{ "paper_id": "W98-0113", "header": { "generated_with": "S2ORC 1.0.0", "date_generated": "2023-01-19T06:04:08.039670Z" }, "title": "Describing Discourse Semantics", "authors": [ { "first": "Claire", "middle": [], "last": "Gardent", "suffix": "", "affiliation": { "laboratory": "", "institution": "Computational Linguistics University of the Saarland Saarbr\u00fccken", "location": { "settlement": "Gerrnany" } }, "email": "claire@coli.uni-sb.de" }, { "first": "Bonnie", "middle": [], "last": "Webber", "suffix": "", "affiliation": { "laboratory": "", "institution": "University of Pennsylvania", "location": { "postCode": "19104-6389", "settlement": "Philadelphia", "region": "PA", "country": "USA" } }, "email": "bonnie@central.cis.upenn.edu" } ], "year": "", "venue": null, "identifiers": {}, "abstract": "", "pdf_parse": { "paper_id": "W98-0113", "_pdf_hash": "", "abstract": [], "body_text": [ { "text": "In recent years, both formal and computational linguistics have been exploiting descriptions of structures where previously the structures themselves were used.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "Tue practice started with (Marcus et al., 1983) , who demonstrated the value of (syntactic) tree descriptions for near-deterministic incremental parsing. Vijay-Shankar (Vijay-Shankar and Joshi, 1988; Vijay-Shankar, 1992 ) used descriptions to maintain the monotonicity of syntactic derivations in the framework ofFeature-Based Tree Adjoining Grammar. In semantics, both (Muskens, 1997) and (Egg et al., 1997) have shown the value of descriptions as an underspecified representation of scope ambiguities.", "cite_spans": [ { "start": 26, "end": 47, "text": "(Marcus et al., 1983)", "ref_id": "BIBREF9" }, { "start": 154, "end": 199, "text": "Vijay-Shankar (Vijay-Shankar and Joshi, 1988;", "ref_id": "BIBREF11" }, { "start": 200, "end": 219, "text": "Vijay-Shankar, 1992", "ref_id": "BIBREF12" }, { "start": 370, "end": 385, "text": "(Muskens, 1997)", "ref_id": "BIBREF10" }, { "start": 390, "end": 408, "text": "(Egg et al., 1997)", "ref_id": null } ], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "Tue current paper further extends the use of descriptions, from individual sentences to discourse, showing their benefit for incremental, near-detenninistic discourse processing. In particular, we show that using descriptions to desc~be the semantic representation of discourse penn1ts:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "(1) a monotone treatment of local ambiguity; (2) a detenninistic treatment of global ambiguity; and (3) a distinction to be made between \"simple\" local ambiguity and \"garden-path\" local ambiguity.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "Discourse descriptions. Suppose we have the discourse:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "(1) a. Jon. and Mary only go to the cinema b. when an Islandic film is playing On hearing the second sentence, the hearer infers a CONDITIONAL relation (CDN) to hold between the event partially specified in (la) and the event partially specified in (1 b ). We associate this with the following description of structure and semantics:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "(2) cdn(A,B) ~ A B 1 1 1 a b", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "The dashed lines indicate domination, the plain lines immediate domination. Labels on the nodes are first-order terms abbreviating their associated semantic infonnation. Capital letters indicate variables, lower letters indicate constants, and shared variables indicate re-entrancy. Whenever two node descriptions are identified and taken to refer to the same node, their labels must unify.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "Tue description licenses a local tree whose root semantics is CDN(A,B), where A and B are the semantics of nodes dominating the nodes whose semantics is a and b, respectively. Intuitively, A and B represent the final arguments of the CON-DITIONA L relation, whereas a and b stand for its current arguments. 1 Formally, A/a and B/b nodes are quasi-nodes in the sense of Vijay-Shankar: they are related by dominance and therefore can (but need not) be identical. Local\u2022 ambiguity. As (Marcus et aI.; 1983) has noted, descriptions facilitate a near-deterministic treatment of local attachment ambiguities in incremental parsing. This is also true at the discou!