A dual contrastive learning-based graph convolutional network with syntax label enhancement for aspect-based sentiment classification

Abstract

Introduction: Aspect-based sentiment classification is a fine-grained sentiment classification task. State-of-the-art approaches in this field leverage graph neural networks to integrate sentence syntax dependency. However, current methods fail to exploit the data augmentation in encoding and ignore the syntactic relation in sentiment delivery.

Methods: In this work, we propose a novel graph neural network-based architecture with dual contrastive learning and syntax label enhancement. Specifically, a contrastive learning-based contextual encoder is designed, integrating sentiment information for semantics learning. Moreover, a weighted label-enhanced syntactic graph neural network is established to use both the syntactic relation and syntax dependency, which optimizes the syntactic weight between words. A syntactic triplet between words is generated. A syntax label-based contrastive learning scheme is developed to map the triplets into a unified feature space for syntactic information learning.

Results: Experiments on five publicly available datasets show that our model substantially outperforms the baseline methods.

Discussion: As such, the proposed method shows its effectiveness in aspect-based sentiment classification tasks.

1 Introduction

Aspect-based sentiment classification (ABSC) is a fundamental task in sentiment analysis [1]; [2], which aims to infer the sentiment of a specific aspect in sentences [3]. Generally, the sentiment of each aspect is classified according to a predefined set of sentiment polarities, i.e., positive, neutral, or negative. For example, in the comment “the price is reasonable, although the service is poor,” the sentiment toward aspects “price” and “service” is positive and negative, respectively.

In general, an ABSC process involves two steps: the identification of sentiment information toward the aspect from the context and the classification of the expressed sentiment from predefined sentiment polarities [4]. Comprehensively, the first step contains key contextual information learning and aspect–context word relation establishment. To capture important contextual words and prevent redundant information, recent publications reveal that encoders and attention networks are taken to encode the sequential information and determine the attentive weights of contexts, respectively [5]. Typically, these deep learning methods are trained via a large amount of textual data to improve their working performance. Notwithstanding, the existing manually annotated data resources are still limited, which causes issues such as model overfitting. As a result, the precise capturing of key contextual words remains challenging. More recently, contrastive learning shows its superiority under the condition of limited training samples. Based on data augmentation, both positive and negative samples are generated. By setting contrastive loss of training models, the representations of positive samples are brought closer, while those of negative samples are pushed apart. In line with the contrastive learning, the model training can be improved, which paves a way for key contextual information learning in ABSC tasks.

On the basis of key contextual information, the aspect–opinion word relation mainly lies in syntax dependency of the sentence [6]. With the parsing of syntax dependency, the relation between the aspect and context words is built. Ongoing studies substantially focus solely on the distance of words while neglecting the syntax label of specific context words toward the aspect. That is, all syntactic relations are interpreted as the same. Figure 1 shows the syntax structure of a given sentence. The establishment of the subject–predicate syntactic relation (nsubj) and adjective modifier syntactic relation (attr) plays a dominate role in sentiment classification, especially compared with other syntactic information. Moreover, the syntax label is also the foundation of textual logical reasoning due to its effects in distinguishing the importance among syntactic relations. So much is the significance of the syntax label that it can be further applied to the aspect–opinion word relation establishment in ABSC.

FIGURE 1

To address the above issues in ABSC, we propose a graph convolutional network (GCN) based on dual contrastive learning and syntax label enhancement (i.e., DCL-GCN). First, a contrastive learning-based encoder is devised, which brings the context representations of the same sentiment closer and pushes those of different sentiments apart. Furthermore, a weighted label-enhanced syntactic GCN is put forward, dealing with not only the syntactic relation but also the syntax dependencies among words. Lastly, a contrastive learning scheme that focuses on the sentence syntax label is developed. A syntactic triplet between words is constructed. The same syntax label-based triplets are given similar semantic representations, while different syntax label-based triplets are distinguished. Thereby, the syntax and semantics are integrated, which contributes to the sentiment classification.

2 Related work

Owing to the advancement of deep learning networks, current methods with various structures are widely developed, aiming to identify their superiority in ABSC tasks [7]. ABSC models are devised to deal with either semantics [8], syntax [9], or both [10] from the given text. In this section, these two major issues in the field of ABSC are presented. In order to achieve better working performance, previous work and their findings about these two focuses are dedicatedly investigated and depicted.

