The notions of attributes and relations are central to Artificial Intelligence. In Knowledge Representation (KR) [Brachman and Levesque2004], a concept is described using its attributes and the relations it has with other concepts in a domain. If we already know a particular concept such as pets, we can describe a new concept such as dogs by stating the semantic relations that the new concept shares with the existing concepts such as dogs belongs-to pets. Alternatively, we could describe a novel concept by listing all the attributes it shares with existing concepts. In our example, we can describe the concept dog by listing attributes such as mammal, carnivorous, and domestic animal that it shares with another concept such as the cat. Therefore, both attributes and relations can be considered as alternative descriptors of the same knowledge. This close connection between attributes and relations can be seen in knowledge representation schemes such as predicate logic, where attributes are modelled by predicates with a single argument whereas relations are modelled by predicates with two or more arguments.
Learning representations of words is an important task with numerous applications [Bengio et al.2013]
. Better representations of words can improve the performance in numerous natural language processing tasks that require word representations such as language modelling[Collobert et al.2011, Bengio et al.2003], part-of-speech tagging [Zheng et al.2013], sentiment classification [Socher et al.2011b], and dependency parsing [Socher et al.2013a, Socher et al.2011a]
. For example, to classify a novel word into a set of existing categories one can measure the cosine similarity between the words in each category and the novel word to be classified. Next, the novel word can be assigned to the category of words that is most similar to[Huang et al.2012]. However, existing methods for learning word representations only consider the co-occurrences of two words within a short window of context, and ignore any semantic relations that exist between the two words.
Considering the close connection between attributes and relations, an obvious question is, can we learn better word representations by considering the semantic relations that exist among words?
More importantly whether word representations learnt by considering the semantic relations among words could outperform methods that focus only on the co-occurrences of two words, ignoring the semantic relations. We study these problems in this paper and arrive at the conclusion that it is indeed possible to learn better word representations by considering the semantic relations between words. Specifically, given as input a relational graph, a directed labelled weighted graph where vertices represent words and edges represent numerous semantic relations that exist between the corresponding words, we consider the problem of learning a vector representation for each vertex (word) in the graph and a matrix representation for each label type (pattern). The learnt word representations are evaluated for their accuracy by using them to solve semantic word analogy questions on a benchmark dataset.
Our task of learning word attributes using relations between words is challenging because of several reasons. First, there can be multiple semantic relations between two words. For example, consider the two words ostrich and bird. An ostrich is-a-large bird as well as is-a-flightless bird. In this regard, the relation between the two words ostrich and bird is similar to the relation between lion and cat (lion is a large cat) as well as to the relation between penguin and bird (penguin is a flightless bird). Second, a single semantic relation can be expressed using multiple lexical patterns. For example, the two lexical patterns X is a large Y and large Ys such as X represent the same semantic relation is-a-large. Beside lexical patterns, there are other representations of semantic relations between words such as POS patterns and dependency patterns. An attribute learning method that operates on semantic relations must be able to handle such complexities inherent in semantic relation representations. Third, the three-way co-occurrences between two words and a pattern describing a semantic relation is much sparser than the co-occurrences between two words (ignoring the semantic relations) or occurrences of individual words. This is particularly challenging for existing methods of representation learning where one must observe a sufficiently large number of co-occurrences to learn an accurate representation for words.
Given a relational graph, a directed labeled weighted graph describing various semantic relations that exist between words denoted by the vertices of the graph, we propose an unsupervised method to factorise the graph and assign latent attributional vectors for each vertex (word) and a matrix to each label (pattern) in the graph. Then, the co-occurrences between two words and in a pattern that expresses some semantic relation is modelled as the scalar product . A regression model that minimises the squared loss between the predicted co-occurrences according to the proposed method and the actual co-occurrences in the corpus is learnt. In the relational graphs we construct, the edge between two vertices is labeled using the patterns that co-occur with the corresponding words, and the weight associated with an edge represents the strength of co-occurrence between the two words under the pattern.
