# From Discourse Relations to Network Edges: A Network Theory Approach to Discourse Analysis
## 2 Discourse Relations and Discourse Units
### Elementary Event Units  
The third kind of node admitted in a discourse representation graph with equal status to EDUs is the EEU. EEUs represent nonlinguistic events in the discourse, especially in the context of situated dialogues, with little to no descriptive content. By nonlinguistic events, in the context of situated dialogues, we mean the different moves in the recorded game. For instance, \(\pi_{2}\) and \(\pi_{3}\) in ref. [28] refer to two moves by the server and the UI that represent two nonlinguistic events. On the other hand, EDUs represent linguistically uttered events, namely events that have been explicit realizations, e.g., through a verb or an event nominal. EEUs can be either related with other EEUs or with other EDUs, opening the window for relating linguistic with nonlinguistic events.  
Although the use of nonlinguistic events in discourse is pervasive, for good reasons, pragmatically driven theories of discourse have focused mainly on the way that reference to nonlinguistic events is realized through the use of indexical and demonstrative expressions, ignoring the complex ways that nonlinguistic events affect discourse structure. Contrary to linguistic events, due to EEUs' lack of propositional content, it is challenging for the interpreters to individuate and conceptualize them. Different interpreters may assign different content to a nonlinguistic event. However, speakers engage in situated dialogues on a daily basis, and they fully integrate nonlinguistic events throughout discourse inference and interpretation. Allowing EEUs to interact with EDUs as well as with CDUs opens up a way to enrich the discourse graph and study in depth the interaction between linguistic and nonlinguistic events. Refs. [28; 37] offer a formal model of situated dialogue to capture these effects and the study of STAC. The first discourse-annotated corpus that includes EEUs by [29] has already pointed out interesting ways that discourse is affected by the presence of EEUs. For instance, Figure 4, which displays the discourse graph of (4), taken from [29], shows the direct relation between the CDUs. The labels \(\pi_{1}\), \(\pi_{2}\) and  
Figure 3: The discourse graph of (3).  
represent three EEUs generated during an online game session that was recorded during the compilation of the STAC corpus (for more on the description and the basic features of the STAC corpus, go to Section 3.1). \(\pi_{4}\) is clearly related to \(\pi_{3}\) with commentary, and the graph encloses a rich repository of linguistic and nonlinguistic information that explains in a more complete fashion the current discourse.  
Additionally, \(\pi_{2}\) and \(\pi_{3}\) are subconstituents of \(\pi^{\prime}\). As [29] points out, CDUs that have EEUs as their subconstituents, such as \(\pi^{\prime}\), serve very often as arguments to discourse relations and are essential in a situated discourse, since speakers frequently refer back to grouped nonlinguistic events in various ways.  
Lastly, according to [28], EEUs differ from EDUs in terms of their semantics, due to the fact that they always denote actual events, and their order matches the order of the events they describe. However, although one would expect that EEUs always describe a sequence of events, there is a wide range of relations between EEUs and between EEUs and EDUs. Ref. [29] describes in detail the frequency distribution of the discourse relations that EEUs are assigned as their arguments, and they include _Result_, _Elaboration_, _Correction_, _Background_, _Sequence_, _Continuation_ and _Question-Answer Pair_.