2.1 Contextual information learning

One bottleneck in ABSC comes from capturing key contextual words, which considerably affects the aspect–opinion word relation modeling. Much recent work uses neural networks, attention networks, or both to concentrate on useful contextual information [11]; [12]. Tang et al. focused on different contextual parts based on LSTM, targeting at obtaining valuable information [13]. In addition, attention-based neural networks are proposed to discriminate more relevant features toward the aspect [14]; [15]. Sun et al. used a BERT-based model to capture semantic features from contexts via fine-tuning, which significantly improves the working performance [16]. Text encoders are widely applied to various tasks [17,18]. Encouragingly, advances in contrastive learning hold great potential in natural language processing (NLP) tasks. Suresh et al. integrated contrastive learning strategy into the pre-training of Bidirectional Encoder Representations from Transformers (BERT) to improve the model efficacy [19]. A contrastive loss among different input categories is introduced, while a weight network refines the differences between each sample pair. In our work, contrastive learning can be taken to distinguish the contextual representations during sentence encoding.

2.2 Syntax dependency parsing

The parsing of syntax dependency plays a pivotal role in the field of ABSC due to its relation establishment between the aspect and contextual words. Previous work primarily tackles the syntactic relation of either single or multiple word pairs. In recent years, the application of a GCN in NLP gave rise to new opportunities in a number of fields [20]; [21]. Regarding sentence syntax parsing, Sun et al. transformed the syntax dependency into an adjacency matrix and propagated the syntactic information using the GCN [22]. Furthermore, Zhang et al. incorporated the aspect-oriented attention mechanism to benefit the contextual information extraction toward a specific aspect [23]. To extract both aspect-focused and inter-aspect sentiment information, an interactive graph convolutional network (InterGCN) is built to leverage the sentiment dependencies of the context [24]. Wang et al. reconstructed the syntax dependency tree rooted at an aspect. A relational graph attention network (R-GAT) is then proposed to encode the aspect-oriented dependency tree and to establish the syntactic relation between the aspect and its opinion words [25].

3 Methodology

A dual contrastive learning GCN (DCL-GCN) is devised on the task of ABSC. Figure 2 shows the framework of the proposed model. A pretrained BERT model is used as the sentence encoder. A contrastive learning scheme is incorporated into contextual encoding during model training, which enhances the semantic information via sentiment labels to obtain differentiated contextual representations. Then, both the semantic and syntactic features are integrated within a weighted label GCN, aiming at addressing the syntactic relation of context words with the aspect. In line with contrastive learning, the syntax labels of words are used for learning the sentence syntax at a higher level. The sentiment polarity is predicted by sending the final sentence representation into a sentiment classifier. More details of the proposed model are given as follows:

FIGURE 2

3.1 Contextual encoder with contrastive learning

The architecture of the contrastive learning-based contextual encoder is shown in Figure 3. Let X = [w₁, …, w_a, …, w_a+m−1, …, w_n] be a sentence of n words and A = [w_a, …, w_a+m−1] be the aspect of m words within S. The contrastive learning scheme during sentence encoding is implemented via data augmentation, feature extraction, and contrastive loss construction. Inspired by the data augmentation in image recognition [26]; [27], positive samples of the same polarities are generated using synonym substitution and random noise injection. Specifically, synonym substitution refers to randomly replacing words within the sentence with their synonyms from WordNet, while noise injection indicates introducing more aspect words and neutral sentiment words to the sentence. The sentiment is enhanced in (1):

FIGURE 3

The original sentence X and the data-enhanced sentence X^E are mapped to word vectors within the same feature space. We use the BERT model obtained through large-scale corpus training by Kenton et al to enhance the semantics of word representations. We then train the BERT model in a fine-tuned manner by freezing part of its parameters, which is written in (2):where CLS and SEP are BERT tokens representing the overall representation and the separation of the sentence, respectively. We thus obtain the sentence-level feature representation h^CLS, the word-level feature representation H^X, and the aspect feature representation H^A. Assuming that a batch consisting of k sentences is the model input for training, the sentence set composed of the original and the enhanced sentences is , with the corresponding sentiment polarity set denoted as Y^all = [Y₁, Y₂, …, Y_2k]. We also have the index set of all sentences as I = [1, 2, …, 2k]. For each sentence in X^all, a set of contrastive learning-based sentences with the same sentiment polarity is generated, i.e., P^all = [P₁, P₂…, P_2k], where P_i = {p: p ∈ I (Y_p = Y_i) ∧ (p ≠ i)}. The contrastive learning loss of the contextual encoder is defined in (3):where τ is a hyperparameter, indicating the temperature coefficient of contrastive learning. The higher the temperature coefficient is, the smaller the sum of the loss reaches. The parameter stands for the representation of the ith sentence in X^all after BERT coding. In such a manner, the context representations of the same sentiment can be brought closer, and those of different sentiments are separated, improving the use of contextual and sentiment labels. Based on contrastive learning, abundant semantic information is integrated into the encoder, targeting at deriving context representation with key information.