Our proposed method does not assume any particular pattern extraction method or a co-occurrence weighting measure. Consequently, the proposed method can be applied to a wide-range of relational graphs, both manually created ones such as ontologies, as well as automatically extracted ones from unstructured texts. For concreteness of the presentation, we consider relational graphs where the semantic relations between words are represented using lexical patterns, part-of-speech (POS) patterns, or dependency patterns. Moreover, by adjusting the dimensionality of the decomposition, it is possible to obtain word representations at different granularities. Our experimental results show that the proposed method obtain robust performances over a wide-range of relational graphs constructed using different pattern types and co-occurrence weighting measures. It learns compact and dense word representations with as low as 200 dimensions. Unlike most existing methods for word representation learning, our proposed method considers the semantic relations that exist between two words in their co-occurring contexts. To evaluate the proposed method, we use the learnt word representations to solve semantic analogy problems in a benchmark dataset [Mikolov et al.2013a].
Representing the semantics of a word is a fundamental step in many NLP tasks. Given word-level representations, numerous methods have been proposed in compositional semantics to construct phrase-level, sentence-level, or document-level representations [Grefenstette2013, Socher et al.2012]. Existing methods for creating word representations can be broadly categorised into two groups: counting-based methods, and prediction-based methods.
Counting-based approaches follow the distributional hypothesis [Firth1957] which says that the meaning of a word can be represented using the co-occurrences it has with other words. By aggregating the words that occur within a pre-defined window of context surrounding all instances of a word from a corpus and by appropriately weighting the co-occurrences, it is possible to represent the semantics of a word. Numerous definitions of co-occurrence such as within a proximity window or involved in a particular dependency relation etc. and co-occurrence measures have been proposed in the literature [Baroni and Lenci2010]
. This counting-based bottom-up approach often results in sparse word representations. Dimensionality reduction techniques such as the singular value decomposition (SVD) have been employed to overcome this problem in tasks such as measuring similarity between words using the learnt word representations[Turney and Pantel2010].
Prediction-based approaches for learning word representations model the problem of representation learning as a prediction problem where the objective is to predict the presence (or absence) of a particular word in the context of another word. Each word is assigned a feature vector of fixed dimensionality such that the accuracy of predictions of other words made using
is maximised. Different objective functions for measuring the prediction accuracy such as perplexity or classification accuracy, and different optimisation methods have been proposed. For example, Neural Network Language Model (NLMM)[Bengio et al.2003] learns word representations to minimise perplexity in a corpus. The Continuous Bag-Of-Words model (CBOW) [Mikolov et al.2013b] uses the representations of all the words in the context of to predict the existence of , whereas the skip-gram model [Mikolov et al.2013c, Mikolov et al.2013a] learns the representation of a word by predicting the words that appear in the surrounding context of Mnih and Kavukcuoglu2013].
Given two words and represented respectively by vectors and of equal dimensions, GloVe [Pennington et al.2014]
learns a linear regression model to minimise the squared loss between the inner-productand the logarithm of the co-occurrence frequency of and . They show that this minimisation problem results in vector spaces that demonstrate linear relationships observed in word analogy questions. However, unlike our method, GloVe does not consider the semantic relations that exist between two words when they co-occur in a corpus. In particular, GloVe can be seen as a special case of our proposed method when we replace all patterns by a single pattern that indicates co-occurrence, ignoring the semantic relations.
Our work in this paper can be categorised as a prediction-based method for word representation learning. However, prior work in prediction-based word representation learning have been limited to considering the co-occurrences between two words, ignoring any semantic relations that exist between the two words in their co-occurring context. On the other hand, prior studies on counting-based approaches show that specific co-occurrences denoted by dependency relations are particularly useful for creating semantic representations for words [Baroni and Lenci2010]. Interestingly, prediction-based approaches have shown to outperform counting-based approaches in comparable settings [Baroni et al.2014]. Therefore, it is natural for us to consider the incorporation of semantic relations between words into prediction-based word representation learning. However, as explained in the previous section, three-way co-occurrences of two words and semantic relations expressed by contextual patterns are problematic due to data sparseness. Therefore, it is non-trivial to extend the existing prediction-based word representation methods to three-way co-occurrences.
Methods that use matrices to represent adjectives [Baroni and Zamparelli2010] or relations [Socher et al.2013b] have been proposed. However, high dimensional representations are often difficult to learn because of their computational cost [Paperno et al.2014]. Although we learn matrix representations for patterns as a byproduct, our final goal is the vector representations for words. An interesting future research direction would be to investigate the possibilities of using the learnt matrix representations for related tasks such as relation clustering [Duc et al.2010].
