Reference to Discourse Topics: Introducing “Global” Shell Nouns (2020)

Abstract: Shell nouns (SNs) are abstract nouns like “fact”, “issue”, and “decision”, which are capable of refer- ring to non-nominal antecedents, much like anaphoric pronouns. As an extension of classical anaphora resolution, the automatic detection of SNs alongside their respective antecedents has received a growing research interest in recent years but proved to be a challenging task. This paper critically examines the assumption prevalent in previous research that SNs are typically accompanied by a specific antecedent, arguing that SNs like “issue” and “decision” are frequently used to refer, not to specific antecedents, but to global discourse topics, in which case they are out of reach of previously proposed resolution strategies that are tailored to SNs with explicit antecedents. The contribution of this work is three-fold. First, the notion of global SNs is defined; second, their qualitative and quantitative impact on previous SN research is investigated; and third, implications for previous and future approaches to SN resolution are discussed.

## introduction traditionally, the primary concern of anaphora resolution (ar) has been the systematic identification of coreference between pronouns and nps whereas less prototypical forms of anaphoric relations such as bridging anaphora and reference to abstract objects still remain comparatively unexplored @xcite . one notable exception to this are shell nouns (sns; @xcite , abstract nouns such as fact, decision, or issue, whose idiosyncratic referring properties have received growing research attention in the last couple of years. the defining characteristic of sns lies in their capability to refer to abstract, proposition-like entities usually expressed by non-nominal syntactic constituents such as full sentences (example 1), that-clauses (example 2), or infinitive clauses (example 3). 1 (1) if the subject prefers to look at one stimulus rather than another we can assume that he has detected a difference between them. this idea had a great influence upon the thinking of schopenhauer who followed up its implications more thoroughly than did kant. the way sns are interpreted in unison with syntactic clauses from their context bears a striking resemblance to anaphora, and it is this observation that sparked efforts to approach sns from a computational perspective with the ultimate goal of automatically detecting sns alongside their respective antecedents. this task -termed sn resolution @xcite -is usually approached in the same way as traditional ar, i.e., the context of a sn instance is scanned for (non-nominal) antecedent candidates which are then passed to an ml-based ranking algorithm that determines the best match. however, the wide range of syntactic shapes sn antecedents can take on adds significantly to the complexity of the problem which, along with other factors, makes major simplifications necessary so as to make the task more feasible. part of the difficulty of quantitative approaches to sns has been attributed to the existence of so-called "long-distance antecedents" @xcite . on the grounds of examples like (4-5), kolhatkar argues that sns can occur far away from the antecedents they refer to, and accordingly, allows her system to obtain antecedent candidates from up to four sentences preceding the sn instance plus the sentence hosting the sn. long-distance antecedents do not only make the task of sn resolution significantly more complex while offering only a rather small potential benefit, they are also problematic from a theoretical perspective because they stand in contrast to research on pronominal anaphora that found non-nominal antecedents to be of low salience, and hence, to be accessible for subsequent anaphoric reference for only a limited amount of time. @xcite attributes the seemingly increased anaphoric range of sns to their richer semantics when compared to pronouns, arguing that the additional information conveyed by the nouns makes it possible for less recent antecedents to be identified. however, this does not seem to fully explain the remarkably long distances reported for some sn instances. for example, @xcite mentions a case where the antecedent occurs six sentences prior to the sn, and the longest distance i am aware of is an instance of the sn issue with an (albeit nominal) antecedent eleven sentences back in the data by @xcite foot_0 , which raises the question of whether such examples are adequately analyzed as anaphora. a hint at a possible answer to this question can be found in work by @xcite who found nps that refer to discourse topics (dts), i.e., the central entities the discourse is about, to be viable antecedents for subsequent pronominal reference for longer distances than would normally be expected, indicating that dts are highly accessible even if they have not been mentioned recently. aside from nps, dts can also be expressed by questions or propositions @xcite , and if it is possible for expressions as semantically unspecific as pronouns to refer to non-local antecedents as long as they correspond to dts, it seems likely that sns, which often carry a "topic-like" meaning (e.g., theory, idea, issue, question, and -obviously -topic), are suitable devices to refer to (propositional) dts. thus, the sns question and issue in examples (4) and ( 5 ) can be taken to refer to the topics their respective discourses are about. much unlike the obligatory antecedents of sns like in examples (1-3), dts are accessible for anaphoric reference regardless of whether or not they are explicitly realized in the discourse. even if an adequate description of the dt is present as is the case in examples (4-5), it appears that the marked spans of text lack the direct anaphoric relation present in examples (1-3). accordingly, we can draw a distinction between 1. proper antecedents that are locally available for immediate reference via a sn or pronoun, and 2. apparent antecedents that are merely descriptions of what the sn in question refers to -often, the dt -without standing in a direct anaphoric relation. as realizations of two distinct underlying processes, topic-referring sns (henceforth global sns) on the one hand, and sns with antecedents in their close vicinity (local sns) on the other, have different properties, which renders sn resolution strategies as previously proposed much less, if at all, adequate for the resolution of the former. the