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

2.2. Lexical Knowledge Formalisms and Resources 19
Conceptual graphs are logically precise, humanly readable, and computationally
tractable. Chein and Mugnier (2008) go further and refer that all kinds of knowledge
can be seen as a labelled graph, as the they provide an intuitive and easily
understandable means to represent knowledge. Therefore, most lexical networks
can roughly be seen as conceptual graphs. For this purpose, the rectangles should
denote the lexical nodes, while the ovals denote the labelled edges.
2.2.4 Lexical Ontologies
In the scope of computer science, despite some terminological issues, an ontology is
commonly defined as an explicit specification of a shared conceptualisation (Gruber,
1993; Guarino, 1998). More precisely, ontologies are formalised models that typically
represent knowledge of a specific domain. They are often structured on unambiguous
concepts and on relations between them.
Given that information about the usage of words is irrelevant for ontologies,
and that LKBs are more linguistic conventionalised than well-formed ontologies, it
may seem that ontologies and LKBs are incompatible resources. The latter contain
knowledge about a language and are structured in lexical items and on relations,
including semantic relations. However, semantic relations are not held between
words, but word meanings (word senses). Since meaning is inherently conceptual,
LKBs can be seen as (lexical) ontologies with natural language concepts. Lexical
ontologies are thus resources that have properties of a lexicon as well as properties
of an ontology (Hirst, 2004; Prévot et al., 2010).
The so-called ontolex interface (Prévot et al., 2010) deals with the mapping
between ontologies and linguistic knowledge, as represented in LKBs. This interface
sees the LKB as a special ontology where, in order to represent concepts unambiguously,
synonymous lexical items should be grouped under the same concept.
A lexical ontology can be seen as a more formalised representation of a LKB. For
instance, concepts can be seen as individuals in the ontology and can belong to
classes; relations are well-defined interactions that may occur between classes or
individuals; and relation instances are defined by rules that explicitly refer to the
classes/individuals and to a pre-defined relation type.
A lexical ontology may be represented as a Semantic Web (Berners-Lee
et al., 2001) model, using languages such as RDF (Miller and Manola, 2004) or
OWL (McGuinness and van Harmelen, 2004), the W3C standards for representing
information computationally as triples. An example of such a model is the W3C
WordNet RDF/OWL (van Assem et al., 2006), where concepts are instances of
classes that group synonymous word senses together (synsets). While the previous
model deals only with linguistic knowledge, Lemon (Buitelaar et al., 2009) is a
model for representing lexical information attached to ontologies. The main idea is
that a lexical item may have several senses, and each one may be used to denote a
concept in an ontology. Still on this context, the Lexical Markup Framework (LMF,
Francopoulo et al. (2009)) is a ISO standard for representing lexicons, which covers
not only the morphology and syntax, but also semantics, including words senses and
semantic relations. The Lemon model can be converted to LMF.