Question: What novel structure do they propose?
Evidence: UTF8gkai With the widespread of electronic health records (EHRs) in recent years, a large number of EHRs can be integrated and shared in different medical environments, which further support the clinical decision making and government health policy formulationBIBREF0. However, most of the information in current medical records is stored in natural language texts, which makes data mining algorithms unable to process these data directly. To extract relational entity triples from the text, researchers generally use entity and relation extraction algorithm, and rely on the central word to convert the triples into key-value pairs, which can be processed by conventional data mining algorithms directly. Fig. FIGREF1 shows an example of entity and relation extraction in the text of EHRs. The text contains three relational entity triples, i.e., $<$咳嗽, 程度等级, 反复$>$ ($<$cough, degree, repeated$>$), $<$咳痰, 程度等, 反复$>$ ($<$expectoration, degree, repeated$>$) and $<$发热, 存在情况, 无$>$ ($<$fever, presence, nonexistent$>$). By using the symptom as the central word, these triples can then be converted into three key-value pairs, i.e., $<$咳嗽的程度等级, 反复$>$ ($<$degree of cough, repeated$>$), $<$咳痰的程度等级, 反复$>$ ($<$degree of expectoration, repeated$>$) and $<$发热的存在情况, 无$>$ ($<$presence of fever, nonexistent$>$).
UTF8gkai To solve the task of entity and relation extraction, researchers usually follows pipeline processing and split the task into two sub-tasks, namely named entity recognition (NER)BIBREF1 and relation classification (RC)BIBREF2, respectively.
However, this pipeline method usually fails to capture joint features between entity and relationship types. For example, for a valid relation “存在情况(presence)” in Fig. FIGREF1, the types of its two relational entities must be “疾病(disease)”, “症状(symptom)” or “存在词(presence word)”. To capture these joint features, a large number of joint learning models have been proposed BIBREF3, BIBREF4, among which bidirectional long short term memory (Bi-LSTM) BIBREF5, BIBREF6 are commonly used as the shared parameter layer. However, compared with the language models that benefit from abundant knowledge from pre-training and strong feature extraction capability, Bi-LSTM model has relatively lower generalization performance.
To improve the performance, a simple solution is to incorporate language model into joint learning as a shared parameter layer. However, the existing models only introduce language models into the NER or RC task separately BIBREF7, BIBREF8. Therefore, the joint features between entity and relationship types still can not be captured. Meanwhile, BIBREF9 considered the joint features, but it also uses Bi-LSTM as the shared parameter layer, resulting the same problem as discussed previously.
Given the aforementioned challenges and current researches, we propose a focused attention model based on widely known BERT language model BIBREF10 to jointly for NER and RC tasks. Specifically, through the dynamic range attention mechanism, we construct task-specific MASK matrix to control the attention range of the last $K$ layers in BERT language model, leading to the model focusing on the words of the task. This process helps obtain the corresponding task-specific context-dependent representations. In this way, the modified BERT language model can be used as the shared parameter layer in joint learning NER and RC task. We call the modified BERT language model shared task representation encoder (STR-encoder) in the following paper.
To sum up, the main contributions of our work are summarized as follows:
We propose a focused attention model to jointly learn NER and RC task. The model integrates BERT language model as a shared parameter layer to achieve better generalization performance.
In the proposed model, we incorporate a novel structure, called STR-encoder, which changes the attention range of the last $K$ layers in BERT language model to obtain task-specific context-dependent representations. It can make full use of the original structure of BERT to produce the vector of the task, and can directly use the prior knowledge contained in the pre-trained language model.
For RC task, we proposed two different MASK matrices to extract the required feature representation of RC task. The performances of these two matrices are analyzed and compared in the experiment.
The rest of the paper is organized as follows. We briefly review the related work on NER, RC and joint entity and relation extraction in Section SECREF2. In Section SECREF3, we present the proposed focused attention model. We report the experimental results in Section SECREF4. Section SECREF5 is dedicated to studying several key factors that affect the performance of our model. Finally, conclusion and future work are given in Section SECREF6.
In the proposed model, we incorporate a novel structure, called STR-encoder, which changes the attention range of the last $K$ layers in BERT language model to obtain task-specific context-dependent representations. It can make full use of the original structure of BERT to produce the vector of the task, and can directly use the prior knowledge contained in the pre-trained language model.
Answer: STR-encoder, which changes the attention range of the last $K$ layers in BERT language model to obtain task-specific context-dependent representations.