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Recognition and extraction of definitional contexts in Spanish for sketching a lexical network

In this paper we propose a method to exploit analytical definitions extracted from Spanish corpora, in order to build a lexical network based on the hyponymy/hyperonymy, part/whole and attribution relations. Our method considers the following steps: (a) the recognition and extraction of definitional contexts from specialized documents, (b) the identification of analytical definitions on these definitional contexts, using verbal predications, (c) the syntactic and probabilistic analysis of the association observed between verbal predication and analytical definitions, (d) the identification of the hyponymy/hyperonymy, part/whole and attribution relations based on the lexical information that lies between predications and definitions and other types of phrases, in particular prepositional phrases mapped by the preposition de (Eng. of/from).


Introduction
Nowadays, the possibility of searching and recognizing lexical relations in definitions occurring in specialized text corpora is an important task in computational lexicography and terminology.
In this sense, authors such as Vossen and Copestake (1993), as well as Wilks, Slator & Guthrie (1995) are pioneers in offering a relevant set of experiments and techniques about how to identify hyponymy/hypernymy relations from analytical definitions, taking into account the underlying association that exists between terms and genus terms.
Complementary to these first attempts for identifying such lexical relations, Riloff & Shepherd (2004) argue that while these efforts have been ori-ented to extract lexical relations from corpus of general language, it is necessary to focus on domain-specific corpora, in order to obtain a specialized knowledge that is required for in-depth understanding of the subject matter.
In line with the argument formuled by Riloff & Shepherd, Buiteelar, Cimiano & Magnini (2005) have proposed several methods for building ontologies from text corpora, priorizing the automatic recognition of syntactic patterns that codify hyponymy/hyperonymy relations.
Following all these authors, we sketch here a research project to design a lexical network, focused on classifying scientific and technical concepts extracted from Spanish text corpora. In particular, we obtain these concepts by extracting definitional contexts (DCs) with terms and definitions clearly formulated, according to the theoretical framework developed by Sierra, Alarcon & Aguilar (2006).
After extracted these DCs, we propose a method to identify lexical relations between terms inserted into the DCs. The method considers, on the one hand, a grammatical analysis for detecting syntactic patterns that represent term and genus term, bearing in mind their association through lexical relations such as hyponymy/hyperonymy, part/ whole or attribution relations. On the other hand, we proposed a semi-automatic evaluation to determine the degree of accuracy respect to the results obtained by our method.
The issues that we will deal in this paper are organized as follows: (a) as a starting point, we expose briefly the theoretical framework to extract DCs from Spanish corpora. (b) According to this framework, we describe how analytical definitions linked to terms can be identified, considering the identification of verbal predications that function as connectors between such definitions and terms. (c) Thus, we offer a probabilistic evaluation for determining the degree of association between predications and analytical definitions. (d) After this evaluation, we sketch a method for exploiting this association between predications and definitions, in order to identify lexical relations, specifically hyponymy/hyperonymy, part/whole and attribution relations.