\"Se level. For instance ( 1) can be continued in two ways: additional discourse material can \"close off\" the scope of the relation (3) a. so they rarely go.", "cite_spans": [ { "start": 482, "end": 497, "text": "(Marcus et aI.;", "ref_id": null }, { "start": 498, "end": 503, "text": "1983)", "ref_id": "BIBREF9" } ], "ref_spans": [], "eq_spans": [], "section": "", "sec_num": null }, { "text": "Ioescriptions can be formulated more precisely. using tree logic (Vijay-Shankar, 1992) . For this paper howevcr, we will use a graphic presentation, as in (2) above, which is easier to rcad than conjunclions of logical formulae. or it can extend it:", "cite_spans": [ { "start": 65, "end": 86, "text": "(Vijay-Shankar, 1992)", "ref_id": "BIBREF12" } ], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "(4) a. or the film got a good review in The Nation. b. Semantics: cdn (a,or(b,c)) By using descriptions of trees rather than the trees themselves, we have a representation which is compatible with both continuations: In the first case (continuation 3), addition of the third clause will Iead the hearer to infer a CAUSE relation to hold between 1and 3. This extends the description in (2) to: 1is continued with (4), the initial description is expanded to:", "cite_spans": [ { "start": 70, "end": 81, "text": "(a,or(b,c))", "ref_id": null } ], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "(5) cause(C,Dn ------------ Ds - 1 1 cdn(A,8)3 CtJ ------------ By contrast, if", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "where OR stands for disjunction. Both descriptions are compatible with the initial description (2) and both descriptions can be further constrained to yield the appropriate discourse semantics. Suppose that no further material is added: now the scope of the discourse relation becomes known. This in turn licenses node identifications which conflate final and current arguments. So if the discourse ends in (4), then node 6 is identified with node 5, fixing to b the left-hand argument of the OR relation. Similarly, nodes 8 and 9 can be identified, thereby fixing the right-hand argument to c'. Given these additionai constraints, the minimai tree structure which satisfies the resulting description is:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "cdn(n.or(b,c))3 --------------- a1.1 or(b,c)4,7 --------------", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "In summary, dominance permits underspecifying the syntactic link between nodes, while semantically, quasi-nodes permits underspecifying the arguments of discourse relations. In both cases, monotonicity is preserved by manipulating descriptions of trees rather than the trees themselves.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "GlobaJ ambiguity Discourse exhibits global scope ambiguities in much the same way sentences do: 6a. I try to read a novel b. if I feel bored c. or 1 am unhappy.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "This discourse means either that the speaker tries to read a novel under one of two conditions (boredom or unhappiness), or that the speaker is unhappy if s/he can 't read a novel when bored. Discourse-level scope ambiguities can be captured as in (Muskens, 1997) by leaving the structural relations holding between scope bearing elements underspecified. For example, the (ambiguous) structure and semantics of (6) can be captured in the description:", "cite_spans": [ { "start": 248, "end": 263, "text": "(Muskens, 1997)", "ref_id": "BIBREF10" } ], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "A if\u00df3 ~ A1 f\u00fc 1 ...", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "b. Semantics: cause(cdn(a,b),c)", "sec_num": null }, { "text": "In the absence of additional information (i.e. when the respective scope of the discourse relations remains unspecified), no additional constraints come into play, so that not one but two trees satisfy the description: one with root semantics a if (b or c) and the other with root semantics ( a if b) or c.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "Defaults, underspecification aml preferences. We assume that a cognitive model of incremental discourse processing should distinguish between those cases of \"simple\" local ambiguity which do not trigger repair when they are resolved by information later in the discourse and those cases of \"garden path\" ambiguity that do.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "No\\'.' there is a ccntinuum of ways to dea! \"economically\" with local ambiguity, without generating all the possible readings. At one end is a pure default approach, commiting to one reading and discarding the others. At the other end is a pure underspecifzcation approach, with a single compact representation of all possible readings but no indication of the reading of the text so far.