Lexical Opposition in Discourse Contrast
Authors: Anna Feltracco, Bernardo Magnini, Elisabetta Jezek (Fondazione Bruno Kessler, Università di Pavia, 2018)
Topic: Lexical and Semantic Resources and Analysis

Abstract
We investigate the connection between lexical opposition and discourse relations, with a focus on the relation of contrast, in order to evaluate whether opposition participates in discourse relations.1 Through a corpus-based analysis of Italian documents, we show that the relation between opposition and contrast is not crucial, although not insigniﬁcant in the case of implicit relation. The correlation is even weaker when other discourse relations are taken into account.

Introduction
This paper focuses on lexical opposition and discourse contrast. We deﬁne opposition as the relation between two lexical units that contrast with each other with respect to one key aspect of their meaning and that are similar for all the other aspects (e.g. to increase / to decrease, up / down). On the other end, we consider discourse contrast as the relation between two parts of a coherent 1Part of this research has already been published in the ﬁrst author Ph.D. thesis (Feltracco, 2018). sequence of sentences or propositions (i.e., discourse arguments) that are in conﬂict. Both opposition and contrast hold between contrasting elements: the ﬁrst at the lexical level, the other at the discourse level. In the following example, a contrast relation is identiﬁed between the two arguments in square brackets; two opposite terms are found in the arguments of the relation and are underlined. (1) [The price of this book increased], while [the price of that one decreased.] Despite the two relations are per se independent, the example shows how opposition can participate in contrast; in fact, the opposites to increase / to decrease convey the difference based on which the two mentioned entities (i.e., the books) are compared, leading to a contrast. Indeed, opposition can be found in the context of other discourse relations (e.g. in the temporal relation “Before the decrease of the demand, an increase of the prices was registered”), and discourse contrast can be conveyed through other strategies (e.g. negation and synonyms “Although the price decreased; the demand did not fall” or incompatibility “She has blue eyes, he has green eyes”). However, our analysis focuses on opposition and contrast, and starts with the observation that both linguistic phenomena involve two elements that are similar in many aspects, but that differ in others (Section 2). This similarity have already been considered by works in the computational ﬁeld, in which opposition is used as a feature for identifying contrast, and viceversa (Section 3). In this paper, we investigate the behaviour of opposition in the context of a contrast relation adopting a corpus-based approach (Section 4). In particular, we study the opposition-contrast intersection by observing how frequently opposites are found in the arguments of a contrast relation in ContrastIta Bank (Feltracco et al., 2017), a corpus annotated with the discourse contrast relation. We analyze the cases in which the two phenomena co-occur, in order to understand the contribution of opposition to discourse contrast (Section 5). The investigation lead us to enrich Contrast-Ita Bank with lexical opposition. Enlarging our focus, we also investigate the behaviour of opposition in the context of other discourse relations in the corpus, by examining which are the relations that involve pairs of opposites in their arguments (Section 6). Finally, we report our concluding observations and our hint for further work (Section 7).

Conclusion
Through the annotation of opposites in the arguments of contrast relations in Contrast-Ita Bank, we aim at providing new insights over the role of opposition in discourse contrast. Overall, we register 23 cases of opposition over 372 contrast relations in our dataset. This number is not high and one we can expect the number to be higher in a larger dataset. However, this limited number suggests that the presence of opposites is not an impacting feature for the identiﬁcation of contrast relation in the Italian language. It is, however, quite frequent for implicit relations, suggesting that the use of opposition can be a strategy to convey contrast when there is a lack of a connective (such as but or however) that lexicalizes the relation. Moreover, we show that also the co-occurrence of opposition and other discourse relations is low. Despite, in related work opposition has been used as a feature for identifying contrast, the result of our investigation suggests that opposition does not appear to be a strong informative feature and this can possibly lead to a decrease in precision in the process of identifying contrast (i.e., many false positives are expected). Further and symmetrical work includes the classiﬁcation of the phenomena that can lead to contrast. 16Eng.: On Saturday, [departure at 7.01] and [arrival at 19.36.] 17Eng.: [.. the group has continued the work of corporate reorganization], [putting some order in the shareholdings that not tied to the core business of the single subsidiaries..]