Theoretical framework: DC extraction
We situate our analysis within the framework of definitional contexts (or DCs) extraction. According to Sierra et al. (2008), a DC is a discursive structure that contains relevant information to define a term. DCs have at least two constituents: a term and a definition, and usually linguistic or metalinguistic forms, such as verbal phrases, typographical markers and/or pragmatic patterns. An example is: (1) La cuchilla fusible [Term] se define como [Verbal Phrase] un elemento de conexión y desconexión de circuitos eléctricos [Definition] . (Engl. The fuse-switch disconnector is defined as an element of connection and disconnection of electric circuits).
In (1), the term cuchilla flexible is emphasized by the use of bold font, and it appears linked with the verbal predication se define como, and the definition un elemento de conexión y desconexión de circuitos eléctricos. Following to Sierra et al. (2008), we consider the term, the verbal phrase and the definition as the three main units constituting the syntactic structure of a DC.
This kind of syntactic structure introduces an analytical definition (in the Aristotle's sense), where the genus term is represented by a noun phrase (NP) un elemento and the differentia is represented by a prepositional phrase (PP) de conexión y desconexión de circuitos eléctricos.
In a detailed analysis on these syntactic structures, Aguilar (2009) explains that these structures are predicate phrases (PrP), according to the description proposed by Bowers (1993Bowers ( , 2001. A PrP is a phrase mapped by a functional head, and its grammatical behavior is similar to other functional phrases such as Inflexional Phrase (IP) or Complement Phrase (CP). A graphical tree representation of a PrP is: Figure 1: Tree representation for PrP, according to Bowers (1993: 596) The Figure 1 describes the syntactic configuration of a PrP. We recognise a functional head with the feature +/predicative (Pr). This head maps two subjects: a primary subject in the Specifier position of PrP (represented for a NP); and a secondary subject, in the Specifier position of verbal phrase or VP (often a NP). Finally, both subjects, the VP and the PrP are linked to one or several complements, which assume phrasal representations (e.g.: NP, IP, CP, and other types of phrases).
Based on this description about PrP, Sierra et al. (2008) and Aguilar (2009) Bowers (1993). Our analysis considers both types of predications as regular patterns that codify syntactically sequences of terms, verbal predications and definitions.

Searching analytical definitions in text corpora
We have adapted the predicative patterns deduced from our syntactic analysis, in order to search and find (semi-)automatically analytical definitions linked to these patterns. So, we conducted an experiment of identification of these definitions in two text corpora: • Linguistic Corpus on Engineering (or CLI). The CLI, prepared by Medina and others (2004), is a collection of technical documents in different thematic areas of engineering, with an extension of 500,000 words, approximately.
• Corpus on Informatics for Spanish (or CIE). This corpus was built under the supervision of L' Homme and Drouin (2006). The CIE compiles several documents related to computer science and informatics. For our experiment we took a portion of CIE, which contains articles extracted from Wikipedia. This portion has an extension of 500,000 words.
Following to Aguilar et al. (2004) and Sierra et al. (2008), we selected a set of verbs that function as heads of predicative patterns in Spanish, taking into account the distinction between primary and secondary predications.
In the case of primary predication, the analytical definition is integrated in a sequence Term + Verbal Predication + Definition. This definition does not refer to possible author(s) of a definition. An example is: Having in mind this sequence, we propose a grammatical description model for this relation: The verbs that operate as head of these predications are: referir (to refer to), representar (to represent), ser (to be) and significar (to signify/to mean). In contrast, when a secondary predication introduces an analytical definition, this predication follows the sequence Author + Term + Verbal Predication + Definition, where the Author is equivalent to the primary subject, the Term assumes the position of secondary subject, and the definition is introduced after the Verbal Predication. In this case, the adverbial particle como (Eng. as/like), or the preposition por (Eng. for/by) indicates the place of the definition. An example is: Thus, the formal description of this sequence is: In order to recognize these sequences of predications and analytical definitions, we employed a system developed in Python by Rodríguez (2004). Broadly speaking, the input for this system is a set of previously delimited text fragments. The output is a XML table with a list of patterns, the verb used for searching these patterns, and the frequency of use in both corpora.

