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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>ContextBERT: Contextual Graph Representation Learning in Text Disambiguation</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Mozhgan Saeidi</string-name>
          <email>mozhgan.saeidi@dal.ca</email>
        </contrib>
      </contrib-group>
      <fpage>0000</fpage>
      <lpage>0002</lpage>
      <abstract>
        <p>Word representations derived by neural language models have been shown to effectively carry useful semantic information to improve the final results of various Natural Language Processing tasks. The information provided by these representations encodes the subtle distinction that might occur between different meanings of the same word. However, these representations do not include the input text's information, as the context, and a semantic knowledge base network. This integration of context and semantic network is helpful in NLP tasks, specifically in the lexical ambiguity problem. In this paper, we first analyzed the defects of current state-of-the-art representations learning approaches, and second, we present a word representation learning method, named ContextBERT, that is aware of the semantic knowledge base network and the context. ContextBERT is a novel approach to producing sense embeddings for the lexical meanings within a lexical knowledge base, using pre-trained BERT model The novel difference in our representation is the integration of the knowledge base information and the input text. Our representations enable a simple 1-Nearest-Neighbour algorithm to perform state-of-the-art models in the English Word Sense Disambiguation task.</p>
      </abstract>
      <kwd-group>
        <kwd>Sense Embedding</kwd>
        <kwd>Representation Learning</kwd>
        <kwd>Word Sense Disambiguation</kwd>
        <kwd>Pre-trained Language Models</kwd>
        <kwd>Semantic Networks</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>-</title>
      <p>
        Text disambiguation is one of many problems in Natural Language Processing (NLP)
tasks. In this task, we have an input text including a word with multiple possible
meanings based on a semantic knowledge base network, and the question is which
one of those multiple meanings is the best meaning match for the word in the text,
based on its context [
        <xref ref-type="bibr" rid="ref17 ref32">17,32</xref>
        ]. The context here refers to the input document text. The
text disambiguation task is mostly referred to as Word Sense Disambiguation (WSD)
task in NLP. Knowledge bases are different in nature [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ]; for example, WordNet is a
lexical graph database of semantic relations (e.g., synonyms, hyponyms, and meronyms)
between words. Synonyms are grouped into synsets with short definitions and usage
examples. WordNet can thus be seen as a combination and extension of a dictionary and
thesaurus [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ]. Wikipedia is a hyperlink-based graph between encyclopedia entries1.
1 Copyright © 2021 for this paper by its authors. Use permitted under Creative Commons License
      </p>
      <p>Attribution 4.0 International (CC BY 4.0).</p>
      <p>
        The text ambiguity task is easy for humans by considering the context. The context
enables us to identify the correct meaning of the ambiguous words. In computational
methods, we try to enhance the algorithms to mimic this approach. These methods often
represent their output by linking each word occurrence to an explicit representation of the
chosen sense [
        <xref ref-type="bibr" rid="ref37">37</xref>
        ]. Two approaches to tackle this problem are the machine learning-based
approach and the knowledge-based approach. In the machine learning-based approach,
systems are trained to perform the task [
        <xref ref-type="bibr" rid="ref32">32</xref>
        ]. The knowledge-based approach requires
external lexical resources such as Wikipedia, WordNet [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ], a dictionary, or a thesaurus.
The machine learning-based approaches mainly focus on achieving maximal precision or
recall and have their drawbacks of run-time and space requirement at the time of classifier
training [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ]. So, knowledge-based disambiguation methods still have advantages to study.
Among different knowledge-based methods, coherence-based has been more effective in
explaining it. In the coherence-based approach, one important factor is the coherence of
the whole text after disambiguation, while in other approaches, this factor might change
to considering the coherence of each sentence or paragraph.
      </p>
      <p>
        There are different factors that play important roles in solving the WSD problem,
including word representation. Word representations have been shown to play an important
role in different Natural Language Processing (NLP) tasks, especially in disambiguation
tasks. There are many different approaches to generate word representation embeddings.
