be
  particular to a certain domain
• After attaching the printer to the computer, load
  it with paper.
• After attaching the printer to the computer,
  reboot it.
• The authorities banned the demonstrators because
  they feared violence.
• The authorities banned the demonstrators because
  they advocated violence.
• Might be built in to the linguistic model
• Speakers and hearers beliefs
• might be modified by the preceding discourse

17
Coreference

• Anaphora are not only pronouns
• Coreference with different referring expressions
  also a problem
• The Presidents wife, Laura Bush, Mrs Bush, the
  First Lady, the wife of US President George
  Bush, etc.

18
Final remarks

• Anaphora particularly prevalent in natural
  dialogue
• Resolution necessary in dialogue systems
• And conversely, anaphora should be naturally
  generated
• Also relevant for other natural language
  applications summarization, text mining,
  information retrieval, QA, translation, story
  understanding,