Results
Once we accomplished the process of searching and extracting of fragments with sequences of predication patterns of analytical definitions, we determined values of precision and recall for the CLI and CIE corpora based on the real number of analytical DCs in the corpus. This data was determined by a human expert through an exploration in the corpora mentioned above. In table 3 we showed DC candidates, as well as the real number of true DCs extracted from these candidates.
Thus, from CLI corpora we obtained a total of 1686 candidates. From these candidates, the human expert recognized a set of 111 true DCs to analytical definition linked to primary predication patterns. Our recall was 100% because we obtained all of the DCs with analytical definitions, but the precision achieved was very low (6.6%).
The main cause about this low precision is due to the verb ser (Eng. to be). The verb ser is highly productive in Spanish, however, much of the fragments found are not analytical definitions. In contrast, from secondary predication patterns, our recall was 100% and precision 9.4%. Thus, the CIE corpora showed measures of precision and recall higher than those of CLI corpora because most of documents were extracted from resources as Wikipedia. We suppose this factor is related with a definition scheme more canonical in scientific and technical documents. We derived a frequency distribution of the verbs with type of predication for CLI and CIE corpora. The table 4 shows the relative frequency of use of each verb explored. Most of these verbs do not have been considered in automatic extraction tasks of hyponymy-hyperonymy relations, e. g.: Hearst (1992) or Wilks, Slator & Guthrie (1995). Once established this distribution, we have analyzed the degree of assurance to find a good candidate for analytical definitions. We have applied a method of conditional probabilities for primary and secondary predications. Our conditional probabilities are formulated by the hypothesis that the probability (P) of co-occurrence of predications (Pred) linked to analytical definition (AD) is high. Thus, we apply the following formula of conditional probability: P(AD ∩ Pred)

P(AD|Pred) = P(Pred)
Taking into account the formula mentioned above, we obtained the following results: Therefore, we considered that the possibility to identify a good candidate of analytical definition is high, insofar as we took into account their relationship with primary and secondary predications. In addition, Alarcón, Bach & Sierra (2007), propose a methodology for filtering true DCs from a set of candidates to DCs. An important advance provided for this work is the application of a filter phase that discards those syntactic patterns without true analytical definitions. For example, if we find a particle as no (Eng. not) or tampoco (Eng. either) in the first position before or after of a predication, there is a high probability these pattern do not introduce a good analytical definition. In Table 5 we showed some results in terms of precision and recall reported by authors only for analytical definition patterns.

Sketching a method
In this section, we propose a method for recognizing lexical relations from the previous extraction of DCs. In particular, we assume that a good way to reach these relations is to improve the syntactic association observed between predications and analytical definitions inserted into these DCs. This assumption is in line with the methodology proposed by Buitelaar, Cimiano & Magnini (2005) for building ontologies based on textual information obtained from corpora. These three authors conceive a chain of processes and sub-processes, represented with a layer cake scheme: Figure 2: Ontology learning layer cake (according to Buitelaar, Cimiano & Magnani 2005) Briefly, in this scheme Buitelaar, Cimiano & Magnini establish a sequence of 6 basic tasks for developing a possible ontology. Thus, the first task is the identification of a set of specific terms to a certain knowledge domain (in this case, a medical domain). After that, it is necessary to identify synonyms related to these terms (e.g., disease/illness). Given both sets of terms and synonyms, the following task is to determine concepts in a formal way. For delineating these concepts, in the next task are deduced lexical relations following lexical networks formulated by WordNet (Fellbaum 1998).
Once these lexical relations are established, the semantic relations are proposed, keeping this in mind, for example, first-order logic to represent predicate-arguments structures. The final process of this chain is to derive universal rules for building concepts, joining lexical and semantic relations deduced previously.
Thus, the recognition and extraction of concepts is a step towards the general goal proposed by Buitelaar, Cimiano & Magnini for building ontologies. For this particular phase, our proposal consists on identifying and extracting conceptual information through lexical-syntactic patterns as we mentioned above.

