  Deep learning-based multi-source unsupervised domain adaptation (MUDA) has
been actively studied in recent years. Compared with single-source unsupervised
domain adaptation (SUDA), domain shift in MUDA exists not only between the
source and target domains but also among multiple source domains. Most existing
MUDA algorithms focus on extracting domain-invariant representations among all
domains whereas the task-specific decision boundaries among classes are largely
neglected. In this paper, we propose an end-to-end trainable network that
exploits domain Consistency Regularization for unsupervised Multi-source domain
Adaptive classification (CRMA). CRMA aligns not only the distributions of each
pair of source and target domains but also that of all domains. For each pair
of source and target domains, we employ an intra-domain consistency to
regularize a pair of domain-specific classifiers to achieve intra-domain
alignment. In addition, we design an inter-domain consistency that targets
joint inter-domain alignment among all domains. To address different
similarities between multiple source domains and the target domain, we design
an authorization strategy that assigns different authorities to domain-specific
classifiers adaptively for optimal pseudo label prediction and self-training.
Extensive experiments show that CRMA tackles unsupervised domain adaptation
effectively under a multi-source setup and achieves superior adaptation
consistently across multiple MUDA datasets.