Recently, embeddings based on pre-trained deep language models have attracted much
interest. These models have proved to be superior to classical embeddings for several
NLP tasks, including Word Sense Disambiguation (WSD). Some of most used models
in this category are including ELMO [
        <xref ref-type="bibr" rid="ref22">22</xref>
        ], BERT [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ], and XLNET [
        <xref ref-type="bibr" rid="ref38">38</xref>
        ]. these models
encode several pieces of linguistic information in their word representations. These
representations differ from static neural word embeddings [
        <xref ref-type="bibr" rid="ref21">21</xref>
        ] in that they are
dependent on the surrounding context of the word [
        <xref ref-type="bibr" rid="ref29">29</xref>
        ].This difference makes these vector
representations especially interesting for disambiguation, where effective contextual
representations can be highly beneficial for resolving lexical ambiguity. In addition, these
representations enabled sense-annotated corpora to be exploited more efficiently [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ].
      </p>
      <p>In this study, next section, we overview different current approaches for text
embedding with focusing on the contextualized word representation. We analyzed the
effectiveness of these methods on different types of words. We show the pros and cons of
these state-of-the-art models in word representation learning on parts of speeches are. In
our representation, we enhanced this detected defectiveness to improve representations.
Our novel contribution provides a new representation of words using the context of the
input text and the context of the knowledge base and uses the nearest neighbor heuristic
algorithm to disambiguate ambiguous words. We finally compare the performance of
our proposed approach with our representations with the most current methods in the
disambiguation task.
2</p>
    </sec>
    <sec id="sec-2">
      <title>Related Work</title>
      <p>
        The Word Sense Disambiguation is one core problem in NLP, which addresses the
ambiguity of words in a given context. In this task, we have access to two main sources
of information to disambiguate the ambiguous words. One source is a semantic network,
and the other is sense-annotated corpora. Semantic networks encode a more general
knowledge that is not tied to a specific task, and the information enclosed therein is
usually employed for WSD by knowledge-based approaches. Instead, sense annotated
corpora are tailored to the WSD task and are typically used as training sets for supervised
systems. Therefore, we divide the WSD approaches into two categories of
knowledgebased and supervised approaches [
        <xref ref-type="bibr" rid="ref17">17</xref>
        ].
2.1
      </p>
      <p>
        Knowledge-Based Approaches
In the knowledge-based methods, the semantic network structure of the knowledge base is
used, e.g., Wikipedia [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ], WordNet [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ], BabelNet [
        <xref ref-type="bibr" rid="ref19">19</xref>
        ], to find the correct meaning based
on its context for each input word [
        <xref ref-type="bibr" rid="ref16">16</xref>
        ]. These approaches employ algorithms on graphs
to address the word ambiguity in texts [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. Disambiguation based on Wikipedia has been
demonstrated to be comparable in terms of coverage to domain-specific ontology [
        <xref ref-type="bibr" rid="ref36">36</xref>
        ]
since it has broad coverage, with documents about entities in a variety of domains [
        <xref ref-type="bibr" rid="ref31">31</xref>
        ].
The most widely used lexical knowledge base is WordNet, although it is restricted
to the English lexicon, limiting its usefulness to other vocabularies. BabelNet solves
this challenge by combining lexical and semantic information from various sources in
numerous languages, allowing knowledge-based approaches to scale across all languages
it supports. Despite their potential to scale across languages, knowledge-based techniques
on English fall short of supervised systems in terms of accuracy.
2.2
      </p>
      <p>
        Supervised Approaches
The supervised approaches surpass the knowledge-based ones in all English data sets.
These approaches use neural architectures, or SVM models, while still suffering from
the need of creating large manually-curated corpora, which reduces their usability to
scale over unseen words [
        <xref ref-type="bibr" rid="ref20">20</xref>
        ]. Automatic data augmentation approaches [
        <xref ref-type="bibr" rid="ref33">33</xref>
        ] developed
methods to cover more words, senses, and languages.
      </p>
      <p>In recent years, the contextual representation learning approaches have improved the
performance of WSD models, where they have been employed for the creation of sense
embeddings. Most NLP tasks now use semantic representations derived from language
models. There are static word embeddings and contextual embeddings. This section
covers aspects of the word and contextual embeddings that are especially important to
our work.</p>
      <p>
        Static Word Embeddings Word embeddings are distributional semantic representations
usually with one of two goals: predict context words given a target word (Skip-Gram),
or the inverse (CBOW) [
        <xref ref-type="bibr" rid="ref12">12</xref>
        ]. In both, the target word is at the center, and the context
is considered as a fixed-length window that slides over tokenized text. These models
produce dense word representations. One limit for word embeddings, as mentioned
before, is meaning conflict around word types. This limitation affects the capability of
these word embeddings for the ones that are sensitive to their context [
        <xref ref-type="bibr" rid="ref28">28</xref>
        ].
