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 Bill’s 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. • He’s big, he’s funny, he’s 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 (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 hearer’s 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. Indefinite vs. definite noun phrases

8. Pronouns • Like definite noun phrases, but more strongly constrained: must be salient • John went to Bob’s 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 l’eau 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 quantifierscomplicates 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 can’t be too careful. • John has two cars but I only have one. • John has two cars but I’ve 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 nobody’s 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 > object > prepobj > … • 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 he’d 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 • Speaker’s and hearer’s beliefs • might be modified by the preceding discourse

17. Coreference • Anaphora are not only pronouns • Coreference with different referring expressions also a problem The President’s 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, …