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

3.2. Lexical-Semantic Information Extraction 39
of a language. They are substantial sources of general lexical knowledge (Briscoe,
1991) and “authorities” of word senses (Kilgarriff, 1997), which are described in
textual definitions, written by lexicographers, the experts on the field.
Despite several automatic attempts to the creation of a broad-coverage LKB, for
English, Princeton WordNet, a manual effort, ended up to be the leading resource
of this kind (Sampson, 2000). As discussed in section 3.1.1, the existence of a
wordnet in one language has a positive impact in the development of NLP tools for
that language. Nevertheless, despite the wide acceptance of WordNet, research on
LSIE continues, not only from dictionaries, but especially from corpora and other
unstructured resources, whether it is for the enrichment of WordNet (see section 3.3)
or for the creation of alternative LKBs, including LKBs in non-English languages.
In this section, we start with a brief chronology of LSIE from dictionaries. Then,
we present work on LSIE from corpora and IE from other unstructured textual
resources.
3.2.1 Information Extraction from Electronic Dictionaries
In the beginning
During the 1970s, and throughout the 1980s, electronic dictionaries started to be
the target of empirical studies (e.g. Calzolari et al. (1973); Amsler (1980); Michiels
et al. (1980)), having in mind their exploitation in the automatic construction of
a LKB. This kind of knowledge base would ease the access to morphological and
semantic information about the defined words (Calzolari et al., 1973), which would
then be very useful in the achievement of NLP tasks.
These earlier works confirmed that the vocabulary used in dictionaries is limited,
which makes them easier to process for obtaining semantic or syntactic relations
(Michiels et al., 1980). They concluded that the textual definitions are often
structured on a genus and a differentia (Amsler, 1980):
• The genus identifies the superordinate concept of the definiendum – the
definiendum is an instance or a “type of” the genus, which means there is
a hyponymy relation between the former and the latter.
• The differentia contains the specific properties for distinguishing the definiendum
from other instances of the superordinate concept.
Having in mind that this kind of structure is suitable for being exploited in
the automatic acquisition of taxonomies, Amsler (1981) proposes a taxonomy for
English nouns and verbs. The extracted structures, dubbed tangled hierarchies,
were created after the analysis of dictionary definitions and manual disambiguation
of the head word of each definition. Amsler (1981) concluded that dictionaries
clearly represent two taxonomic relations: is-a (hypernymy) and is-part (part-of).
Calzolari (1984) suggests a set of frequent patterns in dictionary definitions,
and examines the occurrence of the hyponymy and “restriction” relations. She
claims that hyponymy is the most important and evident relation in the lexicon and
confirms it can be easily extracted from a dictionary, after identifying the genus and
the differentia.
Markowitz et al. (1986) identified a set of textual patterns that occur in the beginning
of the definitions of a dictionary. Those patterns are used to denote relations