Contextual Word Embeddings The problem mentioned as a limitation for the static
word embeddings is solved in this type of embeddings. The critical difference is that
the contextual embeddings are sensitive to the context. It allows the same word types to
have different representations according to their context. The first work in contextual
embeddings is ELMO [
        <xref ref-type="bibr" rid="ref22">22</xref>
        ], which is followed by BERT [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ], as the state-of-the-art model.
The critical feature of BERT, which makes it different, is the quality of its
representations [
        <xref ref-type="bibr" rid="ref30">30</xref>
        ]. Its results are task-specific fine-tuning of pre-trained neural language models.
The recent representations which we analyze their effectiveness are based on these two
models [
        <xref ref-type="bibr" rid="ref23 ref24">24,23</xref>
        ].
      </p>
      <p>In our representation, we use different resources to build the vectors. In this section,
we provide information on these resources.
2.3</p>
      <p>
        Wikipedia
is the largest electronic encyclopedia freely available on the Web. Wikipedia organized
its information via articles called Wikipedia pages. Disambiguation based on Wikipedia
has been demonstrated to be comparable in terms of coverage to domain-specific
ontology [
        <xref ref-type="bibr" rid="ref36">36</xref>
        ] since it has broad coverage with documents about entities in a variety of
domains [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ]. Moreover, Wikipedia has unique advantages over the majority of other
knowledge bases, which include [
        <xref ref-type="bibr" rid="ref40">40</xref>
        ]:
– The text in Wikipedia is primarily factual and available in a variety of languages.
– Articles in Wikipedia can be directly linked to the entities they describe in other
knowledge bases.
– Mentions of entities in Wikipedia articles often provide a link to the relevant
Wikipedia pages, thus providing labeled examples of entity mentions and
associated anchor texts in various contexts, which could be used for supervised learning
in WSD with Wikipedia as the knowledge base.
2.4
      </p>
      <p>
        BabelNet
is a multilingual semantic network, which comprises information coming from
heterogeneous resources, such as WordNet, and Wikipedia [
        <xref ref-type="bibr" rid="ref19">19</xref>
        ]. It is organized into synsets, i.e.,
sets of synonyms that express a single concept, which, in their turn, are connected to
each other by different types of relationships. One of Babelnet’s features which is useful
for our representation is hypernym-hyponym relations. In this relation, each concept is
connected to other concepts via hypernym relation (for generalization) and via hyponym
relation (for specification). Semantically-related relation is the other feature that we
use that expresses a general notation of relatedness between concepts. The last feature
of Babelent used in this work is mapping to Wikipedia, which maps its concepts to
Wikipedia pages.
2.5
      </p>
      <p>
        WordNet
is the most widely used lexical knowledge repository for English. It can be seen as
a graph, with nodes representing concepts (synsets) and edges representing semantic
relationships between them. Each synset has a set of synonyms, such as the lemmas
spring, fountain, and natural spring in the synset, A natural flow of groundwater.
is the typical manually-curated corpus for WSD, with about 220K words tagged with
25K distinct WordNet meanings, resulting in annotated contexts for around 15% of
WordNet synsets.
is a Transformer-based language model for learning contextual representations of words
in a text. The contextualized representation of BERT is the key factor that has changed
the performance in many NLP tasks, such as text ambiguity. In our representations, we
use BERT-base-cased to generate the vectors of each sense [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ].
is a modification of the pre-trained BERT network that uses siamese and triplet network
structures to derive semantically meaningful sentence embeddings that can be compared
using cosine-similarity. We use this sentence representation when generating the vector
representations of sense sentences, both in the input text and in the knowledge base text.
