A Theory-Driven Self-Labeling Refinement Method for Contrastive Representation Learning
Authors: Pan Zhou, Caiming Xiong, Xiaotong Yuan, Steven Chu Hong Hoi

For an image  query, unsupervised contrastive learning  labels crops of  the same image as positives,  and other image crops as  negatives. Although intuitive, such a native label assignment strategy cannot reveal the underlying semantic similarity between a  query and  its positives and negatives, and impairs performance,  since some negatives are  semantically similar to  the query or even share the same semantic class as the query.  In this work, we first  prove that for  contrastive learning,  inaccurate label assignment heavily  impairs its generalization for semantic instance discrimination, while accurate labels  benefit its generalization.  Inspired by this theory, we  propose   a novel self-labeling refinement approach for contrastive learning. It improves the label quality via two complementary  modules:  (i)  self-labeling refinery (SLR) to  generate accurate labels and (ii)  momentum mixup (MM)  to enhance similarity between query and its positive. SLR uses a positive of a query to estimate  semantic similarity between  a query and its positive and negatives, and  combines estimated similarity with  vanilla label assignment in contrastive learning to  iteratively generate  more accurate and informative soft labels. We theoretically show that our SLR can exactly recover the true semantic  labels of  label-corrupted  data, and  supervises   networks to achieve zero prediction  error on classification tasks.  MM randomly  combines   queries and  positives to increase  semantic similarity between the generated virtual queries and their positives so as to improves label accuracy.  Experimental results on CIFAR10,  ImageNet, VOC and COCO show the effectiveness of our method.