Exploring the Dissociated Nucleus Phenomenon in Semantic Role Labeling
Authors: Tommaso Bonomo, Simone Conia, Roberto Navigli (Sapienza Università di Roma, 2024)
Topic: Syntax and Dependency Treebanks

Abstract
Dependency-based Semantic Role Labeling (SRL) is bound to dependency parsing, as the arguments of a predicate are identified through the token that heads the dependency relation subtree of the argument span. However, dependency-based SRL corpora are susceptible to the dissociated nucleus problem: when a subclause's semantic and structural cores are two separate words, the dependency tree chooses the structural token as the head of the subtree, coercing the SRL annotation into making the same choice. This leads to undesirable consequences: when directly using the output of a dependency-based SRL method in downstream tasks it is useful to work with the token representing the semantic core of a subclause, not the structural core. In this paper, we carry out a linguistically-driven investigation on the dissociated nucleus problem in dependency-based SRL and propose a novel algorithm that aligns predicate-argument structures to the syntactic structures from Universal Dependencies to select the semantic core of an argument. Our analysis shows that dissociated nuclei appear more often than one might expect, and that our novel algorithm greatly increases the richness of the semantic information in dependency-based SRL. We release the software to reproduce our experiments at .

Introduction
Within the field of Natural Language Processing, Semantic Role Labeling \citep[SRL]{gildea2002} is aimed at recognizing the semantic information conveyed by a sentence, more specifically identifying who did what to whom, when, where and how AUTHOR. Over the years, SRL has split into two main annotation formalisms, namely, span-based and dependency-based. The key difference between the two lies in how they identify the roles of a predicate: span-based SRL directly extracts a span of the input text as the argument of a predicate, whilst dependency-based SRL identifies the word that heads the syntactic dependency relation subtree corresponding to the argument span as the argument. Using dependency-based SRL can be beneficial in real-world settings, as i) dependency-based SRL parsers have achieved better results on standard benchmarks, and ii) the identified token can be directly utilized in several downstream tasks, including Coreference Resolution AUTHOR, Opinion Role Labeling AUTHOR, Argument Mining AUTHOR, and Concept Map Mining AUTHOR, among others. However, the use of role tokens in the above tasks requires them to carry the ``semantic meaning'' of the role. This requirement is often not fulfilled when examining both the output of state-of-the-art dependency-based SRL systems and the corpora they were trained on, such as CoNLL-2009 AUTHOR. In these annotations, it is not uncommon to have an adpositional clause serving as the head word of a semantic role, even though adpositions do not represent the semantic core of that role. In linguistics, this phenomenon is referred to as an instance of dissociated nucleus \citep[ch.~23]{tesniere-etal-2015-elements}. Although this term encompasses many different syntactical constructions, here we focus on adpositional clauses present in the CoNLL-2009 dataset, across all of its languages. In this paper, we carry out a concise, linguistically-driven investigation on dissociated nuclei in dependency-based SRL, uncovering the extent of this problem and how it affects the semantic aspect of this task. In addtion, we introduce SemDepAlign, a simple yet effective algorithm capable of mitigating this phenomenon significantly by aligning predicate-argument structures in SRL with syntactic parses from the Universal Dependencies project, which addresses the dissociated nucleus phenomenon directly in the dependency structures. Applying SemDepAlign to CoNLL-2009 results in a substantial increase in the semantic variety of role tokens, measured through a set of proxy metrics. Finally, we provide a glimpse at how addressing dissociated nuclei simplifies the alignment between Semantic Role Labeling and Semantic Parsing, specifically with Abstract Meaning Representation~\citep[AMR]{banarescu-etal-2013-abstract}. We release SemDepAlign and Aligned-CoNLL09 -- the result of applying SemDepAlign to CoNLL-2009 -- in the hope that our work can encourage a deeper focus on semantics in SRL and foster future integration of this task into downstream applications.

Conclusion
In this paper, we conducted an in-depth investigation on the dissociated nucleus issue in dependency-based SRL. We introduced SemDepAlign, a novel method to align predicate-argument structures in SRL with syntactic parses from the Universal Dependencies project, which addresses the dissociated nucleus phenomenon. Our analyses and experiments in SRL modeling demonstrate that our approach to dissociated nuclei brings more semantic richness whilst remaining competitive on standard benchmarks.