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "Neither of these \"pure\" approaches suffices to distinguish simple local ambiguity from garden path ambiguity in either sentence-level processing or discourse. While defaults can be subsequently overridden, there is no difference between overriding a simple local ambiguity and overriding a garden path. On the other band, underspecification, which does not \"commit\" to any specific choice, provides no indication of the reading of the text so far and thus again, no way of distinguishing simple local ambiguity from cases where the reader garden-paths.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "However, in between these poJls are approaches that combine features of both. One that seems able to meet the cognitive criteria given above is an approach that combines underspecification with a preference system that highlights a specific reading corresponding to the hearer's currently preferred interpretation.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "Such a proposal was suggested in (Marcus et al\" 1983) , and is the one we are currently exploring for discourse. The two aspects of the approach we want to discuss here are: (1) partial underspecification, and (2) biases in choosing a preferred reading.", "cite_spans": [ { "start": 33, "end": 53, "text": "(Marcus et al\" 1983)", "ref_id": "BIBREF9" } ], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "Partial Underspecification. Tue degree of underspecification in a description is usually only partial: there is always something that it commits to. For example, while underspecifying domination, the structural descriptions used above still rigidly distinguish each branch of a tree from its sisters. Similarly, while allowing underspecification in each individual argument to a predicate, the descriptions used here still rigidly distinguish one argument from another. We take this to be a \"feature\" with respect to making a cognitive distinction between simple local ambiguity and local ambiguity that leads to garden paths.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "In particular, we associate simple local ambiguity with domination underspecification, whether it be at the sentence-level or in discourse: the local ambiguity associated with \"my aunt\" after processing \"1 saw my aunt ... \" -whether it continues is purely a matter of how the domination relation eventually resolves itself.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "On the other hand, the ambiguity associated with \"raced\" after processing \"Tue horse raced .\"\"whether it continues 52 (8) a. The horse raced past the bam. b. The horse raced past the bam fell -is a matter of choosing whether \"raced\" takes \"the horse\" as its argument or whether it acts as a modifier of \"the horse\" (in distinguisl.ing this horse from other ones, cf. (Crain and Steedman, 1985) ). This ambiguity cannot be captured by domination underspecification. As such, it can only be represented as a (disjuctive) alternative, a matter of non-deterministic or preferential choice.", "cite_spans": [ { "start": 367, "end": 393, "text": "(Crain and Steedman, 1985)", "ref_id": "BIBREF2" } ], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "If the choice is incorrect, revision is required, thus providing a way of making the desired distinction between simple and garden-path Iocal ambiguity.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "Biases in choosing a preferred reading. In any abductive process, there are many ways of explaining the given data, and biases are used to identify one that is preferred. For example, in plan recognition (identifying the structure of goals and subgoals that give rise to what is usually taken to be a sequence of observed actions), Kautz (Kautz, 1990) suggested a \"goal minimization\" bias that preferred a tree with the fewest goals (non-terminal nodes) able to \"explain\" the sequence of actions. Where goal minimization is known to produce the wrong explanation, some other bias is needed to yield the one that is preferred (Gertner and Webber, I 996) . Similarly, in associating a preferred reading with a compact underspecified representation, (Marcus et al., 1983) proposed a bias towards a tree that minimised the dominance relation . That is, if two node names stand in a dominance relation, they are taken to refer to one and the same node, provided nothing rules it out. Ofcourse, such a \"min.dom\" bias might yield several trees, each of which are equally minimal. Typically, this is true of global ambiguities as in (6) above, where dominance can be minimised by identifying node 5 either with node 4 or with node 6, each .move resulting in an equally minimal tree.", "cite_spans": [ { "start": 338, "end": 351, "text": "(Kautz, 1990)", "ref_id": "BIBREF7" }, { "start": 625, "end": 645, "text": "(Gertner and Webber,", "ref_id": null }, { "start": 646, "end": 652, "text": "I 996)", "ref_id": null }, { "start": 747, "end": 768, "text": "(Marcus et al., 