Graph Convolutional Networks (GCN) is a very powerful multilayer neural network
architecture for machine learning on graphs [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. GCN operates directly on a graph and
induces embedding vectors of nodes based on the properties of their neighborhoods. In
fact, they are so powerful that even a randomly initiated 2− layer GCN can produce useful
feature representations of nodes in networks2. Formally, consider a graph G = (V, E),
where V (|V | = n) and E are sets of nodes and edges, respectively. Every node is
assumed to be connected to itself, i.e., (v, v) ∈ E for any v which the reason for this
assumption is mentioned at the end of this paragraph. Let X ∈ Rn× m be a matrix
containing all n nodes with their features, where m is the dimension of the feature
vectors, each row xv ∈ Rm is the feature vector for v. We introduce an adjacency
matrix A of G and its degree matrix D, where Dii = Pj Aij . Because of self-loops, the
diagonal elements of A are all 1. We now have a graph, its adjacency matrix A, and a
set of input feature X. After applying the propagation rule f (X, A) = AX and X = I,
the representation of each node (each row) is now a sum of its neighbor’s features. In
other words, the graph convolutional layer represents each node as an aggregate of its
neighborhood. The reason for considering the self-loops in the graph is the aggregated
representation of a node to include its own features.
      </p>
      <p>For a one-layer GCN, the new k-dimensional node feature matrix L(1) ∈ Rn× k is
computed as:</p>
      <p>
        L(1) = ρ (AˆXW0)
(1)
2 The notation we used for GCN in this paper are the same as notations in [
        <xref ref-type="bibr" rid="ref39">39</xref>
        ]
where Aˆ is D− 0.5AD− 0.5, the normalized symmetric adjacency matrix and W0 ∈ Rm× k
is the weight matrix. The ρ is the activation function (RELU); ρ (x) = max(0, x). GCN
can capture information only about immediate neighbors with one layer of convolution.
When multiple GCN layers are stacked, information about larger neighborhoods are
integrated;
      </p>
      <p>
        L(j+1) = ρ (AˆLj Wj )
(2)
which j is the layer number and L0 = X. In other words, the size of the second
dimension of the weight matrix determines the number of features at the next layer.
The feature representations can be normalized by node degree by transforming the
adjacency matrix A by multiplying it with the inverse degree matrix D. First we used
the simple propagation rule f (X, A) = D− 1AX, while then improved it. The improved
version is inspired by a recent work [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ] that proposes a fast approximate spectral graph
convolutions using a spectral propagation rule f (X, A) = σ (D− 0.5AˆD− 0.5XW ). They
showed this property is very useful, that connected nodes tend to be similar (e.g. have
the same label).
3
      </p>
    </sec>
    <sec id="sec-3">
      <title>Methodology</title>
      <p>This section presents our novel embedding approach of creating sense representations
of BabelNet senses. Our representation learning is created by combining semantic and
textual information from the first paragraph of each sense’s Wikipedia page and the
input document paragraph, which includes the ambiguous word. our approach uses the
representation power of neural language models, i.e., BERT and SBERT. We divide our
approach into the following steps:
3.1</p>
      <p>
        Context Retrieval
In this step, we collect suitable contextual information from Wikipedia for each given
concept in the semantic network. Similar to [
        <xref ref-type="bibr" rid="ref34">34</xref>
        ], we exploit the mapping between
synsets and Wikipedia pages available in BabelNet, as well as its taxonomic structure,
to collect textual information that is relevant to a target synset s. For each synset s, we
collect all the connected concepts to s through hyponym and hypernym connections of
the BabelNet knowledge base. We show this set of related synsets to s by Rs which is:
      </p>
      <p>Rs = {s′ |(s, s′ ) ∈ E}</p>
      <p>
        Similar to [
        <xref ref-type="bibr" rid="ref34">34</xref>
        ], we use E as the set includes all hyponyms and hypernyms connections.