3.2 Weighted label-enhanced syntactic GCN

The framework of the syntactic GCN via weighted label enhancement is presented in Figure 4. The syntax dependency of the input sentence is derived using the spaCy toolkit. Specifically, the sentence syntax dependency is characterized by a triplet, i.e., (w_i, w_j, r_i,j), where words w_i and w_j are of the relation r_i,j. In line with the sentence syntax, we construct a syntax adjacency matrix that denotes the connecting edges of the syntax dependency tree.

FIGURE 4

To address the effects of various syntactic labels in the sentiment classification, a syntax label learning (SLL) unit is built. The main purpose of the SLL unit is to transform the syntax label matrix to a learnable syntax label score matrix.

A lexicon R = {relation1: 1, relation2: 2, …, relationt: t} that consists of all syntactic relations from the corpus is constructed, from which each syntactic label is mapped to an index number. For each input sentence, all index numbers denote the syntactic relations consisting of a syntax label matrix. Then, the syntax label adjacency matrix is built based on both the syntax dependency tree and the lexicon R. All syntax labels can be mapped into a unified feature space. The weighted score of each syntactic relation is thus resolved in (4), which is written as a syntax label score adjacency matrix :where Emb (⋅) represents transforming the syntax label matrix into a learnable matrix for syntax label characterization, and are learnable parameter matrices, and d_L and d_S are dimensions of A^L and A^S, respectively. We also have , with d_LS standing for the dimension of the syntax label score space.

Likewise, the same syntactic relation type can have different degrees of importance within different semantic contexts. For this reason, the semantics among words are also integrated into the computation of the syntax label score. We take the multi-head self-attention (MHSA) mechanism to learn the semantic features and to revise the syntax label scores based on attentive weights. Notably, the elements in A^LS represent all the syntactic relation scores, which are not zero. To preserve the original syntax dependencies and remove irrelevant syntactic information, the basic syntax adjacency matrix is also used. The weighted syntax label adjacency matrix can be computed in (5):with (6, 7, and 8)where is the syntax adjacency matrix of w_i and w_j parsed from the syntax dependency tree; is the syntax label score adjacency matrix; stands for the semantic weight adjacency matrix derived from the MHSA mechanism; Concat represents the vector concatenation; W^head is the parameter matrix during concatenation; Norm (⋅) is the normalization operation on the attentive weight matrix; and are parameter matrices of the pth attention head in MHSA; d_head denotes the vector dimensions of each head; and h is a hyperparameter indicating the attention head number.

The working principle of the weighted label-enhanced syntactic GCN is shown in Figure 5. The input of the GCN is the weighted syntax label adjacency matrix A^WL and the feature representation H^X from BERT. The learning of syntactic information is derived in (9):where , refers to the word vector of the jth word in the lth layer of the GCN, with l as an integer and l ∈ [0, F], F is the layer number of the GCN, is the learnable parametric matrix of the lth layer, is the bias vector, and σ is an activation function. The output of the weighted label-enhanced syntactic GCN is the output of the last layer, i.e., .

FIGURE 5

3.3 Syntax label-based contrastive learning scheme

Considering the effect of syntactic information in ABSC, the node pairs with the same syntax label indicate similar syntactic features, and those with different syntax labels have differentiated features. As such, a contrastive learning scheme using syntax labels is proposed, aiming to enhance the learning of syntactic features at a higher level.