remainder of this paper will specify the details of the implications of this observation, starting with an overview of related work in sn annotation and resolution (section 2), followed by an examination of the (qualitative and quantitative) impact of global sns on the tasks of sn annotation and resolution (section 3). next, implications for previous and future research are discussed (section 4) and complemented by some concluding remarks (section 5). ## discussion the fact that the global/local distinction has not been acknowledged in previous work appears to be a major cause of misunderstanding of sns which led to flaws in annotation and resolution approaches. the majority of the criteria previously employed to resolve sns are only applicable to local sns, whereas the resolution of global sns is a novel, separate task that needs additional steps to be introduced into the resolution pipeline. this is particularly apparent in the factor of distance between the sn and its antecedent. although distance is very important for evaluating the accessibility of local antecedents, it is in principle irrelevant for the resolution of global sns, as spans of text describing the dt can occur virtually anywhere in the text or even be left implicit, which puts both resolution systems and human annotators in an awkward position: they are expected to look for an antecedent where there is none, resulting in a high uncertainty and variance across annotations. furthermore, it appears that the anaphoric range of sns has been overestimated as a result of topic references being analyzed as long-distance antecedents. the data from previous studies suggest that local sns will only rarely refer to an antecedent further away than one sentence, and any antecedent that occurs further away than that is either still accessible because of properties of the local discourse structure @xcite or it is not an antecedent in the sense of a reflection of an underlying anaphoric process. regarding annotation, it needs to be asked whether a classic annotation setting is appropriate for global sns. annotators will be biased to annotate something in the vicinity of the target noun even if the available context does not provide a stretch of text that perfectly matches their understanding of the noun. in order to avoid such bias, it might be preferable to approach sns by means of a more open question answering task, asking annotators to paraphrase what they think the target noun means in context, and subsequently comparing their answers with each other and with the source text. ideally, such a strategy would provide a more accurate picture of how sns are understood. the evaluation of free text answers is very difficult and not well researched, though, but it bears resemblance, e.g., to the evaluation of answers to reading comprehension tasks @xcite , which could be adapted to sn research. due to their semantics, some sns lend themselves more readily to a global reading than others. such tendencies are both domain-and text-specific, but it is fair to assume that the nouns issue, and, to a lesser extent, decision, are likely to refer to the overall topic in the domain of newspaper articles. the focus of previous work on instances of such nouns seems to have caused an over-representation of topic references in the data. this is further amplified by limiting the considerations to instances of nouns with the demonstrative determiner this, which is associated with a high accessibility of the antecedent @xcite . as shown by the data by @xcite , sns with an accompanying that tend to refer to closer antecedents (average @xmath0.84 sentences; @xmath1.13; @xmath2, indicating that the former might be correlated with local, and the latter with global sns. as nouns with a less topic-like meaning like fact proved to be much easier to annotate and resolve, sn resolution might be better off focusing on such nouns first, expanding the data to determiners other than this to prevent a bias towards topic references. assuming that topic referents compete with locally available, explicit antecedents, the factors that are helpful for the resolution of local sns are probably also helpful for identifying global sns, following the intuition that if there is no suitable antecedent nearby, the sn derives its information elsewhere. thus, what both human annotators and resolution systems are missing is the option to disregard the available local antecedents in favor of a discourse topic. as a tentative strategy to account for such an option, the set of antecedent candidates could be extended by one or more pseudo-antecedents representing the discourse topic(s). the candidate set would then consist of antecedents extracted from the close proximity of the sn and a number of topic descriptions, making for a considerably reduced search space and leaving human annotators and automatic resolvers free to choose among explicit antecedents and topics as equally viable options. future work will need to explore the details of such an approach, addressing questions such as how to extract topic information, how to generate antecedent candidates from topic(s), how to treat features such as distance and phrase type for such antecedents, etc. ## conclusion quantitative approaches to sns have proved difficult both in terms of annotation and automatic resolution, and part of this difficulty seems to be due to misconceptions about the interplay between sns and their context. the prevalent assumption that the occurrence of a sn "typically [emphasis added] involves a full-fledged clausal antecedent" @xcite seems to have caused more harm than good by raising wrong expectations about sn antecedents, which apparently caused references to dts to be mistaken for long-distance antecedents. the evidence from theoretical work on anaphoric reference to non-nominal antecedents, practical approaches to sns, and the annotation experiment outlined above, suggest that some sns, notably issue and decision -which much of previous work on sn resolution is based on -are frequently used to refer to dts that may or may not be explicitly present in the discourse. crucially, the detection of spans of text that match a topic description is an entirely different task than the resolution of local sns. future research will need to find ways to deal with global sns, topic antecedents, and other implicit sources of abstract referents, in order to make manual annotation and automatic resolution systems more reliable.