Limited Attention and Discourse Structure

In computational theories of discourse, there are at least three processes presumed to operate under a limited attention constraint of some type: (1) ellipsis interpretation; (2) pronominal anaphora interpretation; and (3) inference of discourse relations between representations A and B of utterances in a discourse, e.g. B motivates A. In each case, the interpretation of the current element B of a discourse depends on the accessibility of another earlier element A. According to the limited attention constraint only a limited number of candidates need to be considered in the processing of B, for example, only a limited number of entities in the discourse model are potential cospecifiers for a pronoun. The limited attention constraint has been defined by some researchers by linear recency: a representation of an utterance A is linearly recent for a representation of an utterance B if A is linearly adjacent to B. Using linear recency as a model of the limited attention constraint would mean that an antecedent for an anaphor is determined by a linear backward search of the text, or of a discourse model representation of the text (Clark and Sengul 1979, inter alia). In contrast, other work has formulated the limited attention constraint in terms of hierarchical recency (Grosz and Sidner 1986; Hobbs 1985; Mann and Thompson 1987, inter alia). A representation of an utterance A is hierarchically recent for a representation of an utterance B if A is adjacent to B in the tree structure of the discourse. Of all theories based on hierarchical recency, only Grosz and Sidner's theory of discourse structure provides an operationalization of hierarchical recency in terms of their stack model of attentional state (Sidner 1979; Grosz 1977; Grosz and Sidner 1986). Thus, below, the relationship between limited attention and hierarchical recency will be discussed in terms of their stack model, but the discussion should also apply to claims about the role of hierarchical recency in other work. In the remainder of this squib, I will argue that the limited attention constraint must account for three types of evidence: (1) the occurrence of informationally redundant utterances in naturally occurring dialogues (Walker 1993); (2) the infelicity of discourses that depend on accessing discourse entities that are not linearly recent; and (3) experiments that show that humans have limited attentional capacity (Miller 1956; Baddeley 1986).