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

Abstract
The existence of a broad-coverage lexical-semantic knowledge base has a positive
impact on the computational processing of its target language. This is the case
of Princeton WordNet, for English, which has been used in a wide range of natural
language processing (NLP) tasks. WordNet is, however, created manually by
experts. So, despite ensuring highly reliable contents, its creation is expensive, timeconsuming
and has negative consequences on the resource coverage and growth.
For Portuguese, there are several lexical-semantic knowledge bases, but none
of them is as successful as WordNet is for English. Moreover, all of them have
limitations, that go from not handling ambiguity at the word level and having limited
coverage (e.g. only nouns, or synonymy relations) to availability restrictions.
Having this in mind, we have set the final goal of this research to the automatic
construction of Onto.PT, a lexical ontology for Portuguese, structured in a similar
fashion to WordNet. Onto.PT contains synsets – groups of synonymous words which
are lexicalisations of a concept – and semantic relations, held between synsets. For
this purpose, we took advantage of information extraction techniques and focused
on the development of computational tools for the acquisition and organisation of
lexical-semantic knowledge from text.
Our work starts by exploring textual sources for the extraction of relations,
connecting lexical items according to their possible senses. Dictionaries were our
first choice, because they are structured in words and meanings, and cover a large
part of the lexicon. But, as natural language is ambiguous, a lexical item, identified
by its orthographical form, is sometimes not enough to denote a concept. Therefore,
in a second step, we use a synset-based thesaurus for Portuguese as a starting point.
The synsets of this thesaurus are augmented with new synonyms acquired in the
first step, and new synsets are discovered from the remaining synonymy relations,
after the identification of word clusters. In the last step, the whole set of extracted
relations is exploited for attaching the arguments of the non-synonymy relations to
the most suitable synsets available.
In this thesis, we describe each of the aforementioned steps and present the
results they produce for Portuguese, together with their evaluation. Each step is a
contribution to the automatic creation and enrichment of lexical-semantic knowledge
bases, and results in a new resource, namely: a lexical network; a fuzzy and a
simple thesaurus; and Onto.PT, a wordnet-like lexical ontology. An overview of the
current version of Onto.PT is also provided, together with some scenarios where it
may be useful. This resource, which can be further augmented, is freely available for
download and can be used in a wide range of NLP tasks for Portuguese, as WordNet
is for English. Despite the current limitations of an automatic creation approach,
we believe that Onto.PT will contribute for advancing the state-of-the-art of the
computational processing of Portuguese.