Learning Word Representations from Relational Graphs
We define a relational graph as a directed labelled weighted graph where the set of vertices denotes words in the vocabulary, and the set of edges denotes the co-occurrences between word-pairs and patterns. A pattern is a predicate of two arguments and expresses a semantic relation between the two words. Formally, an edge connecting two vertices in the relational graph is a tuple , where denotes the label type corresponding to the pattern that co-occurs with the two words and in some context, and denotes the co-occurrence strength between and the word-pair . Each word in the vocabulary is represented by a unique vertex in the relational graph and each pattern is represented by a unique label type. Because of this one-to-one correspondence between words and vertices, and patterns and labels, we will interchange those terms in the subsequent discussions. The direction of an edge is defined such that the first slot (ie. X) matches with , and the second slot (ie. Y) matches with in a pattern . Note that multiple edges can exist between two vertices in a relational graph corresponding to different patterns. Most manually created as well as automatically extracted ontologies can be represented as relational graphs.
Consider the relational graph shown in Figure 1. For example, let us assume that we observed the context ostrich is a large bird that lives in Africa in a corpus. Then, we extract the lexical pattern X is a large Y between ostrich and bird from this context and include it in the relational graph by adding two vertices each for ostrich and bird, and an edge from ostrich to bird. Such lexical patterns have been used for related tasks such as measuring semantic similarity between words [Bollegala et al.2007]. The co-occurrence strength between a word-pair and a pattern can be computed using an association measure such as the positive pointwise mutual information (PPMI). Likewise, observation of the contexts both ostrich and penguin are flightless birds and penguin is a bird will result in the relational graph shown in Figure 1.
Learning Word Representations
Given a relational graph as the input, we learn dimensional vector representations for each vertex in the graph. The dimensionality of the vector space is a pre-defined parameter of the method, and by adjusting it one can obtain word representations at different granularities. Let us consider two vertices and connected by an edge with label and weight . We represent the two words and respectively by two vectors , and the label by a matrix . We model the problem of learning optimal word representations and pattern representations as the solution to the following squared loss minimisation problem
The objective function given by Eq. 1 is jointly non-convex in both word representations (or alternatively ) and pattern representations . However, if is positive semidefinite, and one of the two variables is held fixed, then the objective function given by Eq. 1 becomes convex in the other variable. This enables us to use Alternating Least Squares (ALS) [Boyd et al.2010]
method to solve the optimisation problem. To derive the stochastic gradient descent (SGD) updates for the parameters in the model, let us denote the squared loss associated with a single edgein the relational graph by , given by,
In Eq. 2. The gradient of the error w.r.t. and are given by,
In Eq. 4, denotes the outer-product between the two vectors and , which results in a matrix. Note that the summation in Eq. 3 is taken over the edges that contain either as a start or an end point, and the summation in Eq. 4 is taken over the edges that contain the label .
The SGD update for the -th dimension of is given by,
Here, denotes the -th dimension of the gradient vector of w.r.t. , and the superscripts and
denote respectively the current and the updated values. We use adaptive subgradient method (AdaGrad)[Duchi et al.2011] to schedule the learning rate. The initial learning rate, is set to in our experiments.
Likewise, the SGD update for the element of is given by,
Recall that the positive semidefiniteness of is a requirement for the convergence of the above procedure. For example, if is constrained to be diagonal then this requirement can be trivially met. However, doing so implies that , which means we can no longer capture asymmetric semantic relations in the model. Alternatively, without constraining to diagonal matrices, we numerically guarantee the positive semidefiniteness of by adding a small noise term after each update to , where is the identity matrix and is a small perturbation coefficient, which we set to in our experiments.
until a pre-defined number of maximum epochs is reached. Finally, the final values ofand are returned.
Creating Relational Graphs
We use the English ukWaC111http://wacky.sslmit.unibo.it/doku.php?id=corpora corpus in our experiments. ukWaC is a 2 billion token corpus constructed from the Web limiting the crawl to .uk domain and medium-frequency words from the British National Corpus (BNC). The corpus is lemmatised and Part-Of-Speech tagged using the TreeTagger222www.cis.uni-muenchen.de/~schmid/tools/TreeTagger. Moreover, MaltParser333www.maltparser.org is used to create a dependency parsed version of the ukWaC corpus.