Towards the (semi-)automatic identification of lexical relations
In agreement with the methodology of Buitelaar, Cimiano & Magnini mentioned above, we propose to extract lexical relations from analytical definitions for covering the next step about hierarchical relations. Hiponymy/hypernymy and meronym/holonymy relations are considered as re-lations organizing a conceptual space in a hierarchical way (Winston, Chaffin & Herrmann 1987).
Additionally, our method provides a way to get more relations from a domain corpus through the application of a bootstrapping technique with the genus terms/wholes set as seed set.
• Part/whole relations: In this case, we consider works such as Charniak & Berland (1999), as well as those results reported by Girju, Badulescu & Modolvan (2006). We propose a method exploiting the pattern with preposition de, due to its use frequency to link parts and wholes in Spanish. Table 6 shows examples about meronymy/holonymy relations using this pattern compared with other patterns worked in the literature. • Attribution relations: Attribution relations play an important role in disciplines involved with conceptual representation as artificial intelligence/knowledge representation, linguistics and psychology (Poesio & Almuhareb, 2005). So, we consider the work about the automatic extraction of attribution relations proposed by Poesio & Almuhareb (2004). They used an approach as that proposed by Charniak & Berland (1999) but to extract attribution relations using the pattern: "the * of the C [is|was]" Here, * represents a potential attribute for the concept C. In Spanish a common pattern to express attribution relations is the use of the preposition de, e.g.: edad del paciente (Eng. age of patient/patient's age), altura del paciente (Eng. height of the patient/ patient's height), and so on.
Summarizing, our methodology to extract lexical relations starts with the extraction of hyponymyhypernymy relations from analytical DCs. For this phase we consider a lexical-syntactic approach due to the regularity of the definition schemes using predication patterns as those mentioned above. Additionally, we propose a bootstrapping technique starting with the set of genus terms as a seedset to extract more lexical relations from a domain corpus. We use the preposition de to link genus term and other potential terms due to its importance to produce lexical relations of our interest.
For example, in a first phase exploring a domain corpus, a genus term as dilatación (Eng. dilation) links with a set of two elements {vena, pupila} (Eng. {vein, pupil}). In a next phase, the element pupila is linked to ojo (Eng. eye), and so on. Thus, on the one hand we have two relations IS-KIND-OF: dilatación de la pupila and dilatación de la vena. On the other hand, we have a meronymy-holonymy relation: pupila-ojo.
Integrating the three relations described above, we will implement a lexical network that allows organizing concepts related to terms. An example of this possible network is: In the figure, we can distinguish a set of sub−terms linked to the main term Ojo (Engl. Eye). These sub−terms operate as nodes, and the possible lexical relations are branches connected with the main term. Thus, based on a lexical Parth/whole relation, we can infer that córnea (Eng. cornea), is a constituent of eye. In contrast, the term enfermedad (Engl. disease) is an attribute of eye. Finally, the glaucoma is a type of disease that affects the eyes.

Work in progress and possible topics of collaborations
In this paper we proposed a method for recognizing lexical relations, taking into account the identification and extraction of analytical definitions situated into DCs in Spanish. This extraction considers verbal predications associated to these definitions. So, in order to explain this extraction, we have showed a formal syntactic analysis, based on the idea that these predications: (a) could be described in terms of predicative phrases, and (b), the association of predications and analytical definitions has a high frequency of use in specialized documents. For evaluating this frequency, we have exposed the results obtained for an experiment of extraction in two technical corpora. Currently, we are situated in the phase to implement and evaluate a new experiment oriented to the detection of lexical relations between the term and the genus term formulated for analytical definitions. In particular, we are interested in discovering three types of relations: hyponym/hyperonymy, part-whole and attribution-entity.
We conclude suggesting some topics of collaborations for our potential colleges: I. The construction of specialized texts corpora with good candidates of DCs, having in mind the basic features for identifying a DC. II. The implementation of new linguistic and statistical methods for detecting and extracting lexical relations from text corpora. III. The improvement of search systems, using these underlying lexical relations in electronic documents. IV. Following to Wilks, Slator & Guthrie (1995), the design of lexical-semantic tags for recognizing and classifying concepts in taxonomies.
Similarly, according to Buitelaar, Cimiano & Magnini, we can use external lexical resources as Spanish WordNet and Spanish FrameNet (Subirats 2009) for determining and evaluating our lexical networks, in order to enrich the results that we could generate.