Multimodal Action Recognition via Causality-Inspired Graph Representation Learning.

Multimodal human activity recognition (HAR) benefits from complementary skeleton, inertial, and visual observations. However, many learning-based models still treat relationships among modalities, joints, and sensor variables as symmetric associations. This limits their ability to represent asymmetric information flow and can weaken the preservation of modality-specific cues during feature fusion. We propose Causality-Inspired Structure Representation Learning (CSRL), a multimodal HAR framework that uses directional dependency modeling as a structural prior for representation learning. CSRL first estimates transfer-entropy-based graphs from temporal entities, including skeleton joints and IMU sensor variables. These graphs provide asymmetric priors that guide recognition-oriented graph learning in the representation space. CSRL further combines hybrid contrastive learning with an encoder-decoder architecture to learn modality-invariant, modality-specific, and structure-aware representations in a unified framework. This design encourages cross-modal alignment while retaining local motion cues that are important for fine-grained action discrimination. Experiments on five public HAR benchmarks, including UTD-MHAD, MMAct, CZU-MHAD, NTU RGB+D, and NTU RGB+D 120, show that CSRL consistently improves accuracy, F1 score, and recall over competitive supervised and contrastive baselines. These results support TE-guided directional structure modeling as a practical and interpretable prior for multimodal action recognition.