To create relational graphs, we first compute the co-occurrences of words in sentences in the ukWaC corpus. For two words and that co-occur in more than sentences, we create two word-pairs and . Considering the scale of the ukWaC corpus, low co-occurring words often represents misspellings or non-English terms.
Next, for each generated word-pair, we retrieve the set of sentences in which the two words co-occur. For explanation purposes let us assume that the two words and co-occur in a sentence . We replace the occurrences of in by a slot marker X and by Y. If there are multiple occurrences of or in , we select the closest occurrences and measured by the number of tokens that appear in between the occurrences in . Finally, we generate lexical patterns by limiting prefix (the tokens that appears before X in ), midfix (the tokens that appear in between X and Y in ), and suffix (the tokens that appear after Y in ) each separately to a maximum length of tokens. For example, given the sentence ostrich is a large bird that lives in Africa, we will extract the lexical patterns X is a large Y, X is a large Y that, X is a large Y that lives, and X is a large Y that lives in. We select lexical patterns that co-occur with at least two word pairs for creating a relational graph.
In addition to lexical patterns, we generate POS patterns by replacing each lemma in a lexical pattern by its POS tag. POS patterns can be considered as an abstraction of the lexical patterns. Both lexical and POS patterns are unable to capture semantic relations between two words if those words are located beyond the extraction window. One solution to this problem is to use the dependency path between the two words along the dependency tree for the sentence. We consider pairs of words that have a dependency relation between them in a sentence and extract dependency patterns such as X direct-object-of Y. Unlike lexical or POS patterns that are proximity-based, the dependency patterns are extracted from the entire sentence.
We create three types of relational graphs using lexical (LEX) patterns , POS (POS) patterns, and dependency (DEP) patterns as edge labels. Moreover, we consider several popular methods for weighting the co-occurrences between a word-pair and a pattern as follows.
RAW: The total number of sentences in which a pattern co-occurs with a word-pair is considered as the co-occurrence strength .
PPMI: The positive pointwise mutual information between a pattern and a word-pair computed as,
Here, denotes the total number of sentences in which , , and co-occurs. The operator denotes the summation of over the corresponding variables.
LMI: The local mutual information between a pattern and a word pair is computed as,
LOG: This method considers the logarithm of the raw co-occurrence frequency as the co-occurrence strength. It has been shown to produce vector spaces that demonstrate vector substraction-based analogy representations [Pennington et al.2014].
ENT: Patterns that co-occur with many word-pairs tend to be generic ones that does not express a specific semantic relation. Turney Turney_CL proposed the entropy of a pattern over word-pairs as a measure to down-weight the effect of such patterns. We use this entropy-based co-occurrence weighting method to weigh the edges in relational graphs.
We use the semantic word analogy dataset first proposed by Mikolov et al. Milkov:2013 and has been used in much previous work for evaluating word representation methods. Unlike syntactic word analogies such as the past tense or plural forms of verbs which can be accurately captured using rule-based methods [Lepage2000], semantic analogies are more difficult to detect using surface-level transformations. Therefore, we consider it is appropriate to evaluate word representation methods using semantic word analogies. The dataset contains word-pairs that represent word analogies covering various semantic relations such as the capital of a country (e.g. Tokyo, Japan vs. Paris, France), and family (gender) relationships (e.g. boy, girl vs. king, queen).
A word representation method is evaluated by its ability to correctly answer word analogy questions using the word representations created by that method. For example, the semantic analogy dataset contains word pairs such as (man, woman) and (king, queen), where the semantic relations between the two words in the first pair is similar to that in the second. Denoting the representation of a word by a vector , we rank all words in the vocabulary according to their cosine similarities with the vector, . The prediction is considered correct in this example only if the top most similar vector is . During evaluations, we limit the evaluation to analogy questions where word representations have been learnt for all the four words. Moreover, we remove three words that appear in the question from the set of candidate answers. The set of candidates for a question is therefore the set consisting of fourth words in all semantic analogy questions considered as valid (after the removal step described above), minus the three words in the question under consideration. The percentage of correctly answered semantic analogy questions out of the total number of questions in the dataset (ie. micro-averaged accuracy) is used as the evaluation measure.
