Onto.PT: Towards the Automatic Construction of a Lexical Ontology ...

Chapter 7
Moving from term-based to
synset-based relations
Typical information extraction (IE) systems are capable of acquiring concept instances
and information about these concepts from large collections of text. Whether
these systems aim for the automatic acquisition of lexical-semantic relations (e.g.
Chodorow et al. (1985); Hearst (1992); Pantel and Pennacchiotti (2006)), of knowledge
on specific domains (e.g. Pustejovsky et al. (2002); Wiegand et al. (2012)),
or the extraction of open-domain facts (e.g. Agichtein and Gravano (2000); Banko
et al. (2007); Etzioni et al. (2011)) they typically represent concepts as terms, which
are lexical items identified by their lemma. This is also how CARTÃO is structured.
There, semantic relations are denoted by relational triples t = {a R b},
where the arguments (a and b) are terms whose meaning is connected by a relation
described by R. As we have done throughout this thesis, we refer to the previous
representation as term-based triples (tb-triples).
The problem is that a simple term is usually not enough to unambiguously refer
to a concept, because the same word might have different meanings and different
words might have the same meaning. On the one hand, this problem is not severe in
the extraction of domain knowledge, where, based on the “one sense per discourse”
assumption (Gale et al., 1992), ambiguity is low. On the other hand, when dealing
with broad-coverage knowledge, if ambiguities are not handled, it becomes impractical
to formalise the extracted information and to accomplish tasks such as inference
for discovering new knowledge.
Therefore, to make IE systems more useful, a new step, which can be seen
as a kind of WSD, is needed. Originally baptised as ontologising (Pantel, 2005),
this step aims at moving from knowledge structured in terms, identified by their
orthographical form, towards an ontological structure, organised in concepts, which
is done by associating the terms to a representation of their meaning.
After the steps presented in the previous chapters, we are left with a lexical
network, CARTÃO, with tb-triples extracted from text (chapter 4), and with a
thesaurus, with synsets (chapter 5 and 6). While the synsets can be seen as concepts
and their possible lexicalisations, the identification of the correct sense(s) of
the arguments of a tb-triple for which the relation is valid is not straightforward.
However, whereas most WSD techniques rely on the context where the words to
be disambiguated occur to find their most adequate sense, the tb-triples do not
provide their extraction context. While we could recover the context for some of