Assuming that K′ triplets are of syntax dependencies within all the K sentences, the node-pair set of these triplets is . The syntax label set of these node pairs is R′ = [r₁, r₂, …, r_K′] with the index set I′ = [1, 2, …, K′]. Moreover, for each node pair in X′, a set of node pairs with the same syntax label for contrastive learning is constructed, i.e., . The syntax label-based contrastive learning loss is defined in (10):

together with (11)where τ′ is the temperature coefficient for contrastive learning and g_m’ represents the semantic feature representation by mapping the node-pair representations from the syntax dependency triplet and is normalized before the contrastive learning loss computation. We define as the feature representation of the first node in the m′th node pair in X′ and as the feature representation of the second node in the m′th node pair. Both and are obtained from the BERT encoder, which convey semantic information. In addition, , , and are learnable parameter matrices, and b^cl is a bias vector.

3.4 Feature fusion

Average pooling is performed on H^out to obtain the syntactic information-enhanced feature representation H^out in (12), which is further concatenated with h^CLS derived from the BERT encoder. The final sentence representation is given in (13).where ⊕ denotes the concatenation operation. The final sentence representation is sent to a Softmax classifier to obtain the sentiment polarity in (14):

The pseudocode of the proposed model is given as follows:

3.5 Model training

Model training is implemented using cross-entropy and regularization as the loss function in (15):where (x, a) represents the vector of a sentence–aspect pair; C refers to the set of sentiment classes; is the ground-truth sentiment distribution of (x, a) with sentiment C, and is the predicted one; and λ is the coefficient of regularization.

On account of the training of contrastive learning in our model, the total loss function is composed of the contrastive learning loss from the contextual encoder, and the contrastive learning loss L_LCL based on the syntax label and the cross-entropy loss L_CE is shown in (16):where α is a learnable coefficient to adjust the weights of contrastive learning losses in loss function.

4 Experiment

4.1 Experimental setup

The working performance of the DCL-GCN is evaluated on five benchmark datasets, which are Restaurant 14, Restaurant 15, Restaurant 16, and Laptop 14 from SemEval [28]; [29,30], and Twitter [31]. The sentiment of each aspect from the datasets is labeled as positive, neutral, or negative.

Following the idea of [15], the sentences labeled as conflicting sentiment or without explicit aspects from Restaurant 15 and Restaurant 16 are removed. Details of each dataset are given in Table 1.

In this experiment, the lexicon size of the BERT model is set to 30,522, the word embedding dimension is 768, and the layer number of the transformer is 12. The head number of the MHSA is 8, and the learning rate is 0.00001. The layer number of the weighted label-enhanced syntactic GCN is 2. Both τ and τ′ in contrastive learning schemes are set to 0.02. The regularization coefficient is 0.00001. An Adam optimizer is adopted during training with a data batch size of 32. All the hyperparameters used in the experiment are given in Table 2.

4.2 Baseline

All results are expressed in percentage values. “-” denotes that the results are not reported in the published research article. The best performance achieved is marked in bold.

4.3 Result analysis

We take two metrics, i.e., accuracy and Macro-F1, to evaluate the working performance of the proposed model. Table 3 shows the results of six different methods on the task of ABSC. One can observe that our model achieves the best and most consistent result among all the evaluation settings. It is clear that the DCL-GCN result is more remarkable than a range of competitive baselines on all five benchmark datasets. In line with these results, the following observations are made.

First, our model achieves the best and most consistent result among all the evaluation settings. The minimum performance gaps between the DCL-GCN and the baselines are 1.33% (against the R-GAT) on Restaurant 14, 1.62% (against the T-GCN) on Restaurant 15, 1.33% (against the T-GCN) on Restaurant 16, 1.54% (against DGEDT) on Laptop 14, and 0.22 (against DGEDT) on Twitter. In addition, the F1 values on Restaurant 15 and Restaurant 16 are 3.64% (against the T-GCN) and 5.04% (against DGEDT), respectively, higher than the best-performing baseline method, which are significant.

Second, the syntax-dependent-method (BERT4GCN) performs worse than models integrated with both syntax dependency and syntactic relations (R-GCT and T-GCN). The main reason is that the deeper-level syntactic information can be neglected by solely exploiting the dependencies among words. By contrast, the syntactic relation encoded in our model benefits the sentiment comprehending to a large extent. The highest accuracy of our model reaches 93.65 on Restaurant 16, indicating the importance of syntax dependency and syntactic relations in ABSC.

Third, compared with other baselines, the basic BERT model has its own distinctiveness in tackling sentence semantic information. By incorporating BERT into state-of-the-art methods, the working performance is substantially improved, which is the outcome of our model. Notably, the proposed model significantly outperforms the baselines, demonstrating that the contextual semantics take full advantage in line with the BERT-based contrastive learning scheme.