To evaluate the performance of the proposed method on relational graphs created using different pattern types and co-occurrence measures, we train dimensional word representations () using Algorithm 1. iterations () was sufficient to obtain convergence in all our experiments. We then used the learnt word representations to obtain the accuracy values shown in Table 1. We see that the proposed method obtains similar results with all pattern types and co-occurrence measures. This result shows the robustness of our method against a wide-range of typical methods for constructing relational graphs from unstructured texts. For the remainder of the experiments described in the paper, we use the RAW co-occurrence frequencies as the co-occurrence strength due to its simplicity.
|Method||capital-common||capital-world||city-in-state||family (gender)||currency||Overall Accuracy|
To study the effect of the dimensionality of the representation that we learn on the accuracy of the semantic analogy task, we plot the accuracy obtained using LEX, POS, and DEP relational graphs against the dimensionality of the representation as shown in Figure 2. We see that the accuracy steadily increases with the dimensionality of the representation up to 200 dimensions and then becomes relatively stable. This result suggests that it is sufficient to use dimensions for all three types of relational graphs that we constructed. Interestingly, among the three pattern types, LEX stabilises with the least number of dimensions followed by POS and DEP. Note that LEX patterns have the greatest level of specificity compared to POS and DEP patterns, which abstract the surface-level lexical properties of the semantic relations. Moreover, relational graphs created with LEX patterns have the largest number of labels (patterns) followed by that with POS and DEP patterns. The ability of the proposed method to obtain better performances even with a highly specified, sparse and high-dimensional feature representations such as the LEX patterns is important when applying the proposed method to large relational graphs.
We compare the proposed method against several word representation methods in Table 2. All methods in Table 2 use dimensional vectors to represent a word. A baseline method is created that shows the level of performance we can reach if we represent each word as a vector of patterns in which occurs. First, we create a co-occurrence matrix between words and patterns , and use Singular Value Decomposition (SVD) to create dimensional projections for the words. Because patterns represent contexts in which words appear in the corpus, this baseline can be seen as a version of the Latent Semantic Analysis (LSA), that has been widely used to represent words and documents in information retrieval. Moreover, SVD reduces the data sparseness in raw co-occurrences. We create three versions of this baseline denoted by SVD+LEX, SVD+POS, and SVD+DEP corresponding to relational graphs created using respectively LEX, POS, and DEP patterns. CBOW [Mikolov et al.2013b], skip-gram [Mikolov et al.2013c], and GloVe [Pennington et al.2014] are previously proposed word representation learning methods. In particular, skip-gram and GloVe are considered the current state-of-the-art methods. We learn dimensional word representations using their original implementations with the default settings. We used the same set of sentences as used by the proposed method to train these methods. Proposed method is trained using dimensions and with three relational graphs (denoted by Prop+LEX, Prop+POS, and Prop+DEP), weighted by RAW co-occurrences.
From Table 2, we see that Prop+LEX obtains the best overall results among all the methods compared. We note that the previously published results for skip-gram and CBOW methods are obtained using a 100B token news corpus, which is significantly large than the 2B token ukWaC corpus used in our experiments. However, the differences among the Prop+LEX, Prop+POS, and Prop+DEP methods are not significantly different according to the Binomial exact test. SVD-based baseline methods perform poorly indicating that de-coupling the 3-way co-occurrences between and into 2-way co-occurrences between and is inadequate to capture the semantic relations between words. Prop+LEX reports the best results for all semantic relations except for the family relation. Comparatively, higher accuracies are reported for the family relation by all methods, whereas relations that involve named-entities such as locations are difficult to process. Multiple relations can exist between two locations, which makes the analogy detection task hard.
We proposed a method that considers not only the co-occurrences of two words but also the semantic relations in which they co-occur to learn word representations. It can be applied to manually created relational graphs such as ontologies, as well as automatically extracted relational graphs from text corpora. We used the proposed method to learn word representations from three types of relational graphs. We used the learnt word representations to answer semantic word analogy questions using a previously proposed dataset. Our experimental results show that lexical patterns are particularly useful for learning good word representations, outperforming several baseline methods. We hope our work will inspire future research in word representation learning to exploit the rich semantic relations that exist between words, extending beyond simple co-occurrences.
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