But What Do They Mean? Modelling Contrast Between Speakers in Dialogue Signalled by “But”

Abstract: Understanding what is being communicated in a dialogue involves determining how it is coherent, that is, how the successive turns in the dialogue are related, what the speakers’ intentions, goals, beliefs, and expectations are and how they relate to each other’s responses. This thesis aims to address how turns in dialogue are related when one speaker indicates contrast with something in the preceding discourse signalled by “but”. Different relations cued by “but” will be distinguished and characterised when they relate material spanning speaker turns and an implementation in a working dialogue system is specified with the aim of enabling a better model of dialogue understanding and achieving more precise response generation.A large amount of research in discourse addresses coherence in monologue, and much of it focuses on cases in which the coherence relation is explicitly signalled via a cue-phrase or discourse marker (e.g., “on the other hand”, “but”, et cetera) which provides an explicit cue about the nature of the underlying relation linking the two clauses. However despite research on Speech Acts, planning research into speakers’ intentions, and semantic approaches to question-answering dialogues, very little work has focused on coherence relations across turns in dialogue even given the presence of a cue-phrase.This thesis will explore what sorts of relations the speaker of the “but” perceives between elements in the dialogue, and in particular, it will focus on “but”s communicating Denial of Expectation, Concession, and Correction by determining what underlying cross-turn expectations are denied in the former two, and what is being corrected in the latter case. We will extend work by Lagerwerf (1998) in monologue which presents a treatment for Denial of Expectation and Concession arguing that “but” implicates a defeasible expectation which is then denied (in Denial of Expectation) or argued against (in Concession). We also follow Knott’s approach (Knott, 1999a) of describing the semantics of a cue-phrase algorithmically from the agent’s mental model of the related utterances.Task-oriented and nontask-oriented spoken dialogues involving turn-initial “but” are examined, motivating a logical scheme whereby Denial of Expectation, Concession and Correction can be distinguished. These relations are then modelled in the PTT  (Poesio and Traum, 1998) Information State (Matheson, Poesio and Traum, 2000) model of dialogue, enabling more relevant response generation in dialogue systems. A systematic response deliberation scheme based on the speakers’ underlying beliefs is proposed based on the analysis and modelling of relations presented in this thesis.