In this work, for each page ps, we consider the first opening paragraph of the page and
compute its lexical vector by summing the SBERT vector representation of the sentences
in this first paragraph. These lexical representations are later used for the similarity score
ifnding between ps and ps′ , for each s′ ∈ Rs by using the weighted overlap measure
from [
        <xref ref-type="bibr" rid="ref25">25</xref>
        ], which is defined as follows:
      </p>
      <p>W O(p1, p2) = ( X
w∈O</p>
      <p>
        1
rwp1 + rwp2 )(X
i=1
|O| 1
where O is the set of overlapping dimensions of p1 and p2 and rwpi is the rank of
the word w in the lexical vector of pi. We preferred the weighted overlap over the more
common cosine similarity as it has proven to perform better when comparing sparse
vector representations [
        <xref ref-type="bibr" rid="ref25">25</xref>
        ]. Similar to [
        <xref ref-type="bibr" rid="ref34">34</xref>
        ], Once we have scored all the (ps, ps′ ) pairs,
we create partitions of Rs, each comprising all the senses s′ connected to s with the
same relation r, where r can be one among: hypernymy, and hyponymy. We then retain
from each partition only the top-k scored senses according to W O(ps, ps′i ), which we
set k = 15 in our experiments.
3.2
      </p>
      <p>Word Embedding
In the second step, we use BERT for the representation of the given concepts from the
input text. For each ambiguous word–which we call this word by mention– of the input,
we extract the BERT representation of the mention. Using the BabelNet relations of
hyponymy and hypernymy, we extract all synsets of mention from BabelNet (set E). For
each one of these senses, use the link structure of BabelNet and Wikipedia; we collect
all the Wikipedia pages for each sense. We use BERT representation for the second time
to generate vector representation for senses. In the settings, each word is represented as
a 300-dimensional vector, as the BERT dimension.
3.3</p>
      <p>Sense Embedding
In this step, we build the final representation of each concept. From the previous step,
we took the representation of mention, R(m), and the representation of each one of its
senses. We show the representations of each k sense of m by R(si) which i varies from
1 to k, based on their similarity scores. Our unique representations combine the mention
representation with sense representation, averaging the vector representations of R(m)
and R(si). If mention m has k senses, our model generates k different representations
of R(m, s1), R(m, s2), ..., R(m, sk). Since the dimension representation of R(m) and
each R(si) is 300, these averaged representation dimensions are 300. Next novelty in our
representations is ranking the k senses of each mention based on their relevancy degree
to the context. To this aim, we average the representations of the first step. In the first step,
we took the representation of the input text paragraph, which contains the ambiguous
mention, show it by R(P D) which stands for representation of the Paragraph of the
input Document. In the first step, we also took the representation of the first paragraph of
the Wikipedia page, which represents it by R(P W ), which stands for representation of
the first Paragraph of the Wikipedia page. Finally, we average these two representations
as R(P D, P W ). In the R(P D, P W ), the context is constant for each sense since the
input text as the context is constant for each possible sense of the ambiguous words. The
dimension of this averaged representation is also equal to the word representation, so it
makes it possible to calculate their cosine similarities. To rank the senses most related to
the context, we use the cosine similarity as follows:</p>
      <p>Sim(m, si) = Cosine(R(m, si), R(P D, P W )) , for i=1 , ... , k
This ranking provides the most similar sense to the context for each mention. This
novelty makes this representation more effective than the previous contextualized-based
embeddings, especially in the task of sense disambiguation. At the end of these three
steps, each sense is associated with a vector that encodes both the contextual information
and knowledge base semantic information from the extracted context of Wikipedia and
its gloss.</p>
      <p>We consider each mention of the document as one node of the graph, and a newly
added node (redirect link) will connect with its nearest neighbor by using cosine
similarity, which makes the edges of the graph. The cosine similarity between two nodes
on the edges makes the weight matrix. The number of nodes in the text graph |V | is the
number of mentions. For each sense s, we use an integrated representation of its mention
m with its own representation, i.e,. R(m, s). We set the feature matrix X as extracted
representation of BERT as input to GCN. The dimension of the feature matrix here is
300, as it is the averaged representation length of two BERT embeddings, one for the
mention and the other for the sense.</p>
      <p>As mentioned, formally, the weights of edge between node i and node j defines as:
(3)
(4)
(5)
Z = sof tmax(AˆRELU (AˆXW0)W1)</p>
      <p>Aˆ = D− 0.5AD− 0.5
sof tmax(xi) =</p>
      <p>exp(xi)
1</p>
      <p>Z
L = − X</p>
      <p>F</p>
      <p>
        X Ydf lnZdf
d∈Y f=1
where YD is the set of mention indices that have labels and F is the dimension of
the output feature. Y is the label indicator matrix. Similar to [
        <xref ref-type="bibr" rid="ref39">39</xref>
        ], the weight
parameters W0 and W1 can be trained via gradient descent. The AˆXW0 contains the first
layer (mention,sense) and embeddings, and AˆRELU (AˆXW0)W1 contains the second
layer (mention,sense) and embeddings. This two-layer GCN performs message passing
between nodes to two steps away, maximum. Therefore, the two-layer GCN allows the
exchange of information between pairs of nodes. This GCN model on our experimental
datasets shows better performance than a one-layer model and models with more than
two layers. This shows the validity of our model, based on similar results in other recent
works [
        <xref ref-type="bibr" rid="ref8 ref9">8,9</xref>
        ].