1983)", "ref_id": "BIBREF9" } ], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "An alternative bias combines \"min.dom\" with \"right-association\" (Frazier, 1995; Chen and Vijay-Shankar, 1997) , yielding a preference for a structure in which the incoming unit attaches \"low in the tree\" and c\u2022m be obtained by minimising the most recent dominance link. In example 6, this means identifying node 6 with node 5 first so that the default reading in this case is the reading where if scopes over or.", "cite_spans": [ { "start": 64, "end": 79, "text": "(Frazier, 1995;", "ref_id": "BIBREF4" }, { "start": 80, "end": 109, "text": "Chen and Vijay-Shankar, 1997)", "ref_id": "BIBREF1" } ], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "Other biases are possible: Crain and Steedman (Crain and Steedman, 1985) argue for a preference for referring forms that distinguish one already-evoked (discourse) entity from possible alternatives. We believe it is worth exploring what bias best models the preferences people have in discourse interpretation, and how it resembles their preference at the sentence level.", "cite_spans": [ { "start": 46, "end": 72, "text": "(Crain and Steedman, 1985)", "ref_id": "BIBREF2" } ], "ref_spans": [], "eq_spans": [], "section": "DJ bs", "sec_num": "1" }, { "text": "A related approach to discourse structure and semantics is presented in (Webber and Joshi, 1998) , where Lexicalised Tree Adjoining Grammar (LTAG) is used to construct the compositional semantics of discourse. Although the basic structures used here are different, we foresee no difficulty in modifiying them in order to integrate the additional information included in the LTAG discourse trees. Essentially, the atomic labels representing the relations should be mapped into the feature structures used in (Webber and Joshi, 1998) and this information used to labe! not the root node of a local tree but its anchor. Second, the LTAG approach has focussed on describing the compositional semantics of discourse -that is, the semantics explicitely given by the text (as opposed to what can be inferred). In contrast, the present approach does not differentiale between compositional and inferred semantics, though again, the difference does not seem essential as the description based approach could be either extended to explicitely distinguish (e.g. by means of features) between compositional and inferential information, or restricted to describe those aspects of discourse semantics that are compositional. Third, the LTAG proposal does not address the focus of the current approach -incrementality and underspecification. On the whole then, the two systems are complementary rather than antagonistic.", "cite_spans": [ { "start": 72, "end": 96, "text": "(Webber and Joshi, 1998)", "ref_id": "BIBREF13" }, { "start": 507, "end": 531, "text": "(Webber and Joshi, 1998)", "ref_id": "BIBREF13" } ], "ref_spans": [], "eq_spans": [], "section": "Comparison with related work.", "sec_num": null }, { "text": "We have argued that a technique developed to handle well-known problems in sentence processing can also benefit the processing of monologic discourse. First, it provides a welldefined framework for monotonically describing the incremental construction of discourse semantics. This departs from approaches in the discourse literature which give up either monotonicity (Asher, 1993) or incrementality (Hob90; MT87). Second, it has a well-understood formal basis in tree logic. Third, it permits a clear-cut distinction between local ambiguities that lead the hearer down the garden path and those that don 't.", "cite_spans": [ { "start": 367, "end": 380, "text": "(Asher, 1993)", "ref_id": "BIBREF0" } ], "ref_spans": [], "eq_spans": [], "section": "Conclusion.", "sec_num": null } ], "back_matter": [ { "text": "Acknowledgements. Tue authors would Iike to 53 thank Mark Steedman and Aravind Joshi for comments and suggestions. An early draft of this paper was presented at the Workshop on Underspecification, Bad Teinach, Germany, May 1998. Claire Gardent is grateful to the Deutsche Forschungsgesellschaft for financial support within the SFB 378, C2.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "acknowledgement", "sec_num": null } ], "bib_entries": { "BIBREF0": { "ref_id": "b0", "title": "Reference to abstract objects in discourse", "authors": [ { "first": "N", "middle": [], "last": "Asher", "suffix": "" } ], "year": 1993, "venue": "", "volume": "", "issue": "", "pages": "", "other_ids": {}, "num": null, "urls": [], "raw_text": "N. Asher. 1993. Reference to abstract objects in dis- course. Kluwer, Dordrecht.", "links": null }, "BIBREF1": { "ref_id": "b1", "title": "Towards a Reduced Commitment, D-Theory Style TAG Parser Proc. 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