Semantic role annotation of instrument subjects
Authors: Rossella Varvara, Elisabetta Jezek (Università di Trento, Università di Pavia, 2014)
Topic: Lexical and Semantic Resources and Analysis

Abstract
Semantic role annotation has be-come widely used in NLP and lexical re-source implementation. Even if attempts of standardization are being developed, discordance points are still present. In this paper we consider a problematic semantic role, the Instrument role, which presents differences in definition and causes prob-lems of attribution. Particularly, it is not clear whether to assign this role to inani-mate entities occurring as subjects or not. This problem is especially relevant 1- be-cause of its treatment in practical annota-tion and semantic role labeling, 2- because it affects the whole definition of seman-tic roles. We propose arguments to sustain that inanimate nouns denoting instruments in subject positions are not instantiations of Instrument role, but are Cause, Agent or Theme. Ambiguities in the annotation of these cases are due to confusion between semantic roles and ontological types.

Introduction
Semantically annotated resources have become widely used and requested in the field of Natural Language Processing, growing as a productive research area. This trend can be confirmed by looking at the repeated attempts in the implementation of annotated resources (FrameNet, VerbNet, Propbank, SALSA, LIRICS, SensoComune) and in the task of automatic Semantic Role Labeling (Gildea and Jurafsky 2002, Surdeanu et al. 2007, M`arquez et al. 2008, Lang and Lapata 2010, Titov and Klementiev 2012 among others). Since their first introduction by Fillmore (1967), semantic roles have been described and defined in many different ways, with different sets and different level of granularity - from macro-roles (Dowty 1991) to frame-specific ones (Fillmore et al. 2002). In order to reach a common standard of number and definition, the LIRICS (Linguistic Infrastructure for Interoperable ResourCes and Systems) project has recently evaluated several approaches for semantic role annotation and proposed an ISO (International Organization for Standardization) ratified standard that enables the exchange and reuse of (multilingual) language resources. In this paper we examine some problematic issues in semantic role attribution. We will highlight a case, the Instrument role, whose definition and designation should be, in our opinion, reconsidered. The topic is particularly relevant since there is difference in its treatment in different lexical resources and since the theoretical debate is still lively. Moreover, this matter highlights aspects of the nature of semantic roles, relevant both for their theoretical definition and for practical annotation, such as the difference between semantic roles and ontological types. The former refer to the role of participants in the particular event described by the linguistic utterance, the latter to the inherent properties of the entity. We argue that this is a main point in the annotation, because, even in the recent past, roles have been frequently tagged according to the internal properties of the entities involved, not, as it should be, because of their role in the particular event described. This analysis arose from the first step of the implementation of the Senso Comune resource (Vetere et al. 2012). With the aim to provide it with semantic roles, a first annotation experiment was conducted to check the reliability of the set and the annotation procedure (Jeˇzek et al. 2014). The dataset was composed of 66 examples without disambiguation, 3 for 22 target verbs, and it was annotated for semantic roles by 8 annotators. They were instructed with a guideline in which a set of 24 coarse-grained roles was defined, with examples and a taxonomy. During the evaluation process, the major cases of disagreement were highlighted. The present study is based on the evidence coming from these data: the Instrument role caused several misunderstandings (see also Varvara 2013). Nevertheless, our analysis will look primarily at examples from literature and other resources in order to rethink this role and to reach a standardization. We propose to consider what are called instrument subjects (Alexiadou and Sch¨afer 2006) as instances of three different roles (Cause, Agent and Theme) rather than as Instrument.

Conclusion
In this paper we have shown how theoretical and data analysis can be mutually improved by each other. Literature has offered critical discussion about the Instrument role and the case of instru-ment subjects, a discussion that can be useful for the definition and annotation of semantic roles in the implementation of lexical resources. More-over, analysis of annotated data can reveal fal-lacies in the reliability of the set, coming back from application to theoretical topics. At last, our study highlights the importance of distinguishing between semantic roles - relational notions be-longing to the level of linguistic representation -and ontological types, which refer to internal qual-ities of real-world entities. We believe that this topic, because of its importance, should be taken into consideration for a more complete treatment in future work.