      </p>
      <p>Wij = cosine sim(R(i), R(j)) =</p>
      <p>R(i).R(j)
||R(i)||||R(j)||
which R(i) is our representation of node i.</p>
      <p>
        After building the graph, we feed it into a simple 2− layers GCN as [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ], the second
layer node (mention,sense) embeddings are fed into a softmax classifier:
and
      </p>
      <p>
        where
with S = Pi exp(xi). The loss function is the one defined in [
        <xref ref-type="bibr" rid="ref39">39</xref>
        ] as:
4. EXPERIMENTAL SETUP
      </p>
    </sec>
    <sec id="sec-4">
      <title>Experimental Setup</title>
      <p>We present the settings of our evaluation of our representation in the English WSD task.
This setup includes the benchmark, our representation setup for disambiguation task and
state-of-the-art WSD models as our comparison systems.</p>
      <p>
        Evaluation Benchmark We use the English WSD test set framework which is
constructed by vfie standard evaluation benchmark datasets 3. It is included of
Senseval2 [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ], Senseval-3 [
        <xref ref-type="bibr" rid="ref35">35</xref>
        ], SemEval-07 [
        <xref ref-type="bibr" rid="ref26">26</xref>
        ], SemEval-13 [
        <xref ref-type="bibr" rid="ref18">18</xref>
        ], SemEval-15 [
        <xref ref-type="bibr" rid="ref15">15</xref>
        ] along with
ALL, i.e., the concatenation of all the test sets [
        <xref ref-type="bibr" rid="ref27">27</xref>
        ].
      </p>
      <p>
        Experiment Setup In our experiments, we use BERT pre-trained cased model.
Similar to [
        <xref ref-type="bibr" rid="ref34">34</xref>
        ], among all the configurations reported by Devlin et al. (2019), we used
the sum of the last four hidden layers as contextual embeddings of the words since
they showed it has better performance. In order to be able to compare our system with
supervised models, we build a supervised version of our representations. This version
combines the gloss and contextual information with the sense-annotated contexts in
SemCor [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ], a corpus of 40K sentences where words have been manually annotated
with a WordNet meaning. We leveraged SemCor for building a representation of each
sense therein. To this end, we followed [
        <xref ref-type="bibr" rid="ref22">22</xref>
        ], given a mention-sense pair (m, s), we
collected all the sentences c1, ..., cn where m appears tagged with s. Then, we fed all
the retrieved sentences into BERT and extracted the embeddings BERT(c1, m), ... ,
BERT(cn, m). The final embedding of s was built by the average of its context and
sense gloss vectors and its representation coming from SemCor, i.e., the average of
BERT(c1, m), ... , BERT(cn, m). We note that when a sense did not appear in SemCor,
and we built its embedding by replacing the SemCor part of the vector with its sense
gloss representation.
