The performance of sentence encoders can be significantly improved through the simple practice of fine-tuning using contrastive loss. A natural question arises: what characteristics do models acquire during contrastive learning? This paper theoretically and experimentally shows that contrastive-based sentence encoders implicitly weight words based on information-theoretic quantities; that is, more informative words receive greater weight, while others receive less. The theory states that, in the lower bound of the optimal value of the contrastive learning objective, the norm of word embedding reflects the information gain associated with the distribution of surrounding words. We also conduct comprehensive experiments using various models, multiple datasets, two methods to measure the implicit weighting of models (Integrated Gradients and SHAP), and two information-theoretic quantities (information gain and self-information). The results provide empirical evidence that contrastive fine-tuning emphasizes informative words.

通过对比损失的简单调优，句子编码器的性能可以得到显著提高。本文理论上和实验上展示出，在对比学习过程中模型获得了怎样的特征，即基于信息论的数量对词汇进行隐式加权。本文还使用多种模型、多个数据集、两种度量模型隐式加权的方法（集成渐变法和SHAP）以及两个信息论数量（信息增益和自信息量）进行了全面实验。结果提供了对对比调优强调信息丰富词汇的经验证据。

基于对比学习的句子编码器隐式加权信息词汇