Presuppositions in Spoken Discourse

Chapter 4
Here I will first give some background information on global discourse
structure and how it is meant to affect the availability of antecedents for anaphoric
expressions. Two well-known proposals use tree structures to represent discourse
structure, Polanyi’s (1988) Linguistic Discourse Model and Webber’s model (1991).
Lascarides & Asher’s (1993) model also uses tree structures to organize rhetorical
relationships. The attentional model of Grosz & Sidner (1986) represents
hierarchical structure as stacks of focus spaces.
One of the types of examples that have been used to support the need for a
hierarchical discourse structure are so-called ‘return pops.’ However, Walker (1996)
argues that the evidence from return pops can be explained by other factors
without the need to refer to a hierarchical discourse structure. I’ll present an
example of a return pop with an anaphoric expression and then one from my own
data with a bound presupposition. Then I’ll discuss whether or not we need to
hypothesize a hierarchical discourse structure to explain these examples. The
answer seems to be no, and I’ll discuss the issue in relation to the rest of the corpus
data.
In a global discourse structure, information is organized into units, called
discourse segments, which are related to each other by coherence or rhetorical
relationships which may be coordinating or subordinating, creating a hierarchical
structure. This structure then supports the use of underspecified forms efficiently
because their resolution can be guided by it. The main distinction in discourse
structure is between what is in focus, attentionally available or related to the current
topic from what is not, as determined by identifying the relevant “focus space”
(Grosz & Sidner, 1986), “context” (Fox 1987) or part of the hierarchical structure
(usually the right frontier, in e.g. Polanyi 1988, Webber 1991, and Lascarides &
Asher 1993).
The search for antecedents of anaphoric expressions will be most efficient if
it begins by examining salient or activated referents, and these should be the
referents found in a pre-defined discourse structural position. The relationship that
holds between discourse structure and an anaphor may differ depending on the
type of anaphor. Generally, abstract object anaphoric reference has been argued to
be more dependent on discourse structure than anaphoric reference to concrete
individuals in the discourse, perhaps in part because abstract object anaphors can
sometimes refer to the information derived from one particular discourse segment.
This is discussed in Webber (1991), who has argued that some abstract object
anaphors can only access discourse segments on the right frontier, because those
are the ones that are in focus or salient.
In one of the first computational models of discourse structure, presented in
Grosz & Sidner (1986), contexts representing topics with discourse referents and
other information are stored in so-called focus spaces, which are “pushed” onto a
stack when introduced. When a subtopic is completed, it is “popped” from the
stack in order to get to the previous focus space and return attention to the earlier
(superordinate) discourse topic. The search for antecedents of anaphora in this
model is restricted to the focus space on top of the stack. If there is no compatible
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