      </p>
      <p>WSD Model For WSD modeling, we employed a 1-nearest neighbor approach– as
previous methods in the literature– to test our representations on the WSD task. For
each target word m in the test set, we computed its contextual embedding by means of
BERT and compared it against the embeddings of our representation associated with the
senses of m. Hence, we took as a prediction for the target word the sense corresponding
to its nearest neighbor. We note that the embeddings produced by our representations
are created by averaging two BERT representations, i.e., context and sense gloss (see
Section 3.3), hence we repeated the BERT embedding of the target instance to match the
number of dimensions.</p>
      <p>
        Comparison Systems We compared our representation against the best recent
performing systems evaluated on the English WSD task. LMMS is one of these systems
which generates sense embedding with full coverage of Wordnet. It uses pre-trained
ELMO and BERT models, as well as the relations in a lexical knowledge base to create
contextual embeddings [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ]. SensEmBERT is the next system that relies on different
resources for building sense vectors. These resources include Wikipedia, BabelNet,
NASARI lexical vectors, and BERT. It computes context-aware representations of
BabelNet senses by combining the semantic and textual information derived from
multilingual resources. This model uses the BabelNet mapping between WordNet senses
and Wikipedia pages which drops the need for sense-annotated corpora [
        <xref ref-type="bibr" rid="ref34">34</xref>
        ]. The next
3 http://lcl.uniroma1.it/wsdeval/
comparison system is ARES, a semi-supervised approach to produce sense embeddings
for all the word senses in a language vocabulary. ARES compensates for the lack of
manually annotated examples for a large portion of words’ meanings. ARES is the most
recent contextualized word embedding system, to our knowledge. In our comparisons,
we also considered BERT as a comparison system since it is at the core of all the
considered methods. BERT also has shown good performance in most NLP tasks by
using pre-trained neural networks.
5
      </p>
    </sec>
    <sec id="sec-5">
      <title>Results</title>
      <p>
        The results of our evaluations on the WSD task are represented in this section. We show
the effectiveness of our representation by comparing it with the existing
state-of-theart models on the standard WSD benchmarks. In Table 1 we report the results of our
representation and compare it against the results obtained from other state-of-the-art
approaches on all the nominal instances of the test sets in the framework of [
        <xref ref-type="bibr" rid="ref27">27</xref>
        ]. All
performances are reported in terms of F1-measure, i.e., the harmonic mean of precision
and recall. As we can see, our model achieves the best results on the datasets when
compared to other precious contextualized approaches. It indicates that our representation
is competitive with these previous models. These results show the novel idea in the
nature of creating this new representation has improved the lexical ambiguity. It is a
good indicator of the dependency of the WSD task to the representation that is aware of
the context and the information extracted from the reference knowledge base.
Analysis by Part-of-Speech One other possible way to analyze the errors that arise in
WSD with each embedding approach is to measure the frequency of mis-disambiguation
in different parts of speech. The considered parts of speech are nouns, verbs, adjectives,
and adverbs, as are the covered types in the datasets. The F-measure performance of the
1-NN WSD of each embedding on All dataset is shown in Table 3 which is categorized
by parts of speech. As it shows, the type in which its disambiguation has been correct
more than other types is adverbs. At the same time, verbs are the ones that are difficult
to disambiguate because they have the lowest mis-disambiguation frequency across all
language models. In each one of the models, disambiguating the nouns is more accurate
than verbs, when the embedding model is BERT. The coverage of verb senses can
explain this disambiguation performance difference between verbs and the other three
parts of speech in WordNet, significantly less than the coverage of noun senses. To be
more specific with our quantitative POS analysis, we tried to find the type of words in
all datasets with more errors when disambiguating with different representations. We
evaluate the effectiveness of our representation on parts of speeches, in comparison with
the recent methods. The parts of speech that we have in the dataset are nouns, verbs,
adjectives, and adverbs. Table 2 shows the number of instances in each category. In our
second evaluation, we examined the effect of our representation against previous ones on
each word category. Table 3 represents the F-Measure performance of the 1-NN WSD of
each one of the contextualized word embeddings which we considered on All datasets
split by parts of speech.
In this paper, we consider the problem of text ambiguity and one of its important
factors, the word representation. We evaluate the pros and cons of current
state-ofthe-art approaches for word embedding, and applied them in parts of speeches on the
standard datasets. By observing the opportunities to improve a word embedding model,
we present a novel approach for creating word embeddings. In our model, we consider
the knowledge base and the context of the input document text, when generating the
representation. We showed that this context-rich representation is beneficial for lexical
ambiguity in English. The results of experiments in the WSD task show the efcfiiency
of our representations compared to other state-of-the-art methods, despite relying only
on English data. We further tested our embeddings on the split data into four parts
of speeches. As the results of our second experiment show, the effectiveness of the
contextualized embeddings in WSD on verbs is not as good as on nous. This defect is
because of the lack of instances in the dataset in each word category. As future work,
one point to improve our representations in the text ambiguity task is by training the
model with data including more verbs than the current one.
      </p>
    </sec>
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