It is worth noting that the DCL-GCN gives rise to the enhancement in both syntax and semantics learning. With the application of the dual contrastive learning scheme, it is reasonable to expect better working performance in ABSC, as it is the case.

4.4 Ablation study

The impact of different components in our model is investigated by conducting an ablation study (Table 4). w/o L_ECL specifies that the contrastive learning scheme of the contextual encoder is removed; w/o L_LCL specifies that the syntax label-based contrastive learning scheme is removed; and w/o WL-GCN indicates that the weighted label-enhanced syntactic GCN is ablated.

As presented in Table 4, the most significant module in our model is the weighted label-enhanced syntactic GCN. The exploiting of syntactic information shows its effectiveness in word sentiment learning. With the sole utilization of semantics, even with a contrastive learning strategy, the working performance is inferior to the syntactic-based methods in all evaluation settings. Clearly, the integration of semantics and syntax has superiority in ABSC tasks. Moreover, the removal of the contrastive learning scheme from the contextual encoder leads to a substantial decrease on all five datasets. The performance decreases of the accuracy and F1 score on Twitter are 2.76% and 2.25%, respectively. As a result, the contrastive learning scheme in the BERT encoder effectively promotes semantic information learning. By contrast, the syntax label-based contrastive learning scheme makes a relatively small contribution to the model. We can infer that the application of syntax labels also enhances the use of syntactic information and, thus, contributes to the sentiment classification.

4.5 Impact of hyperparameters

An experiment is carried out to analyze the effect of the self-attention head number on model working performance. The head number of the self-attention network is set to [1, 2, 3, …, 8]. The model accuracy with different head numbers is presented in Figure 6.

FIGURE 6

Apparently, the DCL-GCN achieves the highest accuracy with a head number of 5 on Laptopt 14 and Restaurant 15 and a head number of 6 on Twitter, Restaurant 14, and Restaurant 16. In line with the multi-head self-attention mechanism, the attention head stands for the vector representation in feature spaces via different mapping methods. When the number of attention heads is reduced, the self-attention mechanism operates within a smaller space with correspondingly fewer semantic features. Accordingly, the proposed model fails to capture sufficient semantic information. On the other hand, when the head number exceeds 6, the model parameter size significantly increases, resulting in overfitting issues during training. In this way, a test accuracy decrease is inevitable.

4.6 Case study

Two samples are selected to visualize the working performance, in order to further validate the distinctiveness of DCL-GCN. Specifically, the representations of the sentence and the words are maintained. We shall define a parameter φ as the contribution of each word for sentiment delivery in the sentence, which is defined in Eq. 17:

The sentiment contribution of each word is shown in Figure 7. For the sample given in Figure 7A, the contextual words “professional,” “courteous,” and “attentive” make the largest contribution toward the aspect “waiters.” Our model is capable of extracting the most informative words for sentiment expressing. The sentence in Figure 7B contains two aspects, i.e., “food” and “waiting.” For the aspect word “food,” the proposed model accurately identifies the top two highest sentiment contribution words as “good” and “so.” Regarding “waiting,” not only is the the sentiment word “nightmare” captured but also the syntactic relation words “so…that…” for resultative adverbial clause establishment. Both semantics and syntax are used for sentiment classification.

FIGURE 7

In our model, the use of contrastive learning enhances the learning of sentence semantics, and the build of the weighted label-enhanced syntactic GCN fully exploits the syntactic information. The integration of semantic information and syntactic information leads to a competitive manner in ABSC.

5 Conclusion

In this work, we propose a GCN based on dual contrastive learning and syntax label enhancement for ABSC tasks. To obtain sentiment information, a contrastive learning scheme is integrated to a BERT encoder to enhance the learning of semantic-related contextual information. Then, our model exploits both the syntax dependency and syntactic relation, based on which a weighted label-enhanced syntactic GCN is established. In addition, the learning of the syntax label is enhanced using contrastive learning. A syntactic triplet between words is mapped into a unified feature space for syntax and semantic integration. The rxperimental results reveal that the proposed model achieves state-of-the-art performance on five benchmark datasets. The ablation study, the hyperparameter analysis experiment, and the case study also obtain superior working performance.

Future work will focus on introducing more information for further improving the accuracy of ABSC and other sentiment analysis tasks, such as background knowledge and part-of-speech information. In addition, the integration of different categories of information into the model is also considered.

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