The Interpretation of Temporal Relations in Narrative

This paper describes an algorithm for the interpretation of temporal relations between events mentioned in narrative (such as which event occurs before another). These relations are decided through three different levels of linguistic concepts: aspectual information for verbs, time relations for tenses, and time relations between clauses and/or sentences. One contribution of this paper is to present a more rigorous description for time relations of tenses, which is able to express all the 16 tenses in English and incorporate the interval properties of events from the aspectual analysis into the tense relations. For interpreting time relations between clauses, we emphasize the use of anaphoric references to events and introduce the concept of a situational description for an event (including the participants, place, time duration, etc.), used to make the interpreting algorithm deterministic, i.e. the set of interpreting rules are applied in a fixed order rather than in parallel. Last, we suggest a tree-like structure for the representation of temporal relations between events, which allows us to include vaguely specified relations (which may be clarified later), to facilitate the interpretation of subsequent temporal relations. One important part of the understanding of narratives is the interpretation of temporal relations between events mentioned in the narrative (e.g. event1 before event2). These temporal relations are often explicitly indicated by some linguistic categories like tense, aspect, and certain temporal adverbials. They may also be implicitly determined using context-dependent default rules. For example, a rule of %mrrative time progression” provides that in narrative, time does not move backward unless an explicit time marker is given [Hirschman and Story, 19811. In this paper, we consider temporal relations in terms of three different levels of linguistic concepts. At the lowest level, we distinguish events as situations, which are a classification for predications, drawn from aspectual information, including states, processes and transition events [Passonneau, 19871. We generally treat events as intervals, interpreted against some reference points on a time line. The situation types and their interpretations are briefly discussed in section 2. At the middle level we consider tense, which is usually described by three abstract times [Reichenbach, 19471: the time of the event (ET), the time of speech production (ST), and the time of reference (RT) from which the event is interpreted. The concept of RT is a theoretical entity used to distinguish different tense structures. For example, simple past is represented on a time line as RT,ET-ST, and past perfect would be ET-RT-ST, where the comma indicates simultaneity and the hyphen “temporally precedes”. In this paper, we present a more rigorous description of tense relations by introducing more than one RT for some tenses so that we are able to describe all the 16 tenses in English. Moreover we treat ET as an interval, which implies that we actually incorporate aspectual information into tense relations. These modifications are the topics of section 3. At the highest level we study time relations between clauses and/or sentences. The key idea in establishing these relations is the management of temporal focus (TF), which is the node on the time line that provides a context for the interpretation of RT and ET of the next clause or sentence [Webber, 19871. In this paper, we give a deterministic algorithm for interpreting these relations, using a set of default rules to manage the change of TF. Included are the concepts of anaphoric reference to a situation and situational description for a situation (including participants, place, and other information). The temporal relations between events are represented in a tree-like structure, which allows some relations between events to be left vaguely specified, to delay interpretation. A detailed account for the representation structures and the interpreting algorithm is given in section 4. Finally in section 5, we summarize the paper and give some suggestions for future research. elations of Situations According to [Passonneau, 19871, events are classified as situations, which are aspectual classes for predications, determined on the basis of lexical aspect (or verb type like: stative, process, or transition event) and grammatical aspect (i.e. progressive tenses) together with suggestions from temporal adverbials. Here we follow [Passonneau, 19871 and consider only four types of situations: States: The pressure is low at 8:O0. Temporally unbounded processes: The alarm is sounding. Temporally unspecified processes: The alarm sounded. Transition events: The engine failed. Song and Cohen 745 From: AAAI-88 Proceedings. Copyright ©1988, AAAI (www.aaai.